edu.cmu.sphinx.frontend.feature

Class DeltasFeatureExtractor

java.lang.Object

edu.cmu.sphinx.util.props.ConfigurableAdapter

edu.cmu.sphinx.frontend.BaseDataProcessor

edu.cmu.sphinx.frontend.feature.AbstractFeatureExtractor

edu.cmu.sphinx.frontend.feature.DeltasFeatureExtractor

All Implemented Interfaces:

DataProcessor, Configurable

public class DeltasFeatureExtractor
extends AbstractFeatureExtractor

Computes the delta and double delta of input cepstrum (or plp or ...). The delta is the first order derivative and the double delta (a.k.a. delta delta) is the second order derivative of the original cepstrum. They help model the speech signal dynamics. The output data is a FloatData object with a float array of size three times the original cepstrum, formed by the concatenation of cepstra, delta cepstra, and double delta cepstra. The output is the feature vector used by the decoder. Figure 1 shows the arrangement of the output feature data array:

Figure 1: Layout of the returned features.

Suppose that the original cepstrum has a length of N, the first N elements of the feature are just the original cepstrum, the second N elements are the delta of the cepstrum, and the last N elements are the double delta of the cepstrum.

Figure 2 below shows pictorially the computation of the delta and double delta of a cepstrum vector, using the last 3 cepstra and the next 3 cepstra.
Figure 2: Delta and double delta vector computation.

Referring to Figure 2, the delta is computed by subtracting the cepstrum that is two frames behind of the current cepstrum from the cepstrum that is two frames ahead of the current cepstrum. The computation of the double delta is similar. It is computed by subtracting the delta cepstrum one time frame behind from the delta cepstrum one time frame ahead. Replacing delta cepstra with cepstra, this works out to a formula involving the cepstra that are one and three behind and after the current cepstrum.

Field Summary

Fields inherited from class edu.cmu.sphinx.frontend.feature.AbstractFeatureExtractor

cepstraBuffer, cepstraBufferEdge, cepstraBufferSize, currentPosition, PROP_FEATURE_WINDOW, window

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

logger

Constructor Summary

Constructors

Constructor and Description
DeltasFeatureExtractor()
DeltasFeatureExtractor(int window)

Method Summary

Modifier and Type	Method and Description
protected Data	computeNextFeature() Computes the next feature.

Methods inherited from class edu.cmu.sphinx.frontend.feature.AbstractFeatureExtractor

getData, initialize, newProperties

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

Constructor Detail

DeltasFeatureExtractor

public DeltasFeatureExtractor(int window)

DeltasFeatureExtractor

public DeltasFeatureExtractor()

Method Detail

computeNextFeature

protected Data computeNextFeature()

Computes the next feature. Advances the pointers as well.

Specified by:

computeNextFeature in class AbstractFeatureExtractor

Returns:

the feature Data computed