[pulseaudio-discuss] How to map 2 channels from a 32-channel USB device to L/R?

[Sean Greenslade]

On Sun, Jul 10, 2022 at 09:41:16PM -0400, Richard Cooke wrote:
> On 2022-07-10 4:14 PM, Sean Greenslade wrote:
> > You should (in theory) be able to use module-remap-sink or
> > module-remap-source to create a virtual 2-channel sink/source using
> > specific channels from the 32 channel one. See:
> > 
> > https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/User/Modules/#module-remap-sink
> > 
> > It may require some experimentation to figure out what names pulse is
> > assigning to each channel, since the first 8 channels are given
> > standardized 7.1-channel names. Take a look here for the string names
> > that module-remap-* is looking for in channel_map:
> > 
> > https://github.com/pulseaudio/pulseaudio/blob/7f4d7fcf5f6407913e50604c6195d0d5356195b1/src/pulse/channelmap.c#L39
>
> Thanks Sean!  I was working on it again today, I posted the latest update to
> another website:
> 
> https://linuxmusicians.com/viewtopic.php?f=27&t=24713&sid=e80ee72d0f2e1b23402e32e62ca0bf6f
> 
> In summary, I can see only two MIDI devices in the signal patch window.  I
> do not see the 32-channels of audio data that I know is there - because
> Audacity can access it.
> 
> So I assume I am missing something in my config to expose those channels.
> 
> I'll read through your links too, in  case the missing piece is in there.
> 
> Part of my frustration is I can only see the QU when its powered up.  The
> streaming PC is on all the time, but we cannot leave the QU powered up
> because of the main power amps.  Bad things could easily happen if I play
> around with signals while the whole system is live!  Or while left
> unattended.

I re-added the mailing list, although this is perhapes straying away
from a pulse-specific discussion.

You mention "Catlia" in your forum post, which I assume is a typo. There
are two similar programs in the KXStudio lineup, "Catia" and "Carla".
I'm assuming you're using catia, which is a patchbay for the JACK audio
server. JACK is similar to Pulseaudio in that it is a sound routing
engine and not a direct soundcard driver. In Linux, the soundcard driver
is ALSA. Both JACK and Pulse make use of ALSA to interact with the
hardware sound devices. Generally speaking you would only use one sound
server. Although there are ways to run both simultaneously and to route
audio between the two, that's definitely an advanced setup.

Note that Catia cannot control Pulseaudio at all, and while Carla
technically has the ability to use Pulse as its backend, it doesn't
expose the Pulse sources / sinks in a useful way to provide the type of
routing you're looking to do.

>From your screenshot, it appears that JACK has not taken control of the
ALSA device for your 32-channel mixer, or it's not configured correctly
to detect and expose the full channel count. You should probably find a
JACK mailing list if you want to go down that route, although I believe
Pulse is fully capable of accomplishing the task you laid out in your
original email.

One side note: MIDI is not the same as audio. If you're only concerned
about routing audio signals around, you can completely ignore MIDI.

To give some more concrete suggestions, can you do a fresh reboot of the
streaming PC, turn on and connect the mixer, then run the following
command and post its output:

$ pactl list sources

--Sean