
L64005 MPEG-2 Audio/Video Decoder Technical Manual
4-9
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
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4.2.3.2 Frame Skips
To assist in resynchronization, it is possible to force the video decoder to
perform a frame skip. Frame skips are typically used when the decoder
is running slow with respect to the System Clock Reference. To force a
frame skip, the external controller must make a frame skip request. The
next time the decoder encounters a frame of the appropriate type, it skips
the frame rather than reconstructing it. Note that frame skips occur in one
byte/cycle of channel data, because the decoder has to read through the
channel buffer to nd the next picture start code. In some situations, the
frame skip may fail—for instance, when the amount of data that needs
to be read is so large that there is no longer sufcient time left in the
Presentation Unit to decode the next frame. This is most likely to happen
if an attempt is made to skip an I frame. Users should monitor the PTS
and SCR counters to ensure that a requested frame skip actually was
There are three types of frame skip requests:
Skip B frames – This forces the skip of the next available B frame.
B frames are the best candidate for skipping because they do not
affect the quality of the prediction and are relatively short.
Skip B or P – This forces the skip of the next available P or B frame.
Care should be taken when skipping P frames because their values
may be used in subsequent prediction. Skipping a P frame that was
going to be used for prediction causes the picture to become cor-
rupted. The best P frames to skip are P frames immediately before
I frames. Unfortunately, there is no way to be certain that an I frame
is going to follow a P frame, because the I frame has not been
decoded yet. However, it may be possible in some systems to predict
the occurrence of an I frame based on their frequency of occurrence,
which may make skipping P frames acceptable.
Skip I, P, B – This forces the skipping of one frame of any type in
the video decoder. This may be used as an attempt to acquire initial
synchronization when the PTS and the DTS are very far apart. It is