edu.cmu.sphinx.frontend.transform

Class DiscreteFourierTransform

java.lang.Object

edu.cmu.sphinx.util.props.ConfigurableAdapter

edu.cmu.sphinx.frontend.BaseDataProcessor

edu.cmu.sphinx.frontend.transform.DiscreteFourierTransform

All Implemented Interfaces:

DataProcessor, Configurable

public class DiscreteFourierTransform
extends BaseDataProcessor

Computes the Discrete Fourier Transform (FT) of an input sequence, using Fast Fourier Transform (FFT). Fourier Transform is the process of analyzing a signal into its frequency components. In speech, rather than analyzing the signal over its entire duration, we analyze one window of audio data. This window is the product of applying a sliding Hamming window to the signal. Moreover, since the amplitude is a lot more important than the phase for speech recognition, this class returns the power spectrum of that window of data instead of the complex spectrum. Each value in the returned spectrum represents the strength of that particular frequency for that window of data.

By default, the number of FFT points is the closest power of 2 that is equal to or larger than the number of samples in the incoming window of data. The FFT points can also be set by the user with the property defined by PROP_NUMBER_FFT_POINTS. The length of the returned power spectrum is the number of FFT points, divided by 2, plus 1. Since the input signal is real, the FFT is symmetric, and the information contained in the whole vector is already present in its first half.

Note that each call to getData only returns the spectrum of one window of data. To display the spectrogram of the entire original audio, one has to collect all the spectra from all the windows generated from the original data. A spectrogram is a two dimensional representation of three dimensional information. The horizontal axis represents time. The vertical axis represents the frequency. If we slice the spectrogram at a given time, we get the spectrum computed as the short term Fourier transform of the signal windowed around that time stamp. The intensity of the spectrum for each time frame is given by the color in the graph, or by the darkness in a gray scale plot. The spectrogram can be thought of as a view from the top of a surface generated by concatenating the spectral vectors obtained from the windowed signal.

For example, Figure 1 below shows the audio signal of the utterance "one three nine oh", and Figure 2 shows its spectrogram, produced by putting together all the spectra returned by this FFT. Frequency is on the vertical axis, and time is on the horizontal axis. The darkness of the shade represents the strength of that frequency at that point in time:

Figure 1: The audio signal of the utterance "one three nine oh".

Figure 2: The spectrogram of the utterance "one three nine oh" in Figure 1.

Field Summary

Fields

Modifier and Type	Field and Description
static java.lang.String	PROP_INVERT The property for the invert transform.
static java.lang.String	PROP_NUMBER_FFT_POINTS The property for the number of points in the Fourier Transform.

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

logger

Constructor Summary

Constructors

Constructor and Description
DiscreteFourierTransform()
DiscreteFourierTransform(int numberFftPoints, boolean invert)

Method Summary

Modifier and Type	Method and Description
Data	getData() Reads the next DoubleData object, which is a data frame from which we'll compute the power spectrum.
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_FFT_POINTS

@S4Integer(defaultValue=-1)
public static final java.lang.String PROP_NUMBER_FFT_POINTS

The property for the number of points in the Fourier Transform.

See Also:

Constant Field Values

PROP_INVERT

@S4Boolean(defaultValue=false)
public static final java.lang.String PROP_INVERT

The property for the invert transform.

See Also:

Constant Field Values

Constructor Detail

DiscreteFourierTransform

public DiscreteFourierTransform(int numberFftPoints,
                                boolean invert)

DiscreteFourierTransform

public DiscreteFourierTransform()

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 DoubleData object, which is a data frame from which we'll compute the power spectrum. Signal objects just pass through unmodified.

Specified by:

getData in interface DataProcessor

Specified by:

getData in class BaseDataProcessor

Returns:

the next available power spectrum DoubleData object, or null if no Spectrum object is available

Throws:

DataProcessingException - if there is a processing error