Stealing VCV Rack's analog "exciter" formula

jamshark70 · August 19, 2021, 3:21am

Credit:

VCVRack/Fundamental/blob/v1/src/VCO.cpp#L23

    
      
          }
          
          
template <typename T>
          T sin2pi_pade_05_5_4(T x) {
          	x -= 0.5f;
          	return (T(-6.283185307) * x + T(33.19863968) * simd::pow(x, 3) - T(32.44191367) * simd::pow(x, 5))
          	       / (1 + T(1.296008659) * simd::pow(x, 2) + T(0.7028072946) * simd::pow(x, 4));
          }
          
          
template <typename T>
          T expCurve(T x) {
          	return (3 + x * (-13 + 5 * x)) / (3 + 2 * x);
          }
          
          

          
template <int OVERSAMPLE, int QUALITY, typename T>
          struct VoltageControlledOscillator {
          	bool analog = false;
          	bool soft = false;
          	bool syncEnabled = false;
          	// For optimizing in serial code

(
a = {
	var saw = Saw.ar(MouseX.kr(50, 8000, 1));
	var x = saw * 0.5 + 0.5;
	var saw2 = LeakDC.ar(((x * 5 - 13) * x + 3) / (x * 2 + 3)).neg;
	var trig = Impulse.kr(1);
	var index = ToggleFF.kr(trig);
	SendReply.kr(trig, '/index', index);
	(Select.ar(index, [saw, saw2]) * 0.1).dup
}.play;

if(v.tryPerform(\isClosed) ? true) {
	v = Button(nil, Rect(800, 200, 200, 50)).states_([
		["Digital", Color.white, Color.gray(0.1)],
		["Analog", Color.black, Color.gray(0.6)]
	]).font_(Font.default.size_(36)).front;
};

OSCdef(\index, { |msg|
	defer { v.value = msg[3] };
}, '/index', s.addr);
)

To my ear it adds some pleasing high-frequency content. Also works well with Pulse.ar.

Comments? Improvements?

EDIT: Rewrote the formula to suit SC’s operator order better.

EDIT: You could try two versions.

VCV’s exact formula LeakDC.ar(((x * 5 - 13) * x + 3) / (x * 2 + 3)).neg – mellower
Or my first typo version LeakDC.ar(((x * (5 - 13)) * x + 3) / (x * 2 + 3)).neg – adds a touch of distortion. TBH I like this sound better though it’s technically wrong. – -7 instead of -8 sounds slightly nicer.

hjh