[sdiy] Stereo VCOs and headphone phase experiments

[Grant Richter]

This has to do with experiments regarding variable phase of two  
fundamental waveforms monitored in stereo.

My question was: How does the ear react to changes in the phase of  
the two fundamental waveforms routed independently to each ear?

I did these experiments to see if a stereo VCO would be a useful  
patch. You can patch this up on most well equipped modulars, but you  
might need to build or modify a comparator circuit. A quadrature  
oscillator does the same thing with a fixed offset of 90 degrees.

There is an old and relatively easy way to create a stable but  
variable phase relationship between two VCOs.

This was invented or a least popularized by Ron Rivera, called "phase  
modulated synch". He used to refit it to ARP 2600s and there was an  
article on it in "Polyphony" magazine. It was also described in  
"Electronotes" magazine.

The patch requires two sawtooth core VCOs with hard synch. Place a  
comparator on the sawtooth output of the "Master" oscillator. By  
varying the comparator reference voltage, you control the point in  
the master oscillator cycle where the comparator switches, which is  
the reset signal for the slave oscillator. Buffer the comparator  
output as needed to drive the hard synch input of the "slave"  
oscillator (perhaps an edge triggered monostable). This allows you to  
voltage control the phase relationship between the master and slave  
VCOs for all output waveforms derived from the sawtooth waveform.

Route the master VCO waveform to one stereo channel in a pair of  
headphones (or speakers). Route the phase modulated waveform to the  
other stereo channel. Start in-phase and adjust to an out of phase  
condition.

My observation was that the ear/brain doesn't react to a phase  
difference until you get to greater than aprox. 10 degrees. At that  
point, the 2-D vertical "wall" of the in-phase waveforms turns into a  
3-D horizontal "plane" which feels like a stereo space that has  
depth. I experimented to see if adjusting the phase further produced  
any effect, but to my ear it did not produce anything dramatic,  
except possibly varying the "size" of the stereo illusion.

I thought I would pass this along as an interesting experiment and  
maybe a useful patch. Further experiments could involve dynamically  
varying the phase using LFOs or envelopes. Mixing the two signals and  
dynamically varying the phase will probably produce phase  
cancellations in the harmonics and sound like a comb filter???

Hope you have fun!

(BTW the basic idea is cited in various forms by many US patents)