MPEG transport stream

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MPEG Transport Stream
Filename extension .ts, .tsv, .tsa
Internet media type video/MP2T[1]
Developed by MPEG
Initial release 1995 (1995)[2]
Type of format Media container
Container for Audio, video, data
Extended to M2TS, TOD
Standard ISO/IEC 13818-1, ITU-T Recommendation H.222.0[3][4][5]

MPEG transport stream (MPEG-TS, MTS or TS) is a standard format for transmission and storage of audio, video, and Program and System Information Protocol (PSIP) data.[6] It is used in broadcast systems such as DVB, ATSC and IPTV.

Transport Stream is specified in MPEG-2 Part 1, Systems (formally known as ISO/IEC standard 13818-1 or ITU-T Rec. H.222.0).[3]

Transport stream specifies a container format encapsulating packetized elementary streams, with error correction and stream synchronization features for maintaining transmission integrity when the signal is degraded.

Transport streams differ from the similarly named program streams in several important ways: program streams are designed for reasonably reliable media, such as discs (like DVDs), while transport streams are designed for less reliable transmission, namely terrestrial or satellite broadcast. Further, a transport stream may carry multiple programs.

Layers of communication[edit]

Multiple MPEG programs are combined then sent to a transmitting antenna. In the US broadcast digital TV system, an ATSC receiver then decodes the TS and displays it. In most other parts of the world, transmission would be accomplished by one or more variants of the modular DVB system.

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:

  1. Composition of the various programs
  2. Elementary stream (ES) — audio or video (the below is for video only)
  3. Encoding block or just block—a DCT encoding block, 8x8 pixels
  4. Group of pictures (GOP) — providing random access points
  5. Macroblock—consisting of 6 to 12 DCT blocks
  6. Packetized elementary stream (PES)
  7. Slice — preventing an error from being propagated through intra prediction

An example of data that is multiplexed or muxed into the transport stream is an electronic program guide.

Important elements of a transport stream[edit]

Packet[edit]

A packet is the basic unit of data in a transport stream. It starts with a sync byte and a header. 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, but the communication medium may add some error correction bytes to the packet. ISDB-T and DVB-T/C/S uses 204 bytes and ATSC 8-VSB, 208 bytes as the size of emission packets (transport stream packet + FEC data). ATSC transmission adds 20 bytes of Reed-Solomon forward error correction to create a packet that is 208 bytes long.[7] The 188-byte packet size was originally chosen for compatibility with ATM systems.[8][9]

Partial Transport Stream Packet Format
Name Number
of bits
32-bit BE
mask
Description
Sync byte 8 0xff000000 Bit pattern from bit 7 to 0 as 0x47 or ASCII char 'G'
Transport Error Indicator (TEI) 1 0x800000 Set when a demodulator can't correct errors from FEC data;
this would inform a stream processor to ignore the packet [10]
Payload Unit Start Indicator 1 0x400000 Boolean flag with a value of true meaning the start of
PES data or PSI otherwise zero only.
Transport Priority 1 0x200000 Boolean flag with a value of true meaning the current packet
has a higher priority than other packets with the same PID.
PID 13 0x1fff00 Packet Identifier
Scrambling control 2 0xc0 '00' = Not scrambled.

For DVB-CSA only:[11]
'01' (0x40) = Reserved for future use
'10' (0x80) = Scrambled with even key
'11' (0xC0) = Scrambled with odd key

Adaptation field exist 1 0x20 Boolean flag
Contains payload 1 0x10
Continuity counter 4 0xf Sequence number of payload packets (0x00 to 0x0F).
Incremented only when a payload is present (i.e., payload value is true)
Note: the total number of bits above is 32 and is called the transport stream 4-byte prefix or Transport Stream Header.
Adaptation field 0 or more If adaption field exist value is true
Payload Data 0 or more If contains payload value is true
Adaptation Field Format
Name Number
of bits
Byte
mask
Description
Adaptation Field Length 8 Number of bytes in the adaptation field immediately following this byte
Discontinuity indicator 1 0x80 Set to 1 if current TS packet is in a discontinuity state with respect to either the continuity counter or the program clock reference
Random Access indicator 1 0x40 Set to 1 if the PES packet in this TS packet starts a video/audio sequence
Elementary stream priority indicator 1 0x20 1 = higher priority
PCR flag 1 0x10 Set to 1 if adaptation field contains a PCR field
OPCR flag 1 0x08 Set to 1 if adaptation field contains an OPCR field
Splicing point flag 1 0x04 Set to 1 if adaptation field contains a splice countdown field
Transport private data flag 1 0x02 Set to 1 if adaptation field contains private data bytes
Adaptation field extension flag 1 0x01 Set to 1 if adaptation field contains an extension
Below fields are optional variable Depends on flags
PCR 33+6+9 Program clock reference, stored in 6 octets in big-endian as 33 bits base, 6 bits padding, 9 bits extension.
OPCR 33+6+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

Packet Identifier (PID)[edit]

Each table or elementary stream in a transport stream is identified by a 13-bit packet identifier (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.

Identifiers in use
Decimal Hexadecimal Description
0 0x0000 Program Association Table (PAT) contains a directory listing of

all Program Map Tables

1 0x0001 Conditional Access Table (CAT) contains a directory listing of

all ITU-T Rec. H.222 entitlement management message streams

used by Program Map Tables

2 0x0002 Transport Stream Description Table contains descriptors

relating to the overall transport stream

3 0x0003 IPMP Control Information Table contains a directory listing of

all ISO/IEC 14496-13 control streams

used by Program Map Tables

4-15 0x0004-0x000F Reserved for future use
16-31 0x0010-0x001F Used by DVB metadata
32-8186 0x0020-0x1FFA May be assigned as needed to Program Map Tables,

elementary streams and other data tables

8187 0x1FFB Used by DigiCipher 2/ATSC MGT metadata
8188-8190 0x1FFC-0x1FFE May be assigned as needed to Program Map Tables,

elementary streams and other data tables

8191 0x1FFF Null Packet (used for fixed bandwidth padding)

Programs[edit]

Transport stream has a concept of programs. Each 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. A transport stream with more than one program is referred to as MPTS - Multi Program Transport Stream. A single program transport stream is referred to as SPTS - Single Program Transport Stream.

Program Specific Information (PSI)[edit]

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[edit]

PAT stands for Program Association Table. It lists all programs available in the transport stream. Each of the listed programs is identified by a 16-bit value called program_number. Each of the programs listed in PAT has an associated value of PID for its Program Map Table (PMT).

The value 0x0000 of program_number is reserved to specify the PID where to look for Network Information Table (NIT). If such a program is not present in PAT the default PID value (0x0010) shall be used for NIT.

TS Packets containing PAT information always have PID 0x0000.

PMT[edit]

Program Map Tables (PMTs) contain information about programs. For each program, there is one PMT. While the MPEG-2 standard permits more than one PMT section to be transmitted on a single PID (Single Transport stream PID contains PMT information of more than one program), most MPEG-2 "users" such as ATSC and SCTE require each PMT to be transmitted on a separate PID that is not used for any other packets. The PMTs provide information on each program present in the transport stream, including the program_number, and list the elementary streams that comprise the described MPEG-2 program. There are also locations for optional descriptors that describe the entire MPEG-2 program, as well as an optional descriptor for each elementary stream. Each elementary stream is labeled with a stream_type value.

PCR[edit]

To enable a decoder to present synchronized content, such as audio tracks matching the associated video, at least once each 100 ms a Program Clock Reference, or PCR is transmitted in the adaptation field of an MPEG-2 transport stream packet. The PID with the PCR for an MPEG-2 program is identified by the pcr_pid value in the associated Program Map Table. The value of the PCR, when properly used, is employed to generate a system_timing_clock in the decoder. The STC or System Time Clock decoder, when properly implemented, provides a highly accurate time base that is used to synchronize audio and video elementary streams. Timing in MPEG2 references this clock. For example, the presentation time stamp (PTS) is intended to be relative to the PCR.The first 33 bits are based on a 90 kHz clock. The last 9 are based on a 27 MHz clock. The maximum jitter permitted for the PCR is +/- 500 ns.

Null packets[edit]

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.

Use in digital video cameras[edit]

Transport Stream had been originally designed for broadcast. Later it was adapted for usage with digital video cameras, recorders and players by adding a 4-byte timecode (TC) to standard 188-byte packets, which resulted in a 192-byte packet.[12][13] This is what is informally called M2TS stream. Blu-ray Disc Association calls it "BDAV MPEG-2 transport stream".[12] JVC called it TOD (possibly an abbreviation for "Transport stream on disc") when used in HDD-based camcorders like GZ-HD7.[14][15] The timecode allows quick access to any part of the stream either from a media player, or from a non-linear video editing system.[16] It is also used to synchronize video streams from several cameras in a multi-camera shoot.

Use in Blu-ray[edit]

Filename extension .m2ts is used on Blu-ray Disc Video for files which contain an incompatible BDAV MPEG-2 transport stream due to the four additional octets added to every packet.[12][17] Blu-ray Disc Video titles authored with menu support are in the BDMV (Blu-ray Disc Movie) format and contain audio, video, and other streams in a BDAV container, which is based on the MPEG-2 transport stream format.[18][19] There is also the BDAV (Blu-ray Disc Audio/Visual) format, the consumer oriented alternative to the BDMV format used for movie releases. The BDAV format is used on BD-REs and BD-Rs for audio/video recording.[19] Blu-ray Disc employs the MPEG-2 transport stream recording method. That enables transport streams of a BDAV converted digital broadcast to be recorded as they are with minimal alteration of the packets.[13] It also enables simple stream cut style editing of a BDAV converted digital broadcast that is recorded as is and where the data can be edited just by discarding unwanted packets from the stream. Although it is quite natural, a function for high-speed and easy-to-use retrieval is built in.[13][20] Blu-ray Disc Video uses these modified MPEG-2 transport streams, compared to DVD's program streams that don't have the extra transport overhead.

See also[edit]

References[edit]

  1. ^ "RFC 3555 - MIME Type Registration of RTP Payload Formats". July 2003. Retrieved 20 March 2010. 
  2. ^ ITU-T (July 1995). "Recommendation H.222.0 (07/95) - Superseded". Retrieved 18 July 2010. 
  3. ^ a b ITU-T (May 2006). "H.222.0 Summary". Archived from the original on 2011-08-24. 
  4. ^ ITU-T. "H.222.0 : Information technology - Generic coding of moving pictures and associated audio information: Systems". Retrieved 3 June 2010. 
  5. ^ ISO (1996). "ISO/IEC 13818-1:1996 - Information technology – Generic coding of moving pictures and associated audio information: Systems". Retrieved 18 July 2010. 
  6. ^ "MPEG-2 Transport Stream". AfterDawn.com. Retrieved 8 June 2010. 
  7. ^ "ATSC transmission". Broadcastengineering.com. 2005-06-20. Retrieved 2012-05-17. 
  8. ^ "MPEG Systems FAQ". Mpeg.chiariglione.org. Retrieved 2012-05-17. 
  9. ^ "ATSC MPEG Transport Stream Monitor". Tek.com. Retrieved 2012-05-17. 
  10. ^ "TSReader". Coolstf.com. 2008-04-07. Retrieved 2012-05-17. 
  11. ^ Support for use of the DVB Scrambling Algorithm version 3 within digital broadcasting systems Page 6
  12. ^ a b c Blu-ray Disc Association (March 2005) BD ROM - Audio Visual Application Format Specifications (PDF) Page 15, Retrieved on 2009-07-26
  13. ^ a b c Blu-ray Disc Association (March 2008) BD-RE - Audiovisual Application Format Specification for BD-RE 2.1 (PDF), Technical White Papers - BD RE, Retrieved on 2009-07-28
  14. ^ "Steve Mullen, M2TS primer". 
  15. ^ Working with JVC Everio MOD & TOD files, Retrieved on 2009-07-27
  16. ^ "How MPEG-TS works, videohelp.com". Forum.videohelp.com. Retrieved 2012-05-17. [self-published source?]
  17. ^ Videohelp.com What is Blu-ray Disc and HD DVD?, Retrieved on 2009-07-26
  18. ^ AfterDawn.com Glossary - BD-MV (Blu-ray Movie) and BDAV container, Retrieved on 2009-07-26
  19. ^ a b AfterDawn.com Glossary - BDAV container, Retrieved on 2009-07-26
  20. ^ Blu-ray Disc Association (August 2004) Blu-ray Disc Format, White paper (PDF) Page 22, Retrieved on 2009-07-28

External links[edit]