L64005 MPEG-2 Audio/Video Decoder Technical Manual
8-15
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
Table 8.4 provides the delay from the start of decode to the start of dis-
play within the L64005.
Table 8.4
Decode to Display
Delay
1. These values are accurate only for closed GOP bit-
streams.
8.4.2
Video
Decoding and
Presentation
Schedule
The L64005 video decoder decodes data from the video channel buffer.
Once the decoder is instructed to start, it continuously decodes pictures
at the rate controlled by the frame rate of the display controller. When
the L64005 is in master mode, it sets this frame rate. When the L64005
is in display slave mode, the external display system sets the frame rate.
Once the external microprocessor instructs L64005 to start decoding, it
reads the picture header, then waits until the next Video Presentation
Unit boundary before it starts to decode the next picture data. This
ensures that video decoding can proceed synchronously with picture dis-
play. The newly reconstructed picture cannot overwrite data from a pro-
ceeding frame that has not yet been displayed.
The reconstruction of a PU should complete inside a presentation inter-
val. A presentation interval is one frame time for frame mode coding or
two eld times for eld mode coding. After completing a presentation unit
(picture), the decoder reads the picture header of the next picture and
then stops until the next presentation boundary begins. This will typically
be the next eld boundary in the middle of a frame.
The L64005 video decoder will skip a frame if the frame skip bits are set
when the decoder reads the picture header, or after the picture header
has been read but the decoder is waiting for the next PU boundary. To
be successful, a frame skip must be completed before the next PU
begins. If the skip does not complete, the next picture will not start on
the PU boundary. This will cause a freeze of the preceding eld in the
display controller, and the decoding will not commence until the next PU
boundary. During this time it is acceptable to request additional frame
skips.
Frame Type
Decode to Display Delay1
I frame
3 Fields
B Frame
1 Field
P Frame
Number of B frames + 1 Field from
the next P or I frame