raspi-synth/oscillator.h

/* PolyBLEP oscillator inspired by: https://www.metafunction.co.uk/post/all-about-digital-oscillators-part-2-blits-bleps
 * Further info about BLEP at: https://pbat.ch/sndkit/blep/ 
 */

#include <math.h>

#define PIx2 (2 * M_PI)

class Oscillator {
public:
    enum Mode { MODE_SINE, MODE_SAW, MODE_SQUARE };

    Mode mode;
    float phase;
    float phaseStep;
    float value;

    Oscillator();

    float polyBlep(float t) {
        double dt = phaseStep / PIx2;

        // t-t^2/2 +1/2
        // 0 < t <= 1
        // discontinuities between 0 & 1
        if (t < dt) {
            t /= dt;
            return t + t - t * t - 1.0;
        }

        // t^2/2 +t +1/2
        // -1 <= t <= 0
        // discontinuities between -1 & 0
        else if (t > 1.0 - dt) {
            t = (t - 1.0) / dt;
            return t * t + t + t + 1.0;
        }

        // no discontinuities 
        // 0 otherwise
        else return 0.0;
    }

    // This class provides a band-limited oscillator
    float tick() {
        float t = phase / PIx2; // Define half phase
        
        if (mode == MODE_SINE) {
            value = sin(phase); // No harmonics in sine so no aliasing!! No Poly BLEPs needed!
        } else if (mode == MODE_SAW) {
            value = (2.0 * phase / PIx2) - 1.0; // Render naive waveshape
            value -= polyBlep(t); // Layer output of Poly BLEP on top
        } else if (mode == MODE_SQUARE) {
            if (phase < M_PI) {     
                value = 1.0; // Flip 
            } else { 
                value = -1.0; // Flop
            }
            value += polyBlep(t); // Layer output of Poly BLEP on top (flip)
            value -= polyBlep(fmod(t + 0.5, 1.0)); // Layer output of Poly BLEP on top (flop)
        }

        phase += phaseStep;
        if(phase >= PIx2) {
            phase -= PIx2;
        }
        
        return value; // Output
    }
};