/**
* Build a PLP filterbank with the parameters given. The center frequencies of the PLP filters will be uniformly
* spaced between the minimum and maximum analysis frequencies on the Bark scale. on the Bark scale.
*
* @throws IllegalArgumentException
*/
private void BuildCriticalBandFilterbank()
{
double minBarkFreq;
double maxBarkFreq;
double deltaBarkFreq;
double nyquistFreq;
double centerFreq;
var numberDFTPoints = (_numberFftPoints >> 1) + 1;
double[] DFTFrequencies;
/* This is the same class of warper called by PLPFilter.java */
var bark = new FrequencyWarper();
_criticalBandFilter = new PLPFilter[_numberFilters];
if (_numberFftPoints == 0)
{
throw new ArgumentException("Number of FFT points is zero");
}
if (_numberFilters < 1)
{
throw new ArgumentException("Number of filters illegal: "
+ _numberFilters);
}
DFTFrequencies = new double[numberDFTPoints];
nyquistFreq = _sampleRate / 2;
for (var i = 0; i < numberDFTPoints; i++)
{
DFTFrequencies[i] = i * nyquistFreq /
(numberDFTPoints - 1);
}
/**
* Find center frequencies of filters in the Bark scale
* translate to linear frequency and create PLP filters
* with these center frequencies.
*
* Note that minFreq and maxFreq specify the CENTER FREQUENCIES
* of the lowest and highest PLP filters
*/
minBarkFreq = bark.HertzToBark(_minFreq);
maxBarkFreq = bark.HertzToBark(_maxFreq);
if (_numberFilters < 1)
{
throw new ArgumentException("Number of filters illegal: "
+ _numberFilters);
}
deltaBarkFreq = (maxBarkFreq - minBarkFreq) / (_numberFilters + 1);
for (var i = 0; i < _numberFilters; i++)
{
centerFreq = bark.BarkToHertz(minBarkFreq + i * deltaBarkFreq);
_criticalBandFilter[i] = new PLPFilter(DFTFrequencies, centerFreq);
}
}
/// <summary>
/// Initializes a new instance of the <see cref="PLPFilter"/> class.
/// <para> Defines a filter according to the following equation, defined piecewise (all frequencies in the equation are Bark
/// frequencies):</para>
/// Filter(f) = 0 if f < -2.5 <br>
/// = 10^(-(f+0.5)) if -2.5 <= f <= -0.5 <br>
/// = 1 if -0.5 <= f <= 0.5 <br>
/// = 10^(2.5(f-0.5)) if 0.5 <= f <= 1.3 <br>
/// = 0 if f > 1.3 <br>
/// The current implementation assumes that the calling routine passes in an array of frequencies, one for each of
/// the DFT points in the spectrum of the frame of speech to be filtered. This is used in conjunction with a
/// specified center frequency to determine the filter.
///
/// </summary>
/// <param name="dftFrequenciesInHz">A double array containing the frequencies in Hertz corresponding to each of the DFT points in the spectrum of the signal to be filtered.</param>
/// <param name="centerFreqInHz">The filter's center frequency.</param>
/// <exception cref="System.ArgumentException">Center frequency for PLP filter out of range</exception>
public PLPFilter(double[] dftFrequenciesInHz, double centerFreqInHz)
{
var bark = new FrequencyWarper();
_numDftPoints = dftFrequenciesInHz.Length;
CenterFreqInHz = centerFreqInHz;
CenterFreqInBark = bark.HertzToBark(centerFreqInHz);
if (centerFreqInHz < dftFrequenciesInHz[0] ||
centerFreqInHz > dftFrequenciesInHz[_numDftPoints - 1])
{
throw new ArgumentException("Center frequency for PLP filter out of range");
}
_filterCoefficients = new double[_numDftPoints];
for (var i = 0; i < _numDftPoints; i++)
{
double barkf = bark.HertzToBark(dftFrequenciesInHz[i]) - CenterFreqInBark;
if (barkf < -2.5)
{
_filterCoefficients[i] = 0.0;
}
else if (barkf <= -0.5)
{
_filterCoefficients[i] = Math.Pow(10.0, barkf + 0.5);
}
else if (barkf <= 0.5)
{
_filterCoefficients[i] = 1.0;
}
else if (barkf <= 1.3)
{
_filterCoefficients[i] = Math.Pow(10.0, -2.5 * (barkf - 0.5));
}
else
{
_filterCoefficients[i] = 0.0;
}
}
}