/// <summary>
/// Designs an IIR filter with given specifications
/// </summary>
/// <param name="filter">filter type (butter, cheby etc.)</param>
/// <param name="band">filter passband type(Lowpass, Highpass etc.)</param>
/// <param name="format">output format (SOS/TF)</param>
/// <param name="order">order of the filter</param>
/// <param name="fc">cut-off frequency</param>
/// <param name="f0">center frequency</param>
/// <param name="Ap">passband ripple(dB)</param>
/// <param name="As">stopband ripple(dB)</param>
/// <returns>IIRFilter object containing filter co-efficients</returns>
public static Filters.IIRFilter DesignIIR(Filters.IIR_FilterType filter, Filters.IIR_BandType band, int order,
float fc, float f0, float Ap, float As)
{
// derived values : compute filter length
int N = order; // effective order
// filter order effectively doubles for band-pass, band-stop
// filters due to doubling the number of poles and zeros as
// a result of filter transformation
if (band == Filters.IIR_BandType.BANDPASS ||
band == Filters.IIR_BandType.BANDSTOP)
{
N *= 2;
}
int r = N % 2; // odd/even order
int L = (N - r) / 2; // filter semi-length
float[] num = { };
float[] den = { };
// validate input
if (fc <= 0 || fc >= 0.5)
{
throw new Exception("cutoff frequency out of range");
}
else if (f0 < 0 || f0 > 0.5)
{
throw new Exception("center frequency out of range");
}
else if (Ap <= 0)
{
throw new Exception("pass-band ripple out of range");
}
else if (As <= 0)
{
throw new Exception("stop-band ripple out of range");
}
unsafe
{
int h_len = N + 1;
float *_B = stackalloc float[h_len];
float *_A = stackalloc float[h_len];
libliquid.liquid_iirdes(filter, band, IIR_Format.TF, order, fc, f0, Ap, As
, _B, _A);
num = new float[h_len];
den = new float[h_len];
num = libliquid.Create <float>(_B, h_len);
den = libliquid.Create <float>(_A, h_len);
}
IIRFilter f = new IIRFilter(num, den);
return(f);
}
internal static extern void liquid_iirdes(Filters.IIR_FilterType _ftype, Filters.IIR_BandType _btype,
Filters.IIR_Format _format, int _n, float _fc, float _f0, float _Ap, float _As, float *_B, float *_A);
