Transport stream

Transport stréam (TS, TP, or MPEG-TS) nyaéta hiji protokol komunikasi keur audio, video, jeung data nu ditetepkeun dina MPEG-2 Bagéan 1, Systems (standar ISO/IEC 13818-1 [1]). Tujuan rancangannana nyaéta keur ngidinan multipleksing vidéo jeung audio digital sarta keur nyinkronisasi kaluaran. Transport stréam nawarkeun fitur keur ngoreksi kasalahan pikeun transportasi ngaliwatan media nu henteu pati dipercaya, sarta dipaké dina aplikasi siaran saperti DVB jeung ATSC. Transport stréam beda jeung program stream, nu dirancang keur media nu bisa dipercaya kaws DVD.

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Lapisan komunikasi

Sarua jeung OSI network protocol stack, hiji transport stréam diproses ku panarima mangrupa lapisan-lapisan. Hiji stréam conto nu ngandung vidéo bisa diproses kawas kieu:

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Komposisi mangrupa-rupa program.
Packetized Elementary Stream (PES)
Elementary stream (ES) — audio atawa vidéo (the below is for vidéo 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 stréam is an electronic program guide. See Program and System Information Protocol for more information.

Unsur penting transport stream

Paket

A packet is the basic unit of data in a transport stréam. 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 stréam 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 créate a packet that is 208 bytes long [2]. The 188-byte packet size was originally chosen for compatibility with ATM systems [3] Archived 2008-07-26 di Wayback Machine [4] Archived 2005-02-09 di Wayback Machine .

Partial Transport Stréam Packet Format
Name	Number of bits	Description
sync byte	8	0x47
Transport Error Indicator (TEI)	1	Set by demodulator if can't correct errors in the stréam [5]
Payload Unit Start Indicator	1	1 méans start of PES data or PSI otherwise zero only .
Transport Priority	1	One méans 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 méans presence of the adaptation field
Payload data exist	1	1 méans presence of data
Continuity counter	4
		Note: the total number of bits above is 32 and is called the transport stréam 4-byte prefix.
Adaption field	0 or more	Depends on flags
Payload Data	0 or more	Depends on flags

Adaptation Field Format
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 vidéo/audio sequence
Elementary stream priority indicator	1	1 = higher priority
PCR flag	1	1 méans adaptation field does contain a PCR field
OPCR flag	1
Splicing point flag	1	1 méans presence of splice countdown field in adaptation field
Transport private data flag	1	1 méans presence of private data bytes in adaptation field
Adaptation field extension flag	1	1 méans 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

PID

éach table or elementary stréam in a transport stréam is identified by a 13-bit PID. A demultiplexer extracts elementary stréams from the transport stréam 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 appéars in the transport stréam.

Program

Transport stréam has a concept of programs. A single program is described by a Program Map Table (PMT) which has a unique PID, and the elementary stréams associated with that program have PIDs listed in the PMT. For instance, a transport stréam used in digital television might contain three programs, to represent three television channels. Suppose éach channel consists of one vidéo stréam, one or two audio stréams, and any necessary metadata. A receiver wishing to decode a particular "channel" méré ly has to decode the payloads of éach PID associated with its program. It can discard the contents of all other PIDs.

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. The PAT lists PIDs for all PMTs in the stréam. TS Packets containing PAT information always have PID 0x0.

PMT

Program Map Tables, or PMTs, contain information about programs. For éach program, there is a PMT, with the PMT for éach program appéaring on its own PID. The PMTs describe which PIDs contain data relevant to the program. PMTs also provide metadata about the stréams in their constituent PIDs. For example, if a program contains an MPEG-2 vidéo stréam, the PMT will list this PID, describe it as a vidéo stréam, and provide the type of vidéo that it contains (in this case, MPEG-2). The PMT may also contain additional descriptors providing data about its constituent stréams.

PCR

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 90 kHz clock, incremented for éach Hertz or cycle. The 9 bit extension is based on a 27 MHz clock.

Paket kosong

Some transmission schemes, such as those in ATSC and DVB, impose strict constant bitrate requirements on the transport stréam. In order to ensure that the stréam 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.