Auto-switching PulseAudio profiles based on input(s) channel count

Question

I'm running audio over HDMI to my receiver, which has a full 7.1 setup connected to it. However, most of the stuff that I listen to has far fewer channels—some is mono, most is stereo, a bit is 5.1. When the receiver is fed with the right number of channels, it has various DSP effects which work fairly well (e.g. in splitting out a center channel, in sending base to the subwoofer, auto-detecting Dolby matrix surround, etc.). This ceases working if you send it a full 7.1 signal with a bunch of silent channels.

The number of output channels in PulseAudio is configured, per-card/sink, by the profile assigned to the card. You can change the assigned profile with pacmd, pactl, and various GUI programs.

How can I set it up to automatically change the profile based on the channel usage in the input stream(s)? E.g., if the input streams are only using front-left and front-right, switch to the stereo profile. If they're using back-left and back-right as well, switch to quadrophonic, etc.

Answer 1

This is just a concept and no complete solution:

You may be able to do your own handling of events with patcl subscribe if you can assign configurations to pulsaudio clients (e.g., if the movie player connects, you switch to 7.1 configuration, otherwise you use the stereo configuratioin). You will get a constant stream of events from patcl subscribe, which you can handle by observing client connections. Then you can just call pactl/pacmd to change the profile.

Example output of pactl subscribe:

$ pactl subscribe 
Event 'change' on sink #0
Event 'remove' on sink-input #93
Event 'change' on sink #0
Event 'change' on source #0
Event 'change' on sink #0
Event 'change' on source #0
Event 'new' on sink-input #98
Event 'change' on sink-input #98

Please note, that you still have the problem to handle cases in which you have multiple clients with a different number of channels.

Furthermore, the above approach is rather limited. The per application settings are not sufficient to detect the real number of used channels of the application (e.g. vlc might play a 5.1 or 7.1 video). I have not found a way to get the number actually used channels of an pulseaudio client over pactl, but I might have missed some options here. Alternatively, you might be also able to hook into the video player application itself and get the missing bits form there.

Answer 2

I worked the pactl subscribe idea into a solution - the first solution that worked for me, finally!

In .xsessionrc I have

pkill --signal 9 -f auto-switch.rb
~/.auto-switch.rb &

And in ~/.auto-switch.rb

#!/usr/bin/env ruby

require 'open3'

RE = /^Event 'new' on sink #(\d+)$/

Open3.popen2('pactl', 'subscribe') do |_stdin, stdout, _status_thread|
  stdout.each_line do |line|
    match = RE.match(line)
    system('pactl', 'set-default-sink', match.captures.first) if match
  end
end

(And chmod +x ~/.auto-switch.rb)

This listens to pactl subscribe and runs pactl set... commands as you like whenever pactl subscribe reports some change in your setup. You could modify this with extra logic for your situation if you pleased. You could write it in some other language besides Ruby too, if you preferred.

I just wanted to come here to report that this has been working perfectly for me for weeks now and is a vastly simpler solution to all the others I could find on StackOverflow related websites. Thank you!