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: Stephen Davis <email@hidden>
- Date: Mon, 21 Jan 2008 00:09:13 -0800
Silly me, you (and another off-list response I got) make a good
point. In an early iteration of the project, I *was* planning on
making it a mixer and allowing multiple inputs to be active
simultaneously. In reality, I only want one additional input to
always be active and that input is the Mac (to allow voice
notifications or other alerts always come through). In that scenario,
I can just use a virtual device a la the AudioReflector driver or
Soundflower for the "Mac input" side. That's what I get for coming
back to the project after a brief hiatus and then posting late at
night. :-)
So, an external multi-port SPDIF switcher is in fact what I'm looking
for. Thanks for the good pointers, those sites look like they'll be a
lot of help. I have used the Crystal parts in the past but it looks
like there are some SPDIF switchers available already, now that I'm
googling on the right search terms.
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.
stephen
On Jan 20, 2008, at 6:28 AM, Mikael Hakman wrote:
OT or not, this is an interesting topic.
Why individually clocked? Are you trying to implement a mixer or a
preamp/prepro/switch? In the latter case you only have one input
active at a time. Consequently you only need one clock, which in
SPDIF input case is recovered from the input signal itself. All
units in a digital audio system have to be synchronized to one and
the same clock; otherwise you would need buffers of infinite size
because no 2 clocks run at exactly the same rate. Also, using 2 or
more clocks, one channel would inevitably fall behind the other more
and more. Therefore in a digital mixer you have to use one clock
only and do SRC (signal rate conversion) when needed before summing
and outputting.
There are plenty of multichannel audio interfaces, USB or FW, $200 -
$2000 and more. I don't know of any that allows you to use more than
one clock source at a time. There is a lot of information at
Tweakheadz Lab site.
All main semiconductor companies have SPDIF receiver and well as
transmitter chips. This is the easy part in a do-it-yourself
project. The hard part will be USB or FW interface and corresponding
software drivers. One place to start is at http://www.diyaudio.com/forums/forumdisplay.php?forumid=9
Another problem in a project like this will be to know when the
digital stream is plain PCM and when it is a multichannel compressed
bitstream (DD or DTS) needing special processing. The decision
algorithm needs to buffer a lot of data and therefore will introduce
unacceptable delays between video and audio (no lip-sync) in cases
when you take in such audio input, process it, and output the result
whereas video is going straight to the monitor.
Regards/Mikael
Stephen Davis <email@hidden> wrote:
For a home project (like I have time for it) I'm implementing a
virtual preamp: TV, Mac, AppleTV, DVD, etc. in --> decoding / efx
processing --> multi-channel out. Things are working pretty
smoothly but I've got a pile of USB SPDIF interfaces hooked up to
a Mac mini which is, shall we say, not very aesthetically pleasing.
Is there any input device out there that has multiple
*independent* SPDIF interfaces on it? When I say independent, I
mean that they are all individually clocked and don't try to do
any fancy tricks to make the audio bits all line up. As you might
expect, for the most part I am trying to receive digital
bitstreams, not PCM, so any clock stretching tricks will totally
destroy the data.
To be honest, I'm not sure it is even possible for a device driver
to present multiple streams running at different sample rates/
clocks so I'm guessing the driver would have to export multiple
AudioDevices. That seems unusual for a product idea but it'd be
sweet if I were able to consolidate all of my inputs into one
box. Has anyone out there seen such a beast?
If not, any hardware folks out there who have a recommendation for
the SPDIF receiver chip-of-the-day? It's been a long time since
I've done any hardware but it'd be nice to have the info for
reference in case I ever need it. :-)
thanks,
stephen
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