edu.cmu.sphinx.frontend.frequencywarp

Class MelFrequencyFilterBank

java.lang.Object

edu.cmu.sphinx.util.props.ConfigurableAdapter

edu.cmu.sphinx.frontend.BaseDataProcessor

edu.cmu.sphinx.frontend.frequencywarp.MelFrequencyFilterBank

All Implemented Interfaces:

DataProcessor, Configurable

public class MelFrequencyFilterBank
extends BaseDataProcessor

Filters an input power spectrum through a bank of number of mel-filters. The output is an array of filtered values, typically called mel-spectrum, each corresponding to the result of filtering the input spectrum through an individual filter. Therefore, the length of the output array is equal to the number of filters created.

The triangular mel-filters in the filter bank are placed in the frequency axis so that each filter's center frequency follows the mel scale, in such a way that the filter bank mimics the critical band, which represents different perceptual effect at different frequency bands. Additionally, the edges are placed so that they coincide with the center frequencies in adjacent filters. Pictorially, the filter bank looks like:

Figure 1: A Mel-filter bank.

As you might notice in the above figure, the distance at the base from the center to the left edge is different from the center to the right edge. Since the center frequencies follow the mel-frequency scale, which is a non-linear scale that models the non-linear human hearing behavior, the mel filter bank corresponds to a warping of the frequency axis. As can be inferred from the figure, filtering with the mel scale emphasizes the lower frequencies. A common model for the relation between frequencies in mel and linear scales is as follows:

melFrequency = 2595 * log(1 + linearFrequency/700)

The constants that define the filterbank are the number of filters, the minimum frequency, and the maximum frequency. The minimum and maximum frequencies determine the frequency range spanned by the filterbank. These frequencies depend on the channel and the sampling frequency that you are using. For telephone speech, since the telephone channel corresponds to a bandpass filter with cutoff frequencies of around 300Hz and 3700Hz, using limits wider than these would waste bandwidth. For clean speech, the minimum frequency should be higher than about 100Hz, since there is no speech information below it. Furthermore, by setting the minimum frequency above 50/60Hz, we get rid of the hum resulting from the AC power, if present.

The maximum frequency has to be lower than the Nyquist frequency, that is, half the sampling rate. Furthermore, there is not much information above 6800Hz that can be used for improving separation between models. Particularly for very noisy channels, maximum frequency of around 5000Hz may help cut off the noise.

Typical values for the constants defining the filter bank are:

Sample rate (Hz)	16000	11025	8000
numberFilters	40	36	31
minimumFrequency(Hz)	130	130	200
maximumFrequency(Hz)	6800	5400	3500

Davis and Mermelstein showed that Mel-frequency cepstral coefficients present robust characteristics that are good for speech recognition. For details, see Davis and Mermelstein, Comparison of Parametric Representations for Monosyllable Word Recognition in Continuously Spoken Sentences, IEEE Transactions on Acoustic, Speech and Signal Processing, 1980 .

See Also:

MelFilter

Field Summary

Fields

Modifier and Type	Field and Description
static java.lang.String	PROP_MAX_FREQ The property for the maximum frequency covered by the filterbank.
static java.lang.String	PROP_MIN_FREQ The property for the minimum frequency covered by the filterbank.
static java.lang.String	PROP_NUMBER_FILTERS The property for the number of filters in the filterbank.

Fields inherited from class edu.cmu.sphinx.util.props.ConfigurableAdapter

logger

Constructor Summary

Constructors

Constructor and Description
MelFrequencyFilterBank()
MelFrequencyFilterBank(double minFreq, double maxFreq, int numberFilters)

Method Summary

Modifier and Type	Method and Description
Data	getData() Reads the next Data object, which is the power spectrum of an audio input frame.
void	initialize() Initializes this DataProcessor.
void	newProperties(PropertySheet ps) This method is called when this configurable component needs to be reconfigured.

Methods inherited from class edu.cmu.sphinx.frontend.BaseDataProcessor

getPredecessor, setPredecessor

Methods inherited from class edu.cmu.sphinx.util.props.ConfigurableAdapter

getName, initLogger, toString

Methods inherited from class java.lang.Object

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

Field Detail

PROP_NUMBER_FILTERS

@S4Integer(defaultValue=40)
public static final java.lang.String PROP_NUMBER_FILTERS

The property for the number of filters in the filterbank.

See Also:

Constant Field Values

PROP_MIN_FREQ

@S4Double(defaultValue=130.0)
public static final java.lang.String PROP_MIN_FREQ

The property for the minimum frequency covered by the filterbank.

See Also:

Constant Field Values

PROP_MAX_FREQ

@S4Double(defaultValue=6800.0)
public static final java.lang.String PROP_MAX_FREQ

The property for the maximum frequency covered by the filterbank.

See Also:

Constant Field Values

Constructor Detail

MelFrequencyFilterBank

public MelFrequencyFilterBank(double minFreq,
                              double maxFreq,
                              int numberFilters)

MelFrequencyFilterBank

public MelFrequencyFilterBank()

Method Detail

newProperties

public void newProperties(PropertySheet ps)
                   throws PropertyException

Description copied from interface: Configurable

This method is called when this configurable component needs to be reconfigured.

Specified by:

newProperties in interface Configurable

Overrides:

newProperties in class ConfigurableAdapter

Parameters:

ps - a property sheet holding the new data

Throws:

PropertyException - if there is a problem with the properties.

initialize

public void initialize()

Description copied from class: BaseDataProcessor

Initializes this DataProcessor. This is typically called after the DataProcessor has been configured.

Specified by:

initialize in interface DataProcessor

Overrides:

initialize in class BaseDataProcessor

getData

public Data getData()
             throws DataProcessingException

Reads the next Data object, which is the power spectrum of an audio input frame. Signals are returned unmodified.

Specified by:

getData in interface DataProcessor

Specified by:

getData in class BaseDataProcessor

Returns:

the next available Data or Signal object, or returns null if no Data is available

Throws:

DataProcessingException - if there is a data processing error