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

[Tom Wiltshire]

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