Question on filter design

[metaphorz]

I am planning on making a few modules, and really like these as examples of how modules can be created (to add to Tutorial in Rack). The "blank" module is nice! Some of these questions I am sure are basic, but hopefully apply to any module. I have questions (labeled as **) underneath each statement in Step() for the Shaper module (which seems an excellent starter because of the functions)

// normalize signal input to [-1.0...+1.0]
float x = clampf(inputs[RESHAPER_INPUT].value * 0.1f, -1.f, 1.f);

** As you point out, this normalizes the signal, but what is the incoming range for a Rack signal? Are modules always sending a signal from -10 to +10v or are input signals arbitrary in voltage?

float cv = inputs[RESHAPER_CV_INPUT].value * params[RESHAPER_CV_AMOUNT].value;

** The top knob is presumably ranging 0 to 1 in value as it is turned ?

float a = clampf(params[RESHAPER_AMOUNT].value + cv, 1.f, 50.f);

** What is the reason for ranging from 1 to 50?

// do the acid!
float out = x * (abs(x) + a) / (x * x + (a - 1) * abs(x) + 1);

** Is this a known filter design or something you've designed? I have not yet delved into the numerical methods for modular filters but I guess there are many books on the subject.

[lindenbergresearch]

Hi metaphorz,

to be honest, I tried a lot of code I found over the years for music dsp und took the well sounding... :) But I did not invented that formula at all - just adapted it a bit ;)
Some of the functions tend to be unstable outside of > -/+1.0 input, so I normalised it and ensured with clampf, that they do not exceed this. Later it's scaled back. I just measured some of the inputs to get the default values, so my scaling is kinda empiric. In generell values could be all range I think.
You can configure the ranges of the knob's, it is not always from 0..1.0.

[metaphorz]

I can see the empirical approach as it makes the most sense from an artistic perspective. When attaching a scope, it seems that your waveform is nicely between -5/+5V if I am reading the
scope correctly.

[lindenbergresearch]

Yes you are right. Should be the same in real life :)