Chapter 2 — Overview and Operation
127
MPEG WRITE (10)
For a partial buffer, e.g., what would be seen at the end of a file, the remaining
data should be padded (with zeroes) to fill up a full 64-K.
MPEG Playback Operation
The MPEG buffer contains fifty-six 64-K blocks arranged in a circular FIFO. The
write pointer is maintained by the SCSI interface firmware, while the read pointer
is maintained by the MPEG playback firmware. When the decoder receives an
MPEG
WRITE
(10) command, the SCSI interface firmware compares the read and
write pointers.
If a 64-K FIFO block is available, then the SCSI data transfer occurs immediately.
If a 64-K block is not available, the decoder disconnects from the SCSI bus and
waits for a block to be freed by the MPEG playback firmware. Once a block is
free, the decoder reconnects to the SCSI bus and starts SCSI data transfer.
This method is intended for applications which require frame-accurate back-to-
back video playback, as in ad-insertion applications. This method requires
1.0MB to 3.5MB of MPEG data to be preloaded before issuing the
MPEG
PLAY
command (A5h).
Three commands are used:
RESET
BUFFER
,
MPEG
WRITE
(2ah), and
MPEG
PLAY
(a5h).
RESET BUFFER
Part of the
DECODER
(45h) command, which marks the beginning of a new video
inside the DRAM circular buffer.
RESET
BUFFER
has a code (0021h) with no data.
MPEG WRITE
The 3.5MB of DRAM is divided into fifty-six 64-K buffers. The transfer length
must be a multiple of 64K.
All SCSI read and write transfer lengths are expressed in logical blocks, not bytes.
A logical block is defined for the Vela SCSI decoder as 512 bytes/block. Thus, the
minimum size which can be transferred in the
MPEG
WRITE
command is 80h (128)
logical blocks. Depending on the host operating system and SCSI driver host
adapter (initiator), a single
WRITE
can transfer 64K to 1MB (or more).
DISCONNECT-RECONNECT
This is a mechanism provided in SCSI-2 which allows a target, after receiving a
command, to free the SCSI bus (
DISCONNECT
) for other I/O processes. Later, when
the target is ready, it arbitrates for the bus (
RECONNECT
) and completes the data
transfer associated with the command.
The Vela CineCast Quad Pro decoder makes use of
DISCONNECT
-
RECONNECT
for
Summary of Contents for CineCast 2000-0422
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