Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1997-09-26
2002-03-12
Nguyen, Chau (Department: 2663)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S535000, C375S376000
Reexamination Certificate
active
06356567
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to digital delivery systems, especially for digital video and digital audio data. More particularly, the invention relates to multiplexors, networks, distribution systems, demultiplexors, and multiplexed bitstreams, and especially to bitstreams carrying a system or transport layer, and one or more data layers of compressed digital video and digital audio data. More particularly, the invention relates to recovering the system clock with minimum demand on a processor.
BACKGROUND OF THE INVENTION
Within the past decade, the advent of world-wide electronic communications systems has enhanced the way in which people can send and receive information. Moreover, the capabilities of real-time video and audio systems have greatly improved in recent years. Real-time video and audio systems require a large bandwidth. In order to provide services such as video-on-demand and videoconferencing to subscribers, an enormous amount of network bandwidth is required. In fact, network bandwidth is often the main inhibitor to the effectiveness of such systems.
In order to minimize the effects of the constraints imposed by the limited bandwidths of telecommunications networks, compression systems and standards have evolved. These standards prescribe the compression of video and audio data and the delivery of several programs and control data in a single bit stream transmitted in a bandwidth that would heretofore only accomodate one analog program.
One video and audio compression standard is the Moving Picture Experts Group (“MPEG”) standard. Within the MPEG-2 standard, video compression is defined both within a given picture, i.e., spatial compression, and between pictures, i.e., temporal compression. Video compression within a picture is accomplished by conversion of the digital image from the time domain to the frequency domain by a discrete cosine transform, quantization, variable length coding, and Huffman coding. Video compression between pictures is accomplished via a process referred to as motion compensation, in which a motion vector is used to describe the translation of a set of picture elements (pels) from one picture to another. Audio compression is as defined in the standard.
The procedure for transporting the compressed bitstream from the transmitting end to the receiving end of the system, and for thereafter decompressing the bitstream at the receiving end, so that one of the many picture sequences is decompressed and may be displayed in real-time is specified in ISO 13818-1. ISO 13818-1 is the systems or transport layer portion of the MPEG-2 standard. This portion of the standard specifies packetization of audio and video elementary bitstreams into packetized elementary streams (PES), and the combinations of one or more audio and video packetized elementary streams into a single time division or packet multiplexed bitstream for transmission and the subsequent demultiplexing of the single bitstream into multiple bitstreams for decompression and display. The single time division or packet multiplexed bit stream is as shown from various architectural and logical perspectives in the FIGURES, especially
FIGS. 1
to
5
, where many packets make up a single bitstream.
The concept of packetization and the mechanism of packet multiplexing are shown in
FIG. 1
, denominated “Prior Art”, where elementary streams are formed in an audio encoder, a video encoder, a source of other data, and a source of systems data. These elementary streams are packetized into packetized elementary streams, as described hereinbelow. The packetized elementary streams of audio data, and video data, and the packets of other data and systems data are packet multiplexed by the multiplexor into a system stream.
The time division or packet multiplexed bitstream is shown, for example, in
FIGS. 2 and 5
, both denominated “Prior Art”, which gives an overview showing the time division or packet multiplexed bitstream. The bitstream is comprised of packets, as shown in FIG.
5
. Each packet, as shown in
FIG. 2
, is, in turn, made up of a packet header, an optional adaptation field, and packet data bytes, i.e., payload.
The MPEG-2 System Layer has the basic task of facilitating the multiplexing of one or more programs made up of related audio and video bitstreams into a single bitstream for transmission through a transmission medium, and thereafter to facilitate the demultiplexing of the single bitstream into separate audio and video program bitstreams for decompression while maintaining synchronization. By a “Program” is meant a set of audio and video bitstreams having a common time base and intended to be presented simultaneously. To accomplish this, the System Layer defines the data stream syntax that provides for timing control and the synchronization and interleaving of the video and audio bitstreams. The system layer provides capability for (1) video and audio synchronization, (2) stream multiplex, (3) packet and stream identification, (4) error detection, (5) buffer management, (6) random access and program insertion, (7) private data, (8) conditional access, and (9) interoperability with other networks, such as those using asynchronous transfer mode (ATM).
An MPEG-2 bitstream is made up of a system layer and compression layers. Under the MPEG-2 Standard (ISO/IEC 13818-1) a time division or packet multiplexed bit-stream consists of two layers, (1) a compression layer, also referred to as an inner layer, a payload layer, or a data layer, and (2) a system layer, also referred to as an outer layer or a control layer. The compression layer or inner layer contains the data fed to the video and audio decoders, and defines the coded video and audio data streams, while the system layer or outer layer provides the controls for demultiplexing the interleaved compression layers, and in doing so defines the functions necessary for combining the compressed data streams. This is shown in
FIG. 3
, denominated “Prior Art.” As there shown a bitstream of, for example, a system layer and compression layer, is the input to a system decoder. In the system decoder the system layer data is demultiplexed into the compressed audio layer, the compressed video layer, and control data. The control data is shown in
FIG. 3
, denominated Prior Art, as the PCR (Program Clock Recovery) data, enable data, and start up values. The compressed data is sent to the respective audio and video data buffers, and through decoder control to the respective audio and video decoders.
The system layer supports a plurality of basic functions, (1) time division or packet multiplexing and demultiplexing of the time division or packet multiplexed multiple bit-streams, (2) synchronous display of the multiple coded bit streams, (3) buffer management and control, and (4) time recovery and identification. The system layer also supports (5) random access, (6) program insertion, (7) conditional access, and (8) error tracking.
For MPEG-2, the standard specifies two types of layer coding, a program stream (PS), for relatively lossless environments, such as CD-ROMs, DVDs, and other storage media, and transport stream (TS), for lossy media, as cable television, satellite television, and the like. The transport stream (TS), shown in
FIG. 2
, denominated Prior Art, consists of a stream of transport stream packets, each of which consists of 188 bytes, divided into 4 bytes of packet header, an optional adaptation field, and up to 184 bytes of the associated packet data, that is, payload. The relationship of the layering of the access units, the PES packets, and the Transport Stream (TS) packets is shown in
FIG. 5
, denominated Prior Art.
The transport stream (TS) is used to combine programs made up of PES-coded data with one or more independent time bases into a single stream. Note that under the MPEG-2 standard, an individual program does not have to have a unique time base, but that if it does, the time base is the same for all of the elements of the individual program.
The packetized elementary stream (PES) layer is an inner layer
Anderson Richard E.
Foster Eric M.
Hodges Mark R.
International Business Machines - Corporation
Nguyen Chau
Nguyen Phuongchau Ba
Steinberg William H.
Whitman, Curtis & Christofferson P.C.
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