[sdiy] VCF model/passband gain compensation

[Martin Fay]

Hi Scott,

I suspect Antti may pick this up, in his Moog filter model a simple FIR 
is used to introduce a ~1/2 sample delay in the feedback loop for phase 
correction. I've also successfully used an allpass filter in this role.

output = (thisInput + lastInput) * 0.5

(iirc from the top of my head)


Martin
> I'm working on a software model of a filter that I plan to build at
> some point- the idea is to make a transistor ladder filter that can
> output different filter types (HP, LP, BP, whatever) by combining the
> outputs of multiple filter stages (as in the Oberheim Matrix 12,
> Doepher A-107, Siel Mono..).  The point of the software model is to
> first see how crazy I can reasonably make it (8 poles, multiple
> feedback loops, audio injected at different stages?), and also to see
> if I can get away with just using potentiometers to mix the signals
> rather than switching between a series of pre-mixed filter types.
>
>
>
>
> Since it's just a proof of concept, the model doesn't need to be
> perfect, but only accurate enough to judge if it's a worthwhile
> project.  I quickly put something together in Pure Data, and it more or
> less does what it's supposed to do, but there's apparently a small
> amount of latency in the feedback loop (between tabsend~ and
> tabreceive~?) that results in decreased resonance at higher cutoff
> frequencies.  I can "sort of" work around this by increasing the sample
> rate, but I'd rather fix it so I can more accurately judge what it's
> doing.  I know Pure Data really isn't meant for this sort of thing
> (unless there's some specific way to do this that I'm not aware of).  I
> can put something together in Matlab/Octave (would prefer Simulink but
> I don't have it), but I would like to have something that I can hear
> and adjust in realtime.  Is there better software to use for a sort of
> "realtime IIR sandbox"?
>
>
>
>
> One thing that quickly became evident in playing with this is that the
> filter needs some sort of passband gain compensation, otherwise the
> coefficients need to be re-adjusted when the resonance level is
> changed.  It seems relatively common in SSM/CEM VCF chips, but I
> haven't seen a schematic for it and don't really know what the "trick"
> is.  Does it make the passband gain really constant, or does it only
> vary less?
>
>
>
>
> Sorry if this is too wordy and vague, but I think I'm verging on pretty
> interesting territory.  I'm surprised that something so seemingly
> exotic is actually pretty simple.
>
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