[sdiy] tanh distortion in a filter

[lanterma at ece.gatech.edu]

On Aug 10, 2011, at 9:46 PM, Scott Nordlund wrote:

>>> Assuming work in the digital domain: Is tanh distortion something that works in a filter I think the Rossum paper is "Making Digital Filters Sound 'Analog'". He suggests clipping (or otherwise distorting) the delay path of a biquad, but not the input or output. It's a pretty dated paper, from 1992, so it's probably not something to rely on unless you're just looking to do something cheaply.

That sounds like the one I was thinking of.

>>> Antti Huovilainen wrote the stuff about emulating the Moog transistor ladder filter by embedding tanh functions (derived from transistor differential pairs). I don't know how much of that would be applicable to state variable filters.

I guess for SVF you'd be modeling the tanh of the OTAs used in something like the Oberheim SEM VCF.

A while ago I worked out where the tanh should go for an OTA-C one-pole stage, like the OTA-C cells used in the typical Prophet 5 type four-stages-with-negative-feedback configuration. The result wound up similar to Antti's Moog ladder formulation, but the tanh functions wound up in different places. Last time I looked for my notes though I couldn't find it...

...I'm guessing this is probably something Antti's already worked out somwhere. ;)

- Aaron