[sdiy] 2 Pole SVF manipulations

[Andrew Simper]

Hi Justin,

Please note that the Chamberlin SVF places the z^-1 in the wrong
place, you really should use this structure instead which places the
z^-1 in the correct place:

http://www.earlevel.com/main/2003/03/02/the-digital-state-variable-filter/

Once the delay is in the right place you can mix the LP HP BP outputs
just like the analog SVF, but it isn't stable for all cutoff and
resonance settings. If you want to get into higher orders the
structure you want is actually called a Leapfrog filter, an SVF is a
particular case of this more general structure. For instance for a 4
pole leapfrog design you can get simultaneous 4 pole high pass and 4
pole low pass at once, which is particularly useful for crossover
applications. You can generate any shapes you want like this, you can
even get 1 pole responses, bell EQ shapes, and shelving EQ shapes, in
this way you can smoothly blend from say a low pass filter, through
high pass shelving filter, to allpass, to low shelf to high pass, any
transition is possible.

Another filter structure is a cascade, of which a Moog Ladder is a
particular implementation of. In the Oberheim Xpander they sum they
take a sum of the different output taps and treat them much like an
FIR to generate different frequency responses, which is conceptually a
very different approach to that of a Leapfrog in which you have more
than one type of response already to sum instead of different slopes
of low pass only like the cascade.

All the best,

Andy


On 28 February 2013 05:07, Justin Owen <juzowen at gmail.com> wrote:
> http://www.sdiy.org/juz/svf_poles.pdf
>
> Part 1 shows a simplified block/signal-flow diagram of the 2 Pole SVF that so many people have worked on over the years.
>
> I was wondering if anyone could answer (or speculate) what would happen at Outputs A through E for variations 2, 3 and 4 with regards to Filter Type (HP, LP etc), Phase and Attenuation/Slope?
>
> My apologies if that's a vast (or an utterly ridiculous) question!
>
> Thanks in advance,
>
> Justin
>
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