VCF Basics?

[joshs]

So I'm in the midst of building a Digital/Analog polysynth. Have made some strides on the oscillators (4 PWM via atmel/Arduino), built a power supply and I built a YUSYNTH VCF that I think works properly, but I really don't understand exactly what the different parameters do. What exactly are all the parameters? What I find most confusing is that there is a cutoff parameter... but isn't the CV controlling cutoff? How do those two relate? Also, what exactly is tracking in relation to a filter. I get it in terms of pitch for an oscillator, but I don't understand what it means in this context. Also, the YUSYNTH VCF has a trimmer that adjusts the tracking and I haven't quite been able to understand how to get it set up right. I plan on controlling the VCF with an LFO and/or ADSR, so maybe tracking isn't that important for me? Donno.

[PHOBoS]

[joshs]

That's really useful info, especially the cutoff stuff. And actually, the self oscillation stuff could be useful, too (I think it does in fact self oscillate, but I haven't messed with that). Maybe that's the best way to tune the trim part of the circuit. Set it into self oscillation and then try and adjust it via pitch...

[rutgerv]

Hi joshs,

That's indeed how you calibrate a VCF. Turn up the resonance to make it self-oscillate. Feed a keyboard CV (following 1V/oct guideline) into a CV input (one that is capable of that same 1V/oct range) and turn the trimmers until you can play the filter 'in tune' on the keyboard. Playing an octave higher on the keyboard should make the filter sound an octave higher.

Most filters have two trimmers for the tracking: offset and scale. The offset trims the cutoff frequency up or down, meaning that the pitch you hear at resonance goes up or down. The scale changes the distance between each note, so you can stretch or shrink the intervals until you can hear that playing an octave sounds like an octave (and not sharp or flat). Usually both trimmers interact, so you have to go back and forth between them to get the desired tuning.

More theoretical answer:
Even without the filter being in self-oscillation, tracking has a meaning. The filter cuts frequencies out starting at the cutoff point you set with the cutoff-control or CV (roughly speaking). This cutoff-point is the same point at which you hear the filter's sine wave when it's resonating, and can be tuned . In some musical applications this is required, for instance when you want the cutoff-frequency to follow your keyboard notes (high notes get a higher filter frequency, keeping the frequency content above the fundamental static).

The relation between the cutoff parameter and the CV input: usually the cutoff knob internally generates a CV that gets summed with the CV from the input before controlling the filter's cutoff frequency.

Rutger