[sdiy] Pole Mixing

[David Moylan]

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!
>>
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-- 
David Moylan
Expedition Electronics
sonic adventures!