[FFmpeg-user] Copying all audio channels and create mixdown at the same time

[Lou]

On Tue, Aug 22, 2017, at 08:15 AM, Steve Dierker wrote:
> Hello list,
> 
> I have an MXF file with one video track and 8 audio track. I'm trying to
> reencode it to MP4 while preserving all existing tracks and also adding
> a mixdown of all audio tracks as the 9th track.
> Right now I'm only able to do this in 2 separate FFmpeg calls and I'm
> wondering whether one can do it in one call.
> 

ffmpeg -i input -filter_complex
"[0:a:0][0:a:1][0:a:2][0:a:3][0:a:4][0:a:5][0:a:6][0:a:7]amerge=inputs=8[mix]"
-map 0 -map "[mix]" output

Or if you want to be lazy and blindly trust amix to choose inputs for
you:
ffmpeg -i input -filter_complex "amerge=inputs=8[downmix]" -map 0 -map
"[mix]" output