/// <summary>
/// Compute Mel Frequency Filters
/// </summary>
/// <param name="winsize">the window size</param>
/// <param name="sampleRate">the sample rate</param>
/// <param name="numberFilters">number of filters in the filterbank</param>
/// <param name="minFreq">lowest frequency in the range of interest</param>
public MelFilter(int winsize, int sampleRate, int numberFilters, int minFreq)
{
// arrays to store the mel frequencies and the herz frequencies
var mel = new double[sampleRate / 2 - minFreq + 1];
var freq = new double[sampleRate / 2 - minFreq + 1];
// Mel Scale from StartFreq to SamplingRate/2, step every 1Hz
for (int f = minFreq; f <= sampleRate / 2; f++)
{
mel[f - minFreq] = MelUtils.LinToMelFreq(f);
freq[f - minFreq] = f;
}
// prepare filters
var freqs = new double[numberFilters + 2];
melScaleFreqsIndex = new int[numberFilters + 2];
for (int f = 0; f < freqs.Length; f++)
{
double melIndex = 1.0 + ((mel[mel.Length - 1] - 1.0) /
(freqs.Length - 1.0) * f);
double min = Math.Abs(mel[0] - melIndex);
freqs[f] = freq[0];
for (int j = 1; j < mel.Length; j++)
{
double cur = Math.Abs(mel[j] - melIndex);
if (cur < min)
{
min = cur;
freqs[f] = freq[j];
}
}
melScaleFreqsIndex[f] = MathUtils.FreqToIndex(freqs[f], sampleRate, winsize);
}
// triangle heights
melScaleTriangleHeights = new double[numberFilters];
for (int j = 0; j < melScaleTriangleHeights.Length; j++)
{
// Let's make the filter area equal to 1.
// filterHeight = 2.0 / (rightEdge - leftEdge);
melScaleTriangleHeights[j] = 2.0 / (freqs[j + 2] - freqs[j]);
}
var fftFreq = new double[winsize / 2 + 1];
for (int j = 0; j < fftFreq.Length; j++)
{
fftFreq[j] = ((sampleRate / 2) / (fftFreq.Length - 1.0)) * j;
}
// Compute the mel filter Weights
filterWeights = new Matrix(numberFilters, winsize / 2);
for (int j = 0; j < numberFilters; j++)
{
for (int k = 0; k < fftFreq.Length; k++)
{
if ((fftFreq[k] > freqs[j]) && (fftFreq[k] <= freqs[j + 1]))
{
filterWeights.MatrixData[j][k] = (melScaleTriangleHeights[j] *
((fftFreq[k] - freqs[j]) / (freqs[j + 1] - freqs[j])));
}
if ((fftFreq[k] > freqs[j + 1]) &&
(fftFreq[k] < freqs[j + 2]))
{
filterWeights.MatrixData[j][k] += (melScaleTriangleHeights[j] *
((freqs[j + 2] - fftFreq[k]) / (freqs[j + 2] - freqs[j + 1])));
}
}
}
}
/// <summary>
/// Creates a new MelFilterBank.
/// </summary>
/// <param name="minFreq">The minimum frequency in hz to be considered, i.e.
/// the left edge of the first TriangleFilter.</param>
/// <param name="maxFreq">The maximum frequency in hz to be considered, i.e.
/// the right edge of the last TriangleFilter.</param>
/// <param name="numMelBands">The number of Mel bands to be calculated, i.e.
/// the number of TriangleFilters to be applied.</param>
/// <param name="numBins">The number of bins that are present in the fft_buffer
/// that will be passed to the MelFilterBank.Apply method. This is also
/// required to properly configure the TriangleFilter instances which
/// operate on array indices only. (half the window size)</param>
/// <param name="sampleRate">The original sample rate the FFT buffer which will
/// be passed to MelFilterBank.Apply is based on.</param>
/// <param name="doNormalizeFilterArea">If set to "true", the area of the
/// created TriangleFilter will be normalized, e.g. the height of the
/// filter's triangle shape will be configured in a way, that the area
/// of the triangle shape equals one.</param>
public MelFilterBank(double minFreq, double maxFreq, int numMelBands, int numBins, int sampleRate, bool doNormalizeFilterArea)
{
this.minFreq = minFreq;
this.maxFreq = maxFreq;
this.numMelBands = numMelBands;
this.numBins = numBins;
this.sampleRate = sampleRate;
this.doNormalizeFilterArea = doNormalizeFilterArea;
// Let's do some argument checking
if ((minFreq >= maxFreq) || (maxFreq == 0))
{
throw new ArgumentException(String.Format("Invalid min/max frequencies for MelFilterBank: min = '{0}' max = '{1}'", minFreq, maxFreq));
}
if (numMelBands == 0)
{
throw new ArgumentException(String.Format("Invalid number of mel bands for MelFilterBank: n = {0}", numMelBands));
}
if (sampleRate == 0)
{
throw new ArgumentException(String.Format("Invalid sample rate for MelFilterBank: s = {0}", sampleRate));
}
if (numBins == 0)
{
throw new ArgumentException(String.Format("Invalid number of bins for MelFilterBank: s = '{0}'", numBins));
}
// 2 * numBins should be the same as window length
double deltaFreq = (double)sampleRate / (2 * numBins);
double melMin = MelUtils.LinToMelFreq(minFreq);
double melMax = MelUtils.LinToMelFreq(maxFreq);
// We divide by #band + 1 as min / max should present the beginning / end
// of beginng up / ending low slope, i.e. it's not the centers of each
// band that represent min/max frequency in mel bands.
double deltaFreqMel = (melMax - melMin) / (numMelBands + 1);
// Fill up equidistant spacing in mel-space
double melLeft = melMin;
for (int i = 0; i < numMelBands; i++)
{
double melCenter = melLeft + deltaFreqMel;
double melRight = melCenter + deltaFreqMel;
double leftHz = MelUtils.MelToLinFreq(melLeft);
double rightHz = MelUtils.MelToLinFreq(melRight);
// align to closest num_bins (round)
int leftBin = (int)((leftHz / deltaFreq) + 0.5);
int rightBin = (int)((rightHz / deltaFreq) + 0.5);
// calculate normalized height
double height = 1.0;
if (doNormalizeFilterArea)
{
height = 2.0 / (rightBin - leftBin);
}
// Create the actual filter
var filter = new TriangleFilter(leftBin, rightBin, height);
filters.Add(filter);
// next left edge is current center
melLeft = melCenter;
}
}