Information Technology Reference
In-Depth Information
Figure 6.7
also shows that the PMTs fully describe each program. The PID of the video elementary stream is
defined, along with the PID(s) of the associated audio and data streams. Consequently when the viewer selects a
particular program, the demultiplexer looks up the program number in the PAT, finds the right PMT and reads the
audio, video and data PIDs. It then selects elementary streams having these PIDs from the transport stream and
routes them to the decoders.
Figure 6.7:
MPEG-2 Program Specific Information (PSI) is used to tell a demultiplexer what the transport stream
contains.
Program 0 of the PAT contains the PID of the Network Information Table (NIT). This contains information about
what other transport streams are available. For example, in the case of a satellite broadcast, the NIT would detail
the orbital position, the polarization, carrier frquency and modulation scheme. Using the NIT a set-top box could
automatically switch between transport streams.
Apart from 0 and 8191, a PID of 1 is also reserved for the Conditional Access Table (CAT). This is part of the
access control mechanism needed to support pay per view or subscription viewing.
6.6 Multiplexing
A transport stream multiplexer is a complex device because of the number of functions it must perform. A fixed
multiplexer will be considered first. In a fixed multiplexer, the bit rate of each of the programs must be specified so
that the sum does not exceed the payload bit rate of the transport stream. The payload bit rate is the overall bit rate
less the packet headers and PSI rate.
In practice the programs will not be synchronous to one another, but the transport stream must produce a constant
packet rate given by the bit rate divided by 188 bytes, the packet length.
Figure 6.8
shows how this is handled.
Each elementary stream entering the multiplexer passes through a buffer which is divided into payload-sized areas.
Note that periodically the payload area is made smaller because of the requirement to insert PCR.
Figure 6.8:
A transport stream multiplexer can handle several programs which are asynchronous to one another
and to the transport stream clock. See text for details.
