8-40
Video Decoder Module
8.5.3 Channel Buffer Underow Panic Repeat
When this feature is enabled and the decoder detects that the Video ES
Channel Buffer is in danger of underowing, it automatically freezes the
display on the last eld of the currently displaying picture. The freeze is
automatically removed when the channel buffer has lled to an adequate
level. During the panic condition, the decoder pauses; it does not request
any bytes from the channel for decoding.
To enable this feature, the host sets the Video Numitems/Pics Panic
Mode Select bits in Register 69 (
page 4-25) to either 0b01 to select
number of items (64-bit words) or 0b10 to select number of pictures. The
host must then enter an item or picture threshold value in Registers 134
through 136
(page 4-42). The Channel Buffer Controller compares the
number of items or pictures in the Video ES Channel Buffer with the
programmed threshold value. If the actual number falls below the
threshold, a “panic” signal is sent to the Video Decoder. The Video
Decoder responds by repeating a frame to let the Video ES Channel
Buffer rell above the threshold. The panic signal is sampled by the
decoder just before reconstruction of the picture is about to begin. Note
that the decoder pauses for the panic signal to clear even if the host has
commanded the decoder to skip a frame.
Note:
This operation can violate the correct Video Buffering
Verier (VBV) model operation and is generally used in
trick modes when the VBV is invalid.
8.5.4 Rip Forward Mode
Setting the Rip Forward Mode Enable bit in Register 238 (
page 4-72)enables the Rip Forward Mode. In this mode, the decoder processes
pictures as fast as it can without regard to the status of the display,
i.e., the rate control for the decode with respect to the Vertical Sync of
the display is turned off. The rate control for the decode is governed by
the Rip Forward Display Single Step Command bit in Register 238
(page 4-73). The on-chip microcontroller monitors the Single Step bit
after it receives both a Picture Start Code and and has processed the
picture header. The decode for that picture only proceeds when the
Single Step bit is set. The Single Step bit is cleared on reading by the
decoder.