What is the right workflow to clean & replicate one-channel mono audio as stereo using ffmpeg?

Question

What would be the right workflow in terms of sequence of steps to take what seems to be really a single channel audio captured from VHS tape (apart from the video track), and have it made available on the other channel. For now, only my right ear hears the audio, but my left ear is a bit lonely.

Is ffmpeg the right tool for this ? Or should I extract the audio, and do all the manipulation in another tool, since I also need to do some noise-reduction, amplify a bit, remove some continuous clicks/glitches etc. as well ? Just that I love the command-line speed of ffmpeg, and I have many such videos from VHS tapes that I recently converted.

Answer 1

Seems like you have a stereo input, but one channel is silent. You can

• Dump the silent channel and make a mono output (recommended method)
• Or copy the same channel to both channels

Mono output from specific channel

Using the pan filter. Example to place the Front Right channel of the stereo input into the mono output:

ffmpeg -i input.mp4 -af "pan=mono|FC=FR" -c:v copy output.mp4

Or use the channelsplit filter:

ffmpeg -i stereo.wav -af "channelsplit=channel_layout=stereo:channels=FR" -c:v copy output.mp4

• The video is stream copied and is therefore untouched.
• See ffmpeg -layouts for accepted layout and channel names.

Copy same channel to both left and right

Using the pan filter. Example to place the Front Left channel of the stereo input into both the Front Left and Front Right channels of the stereo output.

ffmpeg -i input.mp4 -af "pan=stereo|FL=FL|FR=FL" -c:v copy output.mp4

• The video is stream copied and is therefore untouched.
• See ffmpeg -layouts for accepted layout and channel names.

Also see FFmpeg Wiki: Audio Channels.