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

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)

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.