MiniMoog VCF CV sources question

[LeMury]

Hello everyone, new member here:)

I'm trying to emulate the minimoog in software.
Can someone perhaps explain to me how the different VCF control voltage sources
are combined to determine the final Freq. cutoff point of the filter?
Looking at the schematic it seems they are simply summed?

With different CV sources I mean;
- ADSR (for vcf)
- keyboard tracking
- manual freq cutoff control
- LFO

Is it simply something like this?
(assuming all variables are in the range [0..1])

fc = kbFreq + manFreq + adsr*depth + LFO;
fc = fc > 1 ? 1 : fc;

Thank you.

[mystran]

I'm trying to emulate the minimoog in software.
Can someone perhaps explain to me how the different VCF control voltage sources
are combined to determine the final Freq. cutoff point of the filter?
Looking at the schematic it seems they are simply summed?

Typically (there are some exceptions, but Moog is not one of them) in classic analogs VCF control signals are nominally 1V/octave scaling, meaning that one volt increase bumps the cutoff by an octave.

Usually different CV sources are just summed, but this summing is done with 1V/octave voltages before exponentiation, so the actual cutoff is roughly exp2(sum_of_cv) where exp2 is base-2 exponential (ie. pow(2, sum_of_cv) but generic pow tends to be very slow). So in a sense, the summing is done on "pitch" rather than frequency, so that if keyboard tracking says we go two semitones up and envelope says we go a fourth up then together they always add a fifth whatever the manual cutoff knob happens to be.

With analogs though, there might not be so much of a strict cutoff limit though, rather just the exponential current source (or something else) saturating so that additional CV voltage won't increase the actual control current further. Can't remember if this happens in the audible range with the Mini, perhaps someone else can comment on that, but in software if you're not otherwise limiting the cutoff you probably want to (soft?) clip it slightly below Nyquist (eg. 0.45*fs or some such) just to make sure things stay stable.

[Z1202]

With analogs though, there might not be so much of a strict cutoff limit though, rather just the exponential current source (or something else) saturating so that additional CV voltage won't increase the actual control current further. Can't remember if this happens in the audible range with the Mini, perhaps someone else can comment on that, but in software if you're not otherwise limiting the cutoff you probably want to (soft?) clip it slightly below Nyquist (eg. 0.45*fs or some such) just to make sure things stay stable.

I'm pretty sure MiniMoog's VCF (as many analog VCFs in general) goes into ultrasonic range with its cutoff. IMHO it's one of the key features of the analog sound as you hear the resonance peak disappearing or reappearing back (or just the nonresonanting filter opening fully, which in principle requires the cutoff to be a few octaves above 16kHz)