[sdiy] Patching thru-zero oscillators from regular VCOs

[Grant Richter]

Ooooops!

It was Don Tillman who did the very cool thru-zero trapezoid core.

Sorry Don! Sorry Ian!

Also, it was pointed out to me that most West Coast VCOs do not have  
sine wave outputs.

In that case you can use filters in self oscillation mode to get  
reasonably pure sine waves.


On Jan 8, 2008, at 7:26 PM, Grant Richter wrote:

> 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.
>
>
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