/// <summary>
/// Windowd Sinc method
/// </summary>
/// <param name="length">Length</param>
/// <param name="bw">Bandwidth</param>
/// <returns></returns>
public static double[] WindowedSinc_max(int length, double bw)
{
// http://www.dspguide.com/ch16/1.htm
int i = 0;
int bwl = 0; // integer transition bandwidth
double tbw; // double transition bandwidth
double[] h; // kernel
double x; // position in the antiderivate of the Blackman function of the sample we're at
double coef; // coefficient obtained from the function
tbw = bw * (double)(length - 1);
bwl = GlobalMembersUtil.RoundUp(tbw);
h = new double[length];
for (i = 1; i < length; i++)
{
h[i] = 1.0;
}
for (i = 0; i < bwl; i++)
{
x = (double)i / tbw; // position calculation between 0.0 and 1.0
// antiderivative of the Blackman window function
coef = 0.42 * x - (0.5 / (2.0 * PI)) * Math.Sin(2.0 * PI * x) + (0.08 / (4.0 * PI)) * Math.Sin(4.0 * PI * x);
coef *= 1.0 / 0.42;
h[i + 1] = coef;
h[length - 1 - i] = coef;
}
return(h);
}
/// <summary>
/// Downsampling of the signal by a Blackman function
/// </summary>
/// <param name="in">Signal</param>
/// <param name="Mi">Mi is the original signal's length</param>
/// <param name="Mo">Mo is the output signal's length</param>
/// <returns>Downsampled Signal</returns>
public static double[] BlackmanDownsampling(double[] @in, int Mi, int Mo)
{
int i = 0; // general purpose iterators
int j = 0;
double[] @out = new double[Mo];
double pos_in; // position in the original signal
double x; // position of the iterator in the blackman(x) formula
double ratio; // scaling ratio (> 1.0)
double ratio_i; // ratio^-1
double coef; // Blackman coefficient
double coef_sum; // sum of coefficients, used for weighting
/*
* Mi is the original signal's length
* Mo is the output signal's length
*/
ratio = (double)Mi / Mo;
ratio_i = 1.0 / ratio;
for (i = 0; i < Mo; i++)
{
pos_in = (double)i * ratio;
coef_sum = 0;
for (j = GlobalMembersUtil.RoundUp(pos_in - ratio); j <= pos_in + ratio; j++)
{
if (j >= 0 && j < Mi) // if the read sample is within bounds
{
x = j - pos_in + ratio; // calculate position within the Blackman function
coef = 0.42 - 0.5 * Math.Cos(PI * x * ratio_i) + 0.08 * Math.Cos(2 * PI * x * ratio_i);
coef_sum += coef;
@out[i] += @in[j] * coef; // convolve
}
}
@out[i] /= coef_sum;
}
return(@out);
}