Cipher apparatus for multiplex pulse code modulation systems

Abstract

23. In a secret communication system, the combination of: a source of an intelligence signal wave and a source of a noise wave; means for combining said waves; means for periodically sampling said combined wave at predetermined intervals; means for developing from the samples thus obtained a first pulse train comprising sets of marks and spaces; means for randomly developing in accordance with the noise components accompanying said intelligence signal wave a second pulse train of marks and spaces; means for combining said first and second pulse trains to develop a third pulse train; means for transmitting and receiving said third pulse train; means for developing from said third pulse train a fourth pulse train substantially corresponding with said first pulse train, said means including synchronizing means responsive substantially solely to the components of said third pulse train corresponding to said noise wave; and means for developing from said fourth pulse train an output signal wave substantially corresponding with said intelligence signal wave.

Claims

What is claimed is:

1. In a pulse communication system apparatus for generating cipher key pulses for combination with unciphered intelligence conveying pulses comprising, pulse generating apparatus, apparatus for selecting predetermined unciphered intelligence conveying pulses, means for controlling said pulse generating apparatus in accordance with the character of the selected pulses, and equipment for selecting a time interval subsequent to the time of said selected pulses for rendering the selected pulses effective to control said pulse generating apparatus.

2. In a pulse communication system, ciphering apparatus comprising equipment for generating a series of key pulses in sequence, each pulse of which may be of anyone of a plurality of different conditions, equipment for combining the key pulses with signaling pulses to form ciphered pulses, apparatus responsive to selected signaling pulses for controlling the operation of said key pulse generating equipment, deciphering apparatus for generating a second series of key pulses identical with said first series of key pulses, equipment for combining said second series of key pulses with said ciphered pulses to produce deciphered pulses, other apparatus for controlling said equipment for generating a second series of key pulse in accordance with said deciphered pulses.

3. In a pulse signaling system, a transmitting station, a receiving station, a cipher key generator at the transmitting station for generating cipher key pulses comprising a first plurality of double stability circuits or devices arranged to form a continuous ring circuit, a second plurality of similar double stability circuits or devices arranged to form a second ring circuit, apparatus responsive in part at least to pulses representing noise currents for independently advancing the conditions of stability in each of said ring circuits, circuit by circuit, one at a time and in succession, means responsive to the condition of said double stability circuits of each of said rings to form a series of cipher key pulses, apparatus for combining said cipher key pulses with information conveying pulses to form enciphered pulses, means for conveying said enciphered pulses to said receiving station, similar apparatus for generating an identical series of key pulses at the receiving station, equipment for combining said identical series of key pulses with said enciphered signaling pulses to obtain the deciphered or unciphered signaling pulses therefrom.

4. In a pulse signaling system a first station, a second station, apparatus at each of said stations for generating identical series of cipher key pulses comprising a first ring circuit, a second ring circuit each of which comprises a plurality of devices actuated in sequence one after another, means for independently advancing the conditions of actuation of each of said ring circuits, an output circuit, a plurality of settable members for connecting each of said devices with said output circuit, equipment requiring the settable devices at both said transmitting and receiving stations to be set in identical conditions, said equipment comprising means at each station responsive to the advance of at least two of the ring circuits thereat to a predetermined condition of each for controlling the further advance of one of said ring circuits for a predetermined interval of time.

5. In a pulse communication system a first station, a second station, apparatus at said first station for generating a series of key pulses occurring in succession each of which pulses may be any one of a plurality of different signaling conditions and the character of each of which is determined in an essentially random manner, a plurality of settable members for controlling in part at least the character of the successive key pulses, apparatus responsive to the signaling pulses and to said key pulses for transmitting a series of ciphered pulses to said second station, apparatus for generating a second series of key pulses identical with said first series of key pulses including a similar plurality of settable members, and apparatus for combining said ciphered pulses with said second series of key pulses to derive pulses representing the information which is represented by said signaling pulses at said first station.

6. In a pulse communication system a first station, a second station, apparatus at said first station for generating a series of pulses occurring in succession, each of which pulses may be any one of a plurality of different signaling conditions the character of which is determined in an essentially random manner, a plurality of settable members for controlling in part at least the character of the successive key pulses, apparatus responsive to the signaling pulses and to said key pulses for transmitting a series of enciphered pulses to said second station, apparatus for generating a second series of key pulses identical with said first series of key pulses including a similar plurality of settable members, apparatus combining said enciphered pulses with said second series of key pulses to derive pulses representing the information which is represented by said signaling pulses at said first station, apparatus requiring the said settable members at both of said stations to be set in identical conditions to generate identical key pulses comprising equipment responsive to the advance of said devices in at least two of said ring circuits at each of said stations to a predetermined condition for controlling the advance of at least one of said ring circuits.

7. The method of ciphering speech signals which comprises the steps of (1) sampling of speech waves at recurring instants of time, (2) representing the magnitudes of said samples by permutation code pulses, (3) generating cipher key pulses under control of predetermined ones of said permutation code pulses representing speech, and (4) combining the cipher pulses with said code pulses.

8. The method of ciphering speech signals which comprises the steps of (1) sampling of speech waves at recurring instants of time and (2) representing the magnitudes of said samples by permutation code pulses, (3) generating cipher key pulses under control of predetermined ones of said permutation code pulses representing speech (4) combining the cipher pulses with said code pulses, (5) transmitting the resultant ciphered pulses, combining a second series of identical key pulses with them to recover speech pulses at the receiving station, (6) controlling the generation of the key pulses at the receiving station by the deciphered pulses, and (7) reconstructing the speech wave from the deciphered pulses.

9. In a speech communication system, apparatus for sampling speech waves at recurring instants of time, other equipment for representing the magnitude of each sample by means of pulses of a permutation code similar to a binary number, ciphering equipment comprising means for generating a series of cipher key pulses, equipment responsive to the permutation code pulses representing the units digits of the corresponding binary number for controlling the character of pulses generated by said cipher key generating equipment, and means for combining said cipher key pulses with said permutation code pulses to form enciphered pulses.

10. In a speech communication system, apparatus for sampling speech waves at recurring instants of time, other equipment for representing the magnitude of each sample by means of pulses of a permutation code similar to a binary number, ciphering equipment comprising means for generating a series of cipher key pulses, equipment responsive to the permutation code pulse representing the units digits of the corresponding binary number for controlling the character of pulses generated by said cipher key generating equipment and means for combining said cipher key pulses with said permutation code pulses to form enciphered pulses, deciphering equipment comprising equipment for generating a second series of key pulses identical to the first said series of key pulses, means for combining said second series of key pulses with said enciphered pulses to obtain deciphered pulses, means for controlling the character of said cipher key pulses in accordance with selected deciphered pulses, and other equipment for reconstructing the speech wave from said deciphered pulses.

11. A secrecy system comprising two cipher key generators located one remote from the other and each comprising apparatus for generating cipher key signals, means responsive to signaling currents to be enciphered for controlling the character of the cipher key signals generated by one of said devices, means for combining said key pulses and said signaling pulses to form enciphered pulses, means for combining said enciphered pulses with key pulses from said second cipher key generator for obtaining deciphered pulses, and means for controlling the character of the pulses generated by said second cipher key generator in accordance with the deciphered pulses corresponding to the pulse employed to control said first cipher key generator.

12. A secrecy system comprising two cipher key generators located one remote from the other each comprising apparatus for generating cipher key pulses, a plurality of settable elements associated with each of said cipher key generator, means responsive to permutation code signaling pulses for controlling the character of the cipher key pulses generated by one of the said devices, means for combining said key pulses of said signaling pulses to form enciphered pulses, means for comibining said enciphered pulses with key pulses from said second key generator for obtaining deciphered pulses, means for controlling the character of the pulses generated by said second cipher key generator in accordance with the deciphered pulses corresponding to the pulses employed to control said first cipher key generator, equipment associated with each of said key generators for controlling the operation of said key generators when they are at a predetermined condition whereby the settable elements of each of said generators must be positioned identical in order to secure identical key codes therefrom.

13. The method of ciphering speech signals which comprises the steps of (1) sampling of speech waves at recurring instants of time and (2) representing the magnitudes of said samples by permutation code groups of pulses, (3) apparatus for generating cipher key pulses under control of predetermined ones of said permutation pulses representing speech and (4) combining the cipher pulses with the speech pulses.

14. The method of ciphering speech signals which comprises the steps of (1) sampling of speech waves at recurring at instants of time and (2) representing the magnitudes of said samples by binary permutation code groups of pulses, (3) apparatus for generating cipher key pulses under control of predetermined ones of the pulses representing speech, (4) combining the cipher pulses with said permutation code groups of pulses (5) transmitting the enciphered pulses and combining a second series of identical key pulses with them to recover the binary permutation code groups of pulses at the receiving station, (6) controlling the generation of the key pulses at the receiving station by the deciphered pulses and (7) reconstructing the speech wave from the deciphered pulses.

15. In a speech communication system, apparatus for sampling speech waves at recurring instants of time, other equipment for representing the magnitude of each sample by means of permutation code groups of pulses in which each pulse of one character represents a portion of the magnitude of the sample, ciphering equipment comprising means for generating a series of cipher key pulses, equipment responsive to the character of the permutation code pulse representing the smallest portion of the magnitude of the sample for controlling the character of pulses generated by said cipher key generating equipment, and means for combining said cipher key pulses with said permutation code pulses to form enciphered pulses.

16. In a speech communication system, apparatus for sampling speech waves at recurring instants of time, other equipment for representing the magnitude of each sample by means of permutation code groups of pulses in which each pulse of one character represents a portion of the magnitude of the sample, ciphering equipment comprising means for generating a series of cipher key pulses, equipment responsive to the permutation code pulse representing the smallest portion of the magnitude of the sample for controlling the character of the pulses generated by said cipher key generating equipment and means for combining said cipher key pulses with said permutation code pulses to form enciphered pulses, deciphering equipment comprising equipment for generating a second series of key pulses identical to first said series of key pulses, means for combining said second series of key pulses of said enciphered pulses to obtain deciphered pulses, means for controlling the character of said cipher key pulses in accordance with selected deciphered key pulses and other equipment for reconstructing a speech wave from said deciphered pulses.

17. In a speech communication system, apparatus for sampling speech waves at recurring instants of time, other equipment for representing the magnitude of each sample by means of pulses of a permutation code, ciphering equipment comprising means for generating a series of cipher key pulses, equipment responsive to the character of a predetermined pulse of predetermined ones of said code groups which predetermined pulse changes in character in the most random manner of the pulses of each code group for controlling the character of pulses generated by said cipher key generating equipment, and means for combining said cipher key pulses with said permutation code pulses to form enciphered pulses.

18. In a speech communication system, apparatus for sampling speech waves at recurring instants of time, other equipment for representing the magnitude of each sample by means of permutation code groups of pulses, ciphering equipment comprising means for generating a series of cipher key pulses, equipment responsive to the character of a predetermined pulse of predetermined one of said code groups controlling the character of pulses generated by said cipher key generating equipment and means for combining said cipher key pulses with said permutation code pulses to form enciphered pulses, and the deciphering equipment comprising equipment for generating a second series of key pulses identical to first said series of key pulses, means for combining said second series of key pulses with said enciphered pulses to obtain deciphered pulses, means for controlling the character of said cipher key pulses in accordance with selected deciphered pulses, and other equipment for reconstructing a speech wave from said deciphered pulses.

19. In a system for communicating by means of successive groups of two-valued signal pulses, each group representing the instantaneous value of the intelligence wave to be transmitted, a reentry circuit for combining the signal pulses with two-valued cipher key pulses to produce enciphered pulses to be transmitted, and means for generating said cipher key pulses comprising a plurality of electronic ring circuits each comprising a different number of stages of double stability circuits, a driving source for causing said rings to step at the rate of the signal pulses, means responsive to a predetermined pulse of the signal pulse group for separately interrupting the stepping of said rings, means for deriving pulses of one of said two values from certain of the stages of the individual rings and pulses of the other of said two values from other stages, and reentry means to combine the pulses from the rings to produce the cipher key pulses.

20. In a system in accordance with claim 19 means responsive to the positioning of said rings on a corresponding predetermined stage of each for blocking the stepping of one of said rings.

21. In a secret communication system, the combination of an intelligence signal wave including random noise components, means for periodically sampling said wave at predetermined intervals, means for developing from the samples thus obtained a first pulse train comprising sets of marks and spaces, means for randomly developing in accordance with the noise and intelligence components of said wave a second pulse train, means for combining said first and second pulse trains to develop a third pulse train, means for transmitting and receiving said third pulse train, means for developing from said third pulse train a fourth pulse train substantially corresponding with said first pulse train, and means for developing from said fourth pulse train an output signal wave substantially corresponding with said intelligence signal wave.

22. In a secret communication system, the combination of a source of an intelligence signal wave and a source of a noise wave, means for combining said waves and compressing said combined wave, means for periodically sampling said compressed combined wave at predetermined intervals, means for developing from the samples thus obtained a first pulse train comprising sets of marks and spaces, means for randomly developing in accordance with the noise components accompanying said intelligence signal wave a second pulse train, means for combining said first and second pulse trains to develop a third pulse train, means for transmitting and receiving said third pulse train, means for developing from said third pulse train a fourth pulse train substantially corresponding with said first pulse train, means for developing from said fourth pulse train an output signal wave, and means for expanding said output signal to provide a wave substantially corresponding with said intelligence signal wave.

23. In a secret communication system, the combination of: a source of an intelligence signal wave and a source of a noise wave; means for combining said waves; means for periodically sampling said combined wave at predetermined intervals; means for developing from the samples thus obtained a first pulse train comprising sets of marks and spaces; means for randomly developing in accordance with the noise components accompanying said intelligence signal wave a second pulse train of marks and spaces; means for combining said first and second pulse trains to develop a third pulse train; means for transmitting and receiving said third pulse train; means for developing from said third pulse train a fourth pulse train substantially corresponding with said first pulse train, said means including synchronizing means responsive substantially solely to the components of said third pulse train corresponding to said noise wave; and means for developing from said fourth pulse train an output signal wave substantially corresponding with said intelligence signal wave.

Description

This invention relates to communication systems, and more particularly to communication systems capable of transmitting complex waves, such as are employed in the transmission of speech, telegraphy, picture transmission, etc., over a microwave transmission signaling path including such paths operating at the highest frequencies wherein they possess quasi-optical properties.

More specifically, this invention relates to such communication systems in which the intelligence or communication signals are conveyed by permutation code groups of pulses, each pulse being of any one of a plurality of different types of signaling conditions.

An object of this invention is to provide improved methods and apparatus for ciphering such pulses before transmission to render the information conveyed by them secret and then to decipher pulses at the receiving terminal to recover the information conveyed by them.

Another object of this invention is to provide a circuit for generating a cipher key of a series of pulses which may be combined with pulses of the type referred to above to render the ciphered pulses unintelligible and incapable of being decoded by the usual decoding equipment.

Another object of this invention is to provide an improved deciphering arrangement and methods for deciphering ciphered pulses and recovering the original code combinations and then the original intelligence conveyed by these code combinations.

Another object of this invention is to employ selected ones of the code pulses for controlling the operation of the key equipment both at the transmitting and receiving stations.

Another object of this invention is to employ noise currents to control the operation of the keyer and thus further extend the irregularity of the operation of the keying equipment and to increase the random character of the key code and reduce any repetitive tendency.

Another object of this invention is to employ noise to mask the key pulses during idle periods in the transmission over the system and thus increase the difficulty of unauthorized persons attempting to decipher the signals.

In the exemplary embodiment of this invention described herein the equipment for generating a substantially random but reproducible key for ciphering and deciphering the signals conveyed over the transmission medium has been designed to cooperate with and forms a part of the exemplary system described in the copending application of J. O. Edson--T. F. Gleichmann--C. O. Mallinckrodt, filed on even date herewith. Numerous features of that system described herein but not claimed herein are claimed in said copending application.

Briefly, this equipment for generating ciphered key pulses comprises a plurality of electron conduction devices arranged in the form of ring circuits comprising a plurality of stages and control equipment for stepping these ring circuits from one stage or position to the next. The stepping of the rings is substantially independent one from the other and under control of the various portions of the system described in the above-identified copending application of Edson--Gleichmann--Mallinckrodt.

A feature of this invention relates to means for selecting certain of the code pulses for controlling the stepping of either or any of the ring circuits employed.

Another feature of the invention relates to suitable members for connecting the various elements of each ring to an output circuit in different manners.

Another feature of the invention relates to improved methods and equipment for combining the outputs of the respective ring circuits.

Another feature of the invention relates to equipment for storing certain of the code pulses selected for controlling the stepping equipment and then controlling the stepping of the rings during a later selected time interval.

Another feature of this invention relates to improved apparatus and methods of automatically synchronizing the equipment for generating the key pulses at the transmitting and receiving stations.

Another feature of this invention relates to improved methods and apparatus for insuring that the settable members associated with the key equipment at the transmitting and receiving stations have to be exactly the same. This equipment is referred to herein as coincidence equipment and controls the stepping of one or the other of the ring circuits when both rings arrive simultaneously at a given position.

Other features of this invention relate to improved methods and apparatus for generating shaping, limiting and otherwise controlling pulses of short duration.

Inasmuch as the cooperation between the key equipment and the complete system described in the above-identified copending application is involved and requires cooperation between numerous of the elements of said complete system including synchronization, it is necessary to understand the operation of substantially all of the elements of the complete system in order to understand the operation of the key generating equipment in accordance with the present invention and to further understand the manner in which this equipment cooperates with the elements of the complete system. A description of the complete system has accordingly been included herein so that the various features and objects of the present invention may be more readily and more fully understood, it being understood, of course, that novel features thereof are pointed out in the claims appended hereto and further that these novel features are not in any way limited by the description or disclosure of a specific exemplary system herein embodying the present invention.

More specifically, the details of the exemplary ciphered key generating equipment in accordance with the present invention is shown in FIGS. 12 through 18 and 32 through 38 of the present application and of said copending application of Edson--Gleichmann--Mallinckrodt. Other features of the exemplary system described herein which may be novel but are not claimed in this application are claimed in said copending application of Edson--Gleichmann--Mallinckrodt.

Briefly in accordance with the exemplary system embodying the present invention provision has been made for the transmission of eight secured voice frequency communication channels over a microwave radio communication path which may or may not employ one or more relay repeater stations. One of these eight channels is employed for synchronizing purposes leaving seven channels for voice frequency currents. Each or all of these voice frequency channels may be employed to transmit voice frequency carrier telegraph currents, thus providing a large number of telegraph channels. Inasmuch as the voice frequency telegraph transmission equipment operates in its usual manner in cooperation with the other equipment of the exemplary system described herein, the operation of this telegraph equipment will not be described in detail herein.

It is to be understood that all of the voice frequency channels do not necessarily have to be equipped or provided with input signals. Neither is it necessary that signals be sent continuously over any or all of the voice frequency secured channels. Noise currents are however continuously applied to each of the incoming channels at the transmitting stations including the synchronizing channel, for reasons of security as will be explained hereinafter.

The system described is arranged to provide either two-wire or four-wire terminations for each of the incoming channels of communication. In case the incoming transmission is on a so-called two-wire basis the terminating equipment separates the incoming and outgoing transmission and conveys the incoming transmission to the radio transmitter and provides a transmission path from the radio receiving equipment to the two-wire terminating equipment. This terminating equipment may extend through any suitable types of communication, transmission and switching equipment including open wire lines, cable conductors, toll lines and switching equipment manual switching centers, as well as automatic switching centers. It may include repeaters, level or gain regulators, equalizing equipment, phase compensating and regulating apparatus, radio transmission paths, etc. which are known in the art. Inasmuch as all of this equipment operates in its usual manner in combination with the exemplary system described herein a detailed description of the operation of this interconnecting equipment is not repeated herein.

In case the terminations of the incoming channels are on a four-wire basis the transmitting pair is extended to the transmitting equipment and the receiving pair extended to the receiving equipment, thus providing in both the four-wire and two-wire cases a two-way communication path between each of the channels and two ends of the system.

The transmission of the signals will be described in detail in only one direction through the system and the system is shown in detail for only one direction of transmission because a substantially duplicate system is provided for transmission in the opposite direction which operates in substantially the same manner so that the complete operation of the system may be readily understood from a detailed description of the operation for the transmission of messages through the system in one direction.

The incoming signals after being conveyed through the equipment terminating the incoming channels pass first through a low-pass filter which eliminates frequencies not necessary in transmitting the incoming voice channel and thus prevents these frequencies from interfering with the operation of the system. From the low-pass filter the signals are transmitted through a combined amplifier and instantaneous compressor which compresses the range of signal amplitudes. In other words, the amplitude range of the signals is decreased by the compressor equipment.

From the compressor the signals are transmitted through a pulse amplitude modulator. This pulse amplitude modulating equipment is a combined multiplexing and modulating equipment and is controlled from a crystal oscillator through the five-stage electronic ring which is the equivalent of a high speed five-segment distributor and the eight-stage ring which is the equivalent of a high speed eight-segment distributor. The eight-stage ring runs at a frequency of about 8,000 cycles or revolutions per second. During each revolution or cycle of the eight-stage ring which is 125 microseconds long, a time interval of approximately 15.6 microseconds is assigned to each one of the incoming channels. The output terminals of each of the pulse amplitude modulators are then all connected together to a converter unit which changes the pulse amplitude modulation signals to pulse length modulated signals. The pulse length modulated signals are then employed to control a high speed binary counter which counts those half-cycles of a harmonic of the controlling oscillator which occur during the time interval of each of the pulse length modulation signals received from the pulse length modulation converter. At the end of the pulse length modulation signal the counter stops and the position or condition of the various stages controls a high speed distributor, either directly or through some storage device, and pulses representing the setting of the counter are then transmitted from the high speed distributor under control of the five-stage ring mentioned above. The counter is then reset and is free to determine the next length modulation signal.

A bias control circuit is also provided for controlling the pulse length converter so that on the average the incoming signals are properly centered in the middle of the range of amplitudes capable of being transmitted by the pulse code modulation system.

The counter employed is a multistage binary counter having a stage for determining the character of the pulse transmitted in each of the pulse positions of the permutation pulse code groups. Thus the first or high speed counter stage controls the character of the first pulse transmitted for each group. The next counter stage in the chain controls the character of the next pulse transmitted and so on. It is thus apparent that the setting of the counter at the end of the count represents a binary number defining the length of the pulse length modulated signals. Inasmuch as the length of the pulse length modulated signal is a function of the instantaneous amplitude of one of the incoming complex waves at the time it is sampled by the pulse amplitude modulation system the binary number representing the setting of the counter is also representative of the amplitude of one of the complex waves. Consequently the pulses transmitted representing the setting of this counter also represent the amplitude of the complex wave from one of the channels.

If it is assumed that the "on" pulses or marking pulses represent the ones and the "off" pulses represent the zeros of the binary number, then the first pulse transmitted represents the units denominational order of the binary number and each succeeding pulse the successively higher denominational order or digit of the binary number. Thus the first pulse transmitted if of a marking character represents the smallest unit or smallest step of the total possible amplitude of the complex wave to be transmitted. The next pulse transmitted when marking represents in the binary case two units or a step of twice the magnitude of that represented by the first pulse. The next pulse transmitted if marking in character represents four of the units of the amplitude to be transmitted. Thus each succeeding pulse when marking represents twice the portion of the total possible amplitude of the signal represented by the previous pulse.

If it is assumed that the "on" or marking pulses represent the presence of the corresponding portions in the sample under consideration and the "off" or spacing pulses represent the absence of such portions, then the sum of the marking pulses when properly weighted as described above represent the magnitude or quantity which is a function of the amplitude of the complex wave at the specified instant of time. Each of the complete code groups thus completely defines the amplitude of each of the samples obtained from the pulse amplitude modulators.

These pulse code groups of signals are suitable for controlling or modulating the high frequency radio oscillator and thus cause similar pulses of radio frequency energy to be transmitted over the radio path.

The radio frequency pulses are transmitted over the radio path to and through any desired number of radio relay repeaters or repeater stations to a receiving station or terminal where they are demodulated and pulse code groups similar to those applied to the radio equipment at the transmitting station recovered and applied to demodulating and distributing equipment. Decoding and distributing equipment operates in synchronism with the equipment at the transmitting station. The decoding equipment weights each of the pulses of each of the code groups in the manner described above and adds them together to form a pulse of varying amplitude for each of the code groups received and the decoding equipment together with the distributing equipment distributes each of these pulses to the proper channel where the successive pulses for each channel are received at the rate of 8,000 per second to form a complex wave similar to that received from the corresponding incoming low frequency channel at the transmitting station. Expansion apparatus is employed at the receiving station to compensate for the compression at the transmission station.

While as pointed out above and coded pulses are suitable for modulating a microwave radio transmitter it is not at all necessary that they be so employed. These pulses may be used to modulate any carrier current or radio transmitter or they may be transmitted directly without further change over any suitable medium or communication path which provides the frequency range or bandwidth. Any or all of these transmission paths may include suitable repeaters including regenerative equipment, gain controls, equalizers, etc.

At the receiving station these pulses will be decoded and the original signal wave recovered in the manner described above. While the pulse modulation system described above may provide some small degree of privacy it does not provide any secrecy, consequently any one using the proper type of receiving equipment may receive the pulses and recover the complex signaling wave and thus all of the information being transmitted over the system.

In order to provide secrecy features ciphering equipment is employed at both ends of the system. This ciphering equipment comprises apparatus for generating a cipher key and commonly called a keyer. This apparatus is provided with a number of switches or other devices to permit it to be set at any one of a large plurality of different conditions so that the cipher or key generated by this equipment differs for each different setting of each one of the different switches or other conditioning devices. Thus it is necessary to have the keying equipment set in the same condition or position at both ends of the system before the signals can be properly recovered.

In order further to increase the secrecy and security of the transmission over the system the keying equipment is arranged to be so operated or controlled by the signaling currents or by certain of the pulses representing the voice or other complex waves that the keying equipment operates in a substantially random manner. This makes it essential for deciphering the transmitted material to have receiving equipment similar to that employed at the receiving terminal and to have this apparatus conditioned in the same manner. In particular the code pulse corresponding to the unit's digit is used to control the generation of the cipher key pulses and since that code pulse varies in an essentially random manner the cipher key changes in a similar manner. Low level noise is employed in addition to the speech to control the operation of the keying apparatus. This low level noise is impressed on the signal channel thus not only varying the unit's digit pulse to control the generation of the key pulses during pauses or idle periods in the transmission over the system, but also masking the key pulses during such idle periods.

In the exemplary embodiment set forth herein the output of the keying equipment comprises a series of random pulses occurring at substantially the same frequency or rate as the pulses from the coding equipment. These keying pulses are combined with the "code" pulses in a circuit arrangement sometimes called a reentry circuit. This circuit, in the exemplary system described herein, is arranged to transmit a pulse of either one of the two different signaling conditions during each pulse interval. The character of the pulse transmitted is determined by the character of the pulses simultaneously received from the coding equipment and from the keying equipment. If pulses of like character; i.e., both "on" or both "off", are received from the keying equipment and from the coding equipment the reentry circuit will transmit a pulse of one character, say an "off" pulse. Whereas if the pulse received from the coding equipment is of the opposite character to the pulse received from the keying equipment, the reentry circuit will transmit a pulse of the character opposite to that transmitted in the first case, namely, an "on" pulse.

In this manner the coded pulses are translated into a group of pulses which are arbitrarily arranged in a substantially random manner.

These artibrary pulses are then transmitted over the communication path which in the specific arrangement shown in the drawings comprises a microwave radio transmission path.

If these pulses are received by any ordinary radio receiver or by the receiving equipment described above which is suitable for decoding the coded pulses, they are totally unintelligible and the message or signaling wave cannot be reconstructed by such receiving equipment. In order to reconstruct the signaling wave from such a series of coded and enciphered pulses it is necessary first to decipher the pulses and then to decode them. In order to decipher the pulses it is necessary to translate the coded pulses under control of a series of cipher pulses which are identical with the cipher pulses employed at the transmitting terminal. If an identical series of keying or ciphering pulses are combined with the enciphered received pulses by means of a so-called reentry circuit operated in the same manner as the reentry circuit at the transmitter the original coded pulses may be recovered and then these pulses decoded in the manner described above and the original signaling wave recovered from them.

Inasmuch as the keying equipment at the transmitting station is controlled by voice frequency through message currents or the code pulses derived from them it is necessary to similarly control the keying equipment at the receiving station. This means that the pulses from keying equipment must be employed to decipher the received pulses after which these deciphered pulses must be employed to control the keying equipment for generating the cipher key necessary to decipher succeeding pulses.

As pointed out above it is necessary that the equipment at the receiving station be operated synchronously with the equipment at the transmitting station. There are at least three different synchronizing requirements in such a system. In the first place it is necessary that the proper groups of five pulses be combined or decoded so that the sample of the required amplitude may be recovered. It is thus necessary that the receiving decoding equipment decode the proper groups of five pulses. This is one of the synchronizing requirements.

In the second place, after the groups of pulses are properly decoded it is necessary that either the groups of pulses as such or the result of the decoding thereof be conveyed to the proper one of the signaling channels which corresponds to the signaling channel at the transmitting station from which the code was originally derived. It is obvious that if the receiving equipment does not so distribute the pulses in synchronism with the equipment at the transmitting station the signals cannot be properly reconstructed and recovered or else they will be transmitted over the wrong terminating channel.

In the third place it is essential that the keying or ciphering equipment at the receiving equipment be operated in exact synchronism with the equipment at the transmitting station so that the proper keying pulses are available for combining with the received pulses to recover the desired code group of pulses.

In addition the synchronizing of the receiver equipment is controlled by the received pulses, consequently the time delay of transmission over the system is automatically compensated for.

In order to secure the desired synchronization of the equipment at the receiving station the receiving control oscillator is arranged so that it will oscillate at two different frequencies one frequency being substantially identical with the frequency of the controlling oscillator at the transmitting station and the second frequency being only slightly different from the frequency of the oscillator at the transmitting station. In order to synchronize the equipment at both terminals the equipment is arranged so that when the receiving equipment is not in synchronism the oscillator at the receiving station oscillates at the frequency which is slightly different from the frequency of the oscillator at the transmitting station.

As pointed out above, each one of the channels is sampled at a rate of 8,000 times a second so that at the transmitting station every 1/8000 of a second the equipment and circuits will have sampled each channel are I will be in condition to start sampling each channel in succession another time. Inasmuch as there are 8 channels and 5 pulses transmitted for each channel there will be 5.times.8 or 40 pulse time intervals during each 1/8000 of a second, that is during each 125 microseconds. In other words there are 40 different pulse positions during each multiplex cycle or "frame" both at the transmitting station and at the receiving station. Consequently, there are 40 different relative pulse or phase positions between the multiplex equipment at the transmitting station and at the receiving station.

During the time required for one end of the system to advance one frame, i.e., 40 pulse positions relative to the other end, assuming that they operate at slightly different frequencies, the multiplex equipment at the two ends will have been in each of the 40 different relative pulse positions for an interval of time.

If the oscillator at the receiving station oscillates at a frequency of 10 cycles per second different from the frequency of the oscillator at the transmitting station it will require 4 seconds for the receiving equipment to have occupied each one of the 40 possible different relative positions with respect to the transmitting equipment. Every 4 seconds thereafter the receiving equipment will have again occupied each of the 40 different relative positions. If the frequency of the receiving oscillator differs from the oscillator at the transmitting station by 20 cycles then it will require only 2 seconds for the receiving system to either advance or retard one whole multiplex frame or cycle and consequently once during each two second time interval the equipment at the receiving end occupies each one of the 40 different relative positions for a short interval of time.

If it is possible to recognize the proper one of these relative positions during this short interval of time and then to maintain the two systems in the proper conditions it will be possible to automatically synchronize the apparatus at both ends of the system so far as the first and second requirements are concerned. It will still be necessary to properly synchronize the keying equipment.

By employing keying equipment which likewise has a number of different possible positions or phase relations and causing the receiving keying equipment to test these various phase relations rapidly in an irregular manner it is possible to insure an extremely high degree of probability that the transmitting and receiving keying equipments will come into the identical phase relation during the short period of time for which the multiplex phases are such as to permit correct operation.

If the key equipment fails to come into identical phase relationship during the first time that the multiplex equipment is in proper phase the hunting of proper phase continues to the next time the multiplex equipment is again in proper phase at which time the key equipment again hunts for the proper phase relationship. This action continues until proper orientation or phase is simultaneously obtained.

Thus, if the rate of hunting of the multiplex equipment at the receiving station is made sufficiently slow that it remains in each one of the possible different relative positions for sufficient time, it is possible to recognize the proper relative position and then change the frequency of the controlling oscillator at the receiving station so that it is exactly the same as the frequency of the oscillator at the transmitting station. It is then possible to lock the frequency of the controlling oscillator at the receiving station so that it is maintained at the proper frequency under control of received pulses with the result that the equipment at the received station will be maintained in synchronism with the equipment at the transmitting station.

If a distinctive signal is impressed upon one of the channels, as for example, channel number 1, to indicate at the receiving terminal when the receiving equipment is in synchronism with the transmitting equipment, so long as the distinctive signal is not received over channel number 1 at the receiving station the equipment at the receiving station is not in proper phase or synchronism with the equipment at the transmitting terminal. However, when the distinctive signal is received over the proper channel, namely channel number 1, at the receiving terminal the equipment at the receiving terminal will be in proper synchronism with the equipment at the transmitting terminal. Consequently the reception of this signal may be employed to control the change in frequency of the controlling oscillator at the receiving terminal.

Then so long as this signal is received over channel number 1 at the receiving terminal the oscillator is locked in synchronism at substantially the same frequency as the frequency of the oscillator at the transmitting terminal. When this distinctive signal is not received over channel number 1 at the receiving terminal the frequency of the controlling oscillator at the receiving terminal is changed so that the receiving equipment at the receiving terminal hunts over all of the possible relative positions between the equipment at the receiving terminal and the equipment at the transmitting terminal until the distinctive signal is again received over channel number 1 at the receiving terminal.

In the exemplary system described herein the distinctive signal is low level noise on channel number 1. The keying equipment at both terminals is arranged to be controlled by certain of the code pulses at both terminals. Consequently unless the keying equipment at the receiving terminal is similarly controlled it will not be in synchronism with the keying equipment at the transmitting terminal. As a result due to the operation of the keying equipment at both terminals when they are not in synchronism high level noise is present on all of the channels at the receiving terminal. However, if and when the system comes into proper synchronism the noise on all channels will fall to a low level. In the exemplary system described herein the noise level of channel number 1 is employed to indicate proper synchronism of the system.

In addition to the foregoing requirements and conditions of the system it is found necessary to provide an order wire or service communication channel between the various terminals and the intermediate relay repeater points so that the attendants at the terminals and intermediate repeater points may keep in communication over the system to properly adjust and maintain the equipment in its operating condition. In order to provide such a circuit the entire array of coded and ciphered pulses are time or position modulated so that their relative time of transmission is altered in accordance with the signals of this so-called order wire or service channel.

Provision has also been made to permit the service channel to be employed for other communication purposes when it is not required for service signals so that another communication path may be provided over the system. However, in the exemplary system described herein in detail no provision has been made for providing a cipher for the so-called order wire or time modulated signal.

At each of the intermediate relay repeater points further time modulation, up to a maximum, may be applied to the signals so that each of the attendants at these stations may communicate over the order wire. At receiving terminals and at each of the intermediate relay repeater points equipment is provided for demodulating the time or position modulation of the pulses, thus permitting communication between the attendants at each one of the repeater points and terminal stations of the system.

In the event that for any reason the coded and ciphered pulses are not received by any particular relay repeater, means are provided for transmitting an emergency group of pulses from that repeater which may be time or position modulated by the order wire apparatus so that the communication may be maintained between stations on either side of the trouble.

Various alarm and disabling features are also provided which are responsive to improper operation of various portions of the system.

The foregoing and other objects and features of this invention, the novel features of which are set forth in the claims appended hereto may be more readily understood from the following description when read with reference to the attached drawings, in which:

FIG. 1 shows the general arrangement of terminal stations and an intermediate relay repeater station of the system embodying the present invention;

FIG. 2 shows the manner in which FIGS. 3 through 10 are arranged adjacent one another;

FIGS. 3 through 10, inclusive, show in outline form the various elements of an exemplary system embodying the present invention;

FIGS. 3, 4, 5 and 6 show the various elements and the manner in which they cooperate at one terminal;

While FIGS. 7, 8, 9 and 10 show the corresponding equipment and the manner in which it cooperates at the other terminal;

FIG. 11 shows the manner in which FIGS. 5, 6, 9, 10 and 12 through 48, inclusive when arranged as shown in FIG. 11 show in detail the elements of a typical system embodying the present invention and the manner in which these various elements cooperate one with another;

FIGS. 49 through 56 inclusive illustrate graphs representing currents and voltages in various portions of the system so that its operation may be more readily understood;

FIG. 57 shows the manner in which FIGS. 49 through 54 may be arranged adjacent one another.

GENERAL DESCRIPTION

FIG. 1 shows the elements of a typical exemplary system embodying the present invention. The system illustrated in FIG. 1 comprises two terminals A and B and an intermediate relay repeater station C. While a single relay repeater station has been shown in FIG. 1 it will be readily understood that this repeater station may be dispensed with altogether or that any desired number of such repeater stations may be interposed in tandem between the terminals A and B. Inasmuch as these additional relay repeaters will all be similar, only one relay repeater station C is shown in the drawing.

An overall picture or understanding of the operation of the system may be obtained from a consideration of the circuits and apparatus of the equipment shown in FIG. 1. Terminal A includes a common frame 120 at which 8 two-way voice frequency telephone channels terminate. This common frame has suitable terminating equipment for terminating either two-wire or four-wire circuits in the exemplary system described herein. This terminal equipment may include apparatus for terminating any type of communication channel with which it is desired to operate systems similar to the system described herein in detail. The first of these eight channels is reserved for synchronizing purposes as described herein. Each of the other seven channels may extend through any desired types of transmission, repeating, equalizing and switching equipment desired or necessary to connect the desired transmitter 126 and receiver 127 to the common frame 120 at terminal A. The transmitter and receiver 126 and 127 are shown connected to the second voice frequency channel. It is to be understood that similar transmitting and receiving devices will be connected to the other channels when it is desired to employ them for transmitting speech currents.

The transmitter 126 and receiver 127 are shown to illustrate suitable types of equipment for generating and receiving complex wave forms. The exemplary system described herein however is not limited to speech currents. Consequently the transmitter and receiver 126 and 127, respectively, are intended as merely representative of a large group of transmitting and receiving devices including picture transmission equipment, telegraph equipment including printing or automatic telegraphy and other types of apparatus for generating and responding to complex wave forms. As shown in FIG. 1 the seventh voice frequency channel extends to the voice frequency carrier current system 128 which provides equipment for simultaneously transmitting a plurality of telegraph channels over a voice frequency communication path. Printing equipment 129 of FIG. 1 represents terminal equipment of one of these paths and may include both transmitting and receiving telegraph equipment.

Terminal B. similarly includes a common frame at which a corresponding voice frequency transmission path terminates. These paths may be of any of the types described above with reference to terminal A and are arranged so that channel 1 of terminal A corresponds to channel 1 of terminal B, etc. In other words, signals from transmitting device 126, connected to channel 2 at station A cause corresponding signals to be transmitted over the system to station B where signals corresponding to the signals from device 126 are applied to channel 2 extending to receiving device 167. In a similar manner signals from device 166 are conveyed to receiving device 127.

Likewise telegraph signals from the printing equipment 129 are transmitted to the telegraph equipment 169 through the carrier current channel systems 128 and 168 and then over the seventh voice frequency path between terminals A and B. Persons skilled in the art will readily understand that similar or other types of telegraph equipment may likewise be connected to any and all of the other voice frequency paths except the channel or path number 1. It is also evident that incoming lines or channels need not be connected to all of the voice frequency channels. The mode of operation of the system will not be affected by the number of incoming channels actually connected to or delivering signals to the system.

A noise generator 125 is provided at terminal A and a similar generator 165 at terminal B to add noise to the signals to increase their secrecy in the manner described herein.

The signals are conveyed between the common frame 120 and the transmitting modulator frame 122 and receiving demodulator frame 121 at terminal A and between the common frame 160 and the transmitting demodulator frame 162 and the receiving modulator frame 161 at terminal B.

The transmitting modulator frame causes each of the channels to be sampled in succession and represents the magnitude of each of the samples by a uniform number of permutatively coded pulses each of which may have any one of a plurality of different signaling conditions. These code pulses are then suitable for transmission to the other terminal. Persons skilled in the art will readily understand that the output of the modulator frame may be connected over any suitable type of transmission path and related equipment to the receiving modulator frame at the opposite terminal. This communication path may include open wire lines, cable conductors including coaxial cables, wave guides, carrier current or radio channels as well as any and all types of transmission paths through any necessary or required medium.

As shown in FIG. 1 the output of the transmitting modulator 122 extends to radio transmitter 131 where the output pulses from the transmitting modulator 122 are employed to modulate the output of the radio transmitter 131. In the exemplary embodiment described herein these pulses are employed to turn on and off the radio transmitter. From the radio transmitter the pulses are transmitted from an antenna 133. These radio frequency pulses may be transmitted directly to receiving antenna 172, receiving converter 174 and thence to the radio frame 163 and receiving demodulator 161 where the pulses are recovered and converted into a complex wave similar to that applied to the system from the terminal channels at the transmitting terminal.

Transmission in the opposite direction is in substantially the same manner as described above.

Where the transmitting and receiving terminals are widely separated it will be necessary to provide one or more intermediate repeater points or stations similar to the relay repeater C shown in FIG. 1. In this case the signals transmitted from antenna 133 are received by the antenna 144. The signals then pass through a receiving converter 142 and radio frame 140 to repeater 155 which reforms and retransmits the reformed signals to the radio transmitter 151. From the radio transmitter 151 the signals are transmitted from antenna 153 to the receiving antenna 172. Thereafter the signals are transmitted through the equipment in the same manner as described above. Here again the transmission in the opposite direction through the radio repeater station C is in substantially the same manner as described above.

The signals in passing through the amplifier 155 are reformed so that distortions encountered over the transmission path between terminal A and repeater station C are all substantially eliminated.

In addition to sampling in succession the signals applied to the eight channels and representing the magnitude of each sample by a permutation code group of pulses, secrecy features have been added which render the system relatively secure even though the signals may be received by receiving equipment of the type described herein.

In addition to the eight voice frequency channels, an additional order wire channel is provided between the two terminals of the system. Provision has been made for connecting equipment at each of the intermediate relay repeater stations to this order wire circuit. However, due to the fact that a large number of people may be attempting to use the order wire circuit and thus overload it, means have been provided to regulate the total possible amplitude or volume of the signals which may be transmitted over the order wire.

The order wire may terminate in equipment at either of the terminals or at any of the intermediate relay repeater stations or it may extend to more distant communication centers over communication paths and through switching equipment such as described above. The terminal apparatus is illustrated at 130 at terminal A, 148 and 158 at the intermediate relay repeater station and 170 at terminal B.

As shown in FIG. 1 the equipment has been arranged on various frames for easier maintenance and operation. This arrangement of the equipment when desirable has been followed in the description of this specification. It is to be understood however that any other suitable arrangement of the equipment may be employed. The radio apparatus and equipment 131 and 132 at terminal A are usually located on or near the radio tower but need not be so located. The radio transmitting and receiving apparatus at the intermediate repeater station may be similarly located. Both repeater 145 and 155 are shown in the drawing relatively near one another. These repeaters may be located adjacent to one another in the same building or they may be located at points relatively far apart and signal transmission paths of the desired or necessary bandwidth connecting them together.

The novel features of the relay repeater station including the order wire circuits and apparatus shown and described herein but not claimed are claimed in the copending application of Anderson-Edson Ser. No. 675,902 filed on the same date herewith.

Details of a typical radio system suitable for transmitting the coded pulses has been employed by the Army and designated AN/TRC-6. This equipment is described in the technical manual TM 11-631 or TM 11-632 on the radio transmitting set AN/TRC-6, which description is hereby made a part of the specification as if fully set forth herein.

Suitable power alarm and other equipment is provided at terminal stations and also intermediate relay repeater stations. A source of power is illustrated by the rectangle designated 124 at station A of FIG. 1 designated power supply and voltage regulator frame. Similar power equipment is designated 164 at station B of FIG. 1.

FIGS. 3 through 10 when arranged adjacent to one another as shown in FIG. 2 show in outline form the various elements of an exemplary system and the manner in which they are interconnected and cooperate. FIGS. 3 and 4 show the transmitting equipment at terminal A and FIGS. 5 and 6 show the receiving equipment at terminal A. FIGS. 7 and 8 show the receiving equipment at terminal B. FIGS. 9 and 10 show the transmitting equipment at terminal B. No relay repeater stations have been shown between the terminals at FIGS. 3 to 10 inclusive. It is to be understood however that any suitable or desirable number of relay repeater stations of a type such as that described in detail in the above-identified copending application of Anderson-Edson Ser. No. 675,902, may be interposed between the transmitting and receiving terminals. It is also to be understood that in case only one-way circuits are required between the transmitting and receiving terminal that FIGS. 5, 6 and 9 and 10 may be omitted.

At the left-hand side of FIG. 3 even handsets 310, 330 etc. are shown. These handsets are intended to illustrate both a source of suitable signals and a receiving instrument capable of receiving and responding in the proper manner to the corresponding types of signals. These devices 310, 330, etc. are also shown to be similar for simplicity. It is to be understood however that many other suitable types of transmitting and receiving equipment such as enumerated above may be employed for the source of signals to be transmitted and for the receiver to automatically receive and respond to the signals applied to it.

The transmission path from the place of origin of the signals 310 may extend through any suitable type of terminal equipment 311. This terminal equipment may include any type of transmission path enumerated above and may include manual and automatic switching equipment, gain control regulators, and other compensating equipment, suitable amplifiers and repeaters, etc. employed in the respective types of transmission systems and employed in the terminals thereof and between the systems of the various types.

When the signals arrive at terminal A over a four-wire circuit, four-wire terminal equipment 312 is provided at terminal A. As shown in FIG. 3 the incoming paths 331, 332, 335, 336 and 337 are four-wire circuits. Incoming paths 333 and 334 are shown as two-wire circuits. When the direction of transmission over the voice frequency channel is separated, i.e., a so-called "four-wire circuit", the signals to be transmitted over the system described herein are conveyed to the transmitting equipment while the signals received from the transmitting equipment are transmitted over an east to west path as viewed in FIG. 3. When the directions of transmission of the incoming channel are not separated, i.e., the channel is a "two-wire circuit" in paths 333 and 334, the transmission and reception pass through and are separated by the hybrid coils 338 and 339 respectively employed to terminate these circuits. The channels for transmission in the opposite directions may be then connected to the appropriate circuits and apparatus.

From the terminating equipment the incoming signals pass first through a low pass filter designated 313 in the case of channel 5, which limits the frequencies which may be transmitted over the system to those frequencies which the system is arranged to transmit.

The frequency range of a system of the type described herein is largerly controlled by the sampling rate. In other words each of the channels has to be sampled at least twice for each cycle of the highest frequency component desired to be transmitted. For example if the sampling rate is 8,000 cycles per second, the highest possible frequency which may be transmitted from each of the channels is about 4,000 cycles. However, to insure suppression at unwanted frequencies above 4,000 cycles the filters 313 are arranged to cut-off at a somewhat lower frequency around 3,000 to 3,500 cycles. This frequency range is adequate for telephone or voice communication circuits. However, in case it is desired to extend this frequency range it is only necessary to sample the channels more rapidly and make corresponding changes in the frequency range of the filters.

The signals are conveyed from the low pass filter 313 to a voice amplifier 314 and then through a comrressing network 315. The compressors 315 include some non-linear circuit devices such as rectifiers or "thyrite" and operate as an instantaneous compressor of the signal amplitude. Coper oxide rectifiers are employed in the exemplary system described in detail herein. In other words the output of the compressor does not rise to increase as fast as the input. The exact relationship between the magnitudes of the input and output of the compressor may be controlled by suitable design. After the noise and signals have been compressed they are ready for application to the multiplex and coding equipment.

The operation of this equipment may be more readily understood by first referring to the control oscillator 410 in FIG. 4 which in the exemplary system described herein operates at a fundamental frequency of 320 kilocycles per second.

Inasmuch as this oscillator controls the timing and rate of operation of the system it is desirable that it have a high degree of stability so that a crystal-controlled oscillator is preferred.

The crystal oscillator 410 is employed to simultaneously control a number of different circuits and elements of the system. The 320 kilocycle output from oscillator 410 is fed to a five-stage ring circuit 318 which operates as a distributor and steps one stage or position for each cycle of the 320 kilocycle current from oscillator 410. The five-stage ring is illustrated by 318 in FIG. 3. By means of the five-stage ring, five different output pulses are obtained in succession with each pulse one-fifth of the length of a total cycle or revolution around the ring.

Since the five-stage ring is stepped one stage for each cycle of the oscillator it operates at one-fifth of the 320 kilocycles per second or 64,000 revolutions or 64 kilocycles per second. This frequency is applied to an eight-stage ring 317. The eight-stage ring is arranged so that each stage gives an output pulse which has a length substantially one-eighth of the total period or cycle of the eight-stage ring. Thus pulses in each of the eight output leads from the eight-stage ring occur at one-eighth of 64,000 or 8,000 times a second. These pulses are thus one-eighth of 125 microseconds or approximately 15.6 microseconds long.

The output pulses from the different stages of the eight-stage ring are applied to different pulse amplitude modulators individual to incoming channels similar to the modulator 316 of channel 5. The compressed signal wave from the compressor 315 is also applied to the pulse amplitude modulators (PAM) 316. These modulators are arranged to pass the input signals during the time a pulse from the corresponding stage of the eight stage ring is applied to them from the respective stage of the eight-stage ring. As a result each one of these modulators will pass a pulse during the time interval assigned to it. The amplitude or magnitude of the corresponding pulse is a function of the instantaneous amplitude of the respective speech or other complex wave applied to this modulator during the respective pulse interval. Inasmuch as these output pulses all occur in succession, the outputs of the eight pulse amplitude modulators are all connected together to a pulse converter 426. In this manner the input complex waves are sampled in succession and transmitted to the coding equipment shown in FIG. 4.

The pulse converter 426 operates to convert each of the pulse amplitude modulated pulses to a pulse length modulated pulse. In other words, converter 426 changes each of the pulses applied to it which have a constant length or width but which may have different amplitudes into pulses having a constant amplitude but different widths or lengths. The widths or lengths of the output pulses are functions of the amplitudes of the input pulses. Consequently the length or width of the converted pulse from the pulse converter 426 is a function of the magnitude of the incoming complex or speech wave at the instant it is sampled.

A portion of the output of the 320 kilocycle current from oscillator 410 is employed to control a frequency multiplier 411. The frequency multiplier 411 operates under control of the 320 kilocycle current to generate a frequency of 1600 kilocycles. The 1,600 kilocycles together with the pulse length modulated signal from the pulse converter 426 are applied to the balanced gate 412. The pulse length modulated signal operates to condition the balanced gate 412 so that for the duration of the pulse from converter 426 the balanced gate 412 will transmit the 1,600 kilocycle current to the quantizer 413. However, the balance gate 412 will not transmit 1,600 kilocycles in the absence of the pulse from the converter 426. As a result the 1,600 kilocycles will be passed through the balanced gate 412 for a length of time which is a function of the amplitude of the complex wave at the time of sampling. This means that the number of half-cycles transmitted through the balanced gate 412 is also a function of the amplitude of the complex wave at the corresponding instant of sampling.

Inasmuch as the length of the pulse from the converter 426 bears no fixed relationship to the 1,600 kilocycle current it may end any time during any portion of a cycle of the 1,600 kilocicle current. As a result the final half-cycle from the balanced gate 412 may be cut short so that it may not properly control the succeeding circuits. In order to avoid this difficulty, quantizer 413 is provided which lengthens such short pulses so that they will be substantially a full half-cycle long. Thus the output of the quantizer 413 is always an integral number of half cycles of the 1,600 kilocycle current which number represents the amplitude of the complex wave at the time of sampling.

These half cycles are then employed to control a binary counter comprising five stages 414, 415, 416, 417 and 418. Each of the stages comprises a double stability circuit similar to the Eccles-Jordan trigger circuits. The first stage 414 operates at the frequency of the incoming half cycles and responds to the half cycles, that is each half cycle changes the circuit from one position of stability to the other. The second stage 415 counts the number of times the first stage changes in one direction, that it the second stage changes from one position of stability to the other half as often as the first stage. Each of the succeeding stages is thus controlled by the preceding stage and operates at half the frequency of the preceding stage. In this manner it is possible to count 32 half cycles of the 1,600 kilocycle frequency before the counter is returned to its initial condition. Provision has been made to stop the operation of the counter at the count of 31 so that it will not count beyond 31.

At the end of the count and before the pulses representing the next sample are applied through the balanced gate 412, a pulse is received from the five-stage ring 318 by the storage pulser 428.

At the same time a pulse is applied to distributor unit 431 and gate 430. The distributor 431 is also connected to the first stage of the counter 414. Dependent upon the position of first stage a marking or a spacing pulse will be transmitted over conductor 470 at this time. A pulse of spacing or marking character similarly dependent upon the condition of the first stage of the counter 414 will also be transmitted through the gate 430 to the keying circuit which will be described hereinafter.

The pulse applied to the storage pulser 428 at this time causes the storage pulser 428 to condition the storage devices 422, 423, 424 and 425 so that they will in effect store the condition of the succeeding counting stages. These storage devices each comprises a condenser upon which a charge is stored if the counter stage is in one condition but upon which no charge is stored if the counter is in the opposite condition.

The outputs of the storage devices 422 through 425 are connected to distributor elements 432 through 435. Each of these distributor elements is also connected to the succeeding one of the stages of the five-stage ring so that they will receive pulses in succession from the five-stage ring 318. In this manner the distributor stages 431 through 435 are eneegized in sequence from the five-stage ring and depending upon the condition of the corresponding stage of the counter at the end of the count an "on" or "off" pulse is transmitted over conductor 470. These five pulses then comprise a permutation code group of pulses which represent any one of the 32 different possible conditions of the counter stages 414 to 418, inclusive. It should be noted that 0 to 31 is in effect 32 amplitude conditions which may be represented by the five pulses as pointed out above.

An additional counter 419 is provided which comprises two counter stages and is controlled from the final counter stage 418. This counter counts the number of times stage 418 changes from its operated condition to its normal condition.

Stage 418 changes from the "off" to the "on" condition on the 16th count or 16th half cycle received from the quantizer 413. In other words if the amplitude of the sample is less than half of the total possible amplitude which the sample may assume, counter 418 will not be changed from its "off" to its "on" condition. On the other hand if the sample has a magnitude which is greater than half of the possible magnitude which the sample may assume the counter stage 418 is changed from its "off" to its "on" condition, that is from its normal to its operated condition.

Thus the number of times the counter stage 418 changes from its normal to its operated condition indicates the number of samples which are greater than half of the total possible amplitude of the sample. Counter 419 in operating will count the number of times this occurs and send a pulse to the bias control circuit 420 every time counter 418 changes from its "on" to its "off" position 4 times. A pulse is also sent from the 8-ring distributor 317 to the bias control circuit 420. If the samples on the average are properly centered in the center of the total possible magnitude which the system is capable of transmitting the counter stage 418 will be operated from its normal to its operated position half of the time on the average and thus 4 times during each complete multiplex cycle. As a result counter 419 will transmit 1 pulse to the bias control circuit. The eight-stage ring will likewise transmit one pulse during each multiplex cycle. The bias control circuit operates to control the length of the pulse from the pulse converter 426. This bias control circuit 420 is arranged so that each of the pulses received from the eight-stage ring 317 tends to increase the length of the pulse length modulated signal while each of the pulses received from the counter tends to reduce the length of the pulse modulated signal. As a result if the samples as counted by the counter are on the average more than half the total possible amplitude of the signals more pulses will be received from counter 419 than will be received from the eight-stage ring 317. As a result the bias will be changed so as to reduce the length of the pulse modulated signal so that on the average it will be half the total possible variation in length. On the other hand if the count is less than 16 on the average, fewer pulses will be applied to the bias control circuit 420 from the counter 419 than will be received from the eight-stage ring 317. As a result the length of the pulse length modulated signals will be increased until the average length is substantially half the total possible variation in length. In this way the magnitudes of each of the samples is properly centered in the counting range of the counter so that the system is constantly maintained in a condition for transmitting the maximum possible amplitude variation of applied signals without serious distortion or error.

The signals from the distributor are transmitted over conductor 470 to an amplifier or clipper 437 which may shape the signals in any desired manner. In the exemplary system described herein the amplitude of each of the pulses is made substantially the same and they have substantially the same wave shape.

The pulses are then suitable for transmission either with or without further amplification or shaping to the receiving equipment and may be so transmitted. If they are so transmitted they should be applied to conductor 701 of FIGS. 7 and 8 at the receiver. It is within the scope of the present invention to apply the output from either conductor 470 to 477 over any suitable transmission medium such as a radio channel, a coaxial cable, a wave guide or other suitable transmission medium or path to conductor 701 at the receiver. This transmission path may include suitable amplifiers, pulse shaping and reforming apparatus, gating apparatus, etc.

Receiving and Decoding Equipment

The receiving station is provided with a control oscillator 710 similar to the oscillator 410 at the transmitting station. In the specific embodiment described herein in detail oscillator 710 operates at the same frequency as oscillator 410.

Oscillator 710 is employed to drive or control the five-stage ring or distributor circuit 817 similar to the five-stage ring 318 at the transmitting station. A pulse from one of the stages of the five-stage ring is employed to control or drive the eight-stage ring 818 similar to the eight-stage ring 317 at the transmitting station. The oscillator 710 and the five-stage and eight-stage rings at the receiving station operate continuously and are in synchronism with the corresponding equipment at the transmitting station. In other words, stage 1 of the five-stage ring is in its actuated condition at both stations substantially simultaneously, likewise each of the succeeding stages of both the five-stage rings. The eight-stage rings are controlled from the five-stage rings amd must be properly phased at the two ends of the system but stage one for example, is not in the actuated condition at the same time at both ends of the system as will be explained hereinafter. It is of course also understood that the equipment at the receiving station lags behind the equipment at the transmitter station by a fixed increment of time. Thus the pulse from stage 1 of the five-stage ring at the receiving station lags the pulse from stage one of the five-stage ring at the transmitting station by the delay between the stations; likewise each of the succeeding stages of both rings. This time increment includes any or all time delays interposed by amplifiers, repeater stations and equipment, as well as the time delay for the transmission medium or radio paths employed for conveying the signals between various stations of the system. This fixed delay time increment also includes the delay in the pulse reforming reshaping and amplifying circuits at the receiving station. Since this delay is fixed in any given system and does not present any synchronizing problem in the exemplary system, described herein, no further consideration will be given to it herein.

The manner in which the synchronism described above is obtained will be described hereinafter. For the purpose of describing the decoding equipment it will be assumed that the various circuits referred to above are maintained operating in synchronism and the manner in which synchronism is secured will be described later.

It is also assumed for the purpose of description that either the output of the distributor appearing on conductor 470 or the output of a pulse-shaping amplifier 437 appearing on conductor 477 is conveyed through the equipment to the conductor or lead 701 at the receiving station. The manner in which these signals are conveyed from one conductor to the other may include open-wire lines cable circuits, coaxial cables, wave guides or radio paths, the only requirement being that each of the pulses of a given character applied to lead 470 be conveyed and appear as a pulse of the same or of an opposite character and of a suitable polarity upon lead 701. These pulses then are in the same code groups and follow one another in succession on lead 701 in the same manner as on lead 470. It is of course understood that the pulses may be suitably shaped, amplified and otherwise controlled between these two places in any desired manner so long as the pulses applied to lead 701 are of the same character and follow one another in the same order as appearing on lead 470.

The received pulses are transmitted over lead 701 to video amplifier 810 where they are amplified and otherwise shaped or reformed if desired. The output of this video amplifier is then applied to a group of four decoding circuits 811, 812, 813 and 814. Decoding circuit 811 is provided for channels 1 and 5 and each of the other decoding circuits decode the pulse group for two of the other channels. The decoding circuits likewise receive pulses from the eight-stage ring through the combining circuits such as 815 for decoder 811. Similar combining circuits are provided for each of the other decoders. The combining circuit 815 is supplied by pulses from two of the stages of the eight-stage ring. Under the assumed condition combining circuit 815 is supplied with pulses from the first and fifth stages of the eight-stage ring because these pulses are so timed that they coincide with the reception of the groups of pulses from the first and fifth channels. The combining circuit 815 then supplies pulses to the decoder 811 at both of these times so that the groups of five pulses conveying the information for both channels 1 and 5 are decoded by decoder 811. Decoder 812 is similarly supplied with pulses from stages 2 and 6 of the eight-stage ring for permitting this decoder to receive and decode the pulses for channels 2 and 6. The other decoders are similarly supplied with appropriate pulses for decoding the information for the other channels.

The output of decoder 811 is applied to two gate circuits 821 and 825. These gate circuits are supplied with other pulses from the eight-stage ring, from stage 2 for gate 821 and from stage 6 of gate 825, and serve to separate the decoded signals decoded by the decoder 811 and direct them to their proper channels. The output of the gate circuits 821 and 825 then is transmitted through the expandor circuits 831 and 835 respectively and applied to the respective low-pass filters 841 and 845. The low-pass filter removes the high frequency components and in effect permits only the voice or low frequency currents to be transmitted through them, thus in effect regenerating speech or other waves similar to those impressed upon the system at the transmitting station.

The low frequency voice or other waves are then amplified by the amplifiers 851 and 855. As pointed out hereinbefore the first channel is not employed for speech purposes but reserved for synchronizing as will be described hereinafter. The output, however, of the voice amplifier 855 of the fifth channel is applied to a hybrid 1015 and then through the terminal equipment 1025 to the receiver 1035 at the other terminal of the fifth channel.

For transmission over the fifth channel in the opposite direction similar circuits and apparatus are provided as shown in FIGS. 10, 9, 6 and 5 to the receiving equipment 310 associated with the transmitter 310. Thus a two-way circuit is provided between each of these devices. It is also possible to transmit ringing current over the system in the same manner as other signals. At the receiving station, however, special low frequency receiving or ringing circuits 861 and 865 are provided for responding to the ringing current and applying ringing current directly to the line extending from hybrid coil 1015 instead of causing this ringing current to first pass through the low-pass filter and voice-frequency amplifier.

The message contained in the pulse code groups of signals transmitted between the conductors 470 and 477, and 701, while in the form of code groups may be still at least in part understood with difficulty when received by an ordinary radio receiver. This is particularly true if only one or two channels are transmitting speech currents and the remainder of the channels idle or perhaps when the channels are overloaded. While the information on the coded pulses are less intelligible when received by an ordinary radio receiver than the usual voice amplitude modulation or frequency modulation waves, the system still does not provide sufficient privacy in the communication between the terminals of the system and in the case of radio paths between the transmitting and receiving station, probably does not provide as great a degree of secrecy as a line or cable circuit.

In order to improve the privacy or secrecy of the system to secure secrecy equal to or surpassing the usual types of signals applied to transmission lines and cables, ciphering equipment has been provided at the transmitting station and deciphering equipment at the receiving station. This equipment comprises a group of circuits and other equipment arranged to generate key pulses in a substantially random manner which are reproducible at the receiving station. These random pulses are then combined with the coded pulses, first at the transmitting station to cipher the signals transmitted therefrom and then at the receiving station to decipher the signals. The equipment for generating these pulses is called herein a "keyer".

Cipher and Decipher Circuits and Apparatus

A keying unit is employed to generate a series of pulses of any one of a plurality of different signaling conditions such as marking pulses or spacing pulses, "off" pulses or "on" pulses, and the like. While in the general case it is not necessary that these pulses be generated at the same rate as the signaling or coded pulses described above, they are so generated in the exemplary system described herein. It is to be understood however that this invention is not limited to arrangements in which the keying pulses are generated at the same rate at the intelligence conveying impulses.

The keyer employed in the exemplary embodiment of the invention described in detail herein consists essentially of a plurality of ring circuits driven from the 320 kilocycle oscillator 410. In the specific embodiment described herein two ring circuits are employed. Persons skilled in the art however will understand that any suitable number of ring circuits may be employed. The keyer described herein comprises a thirteen-stage ring and en eleventh-stage ring. It is to be understood that the rings may comprise any number of stages but it is desirably that the stages of each ring be different from the number of stages of all the other rings. In addition it is also desirable that the number of stages in any ring be a prime number.

Each of the stages of each of the rings comprises a double stability circuit employing two electronic discharge devices. A plurality of certain of the devices may be enclosed in a common envelope. Thus two of such devices are frequently enclosed in a common envelope. However such an arrangement comprises in effect two electronic discharge devices. Double stability circuits employing electrode discharge devices are sometimes called Eccles-Jordan trigger circuits. The double stability circuits are arranged so that current flows through one or the other of the tubes but not both. Furthermore when current starts to flow through one tube it continues to flow through that tube until interrupted by the application of some potential or signal to the circuit. One of the tubes normally conducts current for the greater portion of the time so when the circuit is in this condition it is said to be in its normal position or condition. When the other tube is conducting the circuit is said to be in its operated or actuated condition. The stages of each ring are arranged in a ring or sequence so that on the application of a pulse to the ring an operated stage is returned to normal and the succeeding stage actuated to its operated position. Means are also provided to insure that only one stage in each ring is operated at a time.

A plurality of switches, such as 465, 476, etc., are provided, one for each stage to permit the connections between the stages of each ring to be changed at will so that as the rings are advanced stage by stage, as described hereinafter, different series of pulses may be obtained in accordance with the different positions of the switches. In the exemplary system described herein each of the switches may be set in either one of two positions so that there will be transmitted to an output circuit common to each ring a pulse of one character say an "on" or marking pulse when the respective stage is operated and the switch operated to one of its positions. A pulse of opposite character however, that is an "off" pulse or spacing pulse is transmitted to the common output circuit each time the respective stage is actuated when the switch is opperated to its opposite position.

Thus in the thirteen-stage ring and the eleven-stage ring there are a total of 24 switches which may be operated independently to either one of two positions. This makes a total of 2.sup.24 possible permutations of the settings in these switches.

As pointed out above both of the rings are driven from the 320 kilocycle oscillator 410. A portion of the output of oscillator 410 is employed to control the pulse generator 429. The output of the pulse generator 429 is applied to gate circuits 464 and 474. These gate circuits are arranged to transmit the pulses from the pulse generator 429 to the thirteen-stage ring and the eleven-stage ring respectively unless prevented from doing so by other pulses or conditions. These other conditions are determined by the eight-stage ring and the five-stage ring of FIG. 3 through a plurality of switches 325, 326, 480 and 490.

The first pulse of certain of the code combinations when of one character will cause a gate circuit to suppress a selected driving pulse for one or the other or both of the ring circuits. If this pulse is of the opposite character the transmission of the selected driving pulse will not be suppressed. The selection of the code combinations and the time at which the selected pulse becomes effective is made by switches 325, 326, 480 and 490. Switch 325 comprising switch arms 319, 320 and 321 selects the particular group of pulses and switch 480 comprising switch arms 461, 462 and 463 selects the pulse interval of the group during which a driving pulse to the thirteen-stage ring may be suppressed. Switches 326 and 490 are employed to make similar selections for the control of the eleven-stage ring. Thus, depending upon the setting of all of these switches and the conditions of the various rings, a pulse will be transmitted or not transmitted by the gate circuits under the joint or combined control of all these factors. Inasmuch as the first pulse of each code group will be of either marking or spacing character in a substantially random manner, due in part to the random character of speech waves and also due in part to the random character of the low level noise added to the speech or other complex waves to be transmitted, stepping or advancement of the thirteen- and eleven-stage rings will also be of a totally unpredictable and essentially random character.

A coincidence circuit 468 is also provided for suppressing the transmitting of a pulse by one or the other of the gate circuits 464 or 474 depending upon the setting of the switches 463 or 473. The transmission of a pulse by the gate circuit is suppressed only when both rings are simultaneously in their number 1 positions.

The coincidence circuit is provided to obtain the maximum number of different settings of the various switches and thus the maximum number of different possible settings of the switches of the ring circuits. By thus employing a coincidence circuit the condition of the rings and also the condition of the switches associated with each stage of the ring must be identical at both ends to recover the message of the receiving station of the system. Without the coincidence circuit the switches associated with each of the stages of each of the rings need only be set in the same sequence. However, by employing a coincidence circuit such a setting is not sufficient for proper operation of the system. Instead with a coincidence circuit it is necessary that the corresponding switches of each stage of each ring have the same positions at both ends of the systems.

The output circuits of each of the ring circuits are combined or added together by a reentry circuit 469. This circuit is arranged to transmit a pulse of one character, say marking or "on" character, when the pulses supplied to it from the thirteen-stage ring and the eleven-stage ring are both the same character, that is when they are both "on" pulses or both "off" pulses. In case the two pulses supplied to the reentry circuit are of opposite character it is arranged to transmit pulses of the opposite i.e. spacing or "off" character. This reentry circuit may be arranged to transmit pulses opposite in character to the character described above in response to the various signal conditions applied to it as described above. The output of the reentry circuit together with the output of the coding circuits are simultaneously applied to an enciphering circuit 438 which accomplishes substantially the same results as the reentry circuit 469 except that unlike pulses produce marking pulses and like pulses produce spacing pulses. The enciphering circuit 438 after combining the two sets of pulses as described above transmits the resultant enciphered pulses to the amplifier and gate circuit 439.

The gate 439 is supplied from the 320 kilocycle oscillator 410 through the pulse forming or generating circuit 436 and a pulse position or time modulator circuit 444. Circuit 444 will be described hereinafter. The pulses from the 320 kilocycle oscillator are supplied at the rate of 320,000 pulses per second and are transmitted through the gate 439, under control of the ciphered pulses, to the radio system 450 which is frequently located on a radio tower and in the specific embodiment described herein comprises a video amplifier 451 and a pulsed oscillator or pulsed amplifier 452. The high frequency radio signals are then radiated from the antenna and/or other directing equipment 453.

These signals may then be transmitted through any number of intermediate relay repeater stations to the receiving terminal where they are received by the directive antenna system 753. The signals together with current from the beating oscillator 733 are applied to the crystal detector 734 which reduces the carrier frequency of the incoming signals after which the lower or intermediate frequency amplifier 732 amplifies the pulses or spurts of intermediate frequency and transmits them to another intermediate frequency and video amplifier 735. The radio equipment 731 is usually located on a tower near the directive antenna system 753. All of this equipment is designated 730 on FIG. 7.

The amplifier 735 includes intermediate frequency amplifier 736, the second detector 737, video amplifier 738 and a pulse limiting or shaping amplifier and equipment 739.

The radio equipment including transmitting equipment 450, receiving equipment 730 and 735 and all of the components thereof mentioned above may be of any suitable design and include any and all suitable types of apparatus and equipment. The exemplary system described herein makes use of the radio equipment of the AN/TRC-6 equipment supplied to the Signal Corps and described in War Department Technical Manual TM11-631 which manual is hereby made a part of the present application as fully included herein.

It is understood of course that any other suitable equipment may be employed for transmitting the signals from the gate circuit 439 to the new pulse generator 711 to be described hereinafter. This other equipment may include open-wire line circuits, cable circuits, coaxial circuits, wave guides or other transmission structure or media arranged to have a sufficiently wide frequency band to transmit the individual pulses described above.

At the receiving station the output of the radio amplifier 735 or the output of any other transmission apparatus comprises pulses similar in character to those delivered by the gate circuit 439. The new pulse generator 711 generates new pulses under control of the received pulses, which new pulses have lengths which are independent of the received pulses and most suitable for controlling the receiving equipment.

The receiving station has a keying circuit for generating key pulses similar to those generated by the key equipment at the transmitting station. The switches 765, 766 etc. of course are set in the same condition as the corresponding switches 465, 466 etc. of the thirteenth-stage ring at the transmitting station. Likewise, switches 755, 776 etc. of the eleven-stage ring at the receiving station are set in the same condition as the switches 475, 476 etc., of the eleven-stage ring at the transmitting station. Likewise, switches 880 and 780 are set in the same position as corresponding switches 325 and 480 at the transmitting station. The same applies to switches 780 and 890 and switches 490 and 326. With these switches all set in the same positions at the two ends of the system and with the key equipment as well as the synchronizing equipment properly synchronized at both ends of the system, the key equipment at the receiving station supplies a key to the deciphering circuit 712 which is identical with the key which the key equipment at the transmitting station supplied to the enciphering circuit 438.

The deciphering circuit 712 at the receiving station combines the pulses from the new pulse generator and the key pulses from the key circuit and recovers the coded pulses substantially in the same manner as the enciphering circuit combines the code pulses and the key pulses to form the enciphered pulses.

The deciphered code pulses are then transmitted over conductor 701 through the video amplifier 810 to the decoding equipment and the complex signaling waves recovered in the manner previously described herein.

Synchronizing

In the foregoing description it has been assumed that the receiving station was synchronized accurately with the equipment at the transmitting station. In other words, it has been assumed that, except for the delays pointed out above the five-stage ring at each station has the same stage actuated at each station and that the eight-stage rings are properly coordinated at the two ends of the system, although not with the same stages simultaneously actuated as will be pointed out hereinafter. In addition, it has been assumed that the keyers are similarly conditioned and that the same stages of the eleven-stage and thirteen-stage rings in the keyers at the two ends of the system are similarly actaated at all times.

Persons skilled in the art will, of course, appreciate that when power is first applied to the two ends of the system the equipment at both the transmitting and receiving station will not be so synchronized unless by mere accident which would not occur very often.

One of the objects and features of the present invention is to provide equipment and circuits to automatically cause the equipment at the receiving end to come into proper synchronism or phase with the equipment at the transmitting station. In order for this to be accomplished it is necessary that the eight-stage ring separate the incoming pulses into the proper code groups of five pulses so that each of the pulses comprising a code group are delivered to the proper decoding equipment. In addition, it is necessary that the five-stage and eight-stage rings be properly synchronized and phased so that they may properly drive the keyer equipment at the receiving station and thus maintain this equipment in proper synchronism and phase with the keyer equipment at the transmitting station. In other words, it is necessary to have both the multiplex equipment and the key equipment at both stations in proper phase in order to decipher and decode the received signal pulses and then distribute the resulting signals to the proper channels so that the complex wave or speech current may be reconstructed and transmitted to the proper destination. Considering the multiplexing and decoding equipment first, it should be pointed out that 40 pulse intervals, that is eight times five, comprise a complete cycle or frame of the entire multiplex system. In other words, five pulses each of which may be of either a marking or a spacing character are transmitted for each of the eight channels, the pulses of each channel comprising a code combination. Then the cycle is repeated with code groups of pulses again transmitted from each of the eight channels and so on. This means that there are 40 different possible relative positions between the multiplex equipment at the receiving station and the multiplex equipment at the transmitting station. In order to properly synchronize this equipment at the receiving station with the equipment at the transmitting station, it must be possible to recognize the correct one of the 40 relative positions and then maintain the equipment at the receiving station in this particular orientation.

If the equipment at the receiving station were initially caused to run slightly faster or slightly slower than the equipment at the transmitting station, then over a period of time the equipment at the receiving station would occupy each one of the 40 different possible periods of time. For example, if the oscillator at the receiving station were to operate at one cycle per second slower than the oscillator at the transmitting station, then in 40 seconds the equipment at the receiving station would have been in each one of the 40 possible positions relative to the corresponding equipment at the transmitting station. Furthermore, if some distinctive signal were applied to one of the channels of the system and then this signal recognized at the receiving station on the channel in question, then this signal could be employed at the receiving station to change the rate of operation of the receiving equipment so that the equipment at the receiving station could be maintained in synchronism with the equipment at the transmitting station.

It is, of course, apparent that the signaling pulses have a relatively large frequency component of 320 kilocycles, which may be obtained from the received pulses and employed to lock or maintain the crystal oscillator at the receiving station in accurate frequency and phase relationship with the incoming pulses and thus with the oscillator equipment at the transmitting station. However, this frequency component cannot be employed to determine when the multiplex equipment, that is, the five-stage and eight-stage ring, are in the proper phase or orientation in the system. However, by employing a distinctive signal to indicate when the multiplex equipment is in proper synchronism at the two ends of the system, this signal may then be employed to control the injection of the 320 kilocycle component from the received pulses into the crystal-controlled oscillator circuit which will thereafter maintain the equipment at the two ends of the system in proper synchronism.

There is, however, the additional requirement that the key equipment at the two ends of the system also be in actual synchronism. For the keying equipment to be in synchronism at the two ends of the system, it is first necessary that the corresponding keys associated both with the eleven- and thirteen-stage ring output circuits as well as the keys associated with the driving circuits for these rings be set in identical positions at the two ends of the system. If they are not so set, it is impossible to obtain the same key at both the transmitting and receiving stations. Consequently, it will be impossible to properly decipher the signals at the receiving station and thus impossible to drive the key equipment at the receiving station in the same manner as it is driven at the transmitting station.

When the keying equipments at the two ends of the system are not in synchronism, the signals on each of the voice frequency channels sound substantially the same as the signals on the high frequency radio channel when received with an ordinary amplitude or frequency modulation radio receiver. In other words, they appear as noise and the noise is of a considerable magnitude or volume. This noise is due to the fact that the key pulses are essentially random in character and when combined with the code pulses produce substantially random inciphered pulses. Such random pulses appear and sound like noise. When the receiving equipment is out of synchronism the pulses applied to any receiving channel are likewise a random character and hence also appear as noise.

This characteristic of the system including the keying equipment has been employed in the synchronizing of the system by employing the change from high level noise on the low frequency channels at the receiving station when the systems are not in synchronism to a low level of noise when the systems become synchronized. Thus by setting aside one channel and applying nothing but low level noise to this channel at the transmitting station and then at the receiving station automatically measuring the level of noise received on the corresponding channel it is possible to cause the systems to become automatically synchronized. The keying equipment, however, has to be properly conditioned so that the high level noise will always be presented on the selected channel at the receiving station so long as the equipment at the receiving station is not properly synchronized with the equipment at the transmitting station. This is accomplished by appropriate setting of the switches associated with the stepping circuits of the keying equipment. If for example, one of the switches associated with the selection of the channel time during which a keyer stepping pulse may be suppressed, as will be described hereinafter, is always set in the zero position at both ends of the system, the predetermined channel No. 1, for example, will always have high level noise at the receiving station, unless the equipment at both stations is properly synchronized.

In the keyer circuit there are 143 possible different relative positions between the eleventh- and thirteen-stage rings at the two ends of the system, in other words, 11 times 13 different possible relative positions between these rings of the transmitting and receiving keyers. However, all of these different positions are not entirely independent one from the other because it can be shown that if the sequence of the setting of the keys were the same for each ring, although displaced in stages one from another, the equipment at the receiving station would operate properly with the equipment at the transmitting station, enen though the switches at the two ends were not identical on each stage. For example, let us assume that in the eleven-stage ring at the transmitting end the keys individual to the odd stages are operated in one direction say, up, and the keys of the even stages operated in the opposite direction, down. This means that the keys are all alternate except for stages eleven and one which are both odd and thus have both keys actuated in the same direction. Without the coincidence circuits it is possible to operate the keys at the receiving station so that the system would operate properly if the same sequence were maintained, that is, the keys operated alternately, but some place in the circuit two successive keys would have to be actuated to their upper position. If this occurred for stages 5 and 6, for example, then the equipment would operate satisfactorily with stage 6 of the receiving ring actuated at the same time that stage 1 of the transmitting ring was actuated. Thereafter the two rings would stay in corresponding relative positions and deliver the proper corresponding output pulses to their respective output circuits. Consequently, even though the switches at the receiving keyer were set differently from the switches at the transmitting keyer the system would operate properly. Thus, the 143 different positions are not all independent or unique relative one to another and in order to avoid this difficulty there was provided the coincidence circuit which delays one ring every time one of the stages, say, the No. 1 stages, are simultaneously actuated. This reduces the number of different relative positions between the transmitting and receiving keyers to 78. However, each one of these 78 positions is unique so that the switches at the two ends have to correspond stage for stage throughout both rings of both keyers.

During the time the multiplex equipment is searching through these relative positions in the manner described above, the key equipment at the receiving station will also be operated in a random manner through the various relative positions which it may assume relative to the key equipment at the transmitting station. The change in the relative positions of the key equipment at the two ends of the system at this time is due to the fact that different driving pulses are suppressed and also due to the fact that the driving pulses are supplied to the two keying generators at different rates. However, once the receiving key equipment has been actuated to the orientation corresponding to that at the transmitting equipment and at the same time the multiplex equipment is in the proper orientation, the keying equipment at the receiving station will thereafter be maintained in synchronism with the keying equipment at the transmitting station because it will receive the stepping or driving pulses at the correct instants of time and will have these driving pulses suppressed at the proper instants of time relative to the received signals and relative to the proper orientation of the multiplex equipment. At this time the noise level of all of the channels will change from a high value to a low value and the equipment responsive to this change in channel 1 will then lock the receiving oscillator in synchronism with the incoming signals and thus maintain the entire receiving equipment in synchronism with the transmitting equipment at the transmitting station.

Such an arrangement is particularly advantageous in systems of the type described herein operating over radio paths because it will automatically synchronize itself after each interruption of the transmission path, as frequently happens on radio paths. Thus the channel time lost due to channel interruption and loss of synchronism due to other extraneous interfering currents is reduced to a minimum.

Order Wire Circuit

In addition to the regular communication channels provided as described above, it is usually desirable to provide a service channel which is frequently called an "order wire" to permit the attendants at the terminals and also at the intermediate repeater stations when such are provided to communicate with each other for maintaining the system in proper adjustment and operating condition. In order to provide such an order wire circuit means have been provided for employing the entire array of enciphered pulses analogous to a carrier current and for time or position-modulating these pulses in accordance with signals desired to be transmitted over the order wire channel. The time or position modulation of the entire array of enciphered pulses which occur in a random manner is somewhat analogous to phase modulation of a carrier current.

The handset 460 represents a source of order wire signals. Any other suitable source other than the handset may be employed and as is described in detail in the above-identified application of Anderson-Edson any suitable communication path may be interposed between the actual source of the signals and the order wire equipment described herein. The order wire signals from the handset 460 for example may be transmitted through a hybrid coil 441 or they may be transmitted directly to a pulse position modulation circuits 444 as described in detail hereinafter. These order wire signals are employed in the pulse position modulation circuits to vary the time of occurrence of the pulses from the 320 kilocycle oscillator in accordance with the order wire signals to be transmitted. In other words, the pulses from the pulse generator 436 are equally spaced one from another and occur in regular succession under control of the output of the 320 kilocycle oscillator. The pulse position modulation system and circuits 444 however vary the time of occurrence of these pulses so that they are irregularly spaced in time by a small amount in accordance with the order wire signals. These modulated pulses are supplied to the gate circuit 439 which transmits them or suppresses them, under control of the enciphering circuit 438 as described above, with the result that the pulses from the gate circuit 439 occur in accordance with the currents of the pulses from the enciphering circuit 438. In other words, the character of each pulse whether it may be marking or spacing is controlled by the enciphering circuit 438. However, the time of occurrence of these pulses is controlled by the order wire signals. It should be pointed out that the time of occurrence of the pulses is limited to a given pulse interval. If the pulses were not so limited the order wire circuit might cause the pulses to be advanced or retarded into a pulse interval assigned to another pulse and thus add noise or distortion to the signals represented by the ciphered pulses. The radio equipment described above is capable of transmitting the pulses when they are both enciphered and position modulated so that similar pulses which are both ciphered and time or position modulated are delivered to the new pulse generator 711. The new pulses from the new pulse generator 711 are similarly modulated. These new pulses have been supplied to the deciphering circuits 712 where they are deciphered and then sent to decoder to be decoded in the manner described above. The new pulses from the new pulse generator 711 are employed to control the phase of an alternating current which is also supplied to the pulse position demodulation circuits 713. This demodulator circuit is also supplied by a reference current from the oscillator 710. The pulse position demodulator circuit 713 compares the current controlled by the pulses from the new pulse generator 711 with the reference current and derives the order wire signals from them and transmits them to the corresponding order wire receiving device 960 through the hybrid coil 941 when it is necessary or desirable to employ such a hybrid coil. In case it is unnecessary to employ a hybrid coil the connections to this coil will changed so it is used as an output transformer from which the signals are transmitted to the receiving device 960 of the order wire circuit.

The opposite direction of transmission over the order wire circuit is provided over the circuits operating in the opposite direction in the same manner as described above for the transmission of signals from the handset 460 to the handset 960.

It is thus apparent that it is possible to transmit the coded pulses from the transmitting station to the receiving station without first enciphering them or time position modulating them. It is also possible to encipher these signals without time position modulating them when it is so desired and then convey the ciphered pulses to the receiving station or it is possible to perform all of these functions simultaneously.

Detailed Description

FIGS. 5, 6, 9, 10 and 12 through 48, inclusive, show in detail the circuits at the transmitting and receiving terminals of an exemplary system embodying the present invention. FIGS. 12 through 31 show in detail the equipment at one terminal for terminating seven incoming voice frequency channels a synchronizing channel and an order wire channel. The transmitting equipment required to convert or translate signals incoming from the seven voice channels and the synchronizing channel into code groups of pulses representing these signals is also shown in detail as is the equipment for generating the cipher key for enciphering the pulse code groups before they are transmitted. FIGS. 32 through 48, inclusive, show the corresponding receiving equipment at the receiving terminal of the system for deciphering the pulses and then decoding them and finally deriving from them signaling waves similar to those applied to the system at the first terminal.

A transmission path in the opposite direction is shown in outline form in FIGS. 9 and 10 at the second terminal and in FIGS. 5 and 6 at the first terminal. Inasmuch as the equipment represented by FIGS. 9 and 10 at the second terminal is substantially the same and operates in substantially the same manner as the corresponding equipment shown in detail in FIGS. 12 and 31 at the first terminal, and inasmuch as the equipment shown in outline form in FIGS. 5 and 6 at the first terminal operates in substantially the same manner and substantially the same as the equipment shown in FIGS. 32 through 48, inclusive, there is no object in duplicating the description of this equipment in any greater detail for the transmission in the opposite direction; the interconnections between the transmissions in both directions being clearly shown in detail at both terminals.

Control Equipment

The operation of the complete system may be more readily understood if the operation of some of the common control equipment is described first so that its operation will be readily understood when reference to it is required in order to explain and understand the operation of the other elements of the system.

The operation of the complete system is under control of an oscillator shown in FIG. 24. Inasmuch as this oscillator controls the timing and operation of the complete system and inasmuch as the receiving equipment must be maintained in synchronism with the equipment at the receiving station, it is desirable that this oscillator have a high degree of stability.

In the exemplary system described herein the oscillator comprises tubes 2410 and 2411. The output of tube 2410 is coupled to the input of tube 2411 through condenser 2412, while the output of tube 2411 is fed back through the crystal 2413 to the input of tube 2410. The crystal 2413 is operated at a constant temperature in oven 2414 in the well-known manner to maintain its frequency characteristics substantially constant and independent of the ambient temperature. The crystal 2413 operates at its series resonant point so that the small series condenser 2415 may be employed to make small adjustments in the frequency of the output wave. As pointed out hereinbefore in the exemplary embodiment described herein this oscillator oscillates at a frequency of 320 kilocycles per second. This oscillator is employed to control a number of different circuits.

Transformer 2416 together with the impedance connected in shunt with it forms the load impedance for tube 2411 and is tuned to 320 kilocycles. As a result a balanced 320 kilocycle voltage is applied to the grid of both sections of tube 2417 which operates as a fifth harmonic generator. The output impedance of this harmonic generator is tuned to 1600 kilocycles by transformer 2418. As a result the fifth harmonic of 320 kilocycles or 1600 kilocycles is applied to both grids of tube 2419 which operates as an amplifier. The output of this tube is applied to conductors 2420 as a balanced 1600 kilocycle voltage through transformer 2421 which is tuned to 1600 kilocycles. The tuned transformer 2418 and 2421 together with the fifth harmonic generator tube 2417 and amplifier tube 2419 produce and amplify the fifth harmonic of the 320 kilocycle voltage obtained from one tube 2411 through transformer 2416 and suppress or eliminate the fundamental and all other harmonic frequencies of this oscillator.

A portion of the output of tube 2411 is coupled through the network comprising resistors 2310 and capacitors 2311 to the grid of the right-hand section of tube 2312. The coupling network comprising resistor 2310 and capacitors 2311 is employed to prevent the operation of the circuits of tube 2312 from interfering with the operation of the oscillator and to control the phase of the 320 kilocycle voltage applied to the control grid of the right-hand section of tube 2312. The grid of the right-hand section of tube 2312 remains positive for about 1 microsecond.

Curve 4901 of FIG. 49 shows the wave form of the output of oscillator tube 2411 and curve 4902 shows the wave form of the potential of the control grid of the right-hand section of the tube 2312. It should be noted that the top of the curve is cut off flat for about 1 microsecond, as shown at 4903, due to the low grid to cathode impedance caused by the positive potential on the control grid of this section at this time.

The plate circuit of the right-hand side of tube 2312 includes inductance coil 2313 which differentiates the voltage wave which appears in the plate circuit as a result of the wave applied to the control grid. Due to the differentiating action the voltage across 2313 appears as a sharp negative pulse at a time corresponding to the beginning of the flat top at 4903 of curve 4902, FIG. 49, and a sharp positive pulse at the end of the flat top. The voltage across the coil is applied to the grid of the left-hand section of 2312 through the coupling condenser 2314.

The positive pulse at the end of the 1 microsecond is suppressed by low grid impedance of the left-hand section of tube 2312 when this control grid is positive.

Due in part to the action of the inductance 2313 at the beginning of the positive microsecond pulse applied to the control grid of the right-hand section of tube 2312 a large negative pulse is applied through the coupling condenser 2314 to the grid of the left-hand secti