ASM1 filter self oscillation

[gstopp at fibermux.com]

     State-variable filters are notoriously unstable in the high-Q regions. 
     I'm not exactly sure what the engineering reasons for this are, but it 
     may have something to do with the fact that the state-variable goes 
     into a higher-Q condition when feedback is *reduced*, but the 4-pole 
     (cascaded integrator) filter goes into a higher-Q condition when 
     feedback is *increased*.
     
     The fact that the state-variable needs more feedback for less Q means 
     that you can add a non-linear circuit that bypasses the Q pot, so that 
     if the level of the bandpass output exceeds a certain level, then more 
     feedback gets added, thereby reducing the Q a little. That's what's in 
     the ASM-1 filter, copied straight out of the Oberheim SEM schematic. 
     Some component value experimentation in that little circuit may be 
     worthwhile.
     
     Since the 4-pole filter needs more feedback to self-oscillate, its Q 
     pot acts like a feedback control on a tape echo. In other words, if 
     the feedback is a tiny bit less than unity, the oscillation will 
     eventually die away, but if the feedback is slightly more than unity, 
     the oscillation will start to "run away". Unlike tape echo (which will 
     deteriorate into VU-meter-crunching distorted echos), the 4-pole 
     filter will tend to distort in the first integrator (the first pole), 
     but the next three stages will round this off to a nice sine shape and 
     the oscillation will maintain itself as a well-containted sine wave. 
     Therefore you can set the resonance pot to a setting slightly over 
     unity gain and have a stable sine wave oscillator.
     
     In the state variable filter, however, the filter's natural mode of 
     operation (without any feedback) is crazy distorted self-oscillation. 
     Feedback is used to "tone down" this out-of-control behavior, but 
     right around the point of nice sine-waviness the Q pot is at such a 
     low setting that the feedback signal is real tiny, and may be 
     influenced by noise factors. I'm guessing here. I think that there are 
     also some phase-shift versus frequency issues at work here as well - 
     the ASM has some phase compensation capacitors at the 3080 inputs to 
     combat these effects (per Electronotes) but alas it's really just a 
     band-aid rather than a cure.
     
     An analogy could be to compare the state-variable filter to those new 
     jet figher prototypes that are inherently unstable but use a computer 
     fly-by-wire system to keep them from flipping out of control at any 
     moment. The performance is very twitchy and quick, but within safe 
     margins if the proper feedback is kept. As the feedback is reduced, 
     the system becomes unstable.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: Re: ASM1 filter self oscillation
Author:  Troy Sheets <tsheets at xanadu.cyborganic.com> at ccrelayout
Date:    12/19/96 10:27 AM


Well, without any special mods, my ASM1 VCF does self oscillate, but it 
is an interesting, "raspy" self oscillation.  It sounds completely 
different than the 4-pole self-oscillation on my Odyssey, which I would 
suspect is a pretty pure sine wave. 
     
The ASM-1 self-ocillation is a real sound though, and it is stable and 
tunalbe to a keyboard CV.
     
> 
> I have been experimenting with a VC Q circuit for my ASM1 VCF I found in 
> Chamberlin's Musical Applications of Microprocessors.  I think this is
> right out of Electronotes.  One of my goals is to get the filter to self 
> oscillate at a high Q setting with no signal input.  I realize that there 
> is a Q limiter in the feedback path.  So if I take it out, does anybody
> know if a state variable filter can self oscillate in a controllable manner 
> like its' 4 pole LPF conterparts?  Or is there something about this circuit 
> that makes this impossible?