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 ). 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.

 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:

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  
, 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 .  The 188-byte packet size was originally chosen for compatibility with ATM systems    .
+ Partial Transport Stréam Packet FormatNameNumberof bitsDescriptionsync byte80x47Transport Error Indicator (TEI)1Set by demodulator if can't correct errors in the stréam  Payload Unit Start Indicator11 méans start of PES data or PSI otherwise zero only .Transport Priority1One méans higher priority than other packets with the same PID.PID13 Packet IDScrambling control2 '00' = Not scrambled.   The following per DVB spec DVB scrambling control bits defined. Page 6:   '01' = Reserved for future use,   '10' = Scrambled with even key,   '11' = Scrambled with odd keyAdaptation field exist11 méans presence of the adaptation fieldPayload data exist11 méans presence of dataContinuity counter4  Note: the total number of bits above is 32 and is called the transport stréam 4-byte prefix.Adaption field0 or moreDepends on flagsPayload Data0 or moreDepends on flags

+ Adaptation Field FormatNameNumberof bitsDescriptionAdaptation Field Length8Number of bytes in the adaptation field immediately following this byteDiscontinuity indicator1Set to 1 if a discontinuity occurred in the continuity counter of the TS packetRandom Access indicator1Set to 1 if the PES packet in this TS packet starts a vidéo/audio sequenceElementary stream priority indicator11 = higher priorityPCR flag11 méans adaptation field does contain a PCR fieldOPCR flag1Splicing point flag11 méans presence of splice countdown field in adaptation fieldTransport private data flag11 méans presence of private data bytes in adaptation fieldAdaptation field extension flag11 méans presence of adaptation field extensionBelow fields are optionalvariableDepends on flagsPCR33+9 Program clock referenceOPCR33+9 Original Program clock reference.  Helps when one TS is copied into anotherSplice countdown8Indicates how many TS packets from this one a splicing point occurs (may be negative)stuffing bytesvariable

 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.

Rujukan

Tempo oge
Tuner ATSC
DVB
Elementary stream
IPTV
MPEG-2
Packetized Elementary Stream
Program and System Information Protocol
Unidirectional Lightweight Encapsulation (ULE)
HDV jeung AVCHD, nu duanan nerapkeun stréam TS kana data nu dipakétisasi

 Conto parabot nu aya
Software: 
dvb-snoop
ATSC Transport Stream Tools for Linux
Tools 
Ethereal MPEG-2 TS Dissector 
DVB Stream Explorer
Hardware:
Monitor 
TS Multiplexer 
Assorted 
ODG - Open DVB Gateway
Transport stream analyser

 Tumbu luar 
 MPEG-2 Systems FAQ 
 MPEG-4 Systems FAQ 
 MPEG-1 description 
 Powerpoint MPEG-2 Transport Stream introduction  
 Splicing FAQ 

Kategori:ATSC
Kategori:MPEG