[sdiy] VCR - Voltage Controlled Resistor

[Magnus Danielson]

From: Scott Gravenhorst <music.maker at gte.net>
Subject: Re: [sdiy] VCR - Voltage Controlled Resistor
Date: Mon, 19 Jul 2004 14:15:55 -0700
Message-ID: <200407192115.i6JLFtZ02822 at linux6.lan>

> harrybissell <harrybissell at prodigy.net> wrote:
> >RE: LPF
> >
> >This structure is like one stage of a 2040 style filter... probably not directly
> >applicable to SVF.  Don't be fooled by the APF (pahse shifter) example.
> 
> Pardon my niavete...  I don't know what the hell I'm doing, but I try. I was
> under the (possibly mistaken) impression that a SVF is made of 2 active LPF
> blocks each with a feedback path to a summing node, signal input to the
> summing node and the node feeds the first LPF block.  I sort of tried to put
> 1.94 and 1.83 together to get 4...  
> 
> Why is this LPF block not compatible with a SVF configuration?

A state-variable filter is not built around low pass filters, it's built around
integrators. The standard (for synth cases) SVF is built from two integrators
and an input summer, you need the feedback paths to acheive the pole-pattern
control. The outputs of the summer and the integrators can then be summed to
form whatever zero-pattern you fancy.

Integrators isn't like normal low pass filters, since they ideal lack a "pass"
region with a flat gain-curve. Instead the gain doubles for every octave down
we look. This in reality goes on until parasitics and other non-ideal 
properties of the capacitor, the op-amp etc. cuts in, but that is usually way
below where we bother to check normally for audio purposes.

Doing the full math for a SVF is really very quickly done with pen and paper,
but it has been seen in ASCIImathics here in a post earlier this year I think.
Oh, I think I recall that I was the offender :O)

Cheers,
Magnus