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

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 MPEG program stream 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.

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.

Layers of communication

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), 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.

Partial Transport Stream Packet Format
NameNumber
of bits
32-bit BE
mask
Description
4-byte Transport Stream Header
Sync byte80xff000000Bit pattern of 0x47 (ASCII char 'G')
Transport Error Indicator (TEI)10x800000Set when a demodulator can't correct errors from FEC data; indicating the packet is corrupt.
Payload Unit Start Indicator (PUSI)10x400000Set when a PES, PSI, or DVB-MIP packet begins immediately following the header.
Transport Priority10x200000Set when the current packet has a higher priority than other packets with the same PID.
PID130x1fff00Packet Identifier, describing the payload data.
Transport Scrambling Control (TSC)20xc0'00' = Not scrambled.

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

Adaptation field control20x3001 – no adaptation field, payload only,

10 – adaptation field only, no payload,
11 – adaptation field followed by payload,
00 - RESERVED for future use

Continuity counter40xfSequence 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 fieldvariableIf Adaptation field flag is set in the Adaptation field control, see below.
Payload DatavariableIf Payload flag is set in the Adaptation field control. Payload may be PES packets, program specific information (below), or other data.
Adaptation Field Format
NameNumber
of bits
Byte
mask
Description
Adaptation Field Length8Number of bytes in the adaptation field immediately following this byte
Discontinuity indicator10x80Set if current TS packet is in a discontinuity state with respect to either the continuity counter or the program clock reference
Random Access indicator10x40Set when the stream may be decoded without errors from this point
Elementary stream priority indicator10x20Set when this stream should be considered "high priority"
PCR flag10x10Set when PCR field is present
OPCR flag10x08Set when OPCR field is present
Splicing point flag10x04Set when splice countdown field is present
Transport private data flag10x02Set when private data field is present
Adaptation field extension flag10x01Set when extension field is present
Optional fields
PCR48Program clock reference, stored as 33 bits base, 6 bits reserved, 9 bits extension.
The value is calculated as base * 300 + extension.
OPCR48Original Program clock reference. Helps when one TS is copied into another
Splice countdown8Indicates how many TS packets from this one a splicing point occurs (Two's complement signed; may be negative)
Transport private data length8The length of the following field
Transport private datavariablePrivate data
Adaptation extensionvariableSee below
Stuffing bytesvariableAlways 0xFF
Adaptation Extension Field Format
NameNumber
of bits
Byte
mask
Description
Adaptation extension length80xff00The length of the header
Legal time window (LTW) flag10x0080
Piecewise rate flag10x0040
Seamless splice flag10x0020
Reserved50x001f
Optional fields
LTW flag set (2 bytes)
Legal time window valid flag10x8000
Legal time window offset150x7fffExtra information for rebroadcasters to determine the state of buffers when packets may be missing.
Piecewise flag set (3 bytes)
Reserved20xc00000
Piecewise rate220x3fffffThe rate of the stream, measured in 188-byte packets, to define the end-time of the LTW.
Seamless splice flag set (5 bytes)
Splice type40xf000000000Indicates the parameters of the H.262 splice.
DTS next access unit360x0efffefffeThe 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
DecimalHexadecimalDescription
00x0000Program Association Table (PAT) contains a directory listing of all Program Map Tables
10x0001Conditional Access Table (CAT) contains a directory listing of all ITU-T Rec. H.222 entitlement management message streams used by Program Map Tables
20x0002Transport Stream Description Table contains descriptors relating to the overall transport stream
30x0003IPMP Control Information Table contains a directory listing of all ISO/IEC 14496-13 control streams used by Program Map Tables
4-150x0004-0x000FReserved for future use
16-310x0010-0x001FUsed by DVB metadata
32-81860x0020-0x1FFAMay be assigned as needed to Program Map Tables, elementary streams and other data tables
81870x1FFBUsed by DigiCipher 2/ATSC MGT metadata
8188-81900x1FFC-0x1FFEMay be assigned as needed to Program Map Tables, elementary streams and other data tables
81910x1FFFNull Packet (used for fixed bandwidth padding)

Programs

Transport stream has a concept of programs. Every 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. This is what is informally called M2TS stream. Blu-ray Disc Association calls it "BDAV MPEG-2 transport stream". JVC called it TOD (possibly an abbreviation for "Transport stream on disc") when used in HDD-based camcorders like GZ-HD7. The timecode allows quick access to any part of the stream either from a media player, or from a non-linear video editing system. 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. 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. 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. 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. 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. 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.