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

I have an old "stereo" movie as a mp4 file.

The sound is only in the left channel, the right channel is silent.

I'd like to use ffmpeg to copy the video stream :

ffmpeg -hide_banner -i video.mp4 -c:v copy ...

and convert the audio to mono by selecting only the left channel :

-map_channel 0.1.0 -map_channel -1

But that didn't change anything :-(

What is the correct ffmpeg command line for that ?

Is it also possible to do that whithout re-encoding ?

Answer

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

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.

  
  

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