BiquadFilter.cpp

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#include "BiquadFilter.h"
 
//=====================================================================================
krotos::BiquadFilter::BiquadFilter() : a1(0), a2(0), b0(0), b1(0), b2(0), d0_1(0), d1_1(0)
{
    //
}
 
//=====================================================================================
void krotos::BiquadFilter::configure(double rate, FilterType filter_type)
{
 
    switch (filter_type)
    {
    case krotos::BiquadFilter::FilterType::HighShelfDeMan: {
        const double G = 3.99984385397;
        const double Q = 0.7071752369554193;
        const double fc = 1681.9744509555319;
 
        const double K = std::tanh(M_PI * fc / rate);
        const double Vh = std::pow(10.0, G / 20.0);
        const double Vb = std::pow(Vh, 0.499666774155);
        const double a0_ = 1.0 + K / Q + K * K;
 
        b0 = static_cast<float>((Vh + Vb * K / Q + K * K) / a0_);
        b1 = static_cast<float>(2.0 * (K * K - Vh) / a0_);
        b2 = static_cast<float>((Vh - Vb * K / Q + K * K) / a0_);
 
        a1 = static_cast<float>(2.0 * (K * K - 1.0) / a0_);
        a2 = static_cast<float>((1.0 - K / Q + K * K) / a0_);
 
        configured = true;
 
        break;
    }
    case krotos::BiquadFilter::FilterType::HighPassDeMan: {
        const double Q = 0.5003270373253953;
        const double fc = 38.13547087613982;
 
        const double K = std::tanh(M_PI * fc / rate);
 
        b0 = 1.f;
        b1 = -2.f;
        b2 = 1.f;
 
        a1 = static_cast<float>(2.0 * (K * K - 1.0) / (1.0 + K / Q + K * K));
        a2 = static_cast<float>((1.0 - K / Q + K * K) / (1.0 + K / Q + K * K));
 
        configured = true;
 
        break;
    }
    }
}
 
//=====================================================================================
float krotos::BiquadFilter::processSample(float x)
{
    assert(configured);
 
    // direct II transposed structure
    // https://docs.scipy.org/doc/scipy/reference/generated/scipy.signal.lfilter.html
 
    const float y = b0 * x + d0_1;
    const float d0 = b1 * x - a1 * y + d1_1;
    const float d1 = b2 * x - a2 * y;
 
    d0_1 = d0;
    d1_1 = d1;
 
    return y;
}