applying saturation through the sidechain

This short article gives a brief introduction on applying waveshaping algorithms not directly in the audio path but via a transformed equation through the sidechain and a VCA instead.

Typical saturation curves

Typical saturation curves

As mentioned earlier before in this article, waveshaping can be seen and applied quite similar to compression techniques via its computation in a separate sidechain path. This way, the algorithm does not work anymore on the audio path in a direct fashion but only on the sidchain part and the outcome then drives the main audio via a VCA. The actual used algorithm has to be properly transformed math wise to work in a sidechain configuration and the transformation depends on if the sidechain operates on log encoded audio or does work in linear scale.

So, why could this be more favourable compared to a direct implementation?

Some experimental transformed curves (linear scale)

Some experimental transformed curves (linear scale)

One big advantage could be that filter and phase alterings upfront the waveshaping (to shape its frequency response) does not translate back into the originial audio source since due to the VCA only amplitude modulation (by a DC signal) is performed in the end.  Given this method a circuit designer is able to e.g. setup frequency dependent saturation behaviour without the need to filter and compromise the original signal. Even some artifacts of oversampling can be reduced to some extend if its specific use is limited to just parts of the sidechain and not the entire signal chain. However, normally the VCA would be oversampled as well to minimize the intermodulation distortion.

Even more sophisticated waveshaping processing such as combinations with envelope and transient processing or frequency split techniques are now easily possible without touching the integrity of the main audio path.


  1. It’s always a pleasure to read your articles.

    Especially the ones that explain how it works, and where does the magic come from ;-).

    Keep up the GREAT work Herbert.

  2. Ooooh….are you about to do a “Fatso” type plugin?

    You naughty boy …. 🙂

  3. I’ve already been doing this in one of my plugins, you can check it out at the linked site.

  4. Frank Castle says:

    When you referred to “frequency split techniques…without touching the integrity of the main audio path.”, you meant taking the FFT, using the magnitude of each band to compute the compressor gain for that band and using that value in a gain before doing the inverse-FFT? like the ProAudioDSP’s “Dynamic Spectrum Mapper”…such a lovely technique to achieve frequency dependent compression similar to hardware tape compression in digital domain…

What do you think?

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: