
4-6
Video Data Flow
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
4.2.1.6 Program Stream Operation
When the L64005 is programmed to operate in Program mode, the
L64005 accepts MPEG-1 or MPEG-2 Systems layer syntax. These layers
include the Pack, System, and Packet layers. The bitstream is assumed
to be time-multiplexed and contain Audio and Video packets. In Program
Stream Mode, the data is strobed into the L64005 with the AVALID sig-
nal. Both the Audio and Video header information is stored in the Audio
PES header buffer in DRAM. The elementary Audio and Video data is
stored in their corresponding DRAM channel buffers. The preparser per-
forms bitstream syntax checking when it expects start codes and resyncs
to the bitstream if necessary.
4.2.2
Post-Parser
Operation
The post-parser operates on data as it is read out of the channel buffer.
The post-parser is capable of operating on one MPEG symbol every
cycle. Primary functions are to pass data to the video and audio decod-
ers, User and Auxillary Data FIFO’s, and to perform error checking and
recovery at the different layers of MPEG.
4.2.3
Error Detection
and
Concealment
The L64005 ags two types of bitstream errors:
Decode Errors and
Channel Errors.
Decode Errors – The post-parser and on-chip microcontroller check for
syntax errors in the bitstream grammar to detect decode errors. The
parser or microcontroller generates a maskable interrupt if a decode
error is found. Depending on the context of the error the decoder may
perform error concealment to prevent the erroneous data from reaching
the picture store. Both the audio and video decoders can generate
decode errors independently of each other.
Channel Errors – Channel errors occur because of the overow or
underow of the channel buffer. The L64005 generates a maskable inter-
rupt during normal operation if the channel buffer becomes full or empty.
In a properly designed system, the channel buffer should never become
full, and should only become empty if the decoder loses synchronization.
The audio and video decoders have independent channel buffers, and
they generate separate channel error interrupts. Also, if a channel buffer
becomes full, the decoder stops asserting the AREQ or VREQ data
request pin. This allows a closely coupled transmitting device to stop
sending data.