How to swap stereo channels in Ubuntu?

Question

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

Comments

[quack quixote]

+1 beat me by seconds... note Ubuntu 9.04 installs PulseAudio by default which probably has its own way to do it.

[quack quixote]

yes, just sayin' PA can probably do the swap on its own, on the There's More Than One Way To Do It principle. :)

[Amir Uval]

Initially it seemed to do the trick on 14.04. But after reboot pulseaudio daemon failed to start. had to remove the call to fix it.

[Massey101]

Works via pacmd too (i.e. no need to copy config & restart pulseaudio).

[Auron]

pactl set-default-sink reverse-stereo failed with No valid command specified. on a Ubuntu 12.04.

[Massey101]

@Auron works fine on 14.04.

[Auron]

Hmm, I would have to check it.

[Massey101]

@Auron, see update; you can also switch manually via the sound panel.

[user1182474]

If you have more sinks, so you can use pacmd list-sinks to display a list of existing sinks and their indexes. than replace master=0 with the master=IndexOfTheSinkToSwap

[Elder Geek]

This works perfectly under Ubuntu 16.04 as well. Persists through reboot as expected.

[srfrnk]

Great answer! Only problem I'm having is that the volume of the remapped sink is based on the base sink. That means if I switch to the base sink and turn volume to 50% the remapped sink will now be able to go up to only 50% volume (it's 100% will be the 'real' 50%). Any way around that?

[tanius]

Amazingly, this solution still works as of Ubuntu 19.10. Just that after pactl set-default-sink reverse-stereo you have to restart sound-playing applications (like Firefox) so that they pick up the new default on restart.

[Toto]

This should be a comment.

[Aulis Ronkainen]

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review

[Jānis Elmeris]

If the remapped output is selected, in the beginning of a playback (when I double-click on an MP4 file to open it in VLC), there are two small pauses in audio until it starts playing back smoothly. If I rewind the video back, then it is all right from the beginning. The non-remapped output sounds all right also the first time I open a video in VLC.

[Admin]

Worked perfectly for me on PopOS 5.16.11, which derives from Ubuntu! Thanks!