
6-26
Channel Interface
Figure 6.16 Preparsing an MPEG-1 System Stream
The data ow is shown in
Figure 6.16. Since audio and video packets
are multiplexed in the stream, only the AREQn and AVALIDn signals are
used. When the Preparser recognizes a Packet Start Code, it checks to
see whether the packet contains audio or video data, and whether the
Stream Select eld matches the Stream Select code written into
Register 7
(page 4-13) by the host. If the Preparser does not nd a
match, it discards the packet. For accepted packets, the Preparser uses
the Packet Length eld to determine where the packet ends. This is
necessary to avoid mistakenly parsing the possible emulation of start
codes in audio packet data.
If the stream ID indicates an audio stream, the Preparser skips any
packet stufng bytes and moves the remainder of the packet header into
the System PES Channel Buffer. The chip sets the Audio PES Data
Ready Interrupt (Register 2,
page 4-6) and asserts the INTRn output
signal, if the interrupt is not masked, to indicate to the host that the
packet header is in the System Channel Buffer. The Preparser then
samples the current write pointer for the Audio ES Channel Buffer and
moves its value into the System Channel Buffer after the packet header.
The host can subsequently use this value for system synchronization.
The Preparser then moves the packet payload into the Audio ES Channel
Buffer. The Preparser uses the Packet Length eld in the packet header
to determine the end of the audio data payload.
If the stream ID is a video stream, the Preparser skips any packet
stufng bytes and moves the remainder of the packet header into the
System Channel Buffer. INTRn is asserted if not masked and the Video
MPEG-1
System Stream
Preparser and
AVALIDn
Audio ES Channel Buffer
System
L64021
SDRAM
Video ES Channel Buffer
Write FIFO
AREQn
Channel Buffer