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I have an AVI file with audio recorded only to the left channel.

Is there a setting, audio filter, or whatever to allow me to duplicate the left channel to the right channel, thus converting from stereo to monaural (mono)?

This is an issue mainly because my laptop suffers from a bug where audio won't play out of the left speaker.

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up vote 6 down vote accepted

To convert the audio from two channel stereo to mono without changing the video part, you can use FFmpeg:

ffmpeg -i input.avi -c:v copy -c:a libmp3lame -ac 1 -q:a 2 output.avi

The important option is -ac 1, which downmixes the signal to one channel. Note that this will re-encode the audio, so expect some quality loss.

To change the quality for MP3, choose a different value (from 0 to 9), where lower means better. 2 corresponds to around 95 kBit/s per channel IIRC.

In Ubuntu, an old version of FFmpeg is probably supplied in the packages. Don't do apt-get install ffmpeg and rather download a static build from the download page, or use Libav, which provides the avconv command. It's a fork of FFmpeg with similar functionality and usage and available in the Ubuntu packages.

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If I used some type of video editor, would it be possible to do this without loss of audio quality? – LonnieBest Jan 16 '13 at 10:43
Downmixing always requires you to decode the file, combine the two channels, and write them to one channel. At this point you have raw audio data, and in the final writing stage, you can only keep the raw data (e.g. PCM audio for a WAV file) or you will have to use an encoder again. – slhck Jan 16 '13 at 10:49
Very thorough, thanks. – LonnieBest Jan 16 '13 at 10:50
It's amazing how many current YouTube videos suffer from this problem. Just yesterday, I had to fix 3 downloaded videos to make them watchable with headphones. Thanks for your answer! – fredoverflow Jan 25 '15 at 12:35

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