[sdiy] Pole Mixing

David Moylan dave at expeditionelectronics.com
Tue Apr 13 04:26:26 CEST 2021


I guess this is a good time to mention I customized the app to look at 
the Xpander 3 pole modes:

https://expeditionelectronics.com/Diy/Polemixing/xpander

I kept the shifted pole in the analysis rather than just thinking of it 
as a buffer.  You can see the effect at higher frequencies on the HP 
modes, but you'd have to have the cutoff pretty low to hear it I think.  
I only included the 3 pole modes in the drop down as that was what I was 
most interested in.  The 4 pole modes from the Xpander can be simulated 
in the main app.

Dave

On 4/12/21 9:49 PM, Andrew Simper wrote:
> 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 
> <http://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 <mailto: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
>     <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
>     <https://expeditionelectronics.com/Diy/Polemixing>
>
>     Cheers.
>
>     --
>     David Moylan
>     Expedition Electronics
>     sonic adventures!
>
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-- 
David Moylan
Expedition Electronics
sonic adventures!

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