[sdiy] VCF model/passband gain compensation

[Scott Nordlund]

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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