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

[Sean Greenslade]

On Tue, Jul 05, 2022 at 03:26:46PM -0400, Richard Cooke wrote:
> This has been driving me crazy for a few weeks!  I do volunteer work around
> Town, and a local church got its hands on a really nice Allen-Heath QU-24
> sound board.  It has a USB-B jack. I recently set up a Video camera with
> control computer for streaming services and events. The new sound board was
> to vastly improve the audio quality.
> 
> When I connect it to the church's "streaming" computer, running Ubuntu 20.04
> it is recognized as both a SOURCE and SINK device. In Audacity, I can select
> any of the 32-channels and see they are working.  We have our streaming mix
> (9) on CH17 and CH18, repeated on CH1 and CH2 for testing.  Again, Audacity
> has no problem selecting any of them.
> 
> They stream using Zoom, Zoom lists the QU-24 but when selected no audio.  We
> use OBS Studio 27.2.4 for the video (virtual camera), it also lists the
> QU-24 but no audio.
> 
> I figured out half of my solution by creating a virtual sink monitor that
> Zoom can use as a microphone:
> 
> # include the default.pa pulseaudio config file
> .include /etc/pulse/default.pa
> 
> # null sink
> .ifexists module-null-sink.so
> load-module module-null-sink sink_name=Source
> .endif
> 
> # virtual source
> .ifexists module-virtual-source.so
> load-module module-virtual-source source_name=VirtualMic
> master=Source.monitor
> .endif
> 
> 
> Which I learned from this post:
> 
> https://www.youtube.com/watch?v=Goeucg7A9qE
> 
> https://github.com/NapoleonWils0n/cerberus/blob/master/pulseaudio/virtual-mic.org
> 
> 
> Which I tested with an analog mic into OBS that came out the Zoom meeting.
> 
> 
> So, if OBS is not going to pickup the QU audio for me, is there a PACTL
> command I can use to path CH1/CH2 to my virtual sink?  Or, even better,
> CH17/CH18?
> 
> 
> Thank you in advance!

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

--Sean