[sdiy] 4-pole SVF theory

[Magnus Danielson]

On 07/08/11 00:39, karl dalen wrote:
>> David G Dixon<dixon at mail.ubc.ca>
>
>>>> Pleez, help me edukate meself!
>>> First, learn to spell!
>
>> So, I guess that course in "Internet Etiquette"
>> at the local Kom Vux was full, eh?  :)
>
> Yes, you and Neil had taken the two last seats. :)
>
>> You keep creating feedback gain matching the pole polynomial
>> you want to create.You keep creating feedforward gain matching
>> the zero polynomial you want to create.
>
>> The booring detail is this... for variable Q setups the polynomials can
>> get messy, unless you try to create a simple polynomical matching that
>> of a normal lowpass-filter with a feedback for Q. So adapting to
>> "musical" filters needs a bit of care.
>
>> State-variables are neat for the theorists, as it allows for simple
>> mapping of polynomials to filter synthesis. This does not necesserilly
>> make it neat for musical use.
>> Cheers, Magnus.
>
> Amen to that, and that's why Neils 3pole sounds so lame Mr Dixon. ;)

The naive approach will get pretty lame results, but ensuring the poles 
in the right places and not completely wrong zeroes, a higher degree 
state variable should be musically usefull too, it's just that you need 
to twist and turn in differently to other filters to get there.

Getting something similar to the Moog filter response (not the 
non-linearity part of it) should not be too difficult.

Cheers,
Magnus