[sdiy] tanh distortion in a filter

[Andrew Simper]

Yes, Antti has already written a paper on it, but he only derives the
allpass response, for an svf you need a one pole integrator instead:

ENHANCED DIGITAL MODELS FOR ANALOG MODULATION EFFECTS
Dafx 05

And pulls the following equations from:
A. Gratz, Operational Transconductance Amplifiers
http://synth.stromeko.net/diy/OTA.pdf

for an OTA you have:
I = ictrl tanh ((v+ - v-) / 2 Vt)

so plonk it into an integrator circuit and integrate that with forward
euler and you get:
vc += g * tanh (in)

Andrew Simper
--
cytomic - sound music software
mobile: +61-450-774-230
skype: andrewsimper



On 11 August 2011 11:30, <lanterma at ece.gatech.edu> wrote:
>
> 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
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