butter and pole locations

[gstopp at fibermux.com]

     Hey there's something about that in an old Electronotes (isn't there 
     always?) as far as going into heavy detail about pole migration in 
     various filter configurations as feedback is added. Many graphs and 
     equations etc. Also there's a schematic for a "high-ripple" VCF which 
     I seem to remember is a Chebyshev but I'm not sure...
     
     - Gene


______________________________ Reply Separator _________________________________
Subject: butter and pole locations
Author:  Haible_Juergen#Tel2743 <HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de>
at ccrelayout
Date:    5/20/96 5:39 PM
     
The butterworth stuff is interesting. But as far as I know, 
there wasn't any commercial synth with butterworth 4pole
filter. Most 4pole filters start from real, negative poles (without 
resonance; with feedback applied they wander apart in an "X"- 
shape.) In case of the Moog, Oberheim 4p, various ARP
4p, Prophet 4p (both old and new), and many others, you start with 
4 identical pole locations. The VCS3 and TB303 have different
start values for their pole locations (but still all of them are real 
and negative, without feedback applied).
(BTW, has anybody yet calculated the exact locations / pole spread 
for the diode ladders?)
The Roland cascaded SV 4p-filters might be different ... anybody 
knows if *they* start on an arc (as in a butterworth)?