synthapp/svf.h

/* CEM3320/Oberheim/Phrophet-style filter as described here: https://arxiv.org/pdf/2111.05592.pdf */

#include <math.h>

inline float clamp(float x, float a, float b) {
    return fminf(b, fmaxf(a, x));
}

class SVF {
protected:
    float as1, as2;
    float kdiv;
    float kK, kQ;

    inline void updateCoeffs() {
        kdiv = 1 + kK/kQ + kK*kK;
    }

public:
    typedef struct {
        float hp;
        float bp;
        float lp;
        float br;
        float ap;
    } Output;

    SVF() {
        as1 = 0;
        as2 = 0;
        kK = tan(M_PI_4);
        kQ = M_SQRT1_2;
        updateCoeffs();
    }

    void setFrequency(float f) {
        kK = tan(M_PI_2 * clamp(f, 0, 1));
        updateCoeffs();
    }

    void setQ(float q) {
        kQ = q;
        updateCoeffs();
    }

    Output tick(float as) {
        Output out;
        out.hp = (as - (1/kQ + kK) * as1 - as2) / kdiv;
        float au = out.hp * kK;
        out.bp = au + as1;
        as1 = au + out.bp;
        au = out.bp * kK;
        out.lp = au + as2;
        as2 = au + out.lp;
        float abr = out.hp + out.lp;
        float aap = out.hp+out.lp + (1/kQ) * out.bp;
        return out;
    }
};

class SVFx2 {
public:
    enum Mode { MODE_HP, MODE_BP, MODE_LP, MODE_BR, MODE_AP };

private:
    Mode mode;
    SVF first, second;

public:
    SVFx2() {
        this->mode = MODE_LP;
    }

    void setMode(Mode mode) {
        this->mode = mode;
    }

    void setFrequency(float f) {
        first.setFrequency(f);
        second.setFrequency(f);
    }

    void setQ(float q) {
        first.setQ(q);
        second.setQ(q);
    }

    float tick(float as) {
        SVF::Output in = first.tick(as);
        switch(mode) {
            case MODE_HP:
                return second.tick(in.hp).hp;

            case MODE_BP:
                return second.tick(in.bp).bp;

            case MODE_LP:
                return second.tick(in.lp).lp;

            case MODE_BR:
                return second.tick(in.br).br;

            case MODE_AP:
                return second.tick(in.ap).ap;
        }
    }
};