Add option for dynamic FFmpeg sound volume normalization for the merge command

[devnoname120]

Particularly useful for voices as they can get farther and closer to the mic.

• Explanations: http://k.ylo.ph/2016/04/04/loudnorm.html
• loudnorm documentation: https://ffmpeg.org/ffmpeg-filters.html#loudnorm

Note: the first loudnorm pass should be run through the collection of all the inputs to make sure that the dynamic normalization is consistent across the whole merged file.

For example for converting MP3 files to M4B:

ffmpeg -i part1.mp3 -i part2.mp3 -i part3.mp3 -filter_complex 'concat=n=3:v=0:a=1[a]; [a]loudnorm=I=-16:TP=-1.5:LRA=11:print_format=json' -f null -

output:

{
    "input_i" : "-22.99",
    "input_tp" : "-6.99",
    "input_lra" : "7.00",
    "input_thresh" : "-34.56",
    "output_i" : "-16.73",
    "output_tp" : "-1.50",
    "output_lra" : "6.80",
    "output_thresh" : "-28.29",
    "normalization_type" : "dynamic",
    "target_offset" : "0.73"
}

ffmpeg -i part1.mp3 -ab 196k -ar 44100 -ac 2 -af loudnorm=I=-16:TP=-1.5:LRA=11:measured_I=-22.99:measured_TP=-6.99:measured_LRA=7:measured_thresh=-34.56:offset=0.73:linear=true:print_format=summary -f mp4 part1.m4b

ffmpeg -i part2.mp3 -ab 196k -ar 44100 -ac 2 -af loudnorm=I=-16:TP=-1.5:LRA=11:measured_I=-22.99:measured_TP=-6.99:measured_LRA=7:measured_thresh=-34.56:offset=0.73:linear=true:print_format=summary -f mp4 part2.m4b

ffmpeg -i part3.mp3 -ab 196k -ar 44100 -ac 2 -af loudnorm=I=-16:TP=-1.5:LRA=11:measured_I=-22.99:measured_TP=-6.99:measured_LRA=7:measured_thresh=-34.56:offset=0.73:linear=true:print_format=summary -f mp4 part3.m4b

m4b-tool merge -vvv --debug --no-conversion --include-extensions=m4b --output-file="merged.m4b" .

[devnoname120]

Ah it looks like there is already an issue about this: https://github.com/sandreas/m4b-tool/issues/121

[sandreas]

Ah it looks like there is already an issue about this: https://github.com/sandreas/m4b-tool/issues/121

Can I close this issue in favor of #121? I think it already contains a detailed description of what would be the best approach to integrate the loudness normalisation feature.

Although I doubt that I will have the time to integrate it soon. Maybe you wanna give it a shot?

[devnoname120]

Can be closed I agree 👍 I'll copy my comment there