[sdiy] SVFs with different gains in the integrators

[Dakota Melin]

>
> I think unrelated but there's an interestingly different approach in a
> THAT app note which I always meant to try..
>
> http://www.thatcorp.com/datashts/AES13-031_Digitally_Controllable_Audio_Filters.pdf
>

To the contrary. That paper is related. DEEPLY related.
It's what I read that got me talking about the idea with Guy!

It's also reference by L1 in some of his designs... the EQ and also maybe
his earlier SVF that only used 2 VCAs?

searching through muffwiggler as we speak...

-dk

On Sun, Dec 13, 2020 at 1:11 PM Tom Bugs <admin at bugbrand.co.uk> wrote:

> I think unrelated but there's an interestingly different approach in a
> THAT app note which I always meant to try..
>
> http://www.thatcorp.com/datashts/AES13-031_Digitally_Controllable_Audio_Filters.pdf
> Figure 10 - VCA Controlled SVFilter - where the expo summing also deals
> with Q control.
> Haven't tried to get my head around it but thought I'd throw it in the
> ring/bucket ;)
>
> Tom
>
> On 12/12/2020 10:09, Guy McCusker wrote:
> > In Chris McDowell's recent thread I made an off-hand remark about
> > setting up an SVF with different gains in the integrators. I don't
> > know how well-known this is but I'm wondering what list members know
> > about the history and use of this idea in synthesizers.
> >
> > The theory, if I have it right, is that with different integrator
> > gains, the natural frequency is given by the geometric mean of the
> > unity gain frequencies, and the Q is enhanced by something like the
> > square root of the ratio of the gains. So you can vary Q without
> > varying the bandpass feedback.
> >
> > The only use of this that I know about in synthesizers is the Serge
> > Variable Slope filter (VCFS). The claimed varying slope is really
> > varying the Q, so that the slope near the natural frequency changes;
> > the asymptotic slope is still 12dB/Oct. Are there any other examples?
> > Does anyone know any more of the history of this idea?
> >
> > Incidentally, thinking about this always makes me smile at the
> > marketing smarts of Serge in the 1970s. He marketed three filters:
> > variable Q filter, variable slope filter, and variable bandwidth
> > filter. Since Q and bandwidth are the same thing (one is the
> > reciprocal of the other), and since the variable slope filter is
> > actually varying the Q, all three of these are in fact variable
> > bandwidth filters... but he managed to distinguish them by calling it
> > three different things. Smart!
> >
> > Guy.
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