L64105 LSI Logic Corporation, L64105 Datasheet - Page 278

• Manufacturer Part Number: L64105
• Description: Mpeg-2 Audio/video Decoder
• Manufacturer: LSI Logic Corporation

the host clears either bits [1:0] or bit 2. If the host clears bit 2, one more
frame is skipped and the internal microcontroller clears bits [1:0]. All
three bits are read/write so the host can check the current skip mode
status.
To skip a frame, the Postparser transfers the picture header information
to the Auxiliary Data FIFO and reads the rest of the picture bytes out of
the Video ES Channel Buffer at top speed without handing them off to
the IDCT Pipeline. When two or more consecutive frames are skipped,
the Postparser still searches for picture start codes, generates an
interrupt at each, and transfers the header information of each to the
Auxiliary Data FIFO.
When two or more consecutive frames are skipped, the display is frozen
on the last field of the picture before the skip until the next unskipped
frame is displayed.
Figure 8.5
shows two cases of decoder operation for
a single frame skip.
In case 1, the host ordered a B frame skip while the first of two B frames
(B0) was being decoded. The Postparser skips by frame B1 in time to
decode frame P2. Since the B0 decode and B1 skip fit into one frame
time, the display continues without freezing.
In case 2, the B1 frame skip time overlaps the next vertical sync and
pushes the decode time for frame P2 to the next vertical sync. The
display is automatically frozen on the last field of B0 for an extra field
display time. This can occur when all of frame B1 is not in the Video ES
Channel Buffer at the time of the skip, slowing down the Postparser. The
host can avoid this situation by reading/managing the number of pictures
in the Video ES Channel Buffer (refer to
Section 6.4.2, “Detecting
Potential Underflow Conditions in the Video Channel,” page 6-29
and
Section 8.5.3, “Channel Buffer Underflow Panic Repeat,” page
8-40).
The host must issue the skip command before the picture start code
interrupt for the picture that is to be skipped. Once the decoding of a
frame starts, it cannot be skipped.
8-36
Video Decoder Module