# FM of Tri-Square VCO

gstopp at fibermux.com gstopp at fibermux.com
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
frequency.

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
well.

- Gene
gstopp at fibermux.com

______________________________ Reply Separator _________________________________
Subject: FM of Tri-Square VCO
Author:  Christopher List <Christopher_List at sonymusic.com> 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,
CList

```