The MPEG Data Stream - Digital Video and Audio Broadcasting Technology - page 45

Since there can be a number of video and audio streams - as for instance
in a Formula-1 broadcast in various languages - the viewer must select the
elementary streams to be decoded. Ultimately he will select exactly 2 PIDs
- one for the video stream and one for the audio stream, resulting e.g. in
the two hexadecimal numbers PID1 = 0x100 and PID2 = 0x110. PID1 is
then e.g. the PID for the video stream to be decoded and PID2 is the PID
for the audio stream to be decoded. From now on, the MPEG-2 decoder
will only be interested in these transport stream packets, collect them, i.e.
demultiplex them and assemble them again to form the PES packets. It is
precisely these PES packets which are supplied to the video and audio de-
coder in order to generate another video-and-audio signal.
The composition of the transport stream can change during the transmis-
sion, e.g. local programs can only be transmitted within certain windows.
A set-top box decoder, e.g. for DVB-S signals must, therefore, continu-
ously monitor in the background the instantaneous structure of the trans-
port stream, read out the PAT and PMTs and adapt to new situations. The
header of a table contains a so-called version management for this purpose
which signals to the receiver whether something has changed in the struc-
ture. It is regrettable that this does still not hold true for all DVB receivers.
A receiver often recognizes a change in the program structure only after a
new program search has been started. In many regions in Germany, so-
called “regional window programs” are inserted into the public service
broadcast programs at certain times of the day. These are implemented by
a so-called “dynamic PMT”, i.e. the contents of the PMT are altered and
signal changes in the PIDs of the elementary streams.
Video PID = 0x100
MPEG2 TS
Audio PID = 0x200
Fig. 3.14. Accessing a program via video and audio PIDs