public static Complex ComputeAt(Complex x, Vector<float> coeffs)
{
Complex val = new Complex(coeffs[coeffs.Count - 1], 0.0f);
Complex xn = x;
for(int c = coeffs.Count - 2; c >= 0; --c)
{
val = new Complex(val.Real + coeffs.At(c) * xn.Real,
val.Imaginary + coeffs.At(c) * xn.Imaginary);
xn *= x;
}
return val;
}
public Complex At(Complex x)
{
Complex val = new Complex(Coefficents.At(Rank), 0.0f);
Complex xn = x;
for(int c = Rank - 1; c >= 0; --c)
{
val = new Complex(val.Real + Coefficents.At(c) * xn.Real,
val.Imaginary + Coefficents.At(c) * xn.Imaginary);
xn *= x;
}
return val;
}
private double[] LowPassFilter(double[] pcm, double cutOffFrequency = 500, double sampleRate = 8000)
{
// it really should be a power of 2
int fft_size = pcm.Length;
// create a complex data object we will use to shuffle data around
MathNet.Numerics.Complex32[] complex = new MathNet.Numerics.Complex32[fft_size];
// prepare the windowing function
int windowSize = 1000;
double[] window = new double[pcm.Length];
for (int i = 0; i < window.Length; i++)
{
if (i < windowSize)
{
int distanceFromEdge = i;
window[i] = (double)distanceFromEdge / windowSize;
}
else if (i > window.Length - windowSize)
{
int distanceFromEdge = window.Length - i;
window[i] = (double)distanceFromEdge / windowSize;
}
else
{
window[i] = 1;
}
}
// load original PCM data into the complex array
for (int i = 0; i < fft_size; i++)
{
float val = (float)(pcm[i] * window[i]);
complex[i] = new MathNet.Numerics.Complex32(val, 0);
}
// perform the forward FFT
MathNet.Numerics.IntegralTransforms.Fourier.Forward(complex);
// blank-out the high frequency stuff
for (int i = 0; i < fft_size / 2; i++)
{
double freq = (double)(i * sampleRate * 2) / fft_size;
if (i == fft_size / 2 - 1)
{
System.Console.WriteLine(freq);
}
if (freq < cutOffFrequency)
{
continue;
}
complex[i] = new MathNet.Numerics.Complex32(0, 0);
complex[fft_size - i - 1] = new MathNet.Numerics.Complex32(0, 0);
}
// perform the inverse FFT
MathNet.Numerics.IntegralTransforms.Fourier.Inverse(complex);
// extract the real component into the original PCM array and return it
for (int i = 0; i < fft_size; i++)
{
pcm[i] = complex[i].Real;
}
return(pcm);
}
