Convert sound inside a video, from 5.1 to stereo, without image modification?

Question

2 days ago I had troubles with video conversion, today that's audio. Here is the problem, the converter I found (see this question) detects all my sources audio channels as different channels (like if they were dvd languages) :

The first chanenl is the left speaker, and the second one is the right speaker, all others are middle's one. When I convert my video, the output is stereo but I loose one speaker. Here is an image to understand :

I gave a different color to each channels of my source : as you can see, after output, only the selected input is used for both speakers.

I suspect this software not to handle 5.1 channels as source. Because this is the only soft I found that supports my image resolution, I need to keep it for video conversion. But I first need to convert dolby to sereo using another software.

Do you know a way to convert dolby to stereo, without altering video ?

edit: ffmpeg -i <video> command gives this output.

Answer 1

You can use the amerge and pan filters in ffmpeg to combine two mono streams into one stereo output:

ffmpeg -i input -filter_complex "[0:3] [0:4] amerge,pan=stereo:c0=c0:c1=c1" -c:v copy -c:a pcm_s16le output

or using -ac instead of pan:

ffmpeg -i input -filter_complex "[0:3] [0:4] amerge" -c:v copy -c:a pcm_s16le -ac 2 output

[0:3] and [0:4] refer to your desired audio streams as shown in your ffmpeg output. I just chose these as examples. There are other audio filters and options too, so there are probably several other ways to do this.