plpfilter.java

It is the Speech recognition software. It is platform independent. To execute the source code,

/*
 * Copyright 1999-2002 Carnegie Mellon University.  
 * Portions Copyright 2002 Sun Microsystems, Inc.  
 * Portions Copyright 2002 Mitsubishi Electric Research Laboratories.
 * All Rights Reserved.  Use is subject to license terms.
 * 
 * See the file "license.terms" for information on usage and
 * redistribution of this file, and for a DISCLAIMER OF ALL 
 * WARRANTIES.
 *
 */


package edu.cmu.sphinx.frontend.frequencywarp;


/**
 * Defines a filter used by the {@link PLPFrequencyFilterBank} class. The
 * filter is defined by a function in the {@link #PLPFilter
 * Constructor}. A set of equally spaced frequencies in a linear scale
 * is passed to the constructor, which returns the weights for each of
 * the frequency bins, such that the filter has the shape defined by
 * this piecewise function in the bark scale.
 *
 * @author <a href="mailto:[e-mail]">rsingh</a>
 * @version 1.0
 * @see PLPFrequencyFilterBank
 */
public class PLPFilter {
    private double[] filterCoefficients;
    private int numDFTPoints;

    /**
     * The center frequency of the filter in Hertz.
     */
    public double centerFreqInHz;

    /**
     * The center frequency of the filter in Bark.
     */
    public double centerFreqInBark;

    /**
     * Constructs a PLP filter around a given center frequency.
     *
     * Defines a filter according to the following equation, defined piecewise
     * (all frequencies in the equation are Bark frequencies):
     *
     * <pre>
     * 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>
     * </pre>
     * 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.
     *
     * @param DFTFrequenciesInHz is 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 centerFreqInHz is the filter's center frequency
     *
     * @throws IllegalArgumentException
     *
     */


    public PLPFilter(double[] DFTFrequenciesInHz, 
		     double centerFreqInHz) throws IllegalArgumentException {

	FrequencyWarper bark = new FrequencyWarper();

	numDFTPoints = DFTFrequenciesInHz.length;
	this.centerFreqInHz = centerFreqInHz;
	centerFreqInBark = bark.hertzToBark(centerFreqInHz);
	
	if (centerFreqInHz < DFTFrequenciesInHz[0] || 
	    centerFreqInHz > DFTFrequenciesInHz[numDFTPoints-1]) {
	    throw new IllegalArgumentException
                ("Center frequency for PLP filter out of range");
	}

	filterCoefficients = new double[numDFTPoints];

	for (int i=0; i<numDFTPoints; i++){
	    double barkf;

	    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;
	}
    }

    /**
     * Compute the PLP spectrum at the center frequency of this filter
     * for a given power spectrum.
     *
     * @param spectrum the input power spectrum to be filtered
     *
     * @return the PLP spectrum value
     *
     * @throws IllegalArgumentException if the input spectrum is of a
     *         different length than the array of filter coefficients
     */
    public double filterOutput(double[] spectrum)
	throws IllegalArgumentException {

	if (spectrum.length != numDFTPoints){
	    throw new IllegalArgumentException
                ("Mismatch in no. of DFT points " + spectrum.length +
                 " in spectrum and in filter "+numDFTPoints);
	}

	double output = 0.0;
	for (int i = 0; i < numDFTPoints; i++) {
	    output += spectrum[i]*filterCoefficients[i];
	}
	return output;
    }
}