[sdiy] standard filters from ladder topology?

[Don Tillman]

   > Date: Sun, 04 Sep 2005 12:33:32 -0400
   > From: harrybissell <harrybissell at prodigy.net>
   > 
   > I could see the ebenfit to making a voltage tuned filter with
   > specific response... but I have some doubts as to how it would
   > sound in a synth application. I'd expect that the difference
   > would be subtle as best ???

Maybe; maybe not.  It's probably worth investigating anyway.

I can imagine a VCF with a new front panel "Mode" switch with
positions:
  1. Moog Ladder 
  2. Bessel
  3. Butterworth
  4. Chebyshev .5dB
  5. Chebyshev 3dB

The regular "Emphasis" knob only applies to position 1.

The Bessel filter is a very smooth response because it's optimized for
flat group delay, yet it is more resonant than the Moog Ladder (at the
minimal "Emphasis" setting) because the Moog Ladder poles are real
while the Bessel poles are split.

The Butterworth is more resonant; the Chebyshevs even more so.

So this is a way to have a significantly sharper filter response than
the Moog Ladder, but without the big peak in response.  That could be
really interesting; at least to be able to compare them side by side.

Also, the overdrive characteristics of these variants would be
different because of the feedback paths.  

Additionally, I don't think there's a VCF available with 4-pole
Bessel, Butterworth or Chebyshev response.  Is there? 

  -- Don

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