[FFmpeg-user] sidechain compression: NOT ducking, but envelope

[Michael Koch]

Am 18.11.2024 um 09:32 schrieb Michael Werzowa via ffmpeg-user:
> Dear List,
>
> I am new to the list. Searched the last two years of the list for „sidechain“, and did not find a hint.
> My usecase:
> Using an audio file to provide an envelope, that is applied to another audio file, modulating the gain of this file.
> With ffmpeg I easily can manage „ducking“, via direct call and using the python interface, but with my use case I run into errors or get the wrong output.
> For music mixing, this would be a typical use of sidecahin compression.
> ChatGPT 4o and Claude 3.5 Sonnet gave me a lot of advice, which all lead to the same: either the wrong effect, or an error.
>
> An example that does NOT work, although it should be the thing I want:
>
> ffmpeg -i data/to_be_modulated.wav -i data/sidechain.wav -filter_complex "[1:a]aformat=sample_fmts=fltp:sample_rates=44100:channel_layouts=stereo,volume=2,highpass=f=20,agate=mode=downward:threshold=0:ratio=1:attack=0.0057:release=0.02:range=60:makeup=1[env];[0:a][env]amultiply" data/output-processed.wav

I think this can only work if [env] is a unipolar (always positive) 
signal. I doubt that it is. Write the output of the agate filter to a 
file and have a look at this signal.

Michael