SV: Re: SV: [sdiy] QVCO schemo up

[Don Tillman]

JH, Ian, SDIYers,

   > From: "JH." <jhaible at debitel.net>
   > Date: Sat, 12 Nov 2005 16:04:55 +0100
   > 
   > > infact this cirquit is nothing more then a variation of the
   > > SVF!
   > 
   > Sure - what else should it be?

Maybe it should be an actual Hopf Bifurcation device?

Ian, would you say that your approximation of the original equations
has changed the circuit from being an actual Hopf Bifurcation device
to a State Variable Filter oscillator?  

Why do a Hofp Bifurcation device at all?  Maybe it would be musically
useful to disturb it from its oscillating state.  Any thoughts?

   > It's a 2nd order differential equation in the most direct
   > realisation.  If it's dampded, it's a filter. If it's not, it's
   > an oscillator. The art lies in keeping it _just_ so undamped,
   > that it oscillates without increasing amplitude.

Right.  There's a large class of circuits made from filters and
feedback that will oscillate when you crank up the resonance.  That
includes the Moog Ladder filter and State Variable Filters.

It also includes the Wein Bridge oscillator in the famous original
Hewlett-Packard audio oscillator, which used the heating of a light
bulb filament to adjust the feedback and stabilize the amplitude.  And
the phase-shift oscillator used in so many guitar amp tremelo
circuits.  And twin-tee based oscillators.  And many more.

So, the State Variable Filter oscillator is well known.  The question
is, does Ian's variation of the stabilizer with the zeners provide
better (or otherwise more musical) performance?  I dunno.

And...

One of the problems with filter-feedback oscillators is that they take
a while to stabilize.  So, for instance, I would be interested in
seing how Ian's circuit reacts to a large sudden step change in
frequency.

  -- Don

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