[A6] A hidden filter configuration :)

[Boele Gerkes]

Did I already say: "welcome to this list"???

:-)))))

Thanks, that's good thinking!

Boele

Op 4-dec-04 om 21:05 heeft Tim Kleinert het volgende geschreven:

> 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
>
>
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