For the longest time I have wanted an SPDIF interface for the Mac
(FireWire?) which allows access to the full 32-bit word. Problem is
that the Crystal part interprets the bits and delivers audio sample
bits (up to 24) on a separate pin from the control bits (up to 16).
This is handy if you don't have a processor available, but it limits
what can be done if you do. It would certainly be possible to just
give the Mac a 32-bit stream and let the Mac split that into audio
and control, but it would require custom silicon - unless there is
already such a chip and I've missed it. Flags and block starts would
be a cinch this way.
Brian Willoughby
Sound Consulting
On Jan 21, 2008, at 04:12, Mikael Hakman wrote:
Stephen Davis <email@hidden> wrote:
As for the auto-detecting of bitstream vs. PCM goes, I have that
working for DD but haven't looked into the audio/video sync issue
very closely yet. In cursory experiments, it seems to be fine.
The DD bitstream lets you decide pretty quickly what mode you're
in but I'm sure it will take some work to get the latency down low
enough.
When I did my experiments, I had to buffer more than one whole SPDIF
block before knowing for sure whether it was PCM or DD or DTS. Then I
needed a whole block before I could decode it, and then there was
decoding time, and then there was some latency in OS and devices
(both in and out). All this together made an unacceptable delay
between audio and video. This is when I gave up. The problem is that
the syncword used in DD and DTS is a valid PCM value and that the DD/
DTS flag in SPDIF is normally not available outside the device, and
not always set, and that you don't see in your code where each SPDIF
block starts - you only see a buffered continuous stream. Perhaps you
found a clever algorithm for this - could you share it with me, please?
Thanks/Mikael
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