[sdiy] Alternative Q control in SVF (was Re: 'Reverse' connected 2164 approach for HP filter stage purpose..)

Tom Wiltshire tom at electricdruid.net
Tue Mar 11 16:28:45 CET 2014


On 11 Mar 2014, at 14:13, rburnett at richieburnett.co.uk wrote:

>> ...and also an extremely "alternative" way
>> of controlling the Q in an SVF by moving the CVs of the two
>> integrators apart. Interesting stuff.
> 
> That technique is sometimes used in SVFs for industrial control purposes.
> 
> The integrators usually have the same gains, and you just feed back differening amounts of the first integrator output and the second integrator output to tailor the cutoff and Q of the resulting closed-loop response to be what you want.  Rather than altering the mix of the states being few back, you can just alter the gains of the integrators themselves and feed back equal amounts of the two states.  It achieves the same thing in the overall closed-loop transfer function.
> 
> The only difference with controlling the Q by altering the integrator gains independently is that it buggers up the level matching between the LP, BP and HP outputs if you are going to switch between them in an audio application.

It saves you a VCA for controlling the resonance, but aside from that, I don't see much advantage.

Do all the output levels alter, or just the ones with integrators?

If the HP output remains at the same level, then perhaps it has some uses.

Thanks,
Tom


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