[sdiy] Patching thru-zero oscillators from regular VCOs

[Grant Richter]

There has been a lot of talk recently about "thru-zero" oscillators.

These oscillators produce both positive and negative frequencies. In  
practical terms a negative frequency is simply 180 degrees out of  
phase relative to a positive frequency.

Some people think that linear FM produced with a thru-zero oscillator  
sounds better than regular FM.

If you want to test this for yourself, you can patch a thru-zero  
oscillator on almost any wide range modular.

The technique is quite simple, you need two sine wave VCOs and a ring  
or balanced modulator. One of the VCOs will need a linear FM input.

Route each VCO to the ring mod inputs (AC coupled). Set one VCO to  
fixed frequency of 20kHz. Set the other VCO so a control voltage  
modulates it from 15 kHz to 25 kHz. The "sum" sideband from the ring  
mod output will be from 35 kHz to 45 kHz i.e. ultrasonic and inaudible.

The "difference" frequency will go from 5 kHz thru zero Hz and back  
up to 5 kHz. If you adjust the initial frequency properly, and adjust  
the linear FM modulation depth so it sweeps the frequency thru the  
zero point, you will be listening to thru-zero linear FM.

Of course this is simple and will not replace a dedicated module like  
Cynthia's "Zeroscillator".

The most useful application of a thru-zero oscillator that I have  
found is for quadraphonic panning. For this application, the thru- 
zero core needs to produce quadrature trapezoid waveforms. The thru- 
zero feature allows you to do "sound spinning" with smooth  
transitions from clockwise to counter-clockwise rotation (slow down  
and reverse direction, a good and useful thing).

Ian Fritz developed a quadrature trapezoid VCO core which is  
perfection for that application (but only one of many uses both audio  
and CV).
Google was unable to find the current link or I would post it.