/// <summary>
/// H(s) = s / (s^2 + s/Q + 1) (constant skirt gain, peak gain = Q)
/// </summary>
public static BiQuadFilter BandPassFilterConstantSkirtGain(float sampleRate, float centreFrequency, float q)
{
double w0 = 2 * Math.PI * centreFrequency / sampleRate;
double cosw0 = Math.Cos(w0);
double sinw0 = Math.Sin(w0);
double alpha = sinw0 / (2 * q);
BiQuadFilter filter = new BiQuadFilter();
filter.b0 = sinw0 / 2; // = Q*alpha
filter.b1 = 0;
filter.b2 = -sinw0 / 2; // = -Q*alpha
filter.a0 = 1 + alpha;
filter.a1 = -2 * cosw0;
filter.a2 = 1 - alpha;
return filter;
}
/// <summary>
/// H(s) = (s^2 - s/Q + 1) / (s^2 + s/Q + 1)
/// </summary>
public static BiQuadFilter AllPassFilter(float sampleRate, float centreFrequency, float q)
{
double w0 = 2 * Math.PI * centreFrequency / sampleRate;
double cosw0 = Math.Cos(w0);
double sinw0 = Math.Sin(w0);
double alpha = sinw0 / (2 * q);
BiQuadFilter filter = new BiQuadFilter();
filter.b0 = 1 - alpha;
filter.b1 = -2 * cosw0;
filter.b2 = 1 + alpha;
filter.a0 = 1 + alpha;
filter.a1 = -2 * cosw0;
filter.a2 = 1 - alpha;
return filter;
}
/// <summary>
/// H(s) = s^2 / (s^2 + s/Q + 1)
/// </summary>
public static BiQuadFilter HighPassFilter(float sampleRate, float cutoffFrequency, float q)
{
double w0 = 2 * Math.PI * cutoffFrequency / sampleRate;
double cosw0 = Math.Cos(w0);
double alpha = Math.Sin(w0) / (2 * q);
BiQuadFilter filter = new BiQuadFilter();
filter.b0 = (1 + Math.Cos(w0)) / 2;
filter.b1 = -(1 + Math.Cos(w0));
filter.b2 = (1 + Math.Cos(w0)) / 2;
filter.a0 = 1 + alpha;
filter.a1 = -2 * Math.Cos(w0);
filter.a2 = 1 - alpha;
return filter;
}
/// <summary>
/// H(s) = (s^2 + s*(A/Q) + 1) / (s^2 + s/(A*Q) + 1)
/// </summary>
public static BiQuadFilter PeakingEQ(float sampleRate, float centreFrequency, float q, float dbGain)
{
double w0 = 2 * Math.PI * centreFrequency / sampleRate;
double cosw0 = Math.Cos(w0);
double sinw0 = Math.Sin(w0);
double alpha = sinw0 / (2 * q);
double A = Math.Pow(10, dbGain / 40); // TODO: should we square root this value?
BiQuadFilter filter = new BiQuadFilter();
filter.b0 = 1 + alpha * A;
filter.b1 = -2 * cosw0;
filter.b2 = 1 - alpha * A;
filter.a0 = 1 + alpha / A;
filter.a1 = -2 * cosw0;
filter.a2 = 1 - alpha / A;
return filter;
}
/// <summary>
/// H(s) = A * (s^2 + (sqrt(A)/Q)*s + A)/(A*s^2 + (sqrt(A)/Q)*s + 1)
/// </summary>
/// <param name="sampleRate"></param>
/// <param name="cutoffFrequency"></param>
/// <param name="shelfSlope">a "shelf slope" parameter (for shelving EQ only).
/// When S = 1, the shelf slope is as steep as it can be and remain monotonically
/// increasing or decreasing gain with frequency. The shelf slope, in dB/octave,
/// remains proportional to S for all other values for a fixed f0/Fs and dBgain.</param>
/// <param name="dbGain">Gain in decibels</param>
public static BiQuadFilter LowShelf(float sampleRate, float cutoffFrequency, float shelfSlope, float dbGain)
{
double w0 = 2 * Math.PI * cutoffFrequency / sampleRate;
double cosw0 = Math.Cos(w0);
double sinw0 = Math.Sin(w0);
double A = Math.Pow(10, dbGain / 40); // TODO: should we square root this value?
double alpha = sinw0 / 2 * Math.Sqrt((A + 1 / A) * (1 / shelfSlope - 1) + 2);
double temp = 2 * Math.Sqrt(A) * alpha;
BiQuadFilter filter = new BiQuadFilter();
filter.b0 = A * ((A + 1) - (A - 1) * cosw0 + temp);
filter.b1 = 2 * A * ((A - 1) - (A + 1) * cosw0);
filter.b2 = A * ((A + 1) - (A - 1) * cosw0 - temp);
filter.a0 = (A + 1) + (A - 1) * cosw0 + temp;
filter.a1 = -2 * ((A - 1) + (A + 1) * cosw0);
filter.a2 = (A + 1) + (A - 1) * cosw0 - temp;
return filter;
}