FM of Tri-Square VCO

gstopp at gstopp at
Fri Sep 6 20:08:56 CEST 1996

     I haven't any experience with through-zero FM VCO's, so I can only 
     share what I have gleaned from reading materials.
     My understanding is that through-zero FM on a VCO is when a descending 
     CV into the VCO FM input causes the VCO to drop in frequency until it 
     hits zero hertz, and then as the CV keeps going more negative the VCO 
     starts up again in the reverse direction and begins increasing its 
     frequency of operation.
     Of course the "normal" VCOs we are probably familiar with (integrator 
     schmidt-trigger, sawtooth reset, Curtis chip, etc.) don't do this but 
     merely drop to a lower and lower frequency without ever even reaching 
     zero hertz. Therefore one may consider approximating the through-zero 
     behaviour described above by building a VCO whose minimum frequency is 
     achieved at zero volts CV sum in (rather than the minus supply) and 
     putting a full-wave rectifier on the CV input. This way a decreasing 
     CV input will cause the VCO to stop (actually just get real slow) at 
     zero volts and as it goes more negative it gets inverted by the FWR 
     and goes positive again, starting up the VCO into increasing 
     Now the important bit here is that apparently this doesn't sound quite 
     the same as a true through-zero FM effect. See a true through-zero VCO 
     will go down to zero hertz and stop dead and then back up and reverse 
     its cycle. Okay I don't feel like drawing an ASCII waveform at the 
     moment so here's a number string representing a non-modulated 
     triangle's voltage in time:
     -5 -4 -3 -2 -1 0 1 2 3 4 5 4 3 2 1 0
     and here's a "fake" through-zero FM'ed triangle:
     -5 -4 -3 -2 -1 0 1 2 2 2 2 2 2 2 3 4 5 4 3 2 1 0
                    (CV going thru zero here)
     and here's a real through-zero FM'ed triangle:
     -5 -4 -3 -2 -1 0 1 2 1 0 -1 -2 -3 -4 -5 -4 -3 -2 -1 0
                   (CV thru zero here)
     Notice how the "fake" through-zero effect just stops and then picks up 
     where it left off. From what I understand, the "back up the waveform 
     and go the other way" type of FM has a sound that is noticeably more 
     profound or harmonically interesting than using the FWR method. It may 
     seem that it is a little trivial waveform anomaly but this has a 
     drastic effect anyway - enough to justify scads of complicated 
     through-zero FM VCO designs.
     Anyway like I said I never played with this so I can't provide a 
     critique of the sonic ramifications of true through-zero VCO FM. I 
     guess I kind of skipped over the whole issue when I built a 
     through-zero frequency shifter, which I suspect can give similar 
     harmonic results, only better, because it involves live signals as 
     - Gene
     gstopp at

______________________________ Reply Separator _________________________________
Subject: FM of Tri-Square VCO
Author:  Christopher List <Christopher_List at> at ccrelayout
Date:    9/6/96 10:24 AM

Howdy DIYers - 
It was mentioned a while back that the Tri-Square VCO (of 
Electronotes/Elctronic Music Circuits) couldn't do "through zero FM". I have a 
vague notion of what this limitation implies - just from looking at the FM'ed 
waveforms on a scope, but I'm 100% sure. 
1. Could someone describe this limitation (or rather, this capablility)?
2. Does anyone know if it would be possible to modify the circuit so that it 
could do through-zero FM? 
3. How? 
- Perhaps Bernie has a suggestion?
- Thanks,

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