/// <summary>
/// Calculate the FIR coefficients realizing the given cutoff-frequency.
/// </summary>
private void CalculateCoeffs()
{
double temp;
Debug.Assert(_length >= 2);
Debug.Assert(_length % 4 == 0);
Debug.Assert(_cutoffFreq >= 0);
Debug.Assert(_cutoffFreq <= 0.5);
ArrayPtr <double> work = new double[_length];
ArrayPtr <TSampleType> coeffs = new TSampleType[_length];
double wc = 2.0 * Math.PI * _cutoffFreq;
double tempCoeff = (Math.PI * 2) / _length;
double sum = 0;
for (int i = 0; i < _length; i++)
{
double cntTemp = i - ((double)_length / 2);
temp = cntTemp * wc;
double h;
if (temp != 0)
{
h = Math.Sin(temp) / temp; // sinc function
}
else
{
h = 1.0;
}
double w = 0.54 + 0.46 * Math.Cos(tempCoeff * cntTemp);
temp = w * h;
work[i] = temp;
// calc net sum of coefficients
sum += temp;
}
// ensure the sum of coefficients is larger than zero
Debug.Assert(sum > 0);
// ensure we've really designed a lowpass filter...
Debug.Assert(work[_length / 2] > 0);
Debug.Assert(work[_length / 2 + 1] > -1e-6);
Debug.Assert(work[_length / 2 - 1] > -1e-6);
// Calculate a scaling coefficient in such a way that the result can be
// divided by 16384
double scaleCoeff = 16384.0f / sum;
for (int i = 0; i < _length; i++)
{
temp = work[i] * scaleCoeff;
// scale & round to nearest integer
temp += (temp >= 0) ? 0.5 : -0.5;
// ensure no overfloods
Debug.Assert(temp >= -32768 && temp <= 32767);
coeffs[i] = (TSampleType)Convert.ChangeType(temp, typeof(TSampleType));
}
// Set coefficients. Use divide factor 14 => divide result by 2^14 = 16384
_firFilter.SetCoefficients(coeffs, _length, 14);
DebugSaveAntiAliasFilterCoefficients(coeffs, _length);
}
