L64005 MPEG-2 Audio/Video Decoder Technical Manual
4-3
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
4.2.1
Pre-Parser
Operation
The pre-parser contains serial-to-parallel synchronizing mechanisms that
synchronize and byte-align a serial MPEG bitstream to a start code. Par-
allel data is assumed to be byte-aligned with respect to the bitstream.
The pre-parser also lters the bitstream to remove unwanted high-level
syntax.
The pre-parser performs channel synchronization. The L64005 performs
synchronization independently for the system streams (packet and sys-
tem header) and for the elementary data stream. The criteria for synchro-
nization is different in each case.
Figure 4.2 shows the concept of system
synchronization.
Figure 4.2
Conceptual
System
Synchronization
4.2.1.1 Synchronization in the System Header and Packet Layer
The L64005 performs system synchronization for an MPEG-1 or MPEG-
2 system layer only. The L64005 parses these layers to separate the sys-
tem header from the elementary video and audio streams. The system
header data is stored in separate DRAM channel buffer(s) and can be
read on demand by the external controller. The system headers contain
the presentation time stamp (PTS) and the system clock reference which
are the essential parameters for audio and video synchronization. Unfor-
tunately, MPEG syntax does not guarantee that the packet data eld of
the Packet Layer will not emulate sync codes. Therefore, the device must
predict the occurrence of start codes in the system bitstream based on
the length eld of the previously occurring header.
The system-level synchronization circuitry may be considered a lter that
separates the elemental data streams and system headers from the
Unsynchronized Channel data with no known alignment
containing any data including MPEG Packet Layer
Elemental Stream for
selected stream only
Stream
Select
System
Headers
System
Synchronizer
Data