Hello community!
In this video from Jakub, we see a VCF used as a slew limiter. Incredible! And so “organic”!
I have only one question: how? Why? 
How the cutoff frequency can modify the slew rate? And why the resonance appears?
Only one question 
Alain
I can’t give you a mathematical or electronic explanation, but maybe it helps to look at the effect on the CV (1/V/Oct for setting the Oscillator pitch) in a scope - compare unfiltered to filtered:
perhaps it has to do with the spectrum. a filter cuts high frequencies. a waveform displayed in frequency domain is a bunch of spectral lines (fourier). as you cut spectral lines and transform the waveform back in time domain then it will be different.
Just go back to the elementary function of a Lowpass filter. It filters out frequencies above the cutoff frequency.
Now, look at some random periodical signal and the concept of frequency. Overal frequency of a periodic signal is determined by how fast a periodical signal changes polarity (‘zero crossings’). Within complex signals (having a spectrum consisting of more than just the one fundament sine), the amplitude can also change (vector)direction over time within one period.
Generalized, a lowpass filter basically limits how fast a signal can change amplitude. Not just for audiorate signals. For any signal at any rate. Does not even have to be a periodical signal.
Extreme examples: A pure saw goes from 0 to max in no time at all. A saw contains all harmonics (each at amplitude inverse to its rank). So a great candidates for testing the effect of a VCF on the shape. Something similar goes for a square, which has 2 instant transitions. A square contains all odd harmonics (each at amplitude inverse to its rank).
When Lowpass filtered you will see that the sharp transitions in amplitude are ‘rounded’ or ‘averaged’ out.
Just feed a saw into a VCF and slowly lower the Cutoff frequency and check the signal with a scope. You will see the ‘rounding’ or ‘averaging out’ the leading edge of the saw.
Resonance introduces ripples in a sort of decaying sine pulse shape, at the frequency of the cutoff. This ‘resonant’ frequency is basically independent of the periodicity (tuned frequency) of the base signal.
Although, in principle, the resonant frequency (band) should present in the base signal, because ‘resonance’ only amplifies existing ferquencies. Unless some noise is intentionally added to the filter to enable selfresonance. But I digress.
Just feed a saw into a resonant VCF and slowly crack up the resonance an check the signal with a scope. You will see these ripples appear from the leading edge of the saw. This is the cause the of the ‘fading wobbling’ when doing this on a low frequency ‘modulation’ signal.
Also works the other way around.
Calculating a running average on signal levels (on consecutive samples) or feeding a signal into a slew limiter (if fast enough for audiorate) will act as a Lowpass filter.
Averaging Lowpass filter: Average | Airwindows
Fast enough slew limiter: Befaco rampage in fastest mode
BTW, here’s another Jukub Ciupinski video where he really exploits just about all imaginable quirks of filters, going back the fundamental behaviours of filters.
VCV Rack Hacks | Most Underrated Module
The “slew rate” is usually the time constant, like “1/4 of a second”. If time constant is “T” and filter frequency is “F”, T = 1/F (depending on measurement units) for a one pole lowpass filter.
So if you want to make a slew limiter with an exponential shape, and attack and release times the same, a one pole lowpass is it, other than being a not-great user interface for a slew limiter.
Most “good” slew limiters offer a choice of shape (linear, exponential, other) as well as independent control of attack and release rate (or positive vs. negative slewing).
« tuning compensation » ?
Fourier series, indeed… Now I remember 
Sometimes, we forget the basics indeed…
Alain
Thank you for this long, precise and interesting summary 
Alain
Yeah. Waveshape and spectrum are interrelated. This is the basis for many synthesis techniques.
Changing the spectrum, and thus the waveshape:
Changing the shape, and thus the spectrum
So, you can either change the harmonic content of a square to get to a triangle or you can directly change the shape to a square to force it into a triangle
As seen from the perspective of the harmonic series:
So, basically a triangle has way less amplitude/energy in frequencies higher up in the spectrum then a square. So a triangle can be achieved by lowpass filtering a square.
Off course you can also achieve this by directly reshaping the wave. E.g. by putting a slew limiter on both rise and fall of a square…
“VCV Rack Hacks | Most Underrated Module“ I saw this video last year. Interesting too!
Alain
“tuning compensation”, my (bad) words = using “drive” to compensate for the lowered levels from applying “res” - to get back “in tune”.
Thank you.
Pitch cv into a resonant filter is my favorite recipe for a generative soloist. It can sound organic rather than machine-like. Works very well for ragas.
I usually quantize both before and after the filter. The filter’s only function is to add ornamentation to the melody.
I sometimes add more slew after the resonant filter.
I prefer to keep the filter frequency very low, near the bottom of the range, and instead tune the resonance and drive to get the best melodies. Usually the resonance is near the middle of its range. I add very little drive, just enough to give the melody the amount of range that I want.
Thank you for these usage explaination. Alain
Understood!
