Dropping QuickTime Recording from Stereo to Mono

TL;DR: ffmpeg -i stereo.mov -codec:v copy -af pan="mono: c0=FL" mono.mov

QuickTime doesn’t let you select the channel for an input source, so if you have a standard mono mic connected to a multi-channel audio interface, then the recording you export from QuickTime will end up with the audio only on the left channel, and nothing on the right channel.

Professional video editing tools would make it easy to fix that, but I’m just using iMovie. Neither iMovie 9 or 10 will let you fix it in the project itself, but iMovie 9 will let you export via QuickTime with the audio set to mono.

I can’t use v9, though, because it severely degrades the video quality during import. iMovie 10 fixes the video quality issue, but it removed the export-via-QuickTime feature, so I’m still stuck with the mono audio problem.

One possible solution that sounded promising was using Soundflower to make an aggregate device, so that QuickTime could use it as the audio source, but I couldn’t get that working. The only other thing that seems like it would work would be to export the audio to a DAW like Audacity, convert it to mono, and then import that into iMovie as a separate track. That would be a big hassle for every project, though.

Luckily, there’s a much better solution. After exporting the recording from QuickTime, you can just use ffmpeg to copy the left channel to the right one:

ffmpeg -i stereo.mov -codec:v copy -af pan="mono: c0=FL" mono.mov

If you want an alias to do it quickly, you can add this to your .bashrc:

Then you can just run qtmono foo.mov and it’ll output foo-mono.mov.

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