character
chebyshev ripple	db (<0.0)
qfactor (resonator)	(>0.0)
frequency	Alpha Low Alpha Up or samplerate: Hz Lower corner Hz Midi note (69=A440) Upper corner Hz Midi note (69=A440)
more	matched z-transform (better off, check frequency response) prewarp (better on)
output	Signal bits (including sign): -1 bit saturation save brutal shift, this will remove shifts up to 4 positions by changing the factor, hence reducing accuracy by maximum 4 bits but more efficient calculation
execution


//Low pass butterworth filter order=2 alpha1=0.0022675736961451 
class FilterBuLp2
{
    const float cf[3] = {0.00005024142299431266, -0.98005250820633649234, 1.97985154251435924166};
  public:
    FilterBuLp2()
    {}
    double step(double x) //class II 
    {
        v[0] = v[1];
        v[1] = v[2];
        v[2] = x * cf[0]  + cf[1] * v[0] + cf[2] * v[1];
        return (v[0] + v[2]) + 2 * v[1];
    }
  private:
    double v[3]{};
};