Multiplexing 4 channels on SPDIF

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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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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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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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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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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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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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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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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services, security?
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sel?cmd=lnk&kid=120709&bid=263057&dat=121642
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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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