
Video Decoder Pacing
8-25
The channel start command causes the Preparser to start accepting data
from the external channel interface device. For details on how to control
the location of the video channel in SDRAM memory and on selecting
Interface.” After the channel start command is issued and the Video ES
Channel Buffer has lled to a sufcient level, the host decode start
command is issued. This signals the decoder to begin decoding the
bitstream and reconstructing the picture in the frame store memory,
which is allocated in SDRAM. The start of video decode can also be
achieved by using the Autostart Video function. The following sections
explain the operation of the various host registers required to program
the functions mentioned previously. The process is illustrated with the
help of a time line that shows how these operations should be
sequenced in time.
8.3.1 Channel Start/Stop and Status Bits
After power-up or chip reset, the channel is in the reset state. At this
time, the various start and end addresses of the channel are assigned
in SDRAM by writing to the appropriate registers (See
Chapter 6,Start/Reset bit (bit 0 of Register 7,
page 4-13) causes the channel to
start. This results in the Video ES Channel Buffer receiving the MPEG
elementary video bitstream from the external channel interface. The host
can monitor the status of the channel by reading the Channel
Start/Reset/Status bit in Register 7. The host can stop the channel by
clearing the Channel Start/Reset/Status.
Note:
The Channel Status bit is updated when the decoder
acknowledges the channel stop command and not when
the host writes a 0 to the bit. The Channel Start/Reset bit
is checked regularly by the decoder.
8.3.2 Video Decoder Start/Stop
The actual start of decoding should be delayed from the start of the
channel. This is done to allow the Video ES Channel Buffer to ll to a
sufcient level so that there is no underow/overow of the buffer while
actually reconstructing pictures. The host may choose one of the
methods described in
ISO/IEC 11172 (MPEG-1) and ISO/IEC 13818
(MPEG-2) to determine how long this delay should be. See also
Section