How to swap stereo channels in Ubuntu?

I'm currently running Ubuntu 9.04. I wanted to swap the stereo channels, but I couldn't find that option in the Volume Control Preferences.

Is there a way to do this without touching any configuration file? (I'm not allowed to log as root in this machine)

Solution 1:

The PulseAudio way (tested on Ubuntu 10.04, should work on 9.04):

Copy /etc/pulse/default.pa to ~/.pulse/default.pa, and add the following two lines to the end:

load-module module-remap-sink sink_name=reverse-stereo master=0 channels=2 master_channel_map=front-right,front-left channel_map=front-left,front-right
set-default-sink reverse-stereo

Restart PulseAudio by running pactl exit at the command line.

Leave out the second line if you don't want to use the reversed stereo by default. You can switch between reversed and normal stereo output in the "Output" tab in Sound Preferences. If you do have root and want this system wide, you can just add the lines to /etc/pulse/default.pa instead of making a user-specific configuration.

This configuration makes a few basic assumptions: that the card you're reversing the channels of is card 0; that you only have to deal with 2 channels; and that those channels are called front-left and front-right.

If you have a videocard with HDMI out, it is likely the analog out of your motherboard will be card 1, not 0; so master=1. (Front Panel headphones are likely on this analog channel)

For more information, see the PulseAudio documentation for module-remap-sink.

Solution 2:

Essentially the same approach as Brian's above, but without touching any configuration file:

pactl load-module module-remap-sink \
    sink_name=reverse-stereo \
    master=0 \
    channels=2 \
    master_channel_map=front-right,front-left \
    channel_map=front-left,front-right

This will create on the fly an additional PA sink with reversed channels. (It will disappear after pulseaudio restart). To switch to it:

pactl set-default-sink reverse-stereo

— OR, you can also switch manually via the sound control panel:

Off course it's possible to switch back and forth, e.g. when experimenting.

This way is better if you want to try something out quickly rather than to save the setup permanently.

Solution 3:

If you're using ALSA, Add this to your ~/.asoundrc file:

pcm.swapped {
    type         route
    slave.pcm    "cards.pcm.default"
    ttable.0.1   1
    ttable.1.0   1
}

pcm.default      pcm.swapped

Via ALSA FAQ