/// <devdoc>
/// Given the z-plane poles and zeros, compute the rational fraction (the top and bottom polynomial coefficients)
/// of the filter transfer function in Z.
/// </devdoc>
private static Polynomials.RationalFraction ComputeTransferFunction(PolesAndZeros zPlane)
{
var topCoeffsComplex = Polynomials.Expand(zPlane.Zeros);
var bottomCoeffsComplex = Polynomials.Expand(zPlane.Poles);
// If the GetRealPart() conversion fails because the coffficients are not real numbers, the poles
// and zeros are not complex conjugates.
return new Polynomials.RationalFraction
{
Top = topCoeffsComplex.Select(x => GetRealPart(x)).ToArray(),
Bottom = bottomCoeffsComplex.Select(x => GetRealPart(x)).ToArray(),
};
}
/// <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));
}
}
/// <summary>
/// Transforms the s-plane poles of the prototype filter into the s-plane poles and zeros for a filter
/// with the specified pass type and cutoff frequencies.
/// </summary>
/// <param name="poles">The s-plane poles of the prototype LP filter.</param>
/// <param name="type">The filter type (supports only low-pass, high-pass, band-pass and band-stop.</param>
/// <param name="fcf1">
/// The relative filter cutoff frequency for low-pass/high-pass, lower cutoff frequency for band-pass/band-stop.
/// The cutoff frequency is specified relative to the sampling rate and must be between 0 and 0.5.
/// </param>
/// <param name="fcf2">
/// Ignored for low-pass/high-pass, the relative upper cutoff frequency for band-pass/band-stop.
/// The cutoff frequency is specified relative to the sampling rate and must be between 0 and 0.5.
/// </param>
/// <param name="preWarp">
/// <see langword="true"/>true to enable prewarping of the cutoff frequencies (for a later bilinear transform from s-plane to z-plane),
/// <see langword="false"/> to skip prewarping (for a later matched Z-transform).
/// </param>
/// <returns>The s-plane poles and zeros for the specific filter.</returns>
private static PolesAndZeros Normalize(Complex[] poles, FilterKind type, double fcf1, double fcf2, bool preWarp)
{
bool fcf2IsRelevant = type == FilterKind.BandPass || type == FilterKind.BandStop;
Debug.Assert(fcf1 > 0 && fcf1 < 0.5);
Debug.Assert(!fcf2IsRelevant || (fcf2 > 0 && fcf2 < 0.5));
int n = poles.Length;
double fcf1Warped = Math.Tan(Math.PI * fcf1) / Math.PI;
double fcf2Warped = fcf2IsRelevant ? Math.Tan(Math.PI * fcf2) / Math.PI : 0;
double w1 = 2 * Math.PI * (preWarp ? fcf1Warped : fcf1);
double w2 = 2 * Math.PI * (preWarp ? fcf2Warped : fcf2);
if (type == FilterKind.LowPass)
{
return new PolesAndZeros
{
Poles = poles.Select(x => x * w1).ToArray(),
Zeros = new Complex[0]
};
}
if (type == FilterKind.HighPass)
{
var sPlane = new PolesAndZeros(n, n);
for (int i = 0; i < n; ++i)
sPlane.Poles[i] = w1 / poles[i];
return sPlane;
}
if (type == FilterKind.BandPass)
{
double w0 = Math.Sqrt(w1 * w2);
double bw = w2 - w1;
var sPlane = new PolesAndZeros(n * 2, n);
for (int i = 0; i < n; ++i)
{
var hba = poles[i] * (bw / 2);
var temp = Complex.Sqrt(1 - ((w0 / hba) * (w0 / hba)));
sPlane.Poles[i] = hba * (temp + 1);
sPlane.Poles[n + i] = hba * (1 - temp);
}
return sPlane;
}
if (type == FilterKind.BandStop)
{
double w0 = Math.Sqrt(w1 * w2);
double bw = w2 - w1;
var sPlane = new PolesAndZeros(n * 2, n * 2);
for (int i = 0; i < n; ++i)
{
var hba = (bw / 2) / poles[i];
var temp = Complex.Sqrt(1 - ((w0 / hba) * (w0 / hba)));
sPlane.Poles[i] = hba * (temp + 1);
sPlane.Poles[n + i] = hba * (1 - temp);
}
for (int i = 0; i < n; i++)
{
sPlane.Zeros[i] = new Complex(0, w0);
sPlane.Zeros[n + i] = new Complex(0, -w0);
}
return sPlane;
}
throw new System.ComponentModel.InvalidEnumArgumentException("type", (int)type, typeof(FilterKind));
}
