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

[Michael Werzowa]

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

Ducking worked with the same files.
I tried different parameter changes, but I am lost. 
Does anyone of you have a solution for my use case? 
 Is there a „tutorial“ for using the -fiter_complex? 

With librosa and numpy I managed to do this, but this is much slower. I would need „nearly realtime“, hopefully about 2sec. My librosa/numpy solution for this takes about 10sec on a MacBook Pro Max M1.

Thank you for your effort,
best regards,
Michael
--
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Michael Werzowa
A-1190 Wien, Siolygasse 20/3B/6
+43 664 302 4511,  fax +43 810 9554 185931
mikey at werzowa.at
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