Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1998-01-14
2001-02-27
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S412000, C370S516000, C370S537000, C370S542000
Reexamination Certificate
active
06195368
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to communication systems. In particular, the invention pertains to selectively multiplexing bit streams containing one or more programs, such as real-time audio-video programs. Program specific and other program related information is adjusted so as to enable identification, extraction and real-time reproduction of the program at the receiving end of the bit streams.
BACKGROUND OF THE INVENTION
Recently, techniques have been proposed for efficiently compressing digital audio-video programs for storage and transmission. See, for example, ISO\IEC IS 13818-1,2,3: Information Technology-Generic Coding of Moving Pictures and Associated Audio Information: Systems, Video and Audio (“MPEG-2”); ISO\IEC IS 11172-1,2,3: Information Technology-Generic Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 Mbits/sec: Systems, Video and Audio (“MPEG-1”); Dolby AC-3; Motion JPEG, etc. Herein, the term program means a collection of related audio-video signals having a common time base and intended for synchronized presentation, as per MPEG-2 parlance.
MPEG-1 and MPEG-2 provide for hierarchically layered streams. That is, an audio-video program is composed of one or more coded bit streams or “elementary streams” (“ES”) such as an encoded video ES, and encoded audio ES, a second language encoded audio ES, a closed caption text ES, etc. Each ES, in particular, each of the audio and video ESs, is separately encoded. The encoded ESs are then combined into a systems layer stream such as a program stream “PS” or a transport stream “TS”. The purpose of the PS or TS is to enable extraction of the encoded ESs of a program, separation and separate decoding of each ES and synchronized presentation of the decoded ESs. The TS or PS may be encapsulated in an even higher channel layer or storage format which provides for forward error correction.
Elementary Streams
Audio ESs are typically encoded at a constant bit rate, e.g., 384 kbps. Video ESs, on the other hand, are encoded according to MPEG-1 or MPEG-2 at a variable bit rate. This means that the number of bits per compressed/encoded picture varies from picture to picture (which pictures are presented or displayed at a constant rate). Video encoding involves the steps of spatially and temporally encoding the video pictures. Spatial encoding includes discrete cosine transforming, quantizing, (zig-zag) scanning, run length encoding and variable length encoding blocks of luminance and chrominance pixel data. Temporal coding involves estimating the motion of macroblocks (e.g., a 4×4 array of luminance blocks and each chrominance block overlaid thereon) to identify motion vectors, motion compensating the macroblocks to form prediction error macroblocks, spatially encoding the prediction error macroblocks and variable length encoding the motion vectors. Some pictures, called I pictures, are only spatially encoded, whereas other pictures, such as P and B pictures are both spatially and motion compensated encoded (i.e., temporally predicted from other pictures). Encoded I pictures typically have more bits than encoded P pictures and encoded P pictures typically have more bits than encoded B pictures. In any event, even encoded pictures of the same type tend to have different numbers of bits.
MPEG-2 defines a buffer size constraint on encoded video ESs. In particular, a decoder is presumed to have a buffer with a predefined maximum storage capacity. The encoded video ES must not cause the decoder buffer to overflow (and in some cases, must not cause the decoder buffer to underflow). MPEG-2 specifically defines the times at which each picture's compressed data are removed from the decoder buffer in relation to the bit rate of the video ES, the picture display rate and certain picture reordering constraints imposed to enable decoding of predicted pictures (from the reference pictures from which they were predicted). Given such constraints, the number of bits produced in compressing a picture can be adjusted (as frequently as on a macroblock by macroblock basis) to ensure that the video ES does not cause the video ES decoder buffer to underflow or overflow.
Transport Streams
This invention is illustrated herein for TSs. For sake of brevity, the discussion of PSs is omitted. However, those having ordinary skill in the art will appreciate the applicability of certain aspects of this invention to PSs.
The data of each ES is formed into variable length program elementary stream or “PES” packets. PES packets contain data for only a single ES, but may contain data for more than one decoding unit (e.g., may contain more than one compressed picture, more than one compressed audio frame, etc.). In the case of a TS, the PES packets are first divided into a number of payload units and inserted into fixed length (188 byte long) transport packets. Each transport packet may carry payload data of only one type, e.g., PES packet data for only one ES. Each TS is provided with a four byte header that includes a packet identifier or “PID.” The PID is analogous to a tag which uniquely indicates the contents of the transport packet. Thus, one PID is assigned to a video ES of a particular program, a second, different PID is assigned to the audio ES of a particular program, etc.
The ESs of each program are encoded in relation to a single encoder system time clock Likewise, the decoding and synchronized presentation of the ESs are, in turn, synchronized in relation to the same encoder system time clock. Thus, the decoder (
FIG. 5
) must be able to recover the original encoder system time clock in order to be able to decode each ES and present each decoded ES in a timely and mutually synchronized fashion. To that end, time stamps of the system time clock, called program clock references or “PCRs,” are inserted into the payloads of selected transport packets (specifically, in adaption fields). The decoder extracts the PCRs from the transport packets and uses the PCRs to recover the encoder system time clock. The PES packets may contain decoding time stamps or “DTSs” and/or presentation time stamps or “PTSs”. A DTS indicates the time, relative to the recovered encoder system time clock, at which the next decoding unit (i.e., compressed audio frame, compressed video picture, etc.) should be decoded The PTS indicates the time, relative to the recovered encoder system time clock, at which the next presentation unit (i.e., decompressed audio frame, decompressed picture, etc.) should be presented or displayed.
Unlike the PS, a TS may have transport packets that carry program data for more than one program. Each program may have been encoded at a different encoder in relation to a different encoder system time clock. The TS enables the decoder to recover the specific system time clock of the program which the decoder desires to decode. To that end, the TS must carry separate sets of PCRs, i.e., one set of PCRs for recovering the encoder system time clock of each program.
The TS also carries program specific information or (PSI) in transport packets. PSI is for identifying data of a desired program or other information for assisting in decoding a program. A program association table or “PAT” is provided which is carried in transport packets with the PID 0×0000. The PAT correlates each program number with the PID of the transport packets carrying program definitions for that program. A program definition: (1) indicates which ESs make up the program to which the program definition corresponds, (2) identifies the PIDs for each of those ESs, (3) indicates the PID of the transport packets carrying the PCRs of that program (4) identifies the PIDs of transport packets carrying ES specific entitlement control messages (e.g., descrambling or decryption keys) and other information. Collectively, all program definitions of a TS are referred to as a program mapping table (PMT). Thus, a decoder can extract the PAT data from the transport packets and use the PAT to identify the PID of the transpor
Elallam Ahmed
Kizou Hassan
Proskauer Rose LLP
SkyStream Corporation
LandOfFree
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