Microcontroller Registers
4-75
B picture is skipped per frame display period. This is
different from merely skipping B pictures which skips
each B picture as fast as possible and does not control
the rate of the skip with respect to the display.
Panic Prediction Enable
R/W 2
Setting this bit causes the motion compensation module
to go into a panic mode. In this mode, the motion
compensation module performs motion compensation
using only the forward vector set and ignores the
backward vectors for B pictures. Similarly for dual-prime
motion vectors, only same parity prediction is performed.
This mode is intended to be used in situations where
limited SDRAM bandwidth slows down picture
reconstruction to the point where the picture starts
tearing since reconstruction is not able to keep pace with
the display. Eliminating the backward vectors reduces the
demand made on the SDRAM bandwidth by reducing
accesses to the reference anchor store in the SDRAM.
This should alleviate any tearing problems. However, this
does not completely guarantee solving any bandwidth
problems that may exist with slow SDRAM devices.
GOP User Data Only
R/W 3
When this bit is set, the Video Decoder recognizes only
user data supplied in the GOP layer of the MPEG-1/
MPEG-2 video stream. User data of other layers, if
present, is discarded by the decoder and is not written to
the user FIFO. This feature is designed to accept only
line 21 data (closed-caption data) in a DVD system. By
discarding other user data layers, the processing
overhead on the host controller in a DVD system is
reduced signicantly. The default value of this register at
start-up is 0, which means user data of all layers is
available to the host in the user FIFO.
Concealment Copy Option
R/W 4
When set, this bit overrides any concealment vectors that
may have been present in the original MPEG video
bitstream. Normally, MPEG species that these vectors
be used to conceal any errors that may be detected by
the decoder. When this bit is set, copying from the
previously decoded valid picture is used instead of
applying the concealment vectors. This register is cleared
by default (i.e., after reset or power-up).