/// <summary>
/// Method returns FIR bandpass (close to trapezoidal) filterbank based on given frequencies.
/// </summary>
/// <param name="fftSize">Assumed size of FFT</param>
/// <param name="samplingRate">Assumed sampling rate of a signal</param>
/// <param name="frequencies">Array of frequency tuples (left, center, right) for each filter</param>
/// <param name="vtln">VTLN frequency warper</param>
/// <param name="mapper">Frequency scale mapper (e.g. herz-to-mel)</param>
/// <returns>Array of rectangular filters</returns>
public static float[][] Trapezoidal(int fftSize,
int samplingRate,
Tuple <double, double, double>[] frequencies,
VtlnWarper vtln = null,
Func <double, double> mapper = null)
{
var filterBank = Rectangular(fftSize, samplingRate, frequencies, vtln, mapper);
for (var i = 0; i < filterBank.Length; i++)
{
var filterTf = new TransferFunction(DesignFilter.Fir(fftSize / 4 + 1, filterBank[i]));
filterBank[i] = filterTf.FrequencyResponse(fftSize).Magnitude.ToFloats();
// normalize gain to 1.0
var maxAmp = 0.0f;
for (var j = 0; j < filterBank[i].Length; j++)
{
if (filterBank[i][j] > maxAmp)
{
maxAmp = filterBank[i][j];
}
}
for (var j = 0; j < filterBank[i].Length; j++)
{
filterBank[i][j] /= maxAmp;
}
}
return(filterBank);
}
/// <summary>
/// Method returns universal triangular filterbank weights based on given frequencies.
/// </summary>
/// <param name="fftSize">Assumed size of FFT</param>
/// <param name="samplingRate">Assumed sampling rate of a signal</param>
/// <param name="frequencies">Array of frequency tuples (left, center, right) for each filter</param>
/// <param name="vtln">VTLN frequency warper</param>
/// <param name="mapper">Frequency scale mapper (e.g. herz-to-mel) used here only for proper weighting</param>
/// <returns>Array of triangular filters</returns>
public static float[][] Triangular(int fftSize,
int samplingRate,
Tuple <double, double, double>[] frequencies,
VtlnWarper vtln = null,
Func <double, double> mapper = null)
{
if (mapper == null)
{
mapper = x => x;
}
Func <double, double> warp = vtln == null ? mapper : x => mapper(vtln.Warp(x));
var herzResolution = (double)samplingRate / fftSize;
var herzFrequencies = Enumerable.Range(0, fftSize / 2 + 1)
.Select(f => f * herzResolution)
.ToArray();
var filterCount = frequencies.Length;
var filterBank = new float[filterCount][];
for (var i = 0; i < filterCount; i++)
{
filterBank[i] = new float[fftSize / 2 + 1];
var left = warp(frequencies[i].Item1);
var center = warp(frequencies[i].Item2);
var right = warp(frequencies[i].Item3);
var j = 0;
for (; mapper(herzFrequencies[j]) <= left; j++)
{
;
}
for (; mapper(herzFrequencies[j]) <= center; j++)
{
filterBank[i][j] = (float)((mapper(herzFrequencies[j]) - left) / (center - left));
}
for (; j < herzFrequencies.Length && mapper(herzFrequencies[j]) < right; j++)
{
filterBank[i][j] = (float)((right - mapper(herzFrequencies[j])) / (right - center));
}
}
return(filterBank);
}
/// <summary>
/// Method creates overlapping triangular mel filters (as suggested by Malcolm Slaney).
/// </summary>
/// <param name="erbFilterCount">Number of mel filters</param>
/// <param name="fftSize">Assumed size of FFT</param>
/// <param name="samplingRate">Assumed sampling rate</param>
/// <param name="lowFreq">Lower bound of the frequency range</param>
/// <param name="highFreq">Upper bound of the frequency range</param>
/// <param name="normalizeGain">True if gain should be normalized; false if all filters should have same height 1.0</param>
/// <param name="vtln">VTLN frequency warper</param>
/// <returns>Array of mel filters</returns>
public static float[][] MelBankSlaney(
int filterCount, int fftSize, int samplingRate, double lowFreq = 0, double highFreq = 0, bool normalizeGain = true, VtlnWarper vtln = null)
{
if (lowFreq < 0)
{
lowFreq = 0;
}
if (highFreq <= lowFreq)
{
highFreq = samplingRate / 2.0;
}
var frequencies = UniformBands(Scale.HerzToMelSlaney, Scale.MelToHerzSlaney, filterCount, samplingRate, lowFreq, highFreq, true);
var filterBank = Triangular(fftSize, samplingRate, frequencies, vtln);
if (normalizeGain)
{
Normalize(filterCount, frequencies, filterBank);
}
return(filterBank);
}