What is cross modulation?
Have a look at the simplified diagram below. We have two Oscillators, and two Amplifiers. Normally with an “FM” VCO we would phase modulate (PM) first oscillator with the audio output of second oscillator, so far nothing out of the ordinary.
If you look closely at the diagram the Oscillators are arranged so that Oscillator 2 phase modulates Oscillator 1, but here comes the “twist”, we now take the output of Oscillator 1 and phase modulate oscillator 2 with this signal.
If this seems a little confusing, I have highlighted each feedback loop below. The loop from Osc 1 to Osc 2 is in Red, and the loop from Osc 2 to Osc 1 is in green.
A you can imagine this opens up a whole new world of sonic possibilities, from musical to abstract noises more like distorted noise.
The only issue here is (and this is why I chose red for the loop from Osc 1 to Osc 2) that Synthedit will see this as a feedback loop and complain. This means we can never get a 100% accurate emulation of Cross Mod in Synthedit. The Feedback loop needs a special module which introduces a very short delay, in this usage I measured about 2mS.
A Cross-Modulation VCO in SynthEdit.
The main thing to note here is the Feedback-Volts module in the PM loop between the output of Oscilator 1 (top) and Oscillator 2 (bottom). This does mean that there is small delay in the feedback loop, so as I have previously said it’s not a truly 100% accurate feedback loop, but close enough for our purposes. The Third Level Adj module is just used as a means of switching between PM from Oscillator 2 to Oscillator 1, and full Cross Modulation between the oscillators.
The AM Level, and Mod Level Adj modules are used to add an AM modultaion facility into the oscillator.
Using the Phase Offset 2 Input we can also introduce a static phase difference between the oscillators to further increase the sounds available.
The Sync inputs are used along with the DCO setting on the oscillators so that triggering a new note starts the oscillators in the same phase relationship every-time a new note is played.
The Test Bed.
I used the setup below to test the concept. To keep the feedback under control I set the the maximum value on the push button to 3 rather than the default 10. The phase offset slider needs to be changed from it’s default 0 to 10 setting to -5 to +5.
There is no waveform selection in use (as with with all FM/PM setups it’s far better to use sine waves…anything else quickly gets out of control). You can if you want, but it’s really not worth it, the sounds of cross-modded pulse or sawtooth is at best ugly.
