Let us know which structure wins!
Also, in my previous email, I meant to say "At 15278 / 48kHz, instability
is expected", just to ward off that potential confusion. Emailing too fast
again . . .
To see it clearly, note that |p| > 1 for fc above 15278 Hz (when
SR=48Khz). The approximation I used is based on a linearization of the
bilinear transform around z = 1.
- Julius
On Sat, May 29, 2021 at 10:32 AM Oleg Nesterov <o...@redhat.com> wrote:
> On 05/28, Till Bovermann wrote:
> >
> > I was thinking to use the fi.svf.lp filter as a building block instead
> > but it is (AFAICS, it is not mentioned in the doc string...) not a 1st
> > order design (which is crucial here, partly for consistency with the to
> > be emulated analog circuitry, partly because the phase shift should be
> > consistent to be able to deal with the resonance feedback).
>
> Yes, fi.svf is 2nd order. You can this 1nd order filter:
>
> lp(fc) = tick ~ _ : !,_ with {
> g = tan(fc * ma.PI/ma.SR);
> tick(ic1eq, v0) = 2*v1 - ic1eq, v1
> with { v1 = (g * v0 + ic1eq) / (1 + g); };
> };
>
> it has the same impulse response as fi.lowpass(1, fc) when fc is constant,
> but probably differs when fc is modulated.
>
> Oleg.
>
>
>
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