L64005 MPEG-2 Audio/Video Decoder Technical Manual
1-23
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
four times; each line is repeated on neighboring lines, and the eld is dis-
played twice.
The L64005 may be programmed to repeat a eld. The full picture can
be displayed during each eld of an interlaced video display system. This
feature is only available when not using Reduced Memory Mode, how-
ever.
1.6.3.5 Field Pulldown
3:2 pulldown may be optionally performed during display. The eld on
which 3:2 pulldown starts may be programmed. Also, the 3:2 pattern may
be modied at any time. The prole of the 3:2 pulldown is communicated
in the MPEG-2 video stream. The L64005 extracts this data directly from
the stream and applies it to the appropriate eld. 3:2 pulldown may only
be performed if the device is not operating in reduced memory mode.
The L64005 may also be programmed to freeze any given eld or frame
whilst operating in normal mode. In reduced memory mode it is not pos-
sible to freeze on a B frame.
1.6.3.6 16:9 Operation
A source image with a 16:9 aspect ratio is compacted into a 4:3 ratio
before image coding and transmission. It is necessary to display a 4:3
aspect ratio window onto this image by interpolating a segment of each
scan line prior to display. The on-chip interpolation lters perform this
function. A pan-and-scan feature is also included, with the appropriate
parameters communicated within the user data or the MPEG-2 stream.
1.6.3.7 Time Base Correction
Variations in system time base can produce long-term drift between the
channel bitstream rate and the video rate. The decoder skips or repeats
the display of certain frames to achieve time base correction. The
L64005 decodes repeated frames once but displays them more than
once. Skipped frames are not decoded or displayed; they are skipped
over in the channel buffer. If B-frames are present in the stream, then
they can be selected for repeating or skipping. If only I and P frames are
in the stream then the next P frame is repeated or skipped. If only I
frames are in the stream then, any I frame may be repeated or skipped.
Note that this correction mechanism may produce undesirable effects if
it is performed too often. Ideally, the decoder clock should be locked to