Frequency-domain impact of non-linear filters

DSP, Plug-in and Host development discussion.
6370 posts since 12 Feb, 2006 from Helsinki, Finland

Post Wed Apr 21, 2021 1:36 pm

Z1202 wrote:
Wed Apr 21, 2021 5:29 am
signalsmith wrote:
Wed Apr 21, 2021 4:29 am
This sounds promising. This implies that if you have a 200Hz input and a very resonant TWCGB with a cutoff at 210Hz, you could expect stuff at 190 and 220, etc.
I'm not sure about the exact math, whether it's exactly IDM, or just IDM-like behavior, that'd require some theoretical model of that, and I'm not aware of such models. But one thing I noticed (not 100% sure) is the "locking" of the input frequency to the resonance in strong resonance nonlinear filters. Basically, once the input frequency gets closer to the resonance it gets "pulled" all the way towards the resonant frequency. But I'm not sure if it wasn't the spectrogram resolution issue or whether it was the other way around (the resonance being pulled towards the input frequency).
The "locking" behaviour in some (popular) topologies is real. I think it's basically the same mechanism that enables a PLL (=phase-locked loop), where the frequency is adjusted depending on whether your PLL leads or lags the target, except here it's the non-linearity doing the modulation in somewhat adhoc manner.
Preferred pronouns would be "it/it" because according to this country, I'm a piece of human trash.


Topic Starter

52 posts since 5 Jul, 2018 from Cambridge, UK

Post Sun May 23, 2021 5:08 pm

Found it! The image I was remembering is actually from the anti-derivative anti-aliasing paper.

Definite demonstration of the "locking" behaviour:
Screenshot 2021-05-24 at 02.07.09.png
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