System and method for multiplexing a plurality of digital program services for transmission to remote locations

Abstract

A system and method for multiplexing a plurality of digital programs for transmission to a plurality of remote locations is claimed. An encoder is provided that first generates a plurality of subframe data streams each comprising a continuous sequence of subframes. Each subframe in each subframe data stream comprises at least a transport layer region and a program data region. For each subframe data stream, portions of the program data region of each of the subframes in that subframe data stream are allocated to selected ones of the plurality of programs. Mapping information is provided in the transport layer region of each subframe for use by a decoder in extracting a selected program from each subframe. A multiplex data stream is then generated at the encoder that comprises a continuous sequence of superframes. Each superframe comprises a superframe transport layer region and a subframe data region. A portion of the subframe data region of each superframe is allocated to at least one subframe from each of the plurality of subframe data streams. A superframe map is then generated and transmitted with each superframe that specifies at least the location of each subframe within the superframe. The multiplex data stream is then transmitted to the plurality of remote locations. According to one embodiment, at an intermediate location, subframe data streams may be extracted from one multiplex data stream for re-routing within other multiplex data streams.

Claims

What is claimed is:

1. A method of combining a plurality of digital programs into a multiplex data stream for transmitting said programs from an origination point to at least one remote location, wherein each program comprises a single bit stream containing a combination of related digital services, said method comprising the steps of:

a) multiplexing selected ones of the digital programs to generate a subframe data stream having a format comprising a continuous sequence of subframes different portions of each subframe being allocated to different ones of said digital programs;

b) generating for each subframe a subframe multiplex map that at least indirectly specifies which portions of that subframe are allocated to which of said selected digital programs, and inserting the subframe multiplex map in the subframe;

c) repeating step (a) and (b) a selected number of times, each time for different selected ones of said digital programs, thereby generating a plurality of subframe data streams each containing different ones of said digital programs and each comprising a continuous sequence of subframes;

d) multiplexing selected ones of the subframe data streams to generate said multiplex data stream, said multiplex data stream having a format comprising a continuous sequence of superframes, each superframe containing at least one subframe from at least one of said selected subframe data streams; and

e) generating, for each superframe in the multiplex data stream, a superframe map that specifies the location of each subframe within that superframe, and inserting the superframe map in that superframe.

2. A method according to claim 1 wherein each subframe has a format comprising a plurality of lines, a first group of lines in each subframe defining a subframe transport layer region and a second group of lines defining a program data region, and wherein step (a) comprises performing the following steps for each subframe:

(i) partitioning the program data region of the subframe into a plurality of program data packets;

(ii) allocating different portions of each program data packet in the subframe to different ones of said programs;

(iii) generating a program multiplex control packet that specifies which portions of each program data packet in the subframe are allocated to which programs; and

(iv) inserting the program multiplex control packet in the transport layer region of the subframe, the subframe multiplex map generated in step (b) for that subframe specifying the location of the program multiplex control packet within the transport layer region.

3. A method according to claim 2 wherein each of said programs has a respective data rate, and wherein step (a)(ii) comprises allocating different portions of each program data packet to different ones of said programs in proportion to the respective data rates of said programs.

4. A method according to claim 3 further comprising the step of adjusting, on a per subframe basis, the size of the portions allocated to each program as the respective data rates of the programs change.

5. A method according to claim 2 further comprising the step of:

(a') generating a plurality of system related data packets including system data packets (SDP's), system teletext packets (STT's), addressable data packets (ADP's), service seed packets (SSP's) and program virtual channel map packets (PVCMP's); and

(b') inserting selected ones of said system related data packets in the transport layer region of each subframe, the subframe multiplex map generated in step (b) for each subframe specifying the location of each system related data packet inserted within the transport layer region of that subframe.

6. A method according to claim 1 wherein each superframe has a format comprising an adjustable number of successive blocks, a first portion of each block comprising a block synchronization byte (BSYNC), a second portion of each block comprising a data portion, and a last portion of each block comprising a block error code generated in accordance with a block error coding scheme.

7. A method according to claim 6 wherein a first group of blocks in each superframe define a superframe transport layer region of that superframe, and a second group of blocks define a subframe data region, the superframe map generated for each superframe being inserted in the superframe transport layer region of that superframe.

8. A method according to claim 7 wherein each subframe has a format comprising a plurality of lines, and wherein the data portion of each block in the subframe data region of each superframe contains a single line of one of the subframes in that superframe.

9. A method according to claim 6 further comprising the step of inserting, in a first block of each superframe, a superframe synchronization word (FSYNC), and wherein the superframe map for each superframe is inserted in the superframe immediately following the superframe synchronization word.

10. A method according to claim 1 wherein each superframe has a format comprising an adjustable number of successive blocks, a first group of blocks in each superframe defining a superframe transport layer region of that superframe, and a second group of blocks defining a subframe data region, said method further comprising the steps of:

(i) generating a subframe virtual channel map packet that contains information that specifies, for each program, which subframe data stream contains that program; and

(ii) periodically inserting the subframe virtual channel map packet in the transport layer region of one of the successive superframes, the superframe map for each superframe into which the subframe virtual channel map is inserted further specifying the location of the subframe virtual channel map packet within the transport layer region of that superframe.

11. A method according to claim 1 further comprising the step of transmitting successive superframes of said multiplex data stream at a constant rate.

12. A method according to claim 6 wherein each block comprises a predetermined number of bits, and wherein said method further comprises the step of transmitting successive superframes of said multiplex data stream at a constant rate such that the multiplex data stream is transmitted at a bit rage equal to F.sub.SF (n)(B) bits per second, where F.sub.SF is the rate at which successive superframes are transmitted, n is the number of blocks in each superframe, and b is the number of bits in each block.

13. A method of decoding a multiplex data stream of the type recited in claim 1, said method comprising the steps of:

a) receiving the multiplex data stream and selecting one of said programs;

b) determining which subframe data stream contains the selected program;

c) for each successive superframe of the received multiplex data stream, performing the following steps:

(i) extracting the superframe map from the superframe and determining from the extracted superframe map the location of each subframe within that superframe; and

(ii) extracting from the superframe the subframe in that superframe that is one of the subframes of the subframe data stream that contains the selected program;

d) for each subframe extracted in step (c)(ii), performing the following steps:

(i) extracting the subframe multiplex map from the subframe and identifying, based at least in-part upon information contained within that multiplex map, which portion of the subframe is allocated to the selected program; and

(ii) extracting the identified portion from the subframe; and

e) combining the portions extracted from each subframe in step (d)(ii) to reconstruct the selected program.

14. A method according to claim 13 wherein the multiplex data stream contains mapping information that specifies, for each program, which subframe data stream contains that program, and wherein step (b) comprises:

(i) extracting the mapping information from the received multiplex data stream; and

(ii) determining from the extracted mapping information which of the subframe data streams contains the selected program.

15. A method according to claim 13 wherein each subframe of each subframe data stream has a format comprising a plurality of successive lines, a first group of lines in each subframe defining a transport layer region of that subframe and a second group of lines defining a program data region, and wherein the program data region of each subframe is partitioned into a plurality of program data packets and different portions of each program data packet are allocated to different ones of said programs, and further wherein the transport layer region of each subframe contains a program multiplex control packet that specifies which portions of each program data packet in that subframe are allocated to which programs and the subframe multiplex map in each subframe specifies the location, within the transport layer region, of the program multiplex control packet for that subframe, and still further wherein step (d) comprises performing the following steps for each subframe:

(i) extracting the subframe multiplex map from the subframe and determining therefrom the location of the program multiplex control packet within the transport layer region of the subframe;

(ii) extracting the located program multiplex control packet from the transport layer region;

(iii) identifying, based upon the extracted program multiplex control packet, which portion of each program data packet of the subframe is allocated to the selected program; and

(iv) extracting the portion identified in step (d)(iii) from each of the program data packets in the subframe.

16. An encoder apparatus for combining a plurality of digital programs into a multiplex data stream, wherein each program comprises a single bit stream containing a combination of related digital services, said encoder comprising:

a control computer for controlling the operation of the encoder;

a plurality of subframe multiplexers, each coupled to the control computer and each being coupled to receive a respective group of said programs, each subframe multiplexer being operative to multiplex its respective group of programs to form a subframe data stream having a format comprising a continuous sequence of subframes wherein different portions of each subframe are allocated to different ones of said respective group of programs, each subframe multiplexer being further operative to insert, in each subframe, a subframe multiplex map that specifies, at least indirectly, which portions of each subframe are allocated to which programs; and

a superframe multiplexer coupled to each of said subframe multiplexers for receiving said subframe data streams and for multiplexing selected ones of said subframe data streams to generate said multiplex data stream, said multiplex data stream having a format comprising a continuous sequence of superframes, each superframe containing at least one subframe from at least one of said selected subframe data streams, said superframe multiplexer being further operative to generate, for each superframe in the multiplex data stream, a superframe map that specifies the location of each subframe contained in that superframe, and to insert the superframe map in that superframe.

17. An encoder according to claim 16 wherein each subframe has a format comprising a plurality of lines, a first group of lines in each subframe defining a subframe transport layer region and a second group of lines defining a program data region, and wherein each subframe multiplexer comprises:

a subframe controller coupled to the control computer for generating, for each subframe, the subframe multiplex map and at least one program multiplex control packet;

a program multiplexer coupled to the subframe controller and responsive to said program multiplex control packets for multiplexing said respective group of programs in a time-division manner to produce a continuous sequence of program data packets, different portions of each program data packet being allocated to different ones of said group of programs; and

a subframe builder coupled to the subframe controller and to said program multiplexer for constructing each subframe in accordance with the subframe multiplex map generated for that subframe, wherein for each subframe, the subframe builder is operative; (i) to insert a number of consecutive program data packets in the program data region of the subframe, (ii) to insert the program multiplex control packet for that subframe in the transport layer region of the subframe at a location specified by the subframe multiplex map for that subframe, and (iii) to insert the subframe multiplex map at a beginning of the subframe.

18. An encoder according to claim 17 wherein, for each program data packet to be inserted in a particular subframe, the program multiplex control packet generated for that subframe by the subframe controller specifies which portions of each program data packet are to be allocated to which programs, and wherein the program multiplexer is responsive to the multiplex control packet for generating each of the program data packets in accordance with the information specified therein.

19. An encoder according to claim 17 wherein the subframe builder is further operative to insert a subframe synchronization word (SFSYNC) in a first line of each subframe and to insert the subframe multiplex map in each frame such that the subframe multiplex map immediately follows the subframe synchronization word (SFSYNC).

20. An encoder according to claim 17 wherein the control computer is further operative to construct a plurality of system related data packets including system data packets (SDP's), system teletext packets (STT's), addressable data packets (ADP's), service seed packets (SSP's) and program virtual channel map packets (PVCMP's), said subframe builder in each subframe multiplexer inserting selected ones of said system related data packets in the transport layer regions of each subframe, the subframe multiplex map generated for each subframe by the subframe controller specifying the number and location of each system related data packet to be inserted in the transport layer region of that subframe.

21. An encoder according to claim 18 wherein each of said digital programs has a respective data rate and wherein said portions of each program data packet are allocated to said programs in proportion to the respective data rates of each program.

22. An encoder according to claim 21 wherein said subframe multiplexer is further operative to adjust, on a per subframe basis, the size of each portion of each program data packet allocated to a particular program as the respective data rates of each program change.

23. An encoder according to claim 16 wherein said superframe multiplexer comprises:

a superframe controller coupled to the control computer for generating said superframe map; and

a superframe builder coupled to the superframe controller and coupled to receive successive subframes from each subframe multiplexer for constructing each superframe in accordance with the superframe map generated by the superframe controller, and for inserting each superframe map in its respective superframe.

24. An encoder according to claim 23 wherein said superframe multiplexer further comprises an interleaver coupled to an output of said superframe builder for interleaving portions of each superframe to reduce burst errors during subsequent transmission of the multiplex data stream to a remote location.

25. An encoder according to claim 23 wherein each superframe has a format comprising an adjustable number of successive blocks, a first group of blocks in each superframe defining a superframe transport layer region of that superframe and a second group of blocks defining a subframe data region, said superframe builder including:

means for inserting a block syncrhonization byte (BSYNC) in a first portion of each block; and

means for inserting a superframe syncrhonization word (FSYNC) in a first block of each superframe.

26. An encoder according to claim 25 wherein the superframe multiplexer further comprises block error coding means for generating a block error code for each block and appending the block error code to the block.

27. An encoder according to claim 23 wherein the control computer is further operative to generate a subframe virtual channel map packet that contains information that specifies, for each program, which subframe data stream contains that program, and wherein the superframe builder of the superframe multiplexer further comprises means for periodically inserting the subframe virtual channel map packet in the transport layer regions of selected superframes, the superframe map for each superframe into which the subframe virtual channel map packet is inserted further specifying the location of the subframe virtual channel map packet within that superframe.

28. A decoder for use at a remote location to decode a multiplex data stream received at that location, said received multiplex data stream containing a multiplexed combination of digital programs and having a format comprising a continuous sequence of superframes each comprising a predetermined number of blocks, a first group of blocks in each superframe defining a transport layer region of that superframe and a second group of blocks defining a subframe data region, the subframe data region of each superframe comprising a multiplexed combination of subframes, different portions of each subframe being allocated to different ones of said digital programs and at least one subframe in each superframe containing data for a different group of said programs than another subframe in that superframe, said decoder comprising:

a receiver for receiving successive superframes of the multiplex data stream;

a selector for selecting at least one of said digital programs for extraction from said received multiplexed data stream;

a superframe demultiplexer comprising:

a superframe map extractor for locating and extracting a superframe map from a received superframe of the received multiplex data stream, said superframe map specifying the location of each subframe contained within that superframe; and

a subframe extractor at least indirectly responsive to the extracted superframe map for identifying which subframe in the received superframe contains data for the selected program and for extracting that subframe from the received superframe; and

a subframe demultiplexer coupled to the superframe demultiplexer for receiving the extracted subframe from the superframe demultiplexer, said subframe demultiplexer comprising:

a subframe multiplex map extractor for extracting a subframe multiplex map from the extracted subframe that specifies, at least indirectly, which portions of the subframe are allocated to which digital programs; and

a program extractor responsive to the extracted subframe multiplex map for identifying, based at least in-part upon information contained in the subframe multiplex map, which portion of the subframe is allocated to the selected program, and for extracting the identified portion from the subframe.

29. A decoder according to claim 28 wherein the multiplex data stream contains mapping information that specifies, for each program, which subframe data stream contains that program, and wherein the superframe demultiplexer further comprises means for extracting the mapping information from the received multiplex data stream and identifying therefrom which subframe in the received superframe contains data for the selected program.

30. A decoder for use at a remote location to decode a multiplex data stream received at that location, said received multiplex data stream containing a multiplexed combination of digital programs and having a format comprising a continuous sequence of superframes each containing a multiplexed combination of subframes, each subframe comprising a transport layer region and a program data region, the program data region of each subframe being partitioned into a plurality of program data packets, different portions of each program data packet being allocated to different ones of said digital programs, at least one subframe in each superframe containing data for a different group of said programs than another subframe in that superframe, the transport layer region of each subframe containing a program multiplex control packet that specifies, for each program data packet within that subframe, which portions of each program data packet are allocated to which programs, the transport layer region of each subframe further containing a subframe multiplex map which specifies, for that subframe, the location of the program multiplex control packet contained in the transport layer region of that subframe, said decoder comprising:

a receiver for receiving successive superframes of the multiplex data stream;

a selector for selecting at least one of said digital programs for extraction from said received multiplexed data stream;

a superframe demultiplexer comprising:

a superframe map extractor for locating and extracting a superframe map from a received superframe of the received multiplex data stream, said superframe map specifying the location of each subframe contained within that superframe; and

a subframe extractor at least indirectly responsive to the extracted superframe map for identifying which subframe in the received superframe contains data for the selected program and for extracting that subframe from the received superframe; and

a subframe demultiplexer coupled to the superframe demultiplexer for receiving the extracted subframe from the superframe demultiplexer, said subframe demultiplexer comprising:

a subframe multiplex map extractor for extracting the subframe multiplex map from the received subframe and for identifying therefrom the location of the program multiplex control packet within the transport layer region of that subframe;

means for extracting the program multiplex control packet from the transport layer region of the received subframe; and

a program extractor responsive to the extracted program multiplex control packet for identifying, based upon information contained in the extracted program multiplex control packet, which portion of each program data packet in the received subframe is allocated to the selected program, and for extracting the identified portion from each program data packet.

31. A decoder according to claim 30 wherein the multiplex data stream contains mapping information that specifies, for each program, which subframe data stream contains that program, and wherein the superframe demultiplexer further comprises means for extracting the mapping information from the received multiplex data stream and identifying therefrom which subframe in the received superframe contains data for the selected program.

32. A method comprising the steps of:

a) receiving a plurality of multiplex data streams, each multiplex data stream containing a multiplexed combination of subframe data streams wherein each subframe data stream comprises a continuous sequence of subframes, said received multiplex data streams each having a format comprising a continuous sequence of superframes, each superframe in a particular multiplex data stream containing at least one subframe from at least one of the subframe data streams contained in that multiplex data stream and each superframe further containing a superframe map that specifies the location of each subframe within that superframe;

b) for each received multiplex data stream, demultiplexing and separating the subframe data streams contained in that multiplex data stream and providing each subframe data stream in a separate channel;

c) selecting a plurality of said demultiplexed and separated subframe data streams, at least one of said selected subframe data streams having been demultiplexed and separated from a different multiplex data stream than others of said selected subframe data streams;

d) multiplexing said selected subframe data streams to generate a new multiplex data stream having a format comprising a continuous sequence of superframes wherein each superframe comprises at least one subframe from at least one of said selected subframe data streams;

e) generating, for each superframe in the new multiplex data stream, a superframe map that specifies the location of each subframe within that superframe, and inserting the superframe map in that superframe; and

f) transmitting the new multiplex data stream to at least one reception site.

33. A method according to claim 32 wherein for each received multiplex data stream step (b) comprises:

(i) for each successive superframe of the received multiplex data stream, extracting the superframe map from the superframe, and based upon information contained therein, locating, extracting and separating the subframes contained in that superframe; and

(ii) combining each subframe extracted from a particular superframe with corresponding subframes extracted from previous superframes to reproduce each subframe data stream and provide each subframe data stream on a separate channel.

34. A method according to claim 32 wherein each superframe of said new multiplex data stream and each of said received multiplex data streams has a format comprising an adjustable number of successive blocks, a first portion of each block comprising a block synchronization byte (BSYNC), a second portion of each block comprising a data portion, and a last portion of each block comprising a block error code generated in accordance with a block error coding scheme.

35. A method according to claim 32 wherein a first group of blocks in each superframe of the new multiplex data stream define a superframe transport layer region of that superframe, and a second group of blocks define a subframe data region, the superframe map generated in step (c) for each superframe of the new multiplex data stream being inserted in the superframe transport layer region of that superframe.

36. A method according to claim 32 further comprising the step of inserting, in a first block of each superframe of the new multiplex data stream a superframe synchronization word (FSYNC), and wherein step (e) comprises inserting the superframe map for each superframe at a location immediately following the superframe synchronization word.

37. A method according to claim 32 wherein step (f) comprises transmitting successive superframes of said new multiplex data stream at a constant rate.

38. A method according to claim 34 wherein each block comprises a predetermined number of bits, and wherein step (f) comprises transmitting successive superframes of said new multiplex data stream at a constant rate such that the new multiplex data stream is transmitted at a bit rate equal to F.sub.SF (n)(B) bits per second, where F.sub.SF is the rate at which successive superframes are transmitted, n is the number of blocks in each superframe, and B is the number of bits in each block.

39. A headend installation comprising:

a plurality of receivers each tuned to receive a different one of a plurality of multiplex data streams transmitted from at least one encoder site, each multiplex data stream containing a multiplexed combination of subframe data streams wherein each subframe data stream comprises a continuous sequence of subframes, said received multiplex data streams each having a format comprising a continuous sequence of superframes, each superframe in a particular multiplex data stream containing at least one subframe from at least one of the subframe data streams contained in that multiplex data stream and each superframe further containing a superframe map that specifies the location of each subframe within that superframe;

a plurality of superframe demultiplexers each coupled to a respective one of said receivers for demultiplexing and separating the subframe data streams contained in the multiplex data streams received by that receiver and for providing each of the separated subframe data stream on a separate channel;

re-routing means for selecting a plurality of said demultiplexed and separated subframe data streams, at least one of said selected subframe data streams having been demultiplexed and separated from a different multiplex data stream than others of said selected subframe data streams; and

a superframe multiplexer coupled to receive each of said selected subframe data streams for multiplexing said selected subframe data streams to generate a new multiplex data stream, said new multiplex data stream having a format comprising a continuous sequence of superframes, each superframe containing at least one subframe from each of said selected subframe data streams, said superframe multiplexer being further operative to generate, for each superframe in the multiplex data stream, a superframe map that specifies the location of each subframe contained in that superframe, and to insert the superframe map in that superframe.

40. A headend installation according to claim 39 wherein said superframe multiplexer comprises:

a superframe controller for generating the superframe map for each superframe; and

a superframe builder coupled to the superframe controller and coupled to receive successive subframes from each subframe multiplexer for constructing each superframe in accordance with the superframe map generated by the superframe controller, and for inserting each superframe map in its respective superframe.

41. A headend installation according to claim 40 wherein said superframe multiplexer further comprises an interleaver coupled to an output of said superframe builder for interleaving portions of each superframe to reduce burst errors during subsequent transmission of the multiplex data stream to a remote location.

42. A headend installation according to claim 40 wherein each superframe of the new multiplex data stream has a format comprising an adjustable number of successive blocks, a first group of blocks in each superframe defining a superframe transport layer region of that superframe and a second group of block defining a subframe data region, said superframe builder including:

means for inserting a block synchronization byte (BSYNC) in a first portion of each block; and

means for inserting a superframe synchronization word (FSYNC) in a first block of each superframe.

43. A headend installation according to claim 42 wherein the superframe multiplexer further comprises block error coding means for generating a block error code for each block and appending the block error code to the block.

44. A headend installation according to claim 39 further comprising a transmitter coupled to said superframe multiplexer for transmitting successive superframes of said new multiplex data stream at a constant rate.

45. A headend installation according to claim 42 wherein each block comprises a predetermined number of bits, and wherein said headend installation further comprises a transmitter coupled to said superframe multiplexer for transmitting successive superframes of said new multiplex data stream at a constant rate such that the multiplex data stream is transmitted at a bit rate equal to F.sub.SF (n)(B) bits per second, where F.sub.SF is the rate at which successive superframes are transmitted, n is the number of blocks in each superframe, and B is the number of bits in each block.

46. A headend installation according to claim 39 wherein each superframe demultiplexer comprises:

a superframe map extractor for locating and extracting a superframe map from a superframe of the received multiplex data stream, said superframe map specifying the location of each subframe contained within that superframe;

a subframe extractor coupled to the superframe map extractor and at least indirectly responsive to the extracted superframe map for determining the location of each subframe within the superframe and for extracting and separating each subframe from the superframe.

47. A digital data stream containing a multiplexed combination of digital programs wherein each program comprises a combination of related digital services, said data stream comprising a continuous sequence of superframes,

each superframe containing a plurality of subframes, and each superframe containing a superframe map that specifies for that superframe the location of each of the subframes in that superframe,

each subframe in each superframe containing a multiplexed combination of data from a subset of said digital programs wherein different portions of each subframe are allocated to different ones of said programs of said subset, at least one of the subframes in a particular superframe containing a multiplexed combination of digital programs from a different subset than another subframe in that superframe, each subframe further containing a subframe multiplex map that at least indirectly specifies which portions of that subframe are allocated to which programs of said subset.

48. A digital data stream according to claim 47 wherein each subframe in each superframe has a format comprising a plurality of lines, a first group of lines in each subframe defining a subframe transport layer region and a second group of lines defining a program data region, the program data region of each subframe being portioned into a plurality of program data packets, and wherein different portions of each program data packet in each subframe are allocated to different ones of the programs of said subset.

49. A digital data stream according to claim 48 wherein the transport layer region of each subframe in each superframe contains a program multiplex control packet that specifies which portions of each program data packet in that subframe are allocated to which programs of said subset.

50. A digital data stream according to claim 48 wherein each of said digital programs has a respective data rate, and wherein different portions of each program data packet in each subframe are allocated to different ones of said programs of said subset in proportion to the respective data rates of said programs of said subset.

51. A digital data stream according to claim 48 wherein the transport layer region of each subframe in each superframe contains a plurality of system related data packets including system data packets (SDP's), system teletext packets (STT's), addressable data packets (ADP's), service seed packets (SSP's) and program virtual channel map packets (PVCMP's), and wherein the subframe multiplex map in each subframe specifies the location of each system related data packet in the transport layer region of that subframe.

52. A digital data stream according to claim 47 wherein each superframe has a format comprising an adjustable number of successive blocks, a first portion of each block comprising a block synchronization byte (BSYNC), a second portion of each block comprising a data portion, and a last portion of each block comprising a block error code generated in accordance with a block error coding scheme.

53. A digital data stream according to claim 52 wherein a first group of blocks in each superframe define a superframe transport layer region of that superframe, and a second group of blocks define a subframe data region, the superframe map of each superframe being located in the superframe transport layer region of said each superframe.

54. A digital data stream according to claim 53 wherein each subframe has a format comprising a plurality of lines, and wherein the data portion of each block in the subframe data region of each superframe contains a single line of one of the subframes in that superframe.

55. A digital data stream according to claim 52 wherein a first block of each superframe contains a superframe synchronization word (FSYNC), and wherein the superframe map for each superframe is located in the superframe immediately following the superframe synchronization word.

Description

FIELD OF THE INVENTION

The present invention relates generally to digital signal transmission, and more particularly, to a system and method of multiplexing a plurality of digital services for transmission to a plurality of remote locations.

BACKGROUND OF THE INVENTION

The background of the present invention is described herein in the context of subscription television systems, such as cable television and direct broadcast satellite (DBS) systems, that distribute a variety of program services to subscribers, but the invention is by no means limited thereto except as expressly set forth in the accompanying claims.

In the subscription television industry, "programmers" produce "programs" for distribution to various remote locations. A "program" is a collection of related video, audio and other services, such as closed-captioning and teletext services. A single programmer may wish to supply many such programs. Typically, a programmer will supply these services via satellite to either individual subscribers (e.g., DBS subscribers), cable television operators, or both. In the case of cable television operators, the services transmitted via satellite are received at the operator's "cable head-end" installations. A cable operator typically receives programs and other services from many programmers and then distributes them to its subscribers. In addition, a cable operator may insert locally produced services at the cable-head end. The selected services and locally produced services are then transmitted to the individual subscribers via a coaxial cable distribution network. In the case of DBS subscribers, each subscriber is capable of receiving a satellite down-link from the programmers directly.

In the past, subscription television systems, including cable and DBS systems, have operated in the analog domain. Recently, however, the subscription television industry has begun to move toward all digital or hybrid analog/digital systems wherein prior to transmission, all or some of the analog signals are converted to digital signals. Digital signal transmission offers the advantage that digital data can be processed at both the transmission and reception ends to improve picture quality. In addition, digital data compression techniques have been developed that achieve high signal compression ratios. Digital compression allows a larger number of individual services to be transmitted within a fixed bandwidth. Bandwidth limitations are imposed by both satellite transponders and coaxial cable distribution networks, and therefore, digital compression is extremely advantageous.

In some prior art digital service transmission systems, various digital services, such as video services and audio services, are multiplexed into a single multiplex data stream for transmission to subscribers and cable operators. Unfortunately, these prior art systems are inflexible in many respects and cannot be easily adapted to meet the continuously changing requirements of the industry. There is a need, therefore, for a system and method of multiplexing a plurality of digital services that provides a high degree of flexibility so that the system can easily be adapted to meet the diverse and ever changing demands of the industry. In particular, such a system should be capable of multiplexing a wide variety of digital services. As those skilled in the art know, digital services vary widely in individual data rates. For example, digital audio service rates differ substantially from digital video service rates. A digital services delivery system must therefore be flexible in service data rate allocation within the system. Also, such a system should have an overall data transmission rate that is "scalable," i.e., that is adjustable over an almost continuous range. As those skilled in the art can appreciate, digital service delivery may take place via satellite, cable distribution networks, or even optical fiber networks. Each of these mediums has a unique bandwidth limitation, and therefore, accommodates a different maximum data rate. It is desirable, therefore, that a digital services delivery system provide adjustability (i.e., scalability) in the overall data transmission rate of the system.

In addition to service rate flexibility and overall data transmission rate scalability, a program services delivery system should provide for easy and cost efficient demultiplexing and re-multiplexing of the individual services being transmitted. Cable operators typically receive "programming" (i.e., related digital services) from a multitude of "programmers." Cable operators rarely provide their subscribers with every service provided by every programmer; more typically, cable operators select various services from each programmer and then provide only those selected services to their subscribers. Consequently, there is a need for a digital services delivery system and method that provides easy and cost-efficient de-multiplexing and re-multiplexing of various services. Regional hub distribution techniques will further emphasize this need.

As described hereinafter, the system and method of the present invention achieves each of the aforementioned objectives.

SUMMARY OF THE INVENTION

Briefly stated, the present invention is directed to a system and method for multiplexing a plurality of digital programs and transmitting the programs from an origination point to a plurality of remote locations. A program may comprise a collection of related digital services including video, audio, teletext, closed-captioning and "other data" services. According to the invention, digital programs to be transmitted are provided to an encoder at the origination point that generates a multiplex data stream which carries the programs to the remote locations via a transmission medium, such as a satellite or a cable distribution network.

According to the method of the present invention, the encoder first generates a plurality of subframe data streams each comprising a continuous sequence of subframes. Each subframe in each subframe data stream comprises at least a transport layer region and a program data region. For each subframe data stream, portions of the program data region of each of the subframes in that subframe data stream are allocated to selected ones of the plurality of programs; the size of each portion allocated to a respective program is related to a data rate of that program. According to the present invention, a single subframe data stream may carry one or more programs, as desired. For each subframe in a given subframe data stream, a program multiplex control packet is generated that specifies for that subframe which portions of the program data region of that subframe are allocated to each program. A plurality of system packets containing system-wide information are also generated for each subframe. The program multiplex control packet and system packets generated for a given subframe are inserted in the transport layer region of that subframe. A subframe multiplex map is then generated for each subframe that specifies the location of the program multiplex control packet and each of the system packets within the transport layer. The subframe multiplex map for a given subframe is inserted at the beginning of the transport layer region of that subframe.

After each subframe data stream has been constructed in the manner described above, a multiplex data stream is generated at the encoder that comprises a continuous sequence of superframes. Each superframe comprises a superframe transport layer region and a subframe data region. In accordance with the present invention, a portion of the subframe data region of each superframe is allocated to at least one subframe from each of the plurality of subframe data streams. A superframe map is then generated for each superframe that specifies at least the location of each subframe within the superframe. The superframe map is inserted in the transport layer region of the superframe. As the foregoing illustrates, the multiplex data stream comprises a hierachical frame structure consisting of superframes and subframes that carry the plurality of programs. In accordance with the present invention, the multiplex data stream is transmitted to a plurality of remote locations.

A decoder is provided at each remote location. In accordance with the present invention, a decoder recieves a multiplex data stream and extracts the superframe map from each incoming superframe. With the superframe map, the decoder is able to extract the individual subframes from each superframe, thereby recovering each subframe data stream from the multiplex data stream. For a selected subframe data stream, the decoder extracts the subframe multiplex map from each successive subframe in that subframe data stream in order to determine the location of the program multiplex control packet within each subframe. With the program multiplex control packets, the decoder is able to determine which portion of the program data region of each subframe is allocated to a selected program. In this manner, the decoder is then able to extract the selected program.

In accordance with a preferred embodiment of the present invention, the superframes of the multiplex data stream are transmitted to the plurality of remote locations at a constant frame rate. Preferably, portions of each superframe are transmitted in a byte-interleaved manner.

According to another aspect of the present invention, each superframe further comprises a plurality of blocks. A first portion of each block comprises a block synchronization code. A second portion of each block is reserved for superframe data which may comprise either a superframe synchronization word, a portion of the superframe transport layer region, or a portion of one of the subframes in the superframe. Each block ends with a block error code. According to a preferred embodiment, each of the blocks comprises 1600 bits. Preferably, the block synchronization code comprises 8 bits and the error code for each block comprises 160 bits.

According to another feature of the present invention, at an intermediate location, subframe data streams may be extracted from one multiplex data stream for re-routing within other multiplex data streams.

Further details and features of the present invention will become evident hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description of the preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments that are presently preferred, it being understood, however, that the invention is not limited to the specific methods and instrumentalities disclosed. In the drawings:

FIG. 1 illustrates an application of the present invention to a subscription television system;

FIG. 2 is a block diagram of an encoder for generating a multiplex data stream in accordance with the method of the present invention;

FIGS. 3 and 3(a) illustrates the general arrangement and contents of an exemplary superframe in accordance with the present invention;

FIG. 4 shows the general arrangement and contents of a superframe ID that is carried in a first block of each superframe in accordance with the present invention;

FIG. 5 illustrates the general arrangement and contents of the superframe transport layer region of FIG. 3;

FIG. 6 shows in detail the general arrangement and contents of an exemplary subframe in accordance with the present invention;

FIG. 7 shows in detail the general arrangement and contents of an exemplary program data packet in accordance with the present invention;

FIG. 8 is a block diagram of a subframe multiplexer employed in the encoder of FIG. 2 in accordance with the present invention;

FIG. 9 shows in detail the general arrangement and contents of a subframe multiplex map in accordance with the present invention;

FIGS. 10 and 11 show in detail the general arrangement and contents of first and second program multiplex control packets that may be transmitted within a subframe in accordance with the method of the present invention;

FIG. 12 is a block diagram of the superframe multiplexer employed in the encoder of FIG. 2 in accordance with the present invention;

FIG 12(a) graphically illustrates the reduction of burst errors during the transmission of an exemplary superframe by interleaving portions of the superframe in accordance with an aspect of the present invention;

FIG. 13 shows in detail the general arrangement and contents of a superframe map in accordance with the present invention;

FIG. 14 is a detailed block diagram of a decoder apparatus that may be employed at remote locations in accordance with the system and method of the present invention;

FIG. 15 shows in detail the general arrangement and contents of a subframe virtual channel map packet in accordance with the present invention;

FIG. 16 is a detailed block diagram of the superframe demultiplexer of FIG. 14 in accordance with the present invention;

FIG. 17 shows in detail the general arrangement and contents of a superframe encryption seed packet in accordance with the present invention;

FIG. 18 is a block diagram showing details of the subframe demultiplexer of FIG. 14 in accordance with the present invention;

FIG. 19(a) shows in detail the general arrangement and contents of an exemplary first service seed packet that may be transmitted in a subframe;

FIG. 19(b) shows in detail the general arrangement and contents of an exemplary service seed packet that may be transmitted subsequent to the service seed packet shown in FIG. 19 (a);

FIG. 20 shows in detail the general arrangement and contents of a program virtual channel map packet in accordance with the present invention;

FIG. 21 shows in detail the general arrangement and contents of an exemplary addressable data packet in accordance with the present invention;

FIG. 22 shows in detail the general arrangement and contents of an exemplary system data packet in accordance with the present invention;

FIG. 23 illustrates the contents of the first 13 blocks of 8 consecutive subframes (i.e., one nominal "cryptocycle") transmitted in accordance With the present invention;

FIG. 24 is a block diagram of an alternate embodiment of the system of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the drawings, like numerals indicate like elements throughout. As with the Background of the Invention, the following detailed description is presented in the context of a subscription television system that distributes a variety of program services to subscribers, but the invention is by no means limited thereto except as expressly set forth in the accompanying claims.

FIG. 1 illustrates an application of the present invention to a subscription television system 10 wherein "programs" comprising a collection of digital services, such as video, audio, teletext, closed-captioning and other digital services, are communicated from a plurality of "programmer" locations 12, 12' to a plurality of remote locations, e.g. locations 25, 34 and 34'. Any number of "programmers" may be present in the system 10. Each programmer provides a variety of programs 14 to a respective encoder 16 that operates to multiplex the individual digital services of each program and thereby generate, in accordance with the method of the present invention, a multiplex data stream for transmission to a plurality of remote locations. Details of the encoders 16 will be provided hereinafter. The multiplexed data stream generated by each encoder 16 is provided to a transmitter 18 and then to a satellite up-link 20. By way of example in FIG. 1, the encoder 16 operated by programmer A generates a multiplexed data stream 22; the encoder 16 operated by programmer B generates a multiplexed data stream 22'. Each stream 22, 22' is transmitted via a satellite 23 to a cable head-end installation 25. It is understood that there may be many programmers in the system of the present invention, and therefore, a plurality of multiplex data streams may be transmitted via satellite 23 to remote locations such as the cable head-end installation 25. Although not shown, multiplex data streams may be received via satellite at locations other than a cable head-end, such as, for example, at the locale of a direct broadcast satellite (DBS) subscriber.

A satellite down-link 24 at the cable head-end installation 25 provides each stream 22, 22' to a respective receiver 26 at the cable head-end 25. As those skilled in the art know, cable head-end installations provide subscription television signals to cable subscribers via a cable distribution network. Coaxial cables used in cable distribution networks have the capacity to carry a plurality of contiguous 6, 7 or 8 MHz channels. In accordance with the preferred embodiment of the present invention, each multiplex data stream received at the cable head-end is fed from its respective receiver 26 to a modulator 28, where it is modulated on a distinct 6 MHz cable channel. Although 6 MHz channels are employed in the preferred embodiment, 7 or 8 MHz channels may be employed. In the preferred embodiment, the modulators 28 employ 4-VSB (vestigial side-band) modulation, however, any suitable modulation technique may be employed. Each of the modulated data streams is then provided to a combiner 30 that combines the individual 6 Mhz channels into a single wide-band signal that is then transmitted via a cable distribution network 32 to a plurality of cable subscriber locations, e.g. 34 and 34'. Each subscriber location 34, 34' has a decoder 36 that functions in accordance with the present invention to extract selected digital services from a selected multiplex data stream for display on a television set 38 or output to an audio device 40. Details of the decoders 36 will be provided hereinafter. As mentioned above, FIG. 1 illustrates an application of the system and method of the present invention to a subscription television system. It is understood, therefore, that the encoders, decoders and other apparatus described hereinafter may be employed in any application that involves the communication of digital services to a plurality of remote locations and devices other than television receivers such as digital computers.

FIG. 2 is a block diagram of an encoder 16 for generating a multiplex data stream in accordance with the method of the present invention. As shown, the encoder 16 receives at respective inputs thereof a plurality of programs (e.g. programs 1 to N). As mentioned in the Background of the Invention, a program comprises a collection of related digital services, including video (V), audio (A), teletext (TT), closed-captioning (CC) and other data (D). For example, a program may be a subscription television program comprising a video service and its related audio and closed-captioning information. Alternatively, a program may simply comprise a single audio service. The "other data" (D) may be computer data or any other type of digital data, such as, for example, panscan control information and/or EIDAK copy protection information related to the video service in a particular program. In many applications, video and audio services will be compressed. As those skilled in the art know, there are many video and audio compression techniques available. For example, the Motion Pictures Expert Group (MPEG) has developed a video compression algorithm that is widely used in the digital video services industry. Vector quantization is another, more recent compression technique for digital video. Those skilled in the art understand that the encoder 16 functions independently of any data compression, and therefore, both compressed and uncompressed services may be input to the encoder 16.

As shown, the services that comprise each program are fed to a channelizer 50 that combines the services into a single packetized data stream referred to herein as a "program data stream" or simply a "program". In the preferred embodiment, the channelizers 50 employ the Motion Picture Expert Group's (MPEG) systems level protocol (ISO 11172) for generating the program data streams. As those skilled in the art understand, the MPEG systems level protocol specifies a synchronized packet syntax for combining multiple digital services, such as video, audio, teletext, closed-captioning and other data. For a detailed explanation of the MPEG systems level syntax, see "Coding of Moving Pictures and Associated Audio for Digital Storage Media--Part One: Systems," ISO 11172, rev. 1 (Nov. 29, 1991). Although the MPEG systems level protocol provides a convenient method for combining related digital service data into a single program data stream, those skilled in the art will understand that the particular protocol used by the channelizers 50 is not critical to the present invention. Any protocol that specifies a syntax for generating a synchronized combination of related digital services within a single bitstream may be employed without deviating from the spirit and scope of the present invention. For example, a method of channelizing individual services that employs variable-length frame formats with headers that describe the length and contents of each frame may be employed.

As shown in FIG. 2, the digital services input to a given channelizer 50 may be encrypted with individual service encryptors 52. According to the method of the present invention, any service may be transmitted encrypted or unencrypted. Thus, as illustrated in FIG. 2, any number of services in a given program may be encrypted. For example, only the video, audio and teletext services of program1 are encrypted, whereas all of the services of program2 are being encrypted; none of the services of program N are being encrypted. Digital service encryptors are well known to those skilled in the art, and there are many encryption techniques and many ways to implement an encryptor. In the present invention, the encryptors 52 are not limited to any one technique or implementation. However, an important feature of the present invention is that each digital service, if encrypted, is independently encrypted.

Data encryptors, such as encryptors 52, typically use a "seed" value to generate a pseudo-random binary sequence which is then convolved, typically via modulo-2 addition, with the service data stream to produce an encrypted service stream. Accordingly, the encoder 16 includes a service seed generator 54 for supplying each encryptor 52 with its own "seed" value. Thus, each encrypted service is individually encrypted using a unique service seed. As those skilled in the art know, a service can be decrypted as long as the decryptor has the "seed" used to encrypt the service. According to the present invention, the seed used to encrypt a given service is changed periodically in order to enhance cryptographic strength. As described hereinafter, the service seeds are transmitted to the remote locations in service seed packets (SSPs). Details of the contents and arrangement of the SSPs are provided hereinafter.

As those skilled in the art will appreciate, time is needed at a remote location to receive the seeds and process them in order to be ready to decrypt the incoming encrypted service data. As described hereinafter in more detail, encryption seeds are transmitted to remote locations in advance of the data the seeds were used to encrypt. This allows the decoder at each remote location enough time to have the seeds ready for the decryption process and avoids unnecessary buffering of the incoming service data stream.

Still referring to FIG. 2, each program data stream generated by a channelizer 50 is fed to at least one subframe multiplexer 56. As shown, there may be a plurality of subframe multiplexers 56 in a single encoder 16. According to the present embodiment, each subframe multiplexer 56 may receive up to 60 program data streams. As illustrated, however, a subframe multiplexer 56 (e.g. subframe multiplexer N) may receive only one program data stream. Subframe control information provided by a control computer 68 via line 57 to each subframe multiplexer 56 specifies, for each multiplexer 56, how many program data streams that multiplexer 56 is receiving. Programmers must supply this information to the control computer prior to operating the encoder 16. As those skilled in the art will appreciate, independent control of each subframe multiplexer 56 via the control computer 68 provides a significant degree of flexibility to the programmer.

As described hereinafter in greater detail, each subframe multiplexer 56 operates in accordance with the method of the present invention to generate a subframe data stream 58 that comprises a continuous sequence of subframes 60. As described hereinafter, each subframe 60 contains a program data region and a "subframe transport layer." A subframe multiplexer 56 allocates portions of the program data region of each subframe 60 to respective ones of the program data streams input to the multiplexer 56. The "transport layer" of each subframe contains certain "system data" necessary for operating the system of the present invention. Because certain types of system data are too numerous to transmit in a single subframe, these types of data are transmitted over a series of subframes referred to herein as a "cryptocycle." According to the present embodiment, a cryptocycle nominally comprises eight (8) subframes; however, a cryptocycle can be defined by any number of subframes. Essentially, cryptocycles define fixed boundaries in the subframe data stream 58 within which a complete set of system and encryption related data is transmitted. Further details of the general arrangement and content of each subframe, as well as details of the subframe multiplexers 56, will be provided hereinafter. Other types of system data, even more numerous than those transitted within a cryptocycle can be sent over an arbitrarily large number of subframes.

As described above, the various services of a given program may be individually encrypted using a unique service seed, and the seeds used to encrypt each service are changed periodically. More specifically, according to the present invention, the service seeds are changed every cryptocycle, i.e., every (8) subframes. Thus, a given seed value is used to encrypt a particular program service over 8 subframes and then changed. Changing the service seeds every cryptocycle enhances the cryptographic strength of the system. As also described above, the service seeds are transmitted to remote locations in service seed packets (SSPs). Service seed packets (SSPs), when needed, are generated within the appropriate subframe multiplexer 56. For example, in FIG. 2, the seeds used to encrypt the services of program1 and program N-x, are provided to subframe multiplexer 56 which generates the SSP(s) needed to carry those seeds to remote locations. Details of the contents and arrangement of an SSP are provided hereinafter.

Referring still to FIG. 2, the individual subframes 60 generated by each subframe multiplexer 56 are successively supplied to a superframe multiplexer 62, as shown. As described hereinafter in greater detail, the superframe multiplexer 62 operates in accordance with the method of the present invention to generate a multiplex data stream 22 comprising a continuous sequence of superframes 66. A superframe 66 contains at least one subframe 60. Preferably, however, a superframe 66 contains multiple subframes 60. For example, a superframe 66 may contain one subframe 60 from each of the subframe data streams 58 input to the superframe multiplexer 62. As those skilled in the art will appreciate, the superframe multiplexer 62 provides an additional level of multiplexing within the encoder 16. As with the subframe multiplexers 56, the superframe multiplexer 62 receives control information from the control computer 68 via line 69.

As best illustrated in FIG. 1, the multiplex data stream 22 generated within the encoder 16 by the superframe multiplexer 62 is sent to a transmitter 18 for transmission to remote locations. In the subscription television context, the multiplex data stream is transmitted via satellite 23, however, those skilled in the art will appreciate that a multiplex data stream 22 generated in accordance with the method of the present invention is by no means limited to transmission via satellite, but may be transmitted via a variety of transmission mediums depending upon the particular application of the present invention. For example, a multiplex data stream 22 generated in accordance with the present invention may be transmitted via a fiber optic cable or a micro-wave link.

FIG. 3 illustrates the general arrangement and contents of a superframe 66. As mentioned, the multiplex data stream generated by the encoder 16 comprises a continuous sequence of superframes 66. At the lowest level, a superframe comprises a plurality of blocks 80 which, in the preferred embodiment are each 200 bytes (i.e., 1600 bits) wide. The fundamental structure of each block is shown in FIG. 3a. As illustrated in FIG. 3a, each block begins with a BSYNC byte. The BSYNC byte 88 is the same for each block, and as described hereinafter, the BSYNC bytes 88 of each block are employed together with an FSYNC word (described hereinafter) to establish frame synchronization at remote locations. Each block 80 of the superframe 66 ends with a block error code 90. In the preferred embodiment, a Reed-Solomon error coding technique is employed that generates 20 Reed-Solomon error code bytes for every block. As each block in the preferred embodiment is 1600 bits wide (i.e., 200 bytes), there are 179 bytes (i.e., 1432 bits) left for data. While a Reed-Solomon error coding technique is employed in the preferred embodiment, those skilled in the art will appreciate that any block-oriented error coding scheme may be employed without deviating from the true spirit and scope of the present invention. Furthermore, although in the preferred embodiment, 20 bytes of error coding are produced for each block, less than or more than 20 bytes may be employed. Moreover, although each block is 1600 bits wide in the preferred embodiment, other block lengths may be employed. Those skilled in the art will appreciate that the amount of data space available in each block will depend upon both the overall length of the block and the number of bytes reserved for error coding within each block 80.

Referring again to FIG. 3, at a higher level, the superframe 66 comprises, in a first block thereof, an FSYNC word and a superframe ID 84. The FSYNC word 82 and superframe ID 84 are followed by a "superframe transport layer" 82. As described hereinafter, the superframe transport layer contains information concerning the overall content of the superframe, as well as system related information necessary for operation of the system of the present invention. As further shown, the superframe transport layer 86 is followed by a plurality of subframes 60. Each subframe 60 comprises a subframe transport layer 61 and a subframe data region 63 (sometimes hereinafter referred to as a "program data region"). Further details of the superframe transport layer 86 and a typical subframe 60 will be provided hereinafter. As mentioned above, each block in the superframe 66 begins with a BSYNC byte 88 and ends with a number of error code bytes 90.

According to the present invention, superframes are transmitted at a constant frame rate, F.sub.SF. With a constant frame rate, the overall transmission rate of a multiplex data stream can be expressed as: ##EQU1## where F.sub.SF is the overall frame rate;

L.sub.B is the length of each block (in bits); and

n is the number of blocks in each superframe.

As can be seen, therefore, the overall data rate of the multiplex data stream generated by an encoder 16 depends upon the frame rate, the number of blocks per frame, and the length of each block. These parameters may be selected according to the requirements of a given application. As those skilled in the art will appreciate, the method of the present invention provides a fixed, but adjustable frame approach. The decoders at each remote location may be uniquely manufactured for a given application based on a frame size, frame rate and block length selected for that application, or alternatively, a re-configurable decoder may be employed that can receive these dimensional parameters from an encoder and automatically adjust to the particular block length, frame size and frame rate chosen. For example, as described hereinafter in greater detail, these dimensional parameters may be provided in a system data packet (SDP) that may be transmitted to each decoder in accordance with an aspect of the present invention.

As mentioned above, in the preferred embodiment, each block is 1600 bits wide (i.e., 200 bytes). Consequently, the overall transmission rate of the multiplex data stream generated by each encoder 16 in the system 10 can be expressed as ##EQU2## Depending upon the selected frame rate F.sub.SF, therefore, each additional block in the superframe 66 increments the overall data rate by 1600 F.sub.SF) bps. Table 1 shows the data rate increments that result for several exemplary frame rates.

                  TABLE 1
    ______________________________________
    INCREMENTS IN OVERALL DATA RATE OF
    MULTIPLEX DATA STREAM VS. FRAME RATE
    ASSUMING 200 BYTES BLOCKS
                  DATA RATE
    FRAME RATE    INCREMENTS (bps)
    ______________________________________
    20 Hz         32,000
    25 Hz         40,000
    30 Hz         48,000
    40 Hz         64,000
    50 Hz         80,000
    60 Hz         96,000
    ______________________________________