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  1. 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.
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  2. 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
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  3. **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.
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