MPEG transport stream

A Transport Stream (English: Transport stream, TS, TP, and MPEG -TS) is the name for a standardized communication protocol for transmission of audio, video, and data. It is specified in MPEG- 2 Part 1, Systems (ISO / IEC 13818-1 ).

With MPEG -TS is transmitted continuously bundled digital video and digital audio. Transport streams are (satellite, terrestrial, cable ) and ATSC used in transmission paths such as DVB. MPEG -TS is the counterpart to the program stream (MPEG -PS), which was developed for media such as DVDs.

Introduction

The transport stream is on channels such as satellite, cable or antenna, ie, those with high error rate, is used. Despite the error correction process of this means of transmission occurs according to reception quality in data loss.

A transport stream consists of a sequence of each 188 -byte packets. The idea is that in a non-secure transmission can easily be lost parts of the data. In a small error then each parcel is lost. Through short packets to ensure that small transmission errors have only small effects. The length of 188 bytes is adjusted to the size of the time of the creation of the MPEG- 2 standards deemed promising transmission technology (ATM a transport stream packet fits into four ATM " cell ").

Transport streams with several television programs are used in the widely-used digital television standard DVB. In the transmission of DVB- T television programs are usually four housed in a transport stream, the so-called bouquet.

In addition to video and audio channels and data channels are provided. About the data channels can be transmitted, for example, encryption information or an "electronic program guide". In the transport stream embedded tables repeatedly ( Program Association Table, Program Map Table ) group video, audio and data channels to television programs. The individual channels are called elementary streams.

Elements of the transport stream

Package

A package is the basic unit of a transport stream. It starts with a synchronization byte, the value 0x47 is followed by three 1-bit flag and a 13 bit wide PID ( Packet Identifier ). Then, a 2 -bit field is used to describe a possible encryption 00 is unencrypted, another 2 -bit field, which indicates whether to follow more optional fields (10) and payload (01 ), or both (11), and even more a 4 -bit continuity counter. Additional optional fields would come next. The rest of the package optionally consists of the actual data of an elementary stream. A 1-bit flag is a start indicator to indicate the logical block structure of the elementary stream.

PID (packet identifier)

Each table or any packetized elementary stream in a transport stream is identified by a 13 bit wide PID in the packet header. By searching for identical PIDs extracts a demultiplexer an elementary stream from the transport stream. In the transmitter, the time-division multiplexing is used to decide when and how often a particular PID in the transport stream appears in most cases. Is nothing to transmit, empty packets are sent to the transport stream not to be cancel.

A PID indicates either a special package (PAT and PMT) or a data packet belonging to a particular elementary stream.

The 4-bit continuity counter in the packet header is one of the transport stream packets of an elementary stream, respectively.

Programs

The basic concept of transport streams are programs that consist of groups of one or more related PIDs. A transport stream for digital television, for example, consists of three channels, each representing a television channel. Suppose a program consists of a video stream, audio streams and two additional metadata, that is, four elementary streams. If a receiver would try to set a specific program ( " channel"), then he should first of all analyze the PAT (indicated by the PID zero ), whereupon he finds the PID of the associated PMT. Now he would have based this PID is assigned to the channel PMT analyze and learns there which elementary streams belonging to this program. Then he must accurately decode all the data that belong to the PIDs given in the PMT.

PMT

The Program Map Table (PMT) contains information about the programs. For each program there is a PMT, associated with its own PID. The PMTs describe which PIDs contain data for the program. PMTs also provide metadata for streams and their individual PIDs ready. For example, in a program, consisting of a containing MPEG-2 video stream, the PID of the video stream and in addition, the type of data, ie, in this case MPEG-2. The PMT may also contain additional descriptors for individual streams.

PAT

PAT stands for Program Association Table. The PAT lists PIDs for all PMTs in the stream. Packets containing the PAT information always have PID 0x0.

PCR

To allow the decoder a time and the right speed representation, the program includes a Program Clock Reference, or PCR, based on the PIDs in the program.

Null packets

Certain transmission protocols such as ATSC and DVB write a constant bit rate against (CBR ). To ensure this, it may happen that a multiplexer must add additional packages. For this, the PID 0x1FFF is reserved, which then contains no data and is ignored by the receiver.

Storage formats

Some hard drive receiver and many computer TV cards can record from transport streams single transmitter and partly also record of complete transponders. The most commonly used container format is the transport stream described above with the file extension *. Ts. Dvb - files start as MPEG transport stream with the byte 0x47, which repeats every 188 bytes.

Vantage uses the TRP container format and Topfield saves them as *. Rec. Some of these transport stream container can be converted into another. Because often several programs are grouped within a transponder transport stream to allow some beneficiaries without double tuner, but to see more programs or take.