[A6] A hidden filter configuration :)

[Tim Kleinert]

Hi list,

Here's something I discovered today. I cross-checked first if it already
was mentioned in the T&T document. But it doesn't seem to be, so here it
is:


1) close all post-filter levels and switch filter configuration to
serial BP.
3) open up filter1 BP output to your desired level (make sure it's NOT
inverted) and tweak its frequency to taste.
5) open up filter2 completely. Make sure its resonance is zero.
6) The important step: slowly open up filter2 output level. As you do
this, you'll hear the BP signal of filter1 progressingly get thinner and
thinner. Proceed with slowly opening until the thinning out hits a
maximum and only a subtle HF residue remains. (If you keep on turning,
the signal gets thicker again. Dial back until you find that maximum
point.)
7) Your done! Your 24db lowpass filter is now a highpass filter. You
have a true BP>HP configuration. :)

How does it work?
Filter2 seems to introduce a 180 degree phaseshift. So if you mix back
an equivalent amount of this signal to the original BP signal,
cancellation occurs, thus turning the lowpass filter into a highpass. If
you look at the post-filter levels, the F2 output will be somewhat
higher than the BP. That's because F2 causes a slight drop in level
which has to be compensated for in order to make the cancellation
effective.

You cannot use filter2s resonance, though. Resonance is negative
feedback, and will therefore directly revert this effect again. But you
can get this HP filter to resonate somewhat by using the filter
feedback.

...man, I love this synth!

Cheers,
tim/shoshin