[pulseaudio-discuss] Per (sink input, sink) volume control / routing

[Andreas Hartmetz]

Hello!

I am currently developing an audio system for an automotive customer,
based on PulseAudio where PulseAudio is applicable. It is part of the
requirements that the volume of every input channel (sink input) to
every output channel (sink) can be configured separately. There are
good reasons for this, for example there can be two backseat info-
tainment "terminals" which could send their audio to a headphone for
each, or to the speakers in the whole car. The user-exposed volume
controls will of course present a simplified vew, but for factory
configuration, the described control is needed, and volume levels are 
expected to change at runtime in response to some external events.

The problem is that PulseAudio does not support that. It could almost
be "hacked" by using one module-combine-sink for every sink input,
which at least allows sending a stream to an arbitrary collection of
sinks. But it does not support volume control at all.

So:
- Is there a way that I didn't think of  to piece together the
  desired volume control using  existing modules and/or other
  configuration? I don't want to resort to anything really weird
  like creating lots of "loopback" devices using local sockets or
  something like that (also for latency reasons), and running
  multiple server instances also won't do it due to PA requiring to
  open devices exclusively. Also, any hacky solution is not allowed
  to significantly increase (or worse, hide from PA so it can't be
  compensated) latency.
- If not, what do you think would be the best way to do it? I'm
  thinking of a potential "module-crossbar" that I would need to
  write, which allows matrix-style (n, m) volume control, i.e.
  set_volume(sink_input_index, sink_index, volume).

Thanks in advance!

Andreas Hartmetz