
6-2
Channel Interface
as a set of elementary streams that share the same system clock
reference and therefore can be decoded synchronously. In MPEG-1,
there are only two levels of hierarchy in the system syntax. In MPEG-2,
there are four levels of hierarchy in the system syntax. The following table
shows the system streams for MPEG-1 and MPEG-2 system syntax.
MPEG-2 introduced the Packetized Elementary Stream (PES) to allow
multiple streams and multiple programs to be combined in a single
stream. An MPEG-2 system may transmit either a program stream that
contains PES packets for a single program, or a transport stream that
contains PES packets for multiple, possibly unrelated, programs. An
MPEG-2 system decoder therefore must be able to accept PES data
from a transport stream or from a program stream. The crucial difference
between these two is that a program stream contains variable-length
PES packets, but a transport multiplexer reforms input PES packets into
fixed, 188-byte packets (184 payload bytes and 4 header bytes). While
program streams are generally used in DVD applications, transport
streams are usually fed into set top boxes.
The Channel Interface, shown in
Figure 6.1
, includes an Input FIFO,
System Synchronizer, Video Layer Synchronizer, Preparser, Channel
Write FIFO, and a Buffer Controller.
Table 6.1
Levels of Hierarchy in MPEG-1 and MPEG-2 System
Syntax
MPEG-1 Streams
MPEG-2 Streams
Transport Stream
System Stream
Program Stream
Packetized Elementary Stream (PES)
Elementary Stream
Elementary Stream