AirAbsorptionFilter.h

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namespace krotos
{
    //==============================================================================
    class AirAbsorptionFilter
    {
      public:
        AirAbsorptionFilter();
 
        void prepare(double sampleRate, int samplesPerBlock);
        float process(float inputSample, int numChannel);
 
        void setDistance(float distance);
 
      private:
        constexpr float linear_interpolate(float y1, float y2, float mu) { return y1 + mu * (y2 - y1); }
 
        float m_gain{1.0f};
 
        // Digital Filter for Modeling Air Absorption in Real Time (Petruzzellis & Zanghieri 2013)
        // Temperature 0, relative humidity 60%
        float m_a1{0.43f};
        float m_a2{0.6f};
        std::array<float, 8> m_b1{0.04f, 0.13f, 0.25f, 0.36f, 0.5f, 0.65f, 0.76f, 0.76f};
        std::array<float, 8> m_b2{0.15f, 0.22f, 0.25f, 0.3f, 0.3f, 0.3f, 0.52f, 0.52f};
 
        struct modified_lowpass
        {
            modified_lowpass(float a = 0.0f, float b = 0.0f);
 
            float operator()(float s);
 
            void config(float a_, float b_);
 
            void reset();
 
            // Coefficients
            float a, b;
 
            // Sample delays
            float x1, y1;
        };
 
        // We need a separate filter for each channel, and two filters in series.
        modified_lowpass m_filter1[2];
        modified_lowpass m_filter2[2];
 
        void test_impulse_response(std::size_t index, String outfile);
 
        bool m_bypass;
    };
} // namespace krotos