[sdiy] Different way of controlling resonance in a State Variable Filter

[Mattias Rickardsson]

Great topic to bring up! :-)

I'm using it in a couple of different configurations:

1) In a "parametric mid"-style EQ circuit with 2 VCAs in the filter
stages & non-VCA constant feedback. The VCAs get their CVs from a uC
and the freq/Q control math is done digitally.

2) In another resonant circuit, probably in BPF configuration IIRC,
where the peak needs to be sweepable over a modest frequency range but
the rest of the filter shape is not extremely important. Here I
actually use just *one* VCA, in one of the filter stages, leaving the
other stage constant. This results in variations in both amplitude and
Q when the filter frequency is swept, but this side effect even suited
this particular application so well that it wasn't a problem. By
choosing which filter stage to put the VCA in, you can get amplitude/Q
variations in either direction.

Limitations of these configurations when used in a regular multimode
VCF must be that it's hard to span the full expected frequency and
resonance ranges with high signal quality. The VCAs need a very wide
control range -- they need to make the full frequency range PLUS the
full Q range of the filter. Say, if you want 12 octaves of frequency
span and need 4 octaves of Q gain control, you would need VCAs with a
16 octave control range.

In case 1) the F & Q ranges were moderate enough for the solution to
work. In case 2) they were even smaller. All in all, very minimalistic
and cheap designs for these applications. If they were more regular
VCF cases, I'd have to do a lot more research and comparisons in order
to ensure proper performance compared to the 3-VCA approach.

/mr


On 17 April 2017 at 14:31, Tom Wiltshire <tom at electricdruid.net> wrote:
> Hi All,
>
> I came across the following interesting ap note form THAT Corp:
>
>         http://www.thatcorp.com/datashts/AES13-031_Digitally_Controllable_Audio_Filters.pdf
>
> Despite the title, it's mostly VCA applications (as might be expected) so it could just as well have been called "Voltage Controllable Audio Filters".
>
> Anyway, there's a particularly interesting circuit in Figure 10, at the bottom of the page numbered 201.
>
> I've usually seen SVFs with a separate VCA to control the resonance, sometimes in the path from the LP output (Roland System 100M, I think) but more commonly in the path from the BP output. This one dispenses with that and uses the Resonance CV to push the two integrators frequencies apart. This reduces the resonance in the circuit, I'm guessing.
>
>  Seems like a pretty good idea for various reasons, not least of which is that it saves a VCA (or swaps a VCA for an op-amp anyway). I'd be interested to know how the circuit affects the resonant behaviour of an SVF and if this method of control makes it more stable across the range (athough I don't see on the face of it why it should). If you had two DAC channels controlling the VCAs, you could do the Freq CV/Res CV maths in the digital domain and just output the voltages you need, saving even more parts.
>
> Has anyone tried this? Has anyone seen it used anywhere?
>
> Thanks,
> Tom
>
>
>
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