Re: I/O device with multiple *independent* SPDIF ports?
Re: I/O device with multiple *independent* SPDIF ports?
- Subject: Re: I/O device with multiple *independent* SPDIF ports?
- From: "Mikael Hakman" <email@hidden>
- Date: Mon, 21 Jan 2008 22:59:57 +0100
- Organization: Datakonsulten AB
I don't know whether such chip exists or not but it certainly would be
possible to have an SPDIF receiver chip that could be programmed or strapped
to work in 2 different modes, first mode as is done currently and the second
mode giving all 32 bits exactly as they are. They already have a zillion of
options for getting various I2S and other formats out of such a chip.
SPDIF and its professional counterpart AES define other useful streams
besides the audio data stream. Having all bits served would allow for using
these low-bandwidth information streams. Furthermore, AES standard requires
a digital device to either mute or correctly process compressed bitstreams.
This is so that you don't mistakenly get un-decoded DD or DTS reproduced,
which at 110 dB is worse than an earthquake, at least sonically. I did it
once when evaluating active digital studio monitors and I don't want to
experience that once more. Not many professional equipment vendors implement
compliant behavior because of the difficulties in doing so. Having easy
access to the flags would make such compliant implementations more frequent.
I have been looking for a computer audio interface that allows you to access
these bits for a long time now. One piece of equipment, a PCI card, in the
middle price segment, allows you to ask the device whether the non-audio bit
is on or off by a system call. I'm not sure that particular call works under
OS X. However it is not much help because you get status information when
you ask and cannot easily relate to your position in audio input buffer. The
other device, an FW interface, in about the same price segment does this
well, at least according to the spec. When requested, the device passes all
the extra bits and flags in the 4th byte of your audio sample (least
significant byte) provided that you also request a 32 bit integer format -
even if you don't do anything special, that byte is far below audibility
limit. I intend to evaluate this equipment very soon.
Regards/Mikael
Brian Willoughby <email@hidden> wrote:
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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