Investigating FM on Neoni VCO in VCV Rack – “Traditional” mode is through zero!

I assumed that most oscillators that support true linear frequency modulation used a constant Hz/volt modulation coefficient, as does the version 2 VCV Fundamental VCO. The VCV VCO does not support through zero.

But I was surprised to learn that Neoni scales the modulation coefficient to the base frequency of the Neoni carrier wave. And beyond that, the “Traditional” mode is also through zero, just not centered at 0 volts.

The formulas for the Neoni output frequency after the FM is applied are as follows (approximately):

Through Zero Freq = |(Volts+2.1)*Base/2.1|
“Traditional” Freq = |(Volts)*Base/1.08|

where Volts is the current attenuated voltage at the FM input, and Base is the base frequency of the Neoni carrier wave. The vertical bars represent absolute value.

Based on those formulas, I’ve discovered that I can get identical results using either mode if I tune the traditional mode Neoni base frequency 1 octave higher (+1 volt), and offset the traditional mode modulation signal by -2.1 volts. Of course the traditional mode must use DC coupled mode in order for the offset to have any effect.

The accompanying video demonstrates some differences between the VCV VCO and Neoni, shows how I derived the formulas, and demonstrates the patch that shows the equivalency between the Neoni Through Zero and “Traditional” modes.

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  • Modified: 5 months ago
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