MPEG Program Specific Information (PSI)
Program Specific Information is the MPEG 2 data that identifies what parts of the transport stream belong to a particular program. This information is carried in a number of PSI tables:
Program Association Table (PAT) (required)
Program Map Table (PMT) (required)
Conditional Access Table (CAT) (optional)
Network Information Table (NIT) (optional)
The Program Association Table (PAT) is the entry point for the Program Specific Information (PSI) tables. It is always carried in packets with PID (packet ID) = 0. For each assigned program number, the PAT lists the PID for packets containing that program's PMT.
The PMT lists all the PIDs for packets containing elements of a particular program (audio, video, aux data, and Program Clock Reference (PCR)).
The PAT also contains the PIDs for the NIT(s). The NIT is an optional table that maps channel frequencies, transponder numbers, and other guide information for programs.
The CAT is always carried in packets with PID = 1. The CAT contains PIDs for Entitlement Management Messages (EMMs), which contain authorization level information for conditional access systems.
Summary: The simple MPEG transport decoder for in-the-clear programs needs to:
read the PAT to find the PMT for a desired program,
demultiplex the packets that carry the desired PMT
read the PMT
demultiplex the packets (with PIDs specified in the PMT) into the various elemental streams
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