[sdiy] Pole Mixing

Andrew Simper andy at cytomic.com
Tue Apr 13 04:14:26 CEST 2021


On Tue, 13 Apr 2021 at 09:48, David Moylan via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Comments inline.
>
> On 4/12/21 9:22 PM, cheater cheater wrote:
> > On Sat, Apr 10, 2021 at 8:18 AM David G Dixon via Synth-diy
> > <synth-diy at synth-diy.org> wrote:
> >> I did more or less the same thing more than 10 years ago, but in
> Excel.  In
> >> that case, the gain factor of the ABCDE are specified as integer
> values, and
> >> the resonance is altered with a slider.  I found this very useful for
> >> finding interesting modes, particularly with the various notches
> (including
> >> the famous double-notch mode, which has the gain factors of
> 1-4-12-16-8).
> > Is that different from 2NT they list in the app note? There's no mode
> > with 1-4-12-16-8.
>
> They don't have this one in the app note.  I didn't know it was famous
> :)  I came across it as well poking around in my app.  I've included it
> as "Double Notch C (Dixon)".  You can get all sorts of double notches
> playing with the mix factors.  I figured out that you can spec the
> distance between the notches by starting with pairs
>
> (s^2+1)  and (s^2+1) for the numerator.  Then scale the s terms by
> inverse factors.  So, to get the notches 1 octave up/down from center
>
> ((s/2)^2 + 1) and ((2s)^2 + 1)  which comes out to
>
> s^4 + 4.25s^2 + 1
>
> I named that one "Double Notch Octaves" but it has non-integer factors.
> The closest integer one is [ 1 4 10 12 6 ] named "Double Notch A (almost
> Octaves)".
>
> <Synth-diy at synth-diy.org>


As noted in my previous email, you can place the two notches at any
location you want, and the best sounding one to me in practice is either an
octave below or an octave above the cutoff. Here are my favourite
responses, what are the ones I included in the original Strobe synth:

www.cytomic.com/files/dsp/cascade-tapped-filter-responses.pdf

Cheers,

Andy
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