[sdiy] SVFs with different gains in the integrators

[Dakota Melin]

> True.  But I'm not sure that's an advantage.
>
> One of the big features of the State Variable Filter is that the frequency
> and Q are independent.  Compared to, say, varying the component values on a
> Sallen Key filter.  And if you want to make them dependent, it's a simple
> matter to patch it in.
>
> Also note that the level of the bandpass output changes.
>

F and Q can still be controlled independently in an offset style VCF. Q is
offsetting the poles away from each other and F is moving them together. A
couple opamp stages set up appropriately accomplishes that. The problem
remains that your VCA or OTA cells have a limited range.

I'm not sure that's true.  Do we have schematics or anything to check?
>

I don't have Serge's permission to post schematics so I won't but... that's
what it does. I started messing around with the offset idea after talking
with Guy some months back and Serge popped up immediately with a "looks
familiar". Simulations of the original circuit afterwards confirmed it.

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!


I think I agree with Guy here, if you look at the later Serge filters they
are all the same core blackmer cell / integrator SVF. But then he figured
out a bunch of ways to configure them that were all attractive!
The VCFS offsets the integrators to create slope / Q changes near the
cuttoff. The asymptote of the cutoff doesn't change but the area of
interest near the cutoff sure SOUNDs like it changes!
The VCFQ uses some tricks from Electro Music and Serge's brain together to
make a high-Q, musical filter that excels at pings with a specialized
triggering circuit.
The VCF2 or variable bandpass uses 2 SVFs in series with the second one fed
by the LP of the first one to give a variable bandpass BETWEEN the two
filters.
Serge also experimented with a Triple Array that used 3 SVFs with each HP
output feeding the input of the next one... I've played around with this a
bit and it's a bit nutty, like a voltage controlled crossover!
One of those prototypes ended up in Toronto near me and it appears to have
used the earlier CA3080 based SVF, however.

-dk

On Sun, Dec 13, 2020 at 12:35 PM Donald Tillman <don at till.com> wrote:

> On Dec 12, 2020, at 2:09 AM, Guy McCusker <guy.mccusker at gmail.com> wrote:
>
> 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.
>
>
> True.  But I'm not sure that's an advantage.
>
> One of the big features of the State Variable Filter is that the frequency
> and Q are independent.  Compared to, say, varying the component values on a
> Sallen Key filter.  And if you want to make them dependent, it's a simple
> matter to patch it in.
>
> Also note that the level of the bandpass output changes.
>
> 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.
>
>
> I'm not sure that's true.  Do we have schematics or anything to check?
>
> 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!
>
>
> I'm not sure that's true, either.  It looks to me like Serge was seriously
> exploring new filter behaviors specifically for their ability to expand
> musical expression, each with different kinds of parameters to vary.
>
> So repurposing a Q parameter is the exact opposite of that.
>
> But I'd love to see the schematics to know for sure.
>
>   -- Don
> --
> Donald Tillman, Palo Alto, California
> http://www.till.com
>
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