Help with Syntax for FFmpeg (sorry... newbie)

[dstewart275]

Hello - I know questions like these are asked a lot, but I have a slightly tricky issue.
I have about 30 video+audio files that have been captured incorrectly. They are mp4 H.264 with stereo AAC audio.
The video is fine and I don't want to re-encode that. The stereo audio is about -20db on the right leg.

What would be syntax of a windows FFmpeg command be to copy the video and left stereo audio leg and add 20db gain to the right leg only to a new file?

Any guidance much appreciated.

Thanks all - Duncan.

[sneaker]

Try:
for %%a in (*.mp4) do ffmpeg -i %%a -c:v copy -af dynaudnorm=n=0 "output\%%~na.mp4"

That should normalize the channels independently (to same level). If you want to do a "stupid" gain you probably have to split the channels and then do the "normal" gain or normalization filter.

Are they "real" stereo files? If they are not I would just convert to mono, or stereo but just by duplicating the left channel.

https://superuser.com/questions/323119/how-can-i-normalize-audio-using-ffmpeg
https://trac.ffmpeg.org/wiki/AudioVolume
https://trac.ffmpeg.org/wiki/AudioChannelManipulation
https://ffmpeg.org/ffmpeg-filters.html#dynaudnorm

[dstewart275]

**really helpful** thanks you. For my purposes this approach worked just great.
For anyone in a similar situation, I had to make one tiny adjustment to increase the gain range to 20db

ffmpeg -i %%a -c:v copy -af dynaudnorm=n=0:m=20.0 "output\%%~na.mp4"

Your expertise is much appreciated.