4-8
Video Data Flow
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
Picture Layer – The L64005 enters the Picture Layer from the Group of
Pictures Layer only if no errors have been detected in the Group of Pic-
tures header.
Errors in the header of the Picture Layer have different propagation
effects depending on the picture coding type. However, all detected
errors in the Picture Layer header are agged as decode errors. Error
recovery requires waiting for a new Picture Layer, Group of Pictures
Layer, or Video Sequence Layer.
If the extra information bit is set, the syntax is decoded and written to the
Auxiliary Data FIFO. No error is agged. If an extension start code is
found in the Picture Layer, the extension record is decoded until the next
legal start code, and written to the auxiliary FIFO. No error is agged.
To conceal errors in the Slice Layer and layers below, the L64005 uses
temporal replacement from the preceding anchor frame. Prediction is
derived from the concealment motion vector if available, or from the
motion vector from the macroblock immediately above the block where
the error occurred. When operating with an MPEG-2 stream, the decoder
uses the MPEG-2 concealment vectors if they are present. To replace
the block in error with the block from the preceding anchor frame, the
decoder uses the motion vector offset from the concealment vector of the
I block immediately above the block in error. The host selectable CMODE
bit, Group 7, Register 55, bit 7, will allow the user to force the device to
use frame copy based concealment on macroblock level errors when it
is set, or to select motion vector based error concealment when cleared.
4.2.3.1 Synchronization and Error Recovery in the Post-Parser
The post-parser uses the resync symbols in the channel buffer to keep
track of the current grammar layer of the bitstream. The post-parser can
usually resynchronize to any byte-aligned resync symbol in the channel
buffer. If the post-parser is not synchronized, it reads symbols at the
maximum rate until either it regains synchronization or the channel buffer
empties. If the post-parser receives a media error code ag or detects
an error during decoding at the Slice Layer and layers below, it attempts
to conceal errors up to the next resync symbol.The post-parser detects
the following errors:
Sequence error in channel
Resync code not in predicted location