[sdiy] Pole Mixing

[Richie Burnett]

Taking the feedback from after the 4th pole makes all of the responses 
become 4th order when feedback is applied.  So even something otherwise 
simple like a 1-pole low-pass filter can display resonance when feedback is 
applied in this way.

-Richie,


-----Original Message----- 
From: David Moylan via Synth-diy
Sent: Friday, April 9, 2021 2:19 PM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] Pole Mixing

Thanks, Tom.   In practice, I do find quite a few interesting.  LP1 +
Notch sounds great.  And just being able to switch between LP4, LP2, LP1
is also useful.  I don't get a lot of mileage out of the HP only modes,
usually want some LP too, but that makes HP3+LP1 mode interesting.
Though I agree, in the universe of all possible transfer functions the
list is relatively small.  It also begs the question "how many of these
could be approximated with 2 state variables in series?".   But that
ignores the feedback paths which I haven't worked into the equations
yet.  In the Xpander the feedback is always from the LP4 tap like it
would be for standard low pass filter and I think I remember someone,
maybe David Dixon, pointing out that it has interesting effects on the
curves as resonance is increased.

If you click on the slider in question you should be able to use arrow
keys to step in .1 increments.  I thought that was fine enough to get
close to some desired curve, after that get out the pencil and paper or
a soldering iron!  All of the presets except the 20db LP shelf use
integers anyway, seems almost necessary to get the cancellations
required to make interesting curves.

On 4/9/21 5:34 AM, Tom Wiltshire wrote:
> Absolutely agree, that is a fantastic piece of work.
>
> It makes all sorts of things about pole-mixing more obvious, including how 
> few of the combinations are actually interesting, and as Richie said, how 
> sensitive some of the combinations are.
>
> If you’re still working on it, would it be possible to add boxes to type 
> in the coefficients as an alternative to the sliders? Getting specific 
> values with the little sliders is quite fiddly.
>
> Thanks very much for this though - really great.
>
> Tom
>
> ==================
>         Electric Druid
> Synth & Stompbox DIY
> ==================
>
>
>
>> On 8 Apr 2021, at 23:24, 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!
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at synth-diy.org
>> http://synth-diy.org/mailman/listinfo/synth-diy
>> Selling or trading? Use marketplace at synth-diy.org
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org


-- 
David Moylan
Expedition Electronics
sonic adventures!

_______________________________________________
Synth-diy mailing list
Synth-diy at synth-diy.org
http://synth-diy.org/mailman/listinfo/synth-diy
Selling or trading? Use marketplace at synth-diy.org 


-- 
This email has been checked for viruses by AVG.
https://www.avg.com