/// <summary>
/// Initialize parameters of the Butterworth filter
/// </summary>
public FilterButterworth(double frequency, int sampleRate, ButterworthPassType passType)
{
//this.resonance = resonance;
/*
* - we used a 1st order, low-pass Butterworth filter set to 0.05 Hz to extract the tonic signal.
*/
this.frequency = frequency;
this.sampleRate = sampleRate;
this.passType = passType;
switch (passType)
{
case ButterworthPassType.Lowpass:
c = 1.0 / Math.Tan(Math.PI * frequency / sampleRate);
a1 = 1.0 / (1.0 + c * c);
//a1 = 1.0 / (1.0 + resonance * c + c * c);
a2 = 2.0 * a1;
a3 = a1;
b1 = 2.0 * (1.0 - c * c) * a1;
//b2 = (1.0 + c * c) * a1;
b2 = (1.0 - resonance * c + c * c) * a1;
break;
case ButterworthPassType.Highpass:
c = Math.Tan(Math.PI * frequency / sampleRate);
a1 = 1.0 / (1.0 + c * c);
//a1 = 1.0 / (1.0 + resonance * c + c * c);
a2 = -2.0 * a1;
a3 = a1;
b1 = 2.0 * (c * c - 1.0) * a1;
//b2 = (1.0 + c * c) * a1;
b2 = (1.0 - resonance * c + c * c) * a1;
break;
}
}
