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

[David G Dixon]

In my SVF design, I do VCQ in the normal way, using a VCA to control damping
between the BP output and the input.  Works very well (but the sine waves
have slightly variable amplitudes in self-oscillation).  With a 2164, there
are a couple of extra VCAs in that circuit anyway, so no worries.  One could
even use one for tempco and still have one left for VCQ.  Of course, unlike
in the Roland-type filters, the exponential quality of the 2164 VCA is
perfect for controlling SVF damping.

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Tom Wiltshire
> Sent: Monday, April 17, 2017 5:31 AM
> To: SYNTH DIY
> Subject: [sdiy] Different way of controlling resonance in a 
> State Variable Filter
> 
> Hi All,
> 
> I came across the following interesting ap note form THAT Corp:
> 
> 	
> http://www.thatcorp.com/datashts/AES13-031_Digitally_Controlla
ble_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
> 
> 
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy