Through-Zero FM

[jhaible]

Hi Dan, you may be right in theory, and I have long considered this method
when I designed my Frequency Shifter.

The main problem for VCO applications is that you don't have V/Oct tracking
anymore ! In your example you would need a variable VCO that goes from
5100 Hz to 5200 Hz with a 1V CV step, inorder to have the output changing
from 100 Hz to 200 Hz. 
Our ordinary exponential VCOs would do something very different; they would
go from 5100 to 10200 Hz with a 1V CV step.

This would not be a problem with V/Hz VCOs of course. And one could 
construct a "hybrid" VCO that could do the trick, too. (Offset current ...)
But you'd have problems with ordinary V/Oct VCOs.

JH.


>     I think that you can get through zero FM with conventional FM
oscillators by a
> relatively simple patching trick.  Setup a standard FM patch with a
fairly high
> center frequency (5 KHz for example). Make sure that non of the sidebands
are
> folded over by having a too large modulation index. Then ring modulate
this signal
> with a 5 KHz carrier. You now get 2 sidebands, one centered at 10 KHz and
the
> other at 0 Hz. Use a low pass filter to separate the lower sideband only.
The
> lower sideband has the through zero FM output. You can vary either the FM
center
> frequency or ring modulator carrier to shift the through zero FM signal
center
> frequency away from zero Hertz.
> 
>     I believe that that this approach provides results identical to a
through zero
> FM oscillator. The advantage is that you can do this on almost any
modular
> synthesizer. Some analog frequency shifters such as the Bode / Moog
include a
> through zero frequency shift capability. The through zero shift is either
obtained
> by a quadrature oscillator with a through zero shift (Serge) or by a
through zero
> hetrodyning approach (Bode). The patch outlined in the first paragraph is
this
> latter method.
> 
> Dan Slater
> 
> 
> 
> WeAreAs1 at aol.com wrote:
> 
> > Hello DIY listmates,
> >
> > I have read the term "through-zero FM" several times, in previous posts
found
> > here.  Usually with the writer referring to it in glowing terms lavish
with
> > praise.  I, too, now wish to wallow 'n' revel in the wholesome goodness
of
> > through-zero FM.   So, could someone please be so kind to explain the
concept
> > to me?  (and to the rest of the list, of course!)  Also, what are some
musical
> > applications and sonic characteristics of through-zero FM?
> >
> > Very best regards,
> > Michael Bacich
> 
> 
> 
>