Vela 2000-0600, Инструкция по установке и эксплуатации

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Chapter 2 — Decoder Command Set Guide 117
MPEG WRITE (10)
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
0.5MB to 1.5MB of MPEG data to be preloaded before issuing the
MPEG PLAY
command (A5h).
Three commands are used:
RESET BUFFER
,
MPEG WRITE
(2
AH
) , and
MPEG PLAY
(
A
5
H
) .
Reset Buffer
Part of the
DECODER
(45
H
)
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 1.5MB of DRAM is divided into twenty-four 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 HD decoder makes use of
DISCONNECT
-
RECONNECT
for main-
taining buffer fullness. This alleviates the need for the host system to keep track of
how much data is in the decoder. The task of the host, after playback has started,
is to keep sending data to the decoder. The decoder utilizes the
DISCONNECT
-
RECONNECT
sequence when it needs more data. The host begins a
WRITE
of
MPEG data, but the decoder decides when it is ready for more data.
When the decoder receives a new
MPEG WRITE
command, it automatically dis-