[pulseaudio-discuss] How to combine microphone audio and music from application to pipe to video conference

[Sean Greenslade]

On Tue, May 26, 2020 at 02:42:18PM -0400, Samir Parikh wrote:
> Sean Greenslade wrote on 5/24/20 1:44 AM:
> Sean,
> 
> Thanks for this!  Very helpful!  I appreciate you taking the time to also
> include the diagrams.  It helps me understand a bit more some fo the
> commands you gave earlier.
> 
> You've been very helpful and generous with your time and explanations so I
> hope I'm not reaching my limit of questions with you or the list but I have
> a few more:
> 
> - In general, what is the purpose of the "loopback module"?  It just looks
> like a pass through.  The docs[1] say that "This allows one to route audio
> from a source directly back to a sink. This module performs adaptive
> resampling to adjust for slight differences in the clock speeds of the
> source and sink devices" but I'm not sure what purpose it is serving in this
> application.

In this case, it's converting the stream type from a source to a sink.
Recall that these different types cannot be directly routed to each
other. This is why I made the distinction between the pointed ends and
the rounded ends in the diagrams.
 
> - In your original configuration, you had me create just one loopback
> module.  In your more complicated setup, there appear to be 3 in total. Do I
> just use the original
> 	pacmd load-module module-loopback sink=virt_mix
> command to create the other 2 but change the sink?

Yes, every time you call that command, you load another loopback module.
The sink= and source= options let you specify the initial state of the
connections.

> - In general, is pavucontrol just used to create the "connections"?

Yes, you can use pavucontrol to modify any of the connections. As
mentioned above, if you know the source/sink names in advance, you can
use the source= and sink= options to preset them when you load the
modules.

> - Within pavucontrol, is my understanding of what the various tabs do
> correct?
> 	Playback:  Shows the application sources of sound
> 	Recording: Shows any application that can take sound as input
> 	Output Devices: Sinks
> 	Input Devices: Sources

Yes, that's correct. Note that the behavior of applications can differ,
so sometimes you will see applications appear and disappear from the
playback and recording tabs as they start and stop recording / playing
back audio. For example, Firefox will create an individial sink input
stream for every separate audio source, and delete it when the source's
tab is closed.

--Sean