Force downmix to mono on Linux

Question

For personal reasons (I'm hearing impaired) I need my linux machine to produce mono downmixed sound. Downmixed, not just single channel.

Is there any possibility to force such behavior in ALSA / PulseAudio server? Hardware mixing (short-circuiting L and R channels with some resistors) is not possible at the moment. I'm looking for a software solution to mix two channels into one.

I know that some media players have this functionality, but I'd like the whole system (ie games, flash applications, etc) to produce downmixed audio.

Solution

I haven't done any extensive testing, but this ~/.asoundrc file (or /etc/asound.conf) should work for anything that uses ALSA. Assumes that you only want to deal with two in-channels (left and right), and that your soundcard is at hw:0.

pcm.!default makemono

pcm.makemono {
    type route
    slave.pcm "hw:0"
    ttable {
        0.0 1    # in-channel 0, out-channel 0, 100% volume
        1.0 1    # in-channel 1, out-channel 0, 100% volume
    }
}

Check out the official documentation for details on how to fine-tune/complicate things further.

Comments

[We Are All Monica]

This got me on the right track, thank you! Now that PulseAudio is a thing, I had to also copy /etc/pulse/default.pa to ~/.config/pulse/default.pa and uncomment the line that says load-module module-alsa-sink, then run pulseaudio -k to pick up the changes. See also: superuser.com/a/1158248/9599