[sdiy] FatMan Hacking ( and I dont mean hitting me with anAxe!)

[Don Tillman]

   Date: Sun, 28 Oct 2001 01:19:19 -0500
   From: harry <harrybissell at prodigy.net>

   BTW a SVF can be configured to have constant peak response by
   putting the input into the Q feedback, rather than the normal
   point.

   So why can't you do the same with the Moog Ladder ???

   Can any "mathy" types put this in "non-guru" form for us ?????

Oh Harry, you know the answer to this one...

The SVF (that's a State Variable Filter, for anybody who's asking) is
a topology that requires more attenuation in a local negative feedback
path to increase the Q, while the ladder filter adds negative feedback
(which happens to be postive at the resonant frequency) to increase
the Q.  So the basic workings are different.

The SVF normally has constant passband gain.  To get constant resonant
frequency gain you just insert the signal before the attenuator
instead of after it.  Cool, easy.

The ladder filter normally has unity resonant frequency gain.  To get
unity passband gain out of a ladder filter you need to do something
like summing the normal output with the signal after the Q attenuator.

Hmmm, I've never heard of anybody do this.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com