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MPEG transport stream
From Wikipedia, the free encyclopedia
Transport stream (TS, TP, or MPEG-TS) is a communications protocol for audio, video, and data which is specified in MPEG-2 Part 1, Systems (ISO/IEC standard 13818-1[1]). Its design goal is to allow multiplexing of digital video and audio and to synchronize the output. Transport stream offers features for error correction for transportation over unreliable media, and is used in broadcast applications such as DVB and ATSC. It is contrasted with program stream, designed for more reliable media such as DVDs.
Contents |
Similar to the OSI network protocol stack, a transport stream is processed by the receiver in layers. An example stream containing video may be processed as follows:
- Composition of the various programs.
- Packetized Elementary Stream (PES)
- Elementary stream (ES) — audio or video (the below is for video only)
- Group of pictures (GOP) — providing random access points
- Slice — preventing an error from being propagated through intra prediction
- Macroblock—consisting of 6 to 12 DCT blocks
- Encoding block or just block—a DCT encoding block, 8x8 pixels
An example of data that is muxed into the transport stream is an electronic program guide. See Program and System Information Protocol for more information.
A packet is the basic unit of data in a transport stream. It consists of a sync byte, whose value is 0x47, followed by three one-bit flags and a 13-bit PID. This is followed by a 4-bit continuity counter. Additional optional transport fields, as signaled in the optional adaptation field, may follow. The rest of the packet consists of payload. Packets are 188 bytes in length [1] , but the communication medium may add some error correction bytes to the packet. DVb-ASI uses 204 bytes and ATSC 208 bytes as transport stream packet. (DVB t=8 and ATSC t=10 i.e. extra bytes = 2*t). ATSC transmission adds 20 bytes of Reed-Solomon forward error correction to create a packet that is 208 bytes long[2]. The 188-byte packet size was originally chosen for compatibility with ATM systems[3] [[4]] .
| Name | Number of bits |
Description |
|---|---|---|
| sync byte | 8 | 0x47 |
| Transport Error Indicator (TEI) | 1 | Set by demodulator if can't correct errors in the stream[5] |
| Payload Unit Start Indicator | 1 | 1 means start of PES data or PSI otherwise zero only . |
| Transport Priority | 1 | One means higher priority than other packets with the same PID. |
| PID | 13 | Packet ID |
| Scrambling control | 2 | '00' = Not scrambled. The following per DVB spec [2]: '01' = Reserved for future use, '10' = Scrambled with even key, '11' = Scrambled with odd key |
| Adaptation field exist | 1 | 1 means presence of the adaptation field |
| Payload data exist | 1 | 1 means presence of data |
| Continuity counter | 4 | |
| Note: the total number of bits above is 32 and is called the transport stream 4-byte prefix. | ||
| Adaption field | 0 or more | Depends on flags |
| Payload Data | 0 or more | Depends on flags |
| Name | Number of bits |
Description |
|---|---|---|
| Adaptation Field Length | 8 | Number of bytes in the adaptation field immediately following this byte |
| Discontinuity indicator | 1 | Set to 1 if a discontinuity occurred in the continuity counter of the TS packet |
| Random Access indicator | 1 | Set to 1 if the PES packet in this TS packet starts a video/audio sequence |
| Elementary stream priority indicator | 1 | 1 = higher priority |
| PCR flag | 1 | 1 means adaptation field does contain a PCR field |
| OPCR flag | 1 | |
| Splicing point flag | 1 | 1 means presence of splice countdown field in adaptation field |
| Transport private data flag | 1 | 1 means presence of private data bytes in adaptation field |
| Adaptation field extension flag | 1 | 1 means presence of adaptation field extension |
| Below fields are optional | variable | Depends on flags |
| PCR | 33+9 | Program clock reference |
| OPCR | 33+9 | Original Program clock reference. Helps when one TS is copied into another |
| Splice countdown | 8 | Indicates how many TS packets from this one a splicing point occurs (may be negative) |
| stuffing bytes | variable |
Each table or elementary stream in a transport stream is identified by a 13-bit PID. A demultiplexer extracts elementary streams from the transport stream in part by looking for packets identified by the same PID. In most applications, Time-division multiplexing will be used to decide how often a particular PID appears in the transport stream.
Transport stream has a concept of programs. A single program is described by a Program Map Table (PMT) which has a unique PID, and the elementary streams associated with that program have PIDs listed in the PMT. For instance, a transport stream used in digital television might contain three programs, to represent three television channels. Suppose each channel consists of one video stream, one or two audio streams, and any necessary metadata. A receiver wishing to decode a particular "channel" merely has to decode the payloads of each PID associated with its program. It can discard the contents of all other PIDs.
There are 4 PSI tables: Program Association (PAT), Program Map (PMT), Conditional Access (CAT), and Network Information (NIT). The MPEG-2 specification does not specify the format of the CAT and NIT.
PAT stands for Program Association Table. The PAT lists PIDs for all PMTs in the stream. TS Packets containing PAT information always have PID 0x0.
Program Map Tables, or PMTs, contain information about programs. For each program, there is a PMT, with the PMT for each program appearing on its own PID. The PMTs describe which PIDs contain data relevant to the program. PMTs also provide metadata about the streams in their constituent PIDs. For example, if a program contains an MPEG-2 video stream, the PMT will list this PID, describe it as a video stream, and provide the type of video that it contains (in this case, MPEG-2). The PMT may also contain additional descriptors providing data about its constituent streams.
To assist the decoder in presenting programs on time, at the right speed, and with synchronization, programs usually periodically provide a Program Clock Reference, or PCR, on one of the PIDs in the program. This is also known as the master clock. Timing in MPEG2 references this clock, for example the presentation time stamp (PTS) is relative to the PCR. The first 33 bits is based on a 90kHz clock, incremented for each Hertz or cycle. The 9 bit extension is based on a 27MHz clock.
Some transmission schemes, such as those in ATSC and DVB, impose strict constant bitrate requirements on the transport stream. In order to ensure that the stream maintains a constant bitrate, a Multiplexer may need to insert some additional packets. The PID 0x1FFF is reserved for this purpose. The payload of null packets may not contain any data at all, and the receiver is expected to ignore its contents.
- ^ (2000-12-01) ISO/IEC 13818-1 Second edition, page xi or 11 according to PDF viewer.
- ^ DVB scrambling control bits defined. Page 6
- ATSC tuner
- DVB
- Elementary stream
- IPTV
- MPEG-2
- Packetized Elementary Stream
- Program and System Information Protocol
- Unidirectional Lightweight Encapsulation (ULE)
- HDV and AVCHD, which both implement TS streams to packetize data
Software:
- dvb-snoop
- ATSC Transport Stream Tools for Linux
- Tools
- Ethereal MPEG-2 TS Dissector
- DVB Stream Explorer
Hardware:
- MPEG-2 Systems FAQ
- MPEG-4 Systems FAQ
- MPEG-1 description
- Powerpoint MPEG-2 Transport Stream introduction [6]
- Splicing FAQ
Categories: ATSC | MPEG