File content as of revision 0:d2f279439aaf:

//----------------------------------------------------------------
//  縦続形 IIR フィルタで使う 1D タイプの 2 次のフィルタ
//  Biquad filter of 1D type for IIR filter of cascade structure
//
//      u[n] = x[n] + a1*u[n-1] + a2*u[n-2]
//      y[n] = u[n] + b1*u[n-1] + b2*u[n-2]
//          x[n] :  input signal
//          y[n] :  output signal
//          b0 = 1
//
//  2015/09/02, Copyright (c) 2015 MIKAMI, Naoki
//----------------------------------------------------------------

#ifndef IIR_BIQUAD_HPP
#define IIR_BIQUAD_HPP

#include "mbed.h"

namespace Mikami
{
    class Biquad
    {
    public:
        struct Coefs { float a1, a2, b1, b2; };

        Biquad() {}     // default constructor

        Biquad(const Coefs ck)
              : a1_(ck.a1), a2_(ck.a2), b1_(ck.b1), b2_(ck.b2)
        { Clear(); }       

        float Execute(float xn)
        {
            float un = xn + a1_*un1_ + a2_*un2_;
            float yn = un + b1_*un1_ + b2_*un2_;
    
            un2_ = un1_;
            un1_ = un;

            return yn;
        }

        void Clear() { un1_ = un2_ = 0; }

    private:
        float a1_, a2_, b1_, b2_;   // coefficients
        float un1_, un2_;           // buffer for delay

        // disallow copy constructor and assignment operator
        Biquad(const Biquad&);
        Biquad& operator = (const Biquad&);
    };
}
#endif  // IIR_BIQUAD_HPP