edu.cmu.sphinx.frontend.frequencywarp

Class MelFilter

java.lang.Object

edu.cmu.sphinx.frontend.frequencywarp.MelFilter

public class MelFilter
extends java.lang.Object

Defines a triangular mel-filter. The MelFrequencyFilterBank creates mel-filters and filters spectrum data.

A mel-filter is a triangular shaped bandpass filter. When a mel-filter is constructed, the parameters leftEdge, rightEdge, centerFreq, initialFreq, and deltaFreq are given to the Constructor. The first three arguments to the constructor, i.e. leftEdge, rightEdge, and centerFreq, specify the filter's slopes. The total area under the filter is 1. The filter is shaped as a triangle. Knowing the distance between the center frequency and each of the edges, it is easy to compute the slopes of the two sides in the triangle - the third side being the frequency axis. The last two arguments, initialFreq and deltaFreq, identify the first frequency bin that falls inside this filter and the spacing between successive frequency bins. All frequencies here are considered in a linear scale.

Figure 1 below shows pictorially what the other parameters mean.

Figure 1: A triangular mel-filter.

See Also:

MelFrequencyFilterBank

Constructor Summary

Constructors

Constructor and Description
MelFilter(double leftEdge, double centerFreq, double rightEdge, double initialFreq, double deltaFreq) Constructs a filter from the parameters.

Method Summary

Modifier and Type	Method and Description
double	filterOutput(double[] spectrum) Compute the output of a filter.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

MelFilter

public MelFilter(double leftEdge,
                 double centerFreq,
                 double rightEdge,
                 double initialFreq,
                 double deltaFreq)
          throws java.lang.IllegalArgumentException

Constructs a filter from the parameters.

In the current implementation, the filter is a bandpass filter with a triangular shape. We're given the left and right edges and the center frequency, so we can determine the right and left slopes, which could be not only asymmetric but completely different. We're also given the initial frequency, which may or may not coincide with the left edge, and the frequency step.

Parameters:

leftEdge - the filter's lowest passing frequency

centerFreq - the filter's center frequency

rightEdge - the filter's highest passing frequency

initialFreq - the first frequency bin in the pass band

deltaFreq - the step in the frequency axis between frequency bins

Throws:

java.lang.IllegalArgumentException - if input is invalid

Method Detail

filterOutput

public double filterOutput(double[] spectrum)

Compute the output of a filter. We're given a power spectrum, to which we apply the appropriate weights.

Parameters:

spectrum - the input power spectrum to be filtered

Returns:

the filtered value, in fact a weighted average of power in the frequency range of the filter pass band