The MPEG Data Stream - Digital Video and Audio Broadcasting Technology - page 44

having a value of 0x47 appears, the decoder will examine the positions of
n times 188 bytes before and after this byte in the transport stream for the
presence of another sync byte. If there is, then this is a sync byte. If not,
then this is simply some code word which has accidentally assumed this
value. It is inevitable that the code word of 0x47 will also occur in the con-
tinuous transport stream. Synchronization will occur after 5 transport
stream packets and the decoder will lose lock after a loss of 3 packets (as
quoted in the MPEG-2 Standard).
3.3.2 Reading out the Current Program Structure
The number and the structure of the programs transmitted in the transport
stream is flexible and open. The transport stream can contain one program
with one video and audio elementary stream, or there can be 20 programs
or more, some with only audio, some with video and audio and some with
video and a number of audio signals which are being broadcast. It is, there-
fore, necessary to include certain lists in the transport stream which de-
scribe the instantaneous structure of the transport stream.
These lists provide the so-called “program specific information”, or PSI
in short (Fig. 3.13.). They are tables which are occasionally transmitted in
the payload part. The first table is the “Program Association Table” (PAT).
This table occurs precisely once per transport stream but is repeated every
0.5 sec.. This table shows how many programs there are in this transport
stream. Transport stream packets containing this table have the value zero
as packet identifier (PID) and can thus be easily identified. In the payload
part of the program association table, a list of special PIDs is transmitted.
There is exactly one PID per program in the program association table
(Fig. 3.13.).
These PIDs are pointers, as it were, to other information describing each
individual program in more detail. They point to other tables, the so-called
“Program Map Tables” (PMT). The program map tables, in turn, are spe-
cial transport stream packets with a special payload part and special PID.
The PIDs of the PMTs are transmitted in the PAT. If it is intended to re-
ceive, e.g. program No.3, PID no. 3 is selected in the list of all PIDs in the
payload part in the program association table (PAT). If this is, e.g. 0x1FF3,
the decoder looks for transport stream packets having PID = 0x1FF3 in
their header. These packets are then the program map table for program no.
3 in the transport stream. The program map table, in turn, contains PIDs
which are the PIDs for all elementary streams contained in this program
(video, audio, data).