[sdiy] SVFs with different gains in the integrators

Tom Bugs admin at bugbrand.co.uk
Sun Dec 13 19:08:08 CET 2020

I think unrelated but there's an interestingly different approach in a 
THAT app note which I always meant to try..
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 ;)


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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