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: Sun, 20 Jan 2008 15:28:40 +0100
- Organization: Datakonsulten AB
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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