“Hello everybody. This is my first simple question. I’d like to understand exactly what the bipolar FM pot does in a VCF or VCO. It is dedicated to FM modulation. Okay! Let’s say I have a VCV Rack VCO set to 261.63 Hz (middle C). If I rotate the pot fully clockwise (100%), does it mean the VCO frequency will increase from 261.63 Hz up to its maximum value (5919.9 Hz) and then back down? On the opposite side: if I turn the knob fully counterclockwise, does it mean the VCO frequency starts at 261.63 Hz and goes down to its minimum value (11.562 Hz) and then back up? In other words: does the percentage of rotation of the pot represent the fraction of the frequency interval from the VCO frequency setting to its maximum (or minimum) value? Thanks!”
Hello and welcome!
First of all, the FM knob does nothing on its own. It acts on the signal you’ve patched in to the FM input. If you turn it all the way up, the FM signal will be applied at full strength, all the way down is the same but with the polarity of the modulating signal inverted. Think of it as multiplying the modulator by a number between -1 and +1. Such controls are called “attenuverters” because they can do two things, attenuate and invert.
As for the frequencies. Since FM usually happens on such a fast scale, often inside of a single cycle of the wave in question, it’s usually not very useful to think of it in terms of Hz. Most of us will simply dial these depths in by ear. If you did want to find out what the range is, you could patch in a constant +10v signal and check the output with the attenuverter at min/max I guess. Though I’m not sure how useful that knowledge is.
Hello and thank you for the quick response! Obviously, I took for granted the presence of a modulating signal input to the VCO. I didn’t explain myself well, I apologize. The first part of your explanation clarified an important concept for me. The attenuverter multiplies the modulating signal by a value between -1 and +1 (from -100% to +100%), meaning it attenuates the AMPLITUDE of the modulating signal. The voltage!
What is not clear to me is the relationship between the amplitude (voltage) of the modulating signal and the frequency variation of the VCF. You say that in FM you normally judge by ear, and I agree with that. But since I’m meticulous, I wanted to understand more deeply. Searching online, I understood that there is a formula Δf = k ⋅ Vm that relates the frequency variation Δf of the VCF to the amplitude Vm of the modulating signal. Here “k” is a constant determined by the manufacturer. Therefore, if I don’t know k, I cannot do calculations, but of course I judge by ear, which is the simplest thing. After all, we are talking about art. Did I get it right? Thanks!
Yup!
On most vco’s and vcf’s in VCV having the fm amount set to 100% correlates to a v/oct ratio, meaning 1V = one octave above the set frequency. So in theory with a ±10V signal you can get a 20 octave wide frequency range. But some modules have their range limited/clipped to a certain range –you can find this in their manual if there is one.