[sdiy] Pole Mixing

Andrew Simper andy at cytomic.com
Tue Apr 13 03:49:12 CEST 2021


You have two degrees of freedom to place notches with this structure, and
when I worked on this stuff around 15 years ago for the synth Strobe I
found the best sounding position was an octave above and an octave below
the cutoff, but you can place the notches anywhere you want, and in
software land it's trivial to have the notch position as a filter parameter
that can be modulated.

For those wanting to work out frequency responses and get it right the
first time, I recommend starting with the nodal / voltage loop idealised
continuous equations, and solve them directly, instead of introducing
confusion by guessing as to where the feedback terms should be. This also
means you can say use the first g term as 330*g and work out the correct
response for the 3 pole version of the x-pander. Here is a pdf of the full
working from first principles for the 4 pole version:

www.cytomic.com/files/dsp/cascade-4pole-freq-resp-working.pdf

Cheers,

Andy

On Sat, 10 Apr 2021 at 14:13, 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).
> The gain factors translate directly into resistor values for pole mixing.
> This was the basis of the Intellijel Polaris.
>
> I'd be happy to send the spreadsheet to anybody who wants it.
>
>
> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of
> David
> Moylan via Synth-diy
> Sent: Thursday, April 08, 2021 3:25 PM
> To: synth-diy at synth-diy org
> Subject: [sdiy] Pole Mixing
>
> [CAUTION: Non-UBC Email]
>
> 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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