There are at least two methods to select the left audio channel and output as mono.

Using `-map_channel`

Assuming the input only contains audio:

ffmpeg -i input -map_channel 0.0.0 output

The first number is the input file id: you only have one input so that will be 0. The second number is the stream specifier: if the audio is the second stream, such is often the case in a typical video file, then you would use 1 instead. The third number is the channel number: in a stereo input the first channel is usually the Front Left, and the second is usually the Front Right.
If you want the right channel instead use -map_channel 0.0.1.
If the input contains video, then the audio will likely (but not always) be listed as the second stream, so you would need to use 0.1.0 instead.

Using pan audio filter

ffmpeg -i input -af "pan=mono|c0=FL" output

Alternatively, you could use pan=mono|c0=c0.
If you want the right channel instead use pan=mono|c0=FR (or pan=mono|c0=c1).

Also see

llogan

Posted 2016-04-08T20:33:32.107

Reputation: 31 929

Hi, you said "Assuming the input only contains audio:". The problem is that my file contains both video and audio :) – SebMa – 2016-04-13T14:45:38.643

@SebMa This one reason why you need to include the complete console output from your commands so the answers can be more accurate and less based on assumptions. I edited the answer mentioning video. – llogan – 2016-04-13T15:53:35.323

Sorry I forgot to give it but ffmpeg didn't give any error anyway. In my question, you can notice I used -map_channel 0.1.0 but this is not enough to get both channels point to the left one. – SebMa – 2016-04-14T16:43:30.420

@SebMa Using -map_channel 0.1.0 works for me to make a mono audio file from the left channel of a stereo input (with video as the first stream). – llogan – 2016-04-14T17:33:38.473

I think it only works for audio files. For videos, you have to add -ac 1 – SebMa – 2016-04-30T08:51:31.573

Hi, I think the best would be that I give the youtube URL of the video that I'm trying to convert to a mono video : https://www.youtube.com/watch?v=l9dPJCFXAes

– SebMa – 2016-05-03T11:25:54.730

@SebMa What are you using to get a local copy of the video? – llogan – 2016-05-03T18:52:00.427

youtube-dl, but don't bother.

I solved the pb. by replacing :

-map_channel 0.1.0 -map_channel -1

which actually muted the second audio channel, instead of just using :

-map_channel 0.1.0

Thanks for your help :) – SebMa – 2016-05-07T01:32:15.887

How to select the left audio channel with ffmpeg and downmix to mono?

Answers

Using `-map_channel`

Using pan audio filter

Also see

How to select the left audio channel with ffmpeg and downmix to mono?

Answers

Using -map_channel

Using pan audio filter

Also see

Using `-map_channel`