Method of and apparatus for scrambled television program addressable subscription selection and decoding4494142Abstract This disclosure is concerned with an improved special program addressable subscription decoding technique in accordance with which ordinary program subscribing decoders continue to receive and decode the scrambled flat rate programs to which they are entitled while being disabled from receiving any of a plurality of special scrambled programs associated with a different control signal that will only enable decoding after pre-set unique address verification has been transmitted, checked and introduced into memory indicative of subscription to particular of such special programs, whereby the special program decoding will be effected compatibly with the continued flat rate program decoding by ordinary program subscribers. Claims What is claimed is: Description The present invention relates to methods of and apparatus for scrambled television transmission and reception, being more particularly directed to addressable multi-special program decoding subscription techniques operable compatibly with ordinary program subscription decoding at receivers that have not subscribed to the special programming.
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Bit rate 5 kilobits/sec., (1 bit = 200 .mu.s)
Character length
24 bits + parity bit
(D.sub.1 -D.sub.20 + C.sub.1 -C.sub.4)
Parity .sub.(P) Odd
Start bit One (logic `1`)
Stop bit One (logic `0`)
Sync bit 11/2 times bit high + 11/2
("Sync characters")
times bit low.
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An address cycle contains thus 30 bits, using non-return-to zero pulses and occupying 6 ms of a transmission time. The address portion of the frame utilizes bits D1 through D20, allowing for over 1 million unique combinations (addresses). A control character portion occupies bits C1 through C4, involving combinations of the same that provide for 16 unique combinations. These combinations, and corresponding commands, are listed in the following table:
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TABLE OF CONTROL CHARACTERS
LOGIC INPUT
OUT-
C4 C3 C2 C1 PUT NAME
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0 0 0 0 X0 DO NOT USE
0 0 0 1 X1 MASTER ON (MON)
0 0 1 0 X2 ADDRESS ON (AON)
0 0 1 1 X3 ADDRESS OFF (AOF)
0 1 0 0 X4 SPECIAL PROGRAM
(SON)
ON
0 1 0 1 X5 SPECIAL PROGRAM
(SOF)
OFF
0 1 1 0 X6 --
0 1 1 1 X7 --
1 0 0 0 X8 PROG: A ON (PAN)
1 0 0 1 X9 PROG: A OFF (PAF)
1 0 1 0 X10 PROG: B ON (PBN)
1 0 1 1 X11 PROG: B OFF (PBF)
1 1 0 0 X12 PROG: C ON (PCN)
1 1 0 1 X13 PROG: C OFF (PCF)
1 1 1 0 X14 --
1 1 1 1 X15 DO NOT USE
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Not all of the possible combinations are presently shown as used. As previously mentioned, the transmission time of one frame equals 6 ms, thus enabling an addressing speed of approximately 10,000 unique addresses (with commands) per minute. It is now in order to describe the constructional and functional details of the preferred LSI address decoder of FIG. 2, as delineated in the circuit diagram of FIGS. 3A-C. The binary data in the format shown in FIG. 4 is fed from the voltage comparator output of FIG. 2 to the input at the LSI Address Decoder, also shown as left-hand lead 10A in FIG. 3A, and from there to a serial-in, parallel-out 27 bit SHIFT REGISTER 2B, and also to a PULSE EDGE DETECTOR 3B. The PULSE EDGE DETECTOR 3B recognizes both leading and trailing edges, with the leading edge used to trigger a SYNC WINDOW GENERATOR 4B which generates a pulse of approximately 90 .mu.s (0.45.times.bit duration), centered on 300 .mu.s trailing edge (1.5.times.bit duration) in respect to triggering the leading edge of logic `1` portion of the sync bit. This pulse is then used to gate a SYNC TRANSITION DETECTOR 5B which senses a negative transition (trailing edge) supplied to it from the PULSE EDGE DETECTOR 3B. Should such a transition occur within the SYNC WINDOW duration, the SYNC WINDOW DETECTOR 5B provides via "or" gate 5C a RESET pulse, so-labelled, to SHIFT REGISTER 2B, and a PARITY CHECKER causing "clear" of all flip-flops; and, also, a RESET pulse via conductor 5D to a SAMPLE CLOCK DOWN COUNTER 6B, which starts generating the following: (i) "RR" clock pulses ("CLK") repeated every 199.76 .mu.s and lasting 1.3969 .mu.s (=MASTER CLOCK.times.5), which are fed to SHIFT REGISTER 2B to cause shifting of the incoming data; (ii) a "PARITY SAMPLE" pulse to PARITY CHECKER 7B, FIG. 3B, (toggle flip-flop); and (iii) strobe pulses "N+1" and "N+2", which are later used by an ADDRESS LATCH AND COMPARATOR 29B and by 25% DUTY CYCLE GENERATOR 8B, FIG. 3C, later fully explained. The clock pulses to the entire system are provided by a crystal-controlled MASTER CLOCK 10B followed by a divide-by-5 COUNTER 11B. The MASTER CLOCK may, for example, utilize a crystal providing f=3.579545 MHz, giving after division, .0.1M and .0.2M (0.degree. and 180.degree., respectively). The SAMPLE CLOCK DOWN COUNTER 6B, after being reset by the SYNC TRANSITION DETECTOR 5B, starts counting down from its initial state, say N=143. Its outputs may give the following:
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"RR" sample pulses counter state N = 143
"parity sample" counter state N = 137
"N + 1" strobe counter state N = 130
"N + 2" strobe counter state N = 129
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The above sequence is repeated every 143 Master Clock pulses (01M, 02M), resulting in the bit rate equal to 5.00627 kilobits/sec. The bit rate can be altered by employing different crystals. After an entire frame has been loaded into the SHIFT REGISTER 2B, a STOP bit (always low), shown to the far left and far right in FIG. 4, and a START bit (always high), will appear in flip-flop No. 27 and No. 1 of the SHIFT REGISTER 2B. Also, the PARITY CHECKER 7B (toggle flip-flop and "0"-"1" detector) will provide logic "1" if the parity check is positive. These conditions will, in turn, produce logic "0" (AFO) at the output of the AFO gate "N AND" 13B, which will cut off the clock pulses to the SHIFT REGISTER 2B, stopping, in this way, the shifting of incoming data. Now the data present in the SHIFT REGISTER 2B is ready for decoding. The next frame will be loaded only after the next SYNC PULSE (i.e., negative transition within SYNC WINDOW) is detected, causing general RESET and repetition of the above-described procedure. The unique address recognition is effected with the aid of each Address Decoder plug-in module 8 (FIG. 1), individually assigned a unique address which is programmed during its manufacturing process. This is done by electrical "fusing" or jumping of one or more of the 16 straps connected to, say, pins 4 through 12 and 20 through 26 of the LSI Address Decoder, forcing data lines (d1 through d16, FIG. 3) either high or low, enforcing required code. These 16 lines give 2.sup.16 =65536 different combinations. The remaining four data lines (d17 through d20) are programmed internally by means of strapping silicon chip pads (E27 through E30) inside the LSI ADDRESS DECODER. This operation provides 16 additional combinations so that the total amount of available combinations (or unique addresses) equals 16=65536=1,048,576. The received data, after being loaded in the SHIFT REGISTER 2B is now available for further processing. The content of flip-flops No. 2 through No. 21 is fed to 20 bit comparator 9B and compared with the pre-programmed data lines d1 through d20. A positive result of this comparison results in logic "1" at the output of the comparator, which, when combined with e,ovs/AFO/ , produces an ADO (Address Detector Output) at the ADDRESS DETECTOR 21B. The presence of ADO acts as a "go ahead" for the processing of a control command accompanying the just-recognized unique address. As for recognition of a common address, a combination of 0, 1, 0, 0 . . . for D1, D2, D3, D4 . . . respectively, has been reserved as so-called "common address", which is to be recognized by all operating decoders, irrespective of their unique addresses. This common address is used with several commands which will be explained later, and the function is performed by a COMMON ADDRESS DETECTOR 22B, FIG. 3B, which senses the 0-1-0-1 . . . train on the data lines D1 through D20 and produces a CDO (Common Address Detector Output) on its output, when enabled by e,ovs/AFO/ , (i.e. when the frame has been accepted and loaded to the SHIFT REGISTER 2B). The presence of CDO acts similarly to ADO, but is used with different command signals. After the frame has been loaded, the AFO enables CONTROL CHARACTER DECODER 23B (at top of FIG. 3B), which receives the data (C1 through C4) from corresponding portions of the SHIFT REGISTER 2B. One of the characters listed in the previously tabulated TABLE OF CONTROL CHARACTERS is being decoded and one of the corresponding lines (X0 through X15) goes high. Depending upon which line is set high, an associated algorithm is executed. An explanation of the functions of the CONTROL CHARACTERS and how they affect the Decoder is now in order. There are basically two ways of controlling the Decoder installed in the field: a real-time addressing and a pre-programmed addressing. Real-time addressng involves the following: MON--MASTER ON (with common address): AON --ADDRESS ON (meaning unique); and AOF--ADDRESS OFF (meaning unique. Any of these commands, when transmitted with appropriate address, will cause an intermediate execution of a requested function, which is also indicated by the MONTHLY PROGRAM LED indicator fed by lines 30A in FIGS. 2 and 3C. The MON is particularly useful in providing an instant service to all customers. The customers who are not supposed to receive the service can then be selectively turned off (by using AOF) within the next minutes. The AON will ensure an instant servicing of individual requests (as over the phone). Practically, the customer will be able to watch his request being executed while still talking to the computer operator, with the data preferably being sent from a computer to the Encoder and TV transmitting in the same instant as the operator keys it into his terminal. The AOF has a counter-effect to the AON in that it simply disables the Decoder whose unique address has been sent with it. As for pre-addressing with future execution, this task is handled by the following commands: SON--SPECIAL PROGRAM ON; SOF--SPECIAL PROGRAM OFF; and group programs; PAN--GROUP PROGRAM A ON; PAF--GROUP PROGRAM A OFF; PBN--GROUP PROGRAM B ON; PBF--GROUP PROGRAM B OFF; PCN--GROUP PROGRAM C ON; and PCF--GROUP PROGRAM C OFF. A special Program feature is intended for broadcasting extra events (e.g., live sports, interviews, shows, etc.) which do not happen on a regular basis. Suppose, for example, that show X has been assigned SPECIAL PROGRAM status. The customer can order it in advance and his or her request is then honored by transmitting SON with the unique address. This will set the SPECIAL PROGRAM MEMORY LATCH 25B in the Decoder and is also indicated by the SPECIAL PROGRAM LED 26B being lit, FIG. 3B. This indication ensures the viewer that his or her request has been processed promptly. The same applies to any number of other customers who decided to watch SPECIAL PROGRAM. This pre-addressing can be done at any time before the SPECIAL PROGRAM starts, and also during FREE MODE transmission and TV Studio's off-service hours. Prior to the commencement of Show X, the TV station transmits SON with common address and turns on the CONTROL TONE (23.6 KHz CW in the aural baseband). This action causes the following: a. the Decoders which were previously pre-programmed for Show X will decode the signal and indicate their status by extinguishing MONTHLY PROGRAM LED, leaving only SPECIAL PROGRAM LED lit; b. the not pre-programmed Decoders will not decode and also indicate their status (i.e., not authorized for this special program-Show X), keeping SPECIAL PROGRAM LED off, bringing MONTHLY PROGRAM LED to half of its normal brightness, and providing a low level, but clearly audible, continuous "beep" from the speaker. In this way, the viewer is kept well informed about the nature of the program being currently transmitted (i.e., Special Program is on, but your Decoder is not authorized to receive it. Should the non-authorized viewer wish to receive the Special Program after it has already commenced, this can be done by contacting the operator who will immediately transmit SON with particular unique address, instantaneously authorizing the Decoder. After the Show X transmission has been completed, the TV station will transmit SOF with common address, meaning SPECIAL PROGRAM OFF, being addressed to all the Decoders in the field and turn CONTROL TONE off. This will bring all of the Decoders to their original status before the Special Program broadcast. The LED indicators will display the status, accordingly. The construction of the LSI Address Decoder of the invention allows for further splitting the broadcasting into up to, say, three groups of certain additional special programs. (Examples: Group A--movies; Group B-13 Sports; Group C--children's programs; etc.). This feature allows a viewer to select the kind of programs which suit personal taste, and purchase it, accordingly. A programming procedure for the GROUP PROGRAMS A, B and C is, for example, as follows. In the beginning of certain time-periods (e.g., every month), the planned programs are assigned their status. This information is available to the viewers in advance, allowing them to make their choices and inform the broadcast station. These requests are then stored in the TV stations's computer memory and, once completed, are transmitted to the customers in the following format: Viwer's choice+his unique address; i.e., PAN+address XX; PCN+address XX; PBN+address YY; etc. This means: Group programs A and C--for viewer XX; Group program B--for viewer YY, etc. The pre-programmed Decoders will store the above commands in their GROUP PROGRAM MEMORY LATCHES A, B or C, (27B). This action is not indicated by any LED on the Decoder. The pre-programming can take place at any time, both during FREE MODE and ENCODED transmissions. The proper programming is ensured by double addressing, later more fully discussed. Prior to the transmission of the special Group Program "A", the TV station turns the CONTROL TONE on to lock-out all the decoders in the field from decoding the program, and then sends PAN with common address; meaning, the Group Program "A" is now due to be decoded (command PAN) and this message is for all Decoders in the field (common address). When the Decoder receives the above, those pre-programmed for the GROUP PROGRAM "A" will decode it, indicating also correct program status by extinguishing the MONTHLY PROGRAM LED. If the Monthly Program has not been previously authorized, the MONTHLY PROGRAMED LED simply remains off. Those decoders which were not pre-programmed for the GROUP PROGRAM "A" will not decode it, indicating also current program status by dimmed MONTHLY PROGRAM LED, and a "beep" for the speaker, regardless of whether the Monthly Program has or has not been previously authorized. The status of the SPECIAL PROGRAM LED remains unchanged during any operation related to GROUP PROGRAM addressing. In general, the dim MONTHLY PROGRAM LED and a "beep" from the speaker indicates that there is a program being currently transmitted, but reception has not been authorized for it. After the broadcasting of the GROUP PROGRAM "A" has been completed, the TV station turns the CONTROL TONE off and transmits the following: PAF + common address, meaning: Reset GROUP PROGRAM "A" DECODE LATCH 27B (PAF) and this information is for all Decoders in the field (common address). The Decoders now assume their status as before PAN command transmission, which is indicated by corresponding LED's. The above operation does not, however, reset the GROUP PROGRAM "A" MEMORY LATCH 27B, so all those Decoders which have been initially authorized for GROUP PROGRAM "A"can be turned on instantly at any time when next GROUP PROGRAM "A" is about to be broadcasted, by a single command PAN + common address; while those non-authorized and not subscribing will not decode, as previously described. In order to cancel GROUP PROGRAM "A" authorization in the particular Decoder, the TV station has to transmit PAF + unique address. This will reset the GROUP PROGRAM "A" MEMORY LATCH 27B (in this particular decoder only), and from this moment on, the Decoder will not be able to receive a GROUP PROGRAM "A" unless authorized again. The operation is identical for GROUP PROGRAM "B" and GROUP PROGRAM "C". In order to increase reliability of the system and protect a subscriber from being turned off accidentally (e.g., transmission error, etc.) a double addressing feature has been incorporated into the LSI Address Decoder of the invention. This requires certain commands (with their accompanying addresses) to be transmitted and received correctly twice, before the algorithm corresponding to the particular command will be executed. Suitable commands, requiring double addressing, are:
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AOF Monthly address OFF;
PAN Group Program A ON;
PAF Group Program A OFF; and
PBN
PBF - PCN Group Programs B and C, the same as
Program A.
PCF
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Monthly Program ON (AON) requires only a single address frame, and this is due to the high transmission speed required when programming customers for AON command. Master ON (MON) is usually transmitted as multiple-frame command (i.e., 10 frames) for higher reliability and hence is not included in the double addressing scheme. Special Program ON and OFF (SON and SOF) need fast addressing, also; thus single frame addressing is applied here as well. It should be noted that the double frame addressing feature has been included as a redundant security and reliability measure. The system, however, works very reliably using single frame only, even under severe multipath and low level signal conditions. A more detailed explanation of the decoded control characters processing is now appropriate. The processing of a particular command starts after: (a) the frame has been loaded to the SHIFT REGISTER 2B and parity checked, which results in AFO; (b) the address (either common or unique) has been recognized, giving either CDO or ADO, respectively; (c) the command has been decoded i.e., one of the X1 through X13 outputs of the CONTROL CHARACTER DECODER 23B is in high state; (d) in case of double addressing, the second frame has been successfully recognized, resulting in one of DX3 thru DX13 (output from the SUCCESSIVE ADDRESS LATCH AND COMPARATOR block 29B) being in high state. Provided that all of the above conditions are met, the command will set one of the memory latches reserved to store this command. The latch can be re-set by counter command (e.g., AON.fwdarw.AOF) at any time after that. Suitable control character algorithms are as follows: MASTER ON (MON). This command causes X1 to go high, and, since it is meant to affect all the customers at the same time, requires presence of the common address, which, in turn, provides CDO. These signals provide PROGRAM DECODE OUT (D) to a MAGNETIC LATCH INTERFACE CIRCUIT 30B. From there, signal D proceeds to the MONTHLY PROGRAM LATCH 31B, setting its output high. This information is now available at output lead 20A (far right in FIG. 3C, shown also as pin 17 (via AND and NOR gates). At the same time, MONTHLY PROGRAM LED (driven from pin 18, conductor 30A in FIGS. 2 and 3C goes ON, indicating that a reception of the monthly program is now authorized. ADDRESS ON (AON): When AON is decoded, X2 goes high. This command is addressed to a particular viewer, with ADO (a unique address presence) required with it. Both AON and ADO cause setting of MONTHLY PROGRAM DECODE LATCH 31B (pin 17 high). Since the unique address has been used here, and only one viewer is affected, the AON provides means for selective "turn on" of Monthly Program. The MONTHLY PROGRAM LED goes on, indicating Monthly Program authorization. ADDRESS OFF (AOF): This command refers to selective "turn-off" and as such will require a unique address to accompany it. Double addressing is also needed for higher reliability. When AOF is decoded, X3 goes high and along with ADO is stored in the SUCCESSIVE ADDRESS LATCH AND COMPARATOR 29B. As soon as the next frame is received and the next AOF decoded, the second combination of X3 ADO is compared to that stored in the SUCCESSIVE ADDRESS LATCH 29B. If the result of comparison is positive, DX3 goes high and together with ADO (DX3-ADO) resets MONTHLY PROGRAM LATCH 31B. This, in turn, causes MONTHLY PROGRAME DECODE/SCRAMBLE OUTPUT at 30A going low and MONTHLY PROGRAMED LED, off. SPECIAL PROGRAM ON (SON)--pre-programming: The decoding of SON forces X4 high which is then fed either to SPECIAL PROGRAM MEMORY LATCH 25B or SPECIAL PROGRAM DECODE LATCH 25C, depending on either ADO or CDO presence. For pre-programming the SON-ADO combination is used, so that the SPECIAL PROGRAM MEMORY LATCH 25B is set. This results in pin 19 going high and SPECIAL PROGRAM LED at conductor 50A being turned on. SPECIAL PROGRAM ON (SON)--Execution: Stored in the SPECIAL PROGRAM MEMORY LATCH 25B, command will be excuted when SON with the common address is decoded. SON-CDO will set SPECIAL PROGRAM DECODE LATCH 25C with the "Q" output there-indicated; when ANDed with the Q output of previously set SPECIAL PROGRAM MEMORY LATCH 25B provides logic `one` to pins 15 nd 17, causing special program execution. At the same time, the "Q" output of SPECIAL PROGRAM DECODE LATCH 25C will cut off the signal from the output of the MONTHLY PROGRAM DECODE LATCH turning also MONTHLY PROGRAMED LED off (at 30A). Had the SPECIAL PROGRAM MEMORY LATCH not been pre-programmed, the SPECIAL PROGRAM DECODE/SCRAMBLE output (pin 15), would have remained low (i.e., program cannot be executed), The MONTHLY PROGRAM DECODE LATCH is cut-off and the output of the 25% DUTY CYCLE GENERATOR 8B is enabled, driving MONTHLY PROGRAM LED with 25% of its normal on-current, and hence giving dim light. A 25% duty cycle square wave of f=Bit Rate:4 (1250 Hz) is also fed to the audio amplifier giving the before-described continuous "beep" signal. SPECIAL PROGRAM OFF (SOF): The decoding of SOF brings X5 high, and since it is to be transmitted to all the Decoders, CDO will appear along with it as a result of the common address transmission. The CDO-X5 will reset both SPECIAL PROGRAM MEMORY 25B and DECODE LATCHES 25C, bringing the Address Decoder to the status it occupied before SON transmission. This will result in SPECIAL PROGRAM LED turning off, pin 15 going down, 25% DUTY CYCLE GENERATOR 8B disabling and MONTHLY PROGRAM DECODE LATCH enabling, which ultimately allows monthly program decoding again.
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GROUP PROGRAM Control Characters
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PAN
PBN Program A, B or C-ON
PCN
PAF
PBF Program A, B or C-OFF
PCF
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Functionally, the operation of these commands is similar to that described for SON and SOF characters, since they are also intended for the pre-programmed type of addressing. Since the PBN-PBF and PCN-PCF pairs are exactly the same as PAN-PAF, operation of the one pair only will be described as illustrative. GROUP PROGRAM A ON (PAN)--PRE-PROGRAMMING: Decoding of this command results first in X8 going high, which is then passed to the SUCCESSIVE ADDRESS LATCH AND COMPARATOR 9B. This command is addressed to a single Decoder and hence requires ADO. After two successive X8-ADO have been successfully recognized, resulting in DX8 going high, PROGRAM "A" MEMORY LATCH 27B is set by DX8-ADO. This action is--contrary to similar operation of SPECIAL PROGRAM MEMORY LATCH--not indicated by any LED, but is also more reliable than SON due to the double-addressing feature. GROUP PROGRAM "A" ON--Execution: The command stored in the GROUP PROGRAM MEMORY LATCH 27B will be excuted after decoding the next PAN and CDO (common address-with execution carried out in all the Decoders simultaneously). The X8-CDO combination will set GROUP PROGRAM "A" DECODE LATCH 27C, whose output Q, when ANDed with output Q at previously set GROUP PROGRAM "A" MEMORY LATCH 27B, provides logic `one` to pin 15 and pin 17. These two, in turn, put the Decoder into DECODE mode and the TV program can now be viewed and heard. In the Decoders which were not pre-programmed for GROUP PROGRAM "A" (i.e., their PROGRAM "A" MEMORY LATCH has not been set) output Q of the GROUP PROGRAM DECODE LATCH 27C, being now low, will cut off logic "one" (if any) from MONTHLY PROGRAM DECODE LATCH and, in the absence of logic "one" from MONTHLY PROGRAM MEMORY LATCH, will result in low levels on both pins 15 and 17. This situation stops all decoding. At the same time, inverted "Q" output from GROUP PROGRAM "A" DECODE LATCH 27C will enable 25% DUTY CYCLE GENERATOR 8B, giving dim light from MONTHLY PROGRAM LED and "beep" from the speaker. GROUP PROGRAM "A" OFF (PAF)--end of broadcasting: At the end of a broadcast of the GROUP PROGRAM "A", the TV station transmits: PAF + common address. This forces X9 high, which, when ANDed with CDO will reset GROUP PROGRAM "A" DECODE LATCH 27C, bringing the Decoder to its previous status. The GROUP PROGRAM "A" MEMORY LATCH remains unaffected, allowing for reiterated execution of the GROUP PROGRAM "A". GROUP PROGRAM "A" OFF (PAF)--Cancellation of Authorization: In order to cancel authorization for a GROUP PROGRAM "A" of the particular Decoder, the TV station must transmit the following: PAF + unique address. Since this command is addressed to one Decoder only, it requires a unique address which, in turn, creates ADO. Double addressing is also required to perform this function, so that the ADO-Dx9 finally resets GROUP PROGRAM "A" MEMORY LATCH 27B, giving logic "zero" at its Q output. From this moment on, the Decoder will be unable to decode a GROUP PROGRAM "A", unless its GROUP PROGRAM "A" MEMORY LATCH is set again. A further important advantage of the invention resides in its power-off memory back-up features. Reliable operation of the control character processing part of the LSI Address Decoder is based on information stored in its memory cells. Should the power fall below a certain voltage level, all data stored in the memory would ordinarily be lost. In order to protect the system against such power failure, two independent back-up circuits have been implemented: one, an active protection for short time memory retention; and secondly, a passive one, for a power failure of an infinite length of time. The active protection utilizes the principle of capacitive storage, employing a high quality (low leakage) capacitor C' with its charging circuit, connected to the pin 16 and a POWER-ON RESET GENERATOR 33B. Once the supply voltage drops, a charge stored in the capacitor C' supplies all the memory latches, allowing them to remain active. Due to a very small current required to keep the memory operative and extremely low leakage of the capacitor C', the voltage at pin 16 decreases very slowly, following an exponential discharge curve. This allows the memory to retain its state for approximately 4 minutes, in this way taking care of short-time power failures. The POWER ON RESET GENERATOR 33B will provide a POCR (POWER-ON CONDITIONAL RESET) pulse to all the memory latches only if a voltage at pin 16 has fallen below, say, +5 V (which is a limit for the memory to remain operative). The memory circuits are otherwise treated as if power failure had never occured. This action insures that the memory content will not be altered by a normal power-on reset circuit. The storage capacitor C', of course can be replaced by a long-life low power battery, of a minimum voltage of about 5 V. Long-term power failure memory back-up is based upon a non-volatile magnetic memory, using a latching relay as its elementary cell. This kind of memory is able to retain its state for an infinite period of time with no need for supply power. This kind of protection is provided for the AON/AOF commands only, which takes care of the MONTHLY PROGRAM DECODE LATCH. Information of the MONTHLY PROGRAM DECODE LATCH state is provided, via pin 13 to the magnetic latch driving circuitry 30B which sets it in corresponding position. When the power is interrupted for a time longer than the capacitive storage back-up ability, all the semiconductor memory latches become inactive and the information stored there is lost. After the power has been restored again or POR (POWER ON RESET) would clear all circuits to their zero state. The information stored in the magnetic latch, however, has been kept unaffected by the power variations and is now fed, via pin 14 and the MAGNETIC LATCH INTERFACE CIRCUIT 30B to the MONTHLY PROGRAM DECODE LATCH 37B, hence restoring its previous status. This long-term memory retention is particularly useful when the user leaves for a longer period (e.g., weekend, vacation, etc.) and decides to turn the power off for safety reasons. While the invention has been described with reference to particular preferred decoding techniques, it is clearly applicable to a wide variety of scrambling and decoding processes, and as many of its features as may be desired can be employed without departing from the spirit and scope of the invention; further modifications as will occur to those skilled in this art being considered to fall within such scope as defined in the appended claims.
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