SVF resonant?

Development
17

From the Cytomic paper (last page) and kerfuffle’s filter source code, something like:

// Resonant low-pass filter based on Cytomic SVF.
class Filter {
public:
  float sampleRate;

  void updateCoefficientsLP(float cutoff, float Q) {
    g = std::tan(PI * cutoff / sampleRate);
    k = 1.0f / Q;
    a1 = 1.0f / (1.0f + g * (g + k));
    a2 = g * a1;
    a3 = g * a2;
    m0 = 0;
    m1 = 0;
    m2 = 1;
  }

  void updateCoefficientsBP(float cutoff, float Q) {
    g = std::tan(PI * cutoff / sampleRate);
    k = 1.0f / Q;
    a1 = 1.0f / (1.0f + g * (g + k));
    a2 = g * a1;
    a3 = g * a2;
    m0 = 0;
    m1 = 1;
    m2 = 0;
  }

  void updateCoefficientsHP(float cutoff, float Q) {
    g = std::tan(PI * cutoff / sampleRate);
    k = 1.0f / Q;
    a1 = 1.0f / (1.0f + g * (g + k));
    a2 = g * a1;
    a3 = g * a2;
    m0 = 1;
    m1 = -k;
    m2 = -1;
  }

  void reset() {
    g = 0.0f;
    k = 0.0f;
    a1 = 0.0f;
    a2 = 0.0f;
    a3 = 0.0f;
    m0 = 0.f;
    m1 = 0.f;
    m2 = 0.f;

    ic1eq = 0.0f;
    ic2eq = 0.0f;
  }

  float render(float x) {
    float v3 = x - ic2eq;
    float v1 = a1 * ic1eq + a2 * v3;
    float v2 = ic2eq + a2 * ic1eq + a3 * v3;
    ic1eq = 2.0f * v1 - ic1eq;
    ic2eq = 2.0f * v2 - ic2eq;
    return m0 * x + m1 * v1 + m2 * v2;
  }

private:
  const float PI = 3.1415926535897932f;

  float g, k, a1, a2, a3; // filter coefficients
  float m0, m1, m2;       // filter mixers
  float ic1eq, ic2eq;     // internal state
};