/// <devdoc>
/// Designs an IIR filter and returns the IIR filter coefficients. Cutoff frequencies
/// are relative to sampling rate and must be between 0 and 0.5.
/// Passband ripple is in dB. Must be negative. Only used for Chebyshev filter, ignored for other filters.
/// Both low-pass and high-pass use fcf1, band-pass and band-stop need both.
/// </devdoc>
private IirFilterCoefficients Design(FilterKind type, int filterOrder, double ripple, double fcf1, double fcf2 = 0)
{
var poles = GetPoles(filterOrder, ripple);
SToZMappingMethod sToZMappingMethod = GetSToZMappingMethod();
var sPlane = Normalize(poles, type, fcf1, fcf2, sToZMappingMethod == SToZMappingMethod.BilinearTransform);
var zPlane = MapSPlaneToZPlane(sPlane, sToZMappingMethod);
var tf = ComputeTransferFunction(zPlane);
return ComputeIirFilterCoefficients(tf, ComputeGain(tf, type, fcf1, fcf2));
}
/// <devdoc>
/// Maps the poles and zeros from the s-plane (<c>sPlane)</c>to the z-plane.
/// </devdoc>
private static PolesAndZeros MapSPlaneToZPlane(PolesAndZeros sPlane, SToZMappingMethod sToZMappingMethod)
{
switch (sToZMappingMethod)
{
case SToZMappingMethod.BilinearTransform:
{
return new PolesAndZeros
{
Poles = DoBilinearTransform(sPlane.Poles),
Zeros = Extend(DoBilinearTransform(sPlane.Zeros), sPlane.Poles.Length, new Complex(-1, 0))
};
}
case SToZMappingMethod.MatchedZTransform:
{
return new PolesAndZeros
{
Poles = DoMatchedZTransform(sPlane.Poles),
Zeros = DoMatchedZTransform(sPlane.Zeros)
};
}
default:
throw new System.ComponentModel.InvalidEnumArgumentException("sToZMappingMethod", (int)sToZMappingMethod, typeof(SToZMappingMethod));
}
}
