MPEG transport stream

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[2]

MPEG transport stream (MPEG-TS, MTS or TS) is a standard container format for transmission and storage of audio, video, and Program and System Information Protocol (PSIP) data.[3] 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).[2]

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

Multiple MPEG programs are combined then sent to a transmitting antenna. The receiver parses and decodes one of the streams.

A transport stream encapsulates a number of other substreams, often Packetized elementary streams (PES) which in turn wrap the main data stream of an MPEG codec, as well as any number of non-MPEG codecs (such as AC3 or DTS audio, and MJPEG or JPEG 2000 video), text and pictures for subtitles, tables identifying the streams, and even broadcaster-specific information such as an electronic program guide. Many unrelated streams are often mixed together, such as several different television channels, or multiple angles of a movie. Each stream is chopped into (at most) 188-byte sections and interleaved together; because of the tiny packet size, streams can be interleaved with less latency and greater error resilience compared to program streams and common containers such as AVI, MOV/MP4, and MKV, which generally wrap each frame into one packet. This is particularly important for videoconferencing, where even one large frame may introduce unacceptable audio delay.

Transport streams tend to be broadcast as Constant bitrate (CBR) to maintain a consistent broadcast rate, filled with padding bytes when not enough data exists, although the Blu-Ray format does not require CBR.

Important elements of a transport stream

Packet

A packet is the basic unit of data in a transport stream, and a transport stream is merely a sequence of packets, without any global header. Each packet starts with a sync byte and a header, that may be followed with optional additional headers; the rest of the packet consists of payload. All header fields are read as big-endian. Packets are 188 bytes in length, but the communication medium may add additional information: Forward error correction is added by ISDB & DVB (16 bytes) and ATSC (20 bytes),[4] while the M2TS format prefixes packets with a 4-byte copyright and timestamp tag. The 188-byte packet size was originally chosen for compatibility with ATM systems.[5][6]

Partial Transport Stream Packet Format
Name Number
of bits		 32-bit BE
mask		 Description
4-byte Transport Stream Header
Sync byte 8 0xff000000 Bit pattern of 0x47 (ASCII char 'G')
Transport Error Indicator (TEI) 1 0x800000 Set when a demodulator can't correct errors from FEC data; indicating the packet is corrupt.[7]
Payload Unit Start Indicator 1 0x400000 Set when a PES, PSI, or DVB-MIP packet begins immediately following the header.
Transport Priority 1 0x200000 Set when the current packet has a higher priority than other packets with the same PID.
PID 13 0x1fff00 Packet Identifier, describing the payload data.
Scrambling control 2 0xc0 '00' = Not scrambled.

For DVB-CSA only:[8]
'01' (0x40) = Reserved for future use
'10' (0x80) = Scrambled with even key
'11' (0xC0) = Scrambled with odd key

Adaptation field flag 1 0x20
Payload flag 1 0x10
Continuity counter 4 0xf Sequence number of payload packets (0x00 to 0x0F) within each stream (except PID 8191)
Incremented per-PID, only when a payload flag is set.
Optional fields
Adaptation field variable If Adaptation field flag is set, see below.
Payload Data variable If Payload flag is set. Payload may be PES packets, program specific information (below), or other data.
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 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 when the stream may be decoded without errors from this point
Elementary stream priority indicator 1 0x20 Set when this stream should be considered "high priority"
PCR flag 1 0x10 Set when PCR field is present
OPCR flag 1 0x08 Set when OPCR field is present
Splicing point flag 1 0x04 Set when splice countdown field is present
Transport private data flag 1 0x02 Set when private data field is present
Adaptation field extension flag 1 0x01 Set when extension field is present
Optional fields
PCR 48 Program clock reference, stored as 33 bits base, 6 bits reserved, 9 bits extension.
The value is calculated as base * 300 + extension.
OPCR 48 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 (Two's complement signed; may be negative)
Transport private data length 8 The length of the following field
Transport private data variable Private data
Adaptation extension variable See below
Stuffing bytes variable Always 0xFF
Adaptation Extension Field Format
Name Number
of bits		 Byte
mask		 Description
Adaptation extension length 8 0xff00 The length of the header
Legal time window (LTW) flag 1 0x0080
Piecewise rate flag 1 0x0040
Seamless splice flag 1 0x0020
Reserved 5 0x001f
Optional fields
LTW flag set (2 bytes)
Legal time window valid flag 1 0x8000
Legal time window offset 15 0x7fff Extra information for rebroadcasters to determine the state of buffers when packets may be missing.
Piecewise flag set (3 bytes)
Reserved 2 0xc00000
Piecewise rate 22 0x3fffff The rate of the stream, measured in 188-byte packets, to define the end-time of the LTW.
Seamless splice flag set (5 bytes)
Splice type 4 0xf000000000 Indicates the parameters of the H.262 splice.
DTS next access unit 36 0x0efffefffe The PES DTS of the splice point.
Split up as 3 bits, 1 marker bit (0x1), 15 bits, 1 marker bit, 15 bits, and 1 marker bit, for 33 data bits total.

Packet Identifier (PID)

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

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)

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

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

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

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

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

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.[9][10] This is what is informally called M2TS stream. Blu-ray Disc Association calls it "BDAV MPEG-2 transport stream".[9] JVC called it TOD (possibly an abbreviation for "Transport stream on disc") when used in HDD-based camcorders like GZ-HD7.[11][12] The timecode allows quick access to any part of the stream either from a media player, or from a non-linear video editing system.[13] It is also used to synchronize video streams from several cameras in a multi-camera shoot.

Use in Blu-ray

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.[9][14] 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.[15][16] 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.[16] 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.[10] 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.[10][17] 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

References

  1. ↑ "RFC 3555 - MIME Type Registration of RTP Payload Formats". July 2003. Retrieved 20 March 2010.
  2. 1 2 3 ITU-T (October 2014). "Recommendation H.222.0 (10/14)".
  3. ↑ "MPEG-2 Transport Stream". AfterDawn.com. Retrieved 8 June 2010.
  4. ↑ "ATSC transmission". Broadcastengineering.com. 2005-06-20. Retrieved 2012-05-17.
  5. ↑ "MPEG Systems FAQ". Mpeg.chiariglione.org. Retrieved 2012-05-17.
  6. ↑ "ATSC MPEG Transport Stream Monitor". Tek.com. Retrieved 2012-05-17.
  7. ↑ "TSReader". Coolstf.com. 2008-04-07. Retrieved 2012-05-17.
  8. ↑ Support for use of the DVB Scrambling Algorithm version 3 within digital broadcasting systems Page 6
  9. 1 2 3 Blu-ray Disc Association (March 2005) BD ROM - Audio Visual Application Format Specifications (PDF) Page 15, Retrieved on 2009-07-26
  10. 1 2 3 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
  11. ↑ "Steve Mullen, M2TS primer".
  12. ↑ Working with JVC Everio MOD & TOD files, Retrieved on 2009-07-27
  13. ↑ "How MPEG-TS works, videohelp.com". Forum.videohelp.com. Retrieved 2012-05-17.
  14. ↑ Videohelp.com What is Blu-ray Disc and HD DVD?, Retrieved on 2009-07-26
  15. ↑ AfterDawn.com Glossary - BD-MV (Blu-ray Movie) and BDAV container, Retrieved on 2009-07-26
  16. 1 2 AfterDawn.com Glossary - BDAV container, Retrieved on 2009-07-26
  17. ↑ Blu-ray Disc Association (August 2004) Blu-ray Disc Format, White paper (PDF) Page 22, Retrieved on 2009-07-28

External links

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