Multiplexing 4 channels on SPDIF

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Multiplexing 4 channels on SPDIF

Fons Adriaensen
Hello all,

Core Sound <http://www.core-sound.com/default.php> willsoon
be offering a tetrahedral (Ambisonic) microphone at a very
reasonable price. They are also working on a combined preamp
+ AD converter unit for this mic. This will be able to multiplex
the 4 channels over a single SPDIF link, by using it a the
double sample frequency.

I'm currently working on a software controller unit for this
microphone. It will perform A-format to B-format conversion,
and allow measured impulse responses to be used for calibrating
the four mics. The result should be a very high quality portable
surround recording system at a reasonable price (compared to other
solutions which cost easily five times as much).

The remaining problem is the demultiplexing of the two double
speed SPDIF channels to four channels. It could either be done
within the ALSA layer, or in JACK's ALSA backend. Doing this
in a JACK client will not work unless it would be the only
client - all others would get the wrong idea of the sample
frequency and buffer size.

So here's my question to both the ALSA and JACK teams: what
would be your idea of a solution for this ?

--
FA

Lascia la spina, cogli la rosa.


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Re: Multiplexing 4 channels on SPDIF

Jussi Laako
Fons Adriaensen wrote:
> Core Sound <http://www.core-sound.com/default.php> willsoon
> be offering a tetrahedral (Ambisonic) microphone at a very
> reasonable price. They are also working on a combined preamp
> + AD converter unit for this mic. This will be able to multiplex

Sounds nice :)

> So here's my question to both the ALSA and JACK teams: what
> would be your idea of a solution for this ?

Some ALSA plugin? Of course, doing it in JACK's ALSA backend is also
reasonably simple, but would require small changes in many places to
override samplerates, etc.


        - Jussi

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Re: [linux-audio-dev] Re: Multiplexing 4 channels on SPDIF

Lee Revell
On Mon, 2006-10-30 at 13:40 +0200, Jussi Laako wrote:

> Fons Adriaensen wrote:
> > Core Sound <http://www.core-sound.com/default.php> willsoon
> > be offering a tetrahedral (Ambisonic) microphone at a very
> > reasonable price. They are also working on a combined preamp
> > + AD converter unit for this mic. This will be able to multiplex
>
> Sounds nice :)
>
> > So here's my question to both the ALSA and JACK teams: what
> > would be your idea of a solution for this ?
>
> Some ALSA plugin? Of course, doing it in JACK's ALSA backend is also
> reasonably simple, but would require small changes in many places to
> override samplerates, etc.

I don't think it even needs to be a plugin, just tell ALSA about the
audio format.  Where can the detailed specs be downloaded?

Lee


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Re: [linux-audio-dev] Re: Multiplexing 4 channels on SPDIF

Alfons Adriaensen-2
On Mon, Oct 30, 2006 at 10:07:06AM -0500, Lee Revell wrote:

> > Some ALSA plugin? Of course, doing it in JACK's ALSA backend is also
> > reasonably simple, but would require small changes in many places to
> > override samplerates, etc.
>
> I don't think it even needs to be a plugin, just tell ALSA about the
> audio format.

How can you tell ALSA that an SPDIF input is 4 channels ?

> Where can the detailed specs be downloaded?

According to the Core Sound website this thing is still in development,
and the specs there are incomplete.

What they want to do is trick  hardware 2-ch recorders into accepting
the 4-ch signal. So what is in reality 4 channels at 48 kHz will be
presented as 2 channels at 96 kHz.

 
--
FA

Lascia la spina, cogli la rosa.

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Re: [linux-audio-dev] Re: Multiplexing 4 channels on SPDIF

Lee Revell
On Mon, 2006-10-30 at 17:35 +0100, Alfons Adriaensen wrote:

> On Mon, Oct 30, 2006 at 10:07:06AM -0500, Lee Revell wrote:
>
> > > Some ALSA plugin? Of course, doing it in JACK's ALSA backend is also
> > > reasonably simple, but would require small changes in many places to
> > > override samplerates, etc.
> >
> > I don't think it even needs to be a plugin, just tell ALSA about the
> > audio format.
>
> How can you tell ALSA that an SPDIF input is 4 channels ?
>

The ALSA code would have to be extended... I'm not sure exactly where.

I don't think it should be called SPDIF because SPDIF implies two
channels.

> > Where can the detailed specs be downloaded?
>
> According to the Core Sound website this thing is still in development,
> and the specs there are incomplete.
>
> What they want to do is trick  hardware 2-ch recorders into accepting
> the 4-ch signal. So what is in reality 4 channels at 48 kHz will be
> presented as 2 channels at 96 kHz.
>

I don't think it's useful to discuss further until technical details are
available.

When specs are available, this thread should probably be re-started on
alsa-devel.

Lee


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