[Stk] STK Filters + Third Party Classes

[Ross Clement]

On 6/23/08, Julius Smith <jos at ccrma.stanford.edu> wrote:
> Hi Perry,
>
> Attached are MoogVCF.cpp and MoogVCF.h written by Nick Porcaro based on
> input from Tim Stilson for the RealSimple project. We also plan to submit
> other works from that project for STK consideration (just need time to
> figure out what makes sense to submit).
>
> Cheers,
> Julius

Is there any further documentation on the MoogVCF filter class? I've
tried using it, but cannot seem to work out what values I should give
for "frequency" and "gain".

Looking at the code, it looks like you have four one-pole filters (as
expected), seemingly fitting the approximate structure:

y_i = freq * x_i + (1.0-freq) * y_i-1

with additional steps seemingly involved with clipping and clamping
values in the ladder. This would suggest that the "freq" value should
be between 0 and 1. But adjusting the parameters between these values
doesn't change the sound of the output. I even tried setting the freq
and gain arguments sent to MoogVCF::tick to be zero, and still I got
something out which sounded like the input. However, using the
FormSwep filter gave me something exactly as I would expect from the
FormSwep::setStates() arguments I used. ??????????

I've written the following program to test the filter, using
unmodified MoogVCF.cpp and MoogVCF.h as posted by Professor Smith.
I've been trying to work out how to use the class by editing and
recompiling. Unfortunately, I'm still in the dark.

Could anyone please tell me where I can find more information so that
I can use the class properly?

Thanks in anticipation,

Ross Clement
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