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

[Paul B Mahol]

There is no standalone envelope filter in FFmpeg, but there is one in
Librempeg. (If envelope is standard definition by audio community)

If you already have smooth envelope, you can use in FFmpeg
sidechaincompress filter (takes two input streams) + amix filter.

I could help more if I can look at that py code that works.


Also see lavfi-preview tool, it can help building (complex) filter graphs
within GUI.

On Mon, Nov 18, 2024 at 10:05 AM Michael Koch <astroelectronic at t-online.de>
wrote:

> 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
>
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