MPEG-2 transport streams
There are two basic ways of employing MPEG-2 technology: program streams and transport streams. Program streams are commonly experienced by viewing an audio-video DVD. (Audio-video Compact Discs generally use MPEG-1 technology).
MPEG-2 Transport streams, on the other hand, are employed in terrestrial, cable and satellite broadcasting. The main differences between program streams and transport streams are that transport streams permit more than one program service (or virtual channel) to be transmitted at the same time.
Transport streams consist of packets of 188 bytes in length that are transmitted one after another in rapid succession. Transport packets can contain video, audio or data "essence" and metadata which is used to specify information about the essence streams. "Metadata" means "data about data", the data in this case being program video, audio or plain old data.
So receiving equipment can tell what packet is used for a particular use, packets with the same purpose are assigned the same packet identifier or pid. Pids are integers that range from 0 to 8191. Pids from 0 to 47 (0x0 to 0x2F and 8191 (0x1FFF) are either reserved by MPEG, or have specified uses. Within the remaining numbers, MPEG-2 users (like ATSC, DVB, SCTE or ARIB) have either assigned pids for specific purposes or leave the assingment of pids up to end users.
Essence video, audio or data are subdivided into what are called packetized elementary streams (PES). Metadata, on the other hand, subdivide metadata elements in to data tables using the common "MPEG-2 private table section" structure.
The MPEG-2 systems specification, called ISO/IEC 13818-1, specifies two mandatory and other optional private table section types to transmit metadata.
For transport streams that are transmitting audio, video or data content, the Program Association Table (PAT) is mandatory, and there must be one Program Map Table (PMT) per program service. The PAT lists the program number of every program in the transport stream, and furnishes the pid where each PMT section can be found.
See: transport streams