[sdiy] Pole Mixing

Andrew Simper andy at cytomic.com
Sun Apr 11 06:21:39 CEST 2021


Hi David,

I'm not sure you've got the correct frequency response on your page. You
may have better luck with this:

(g^4 (m0 + m1 + m2 + m3 + m4) + g^3 (4 m0 + 3 m1 + 2 m2 + m3) s +  g^2 (6
m0 + 3 m1 + m2) s^2 + g (4 m0 + m1) s^3 +  m0 s^4) /
(g^4 (1 + k) + 4 g^3 s + 6 g^2 s^2 + 4 g s^3 + s^4)

where g is the cutoff in hz, and k is the resonance feedback with 4 being
self oscillation, and the mixing terms are as follows:

m0*(in - k*lp4) + m1*lp1 + m2*lp2 + m3*lp3 + m4*lp4

Note that for the xpander uses inverting stages, I've used non-inverting
stages, so each other mixing terms needs to be negative, eg:

m0, -m1, m2, -m3, m4

Cheers,

Andy

On Fri, 9 Apr 2021 at 06:24, David Moylan via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Hi All.  I banged together a little web app to play around with filter
> pole mixing, of the Oberheim Xpander type.  You can mix poles in varying
> amounts and see the output magnitude shape as well as the transfer
> function.  Y axis is Db and X axis is log scale based on normalized
> frequency (so basically 1 equals the cutoff frequency).  Haven't done
> phase plot yet.
>
> If you have an interest in this sort of thing check it out:
>
> https://expeditionelectronics.com/Diy/Polemixing
>
> Cheers.
>
> --
> David Moylan
> Expedition Electronics
> sonic adventures!
>
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