// Calculates coefficients for a low-pass FIR filter using Hamming window
void CalculateCoeffs()
{
int i;
double cntTemp, temp, tempCoeff, h, w;
double wc;
double scaleCoeff, sum;
double[] work = new double[length];
float[] coeffs = new float[length];
Debug.Assert(length >= 2);
Debug.Assert(length % 4 == 0);
Debug.Assert(cutoffFreq >= 0);
Debug.Assert(cutoffFreq <= 0.5);
wc = 2.0 * Math.PI * cutoffFreq;
tempCoeff = Math.PI * 2 / (double)length;
sum = 0;
for (i = 0; i < length; i++)
{
cntTemp = (double)i - (double)(length / 2);
temp = cntTemp * wc;
if (temp != 0)
{
h = Math.Sin(temp) / temp; // sinc function
}
else
{
h = 1.0;
}
w = 0.54 + 0.46 * Math.Cos(tempCoeff * cntTemp); // hamming window
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
scaleCoeff = 16384.0f / sum;
for (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] = (float)temp;
}
// Set coefficients. Use divide factor 14 => divide result by 2^14 = 16384
FirFilter.SetCoefficients(coeffs, length, 14);
}
