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njit-etd2003-081 - New Jersey Institute of Technology

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158<br />

5.2 Time Frequency Analysis<br />

One <strong>of</strong> the reasons that the time-frequency analysis (TFA) has enjoyed a tremendous<br />

popularity and phenomenal development in the past three decades is that TFA has<br />

overcome some <strong>of</strong> the shortcomings <strong>of</strong> the widely used Fourier transform <strong>of</strong> spectral<br />

analysis. The Fourier transform describes the spectral properties <strong>of</strong> a time signal [23]. It<br />

provides frequency information that can only be extracted for the complete duration <strong>of</strong><br />

the signal. It gives no information about the local variations in time and requires the<br />

signal to be stationary. There is no way <strong>of</strong> knowing whether the value <strong>of</strong> the power<br />

spectral density <strong>of</strong> a signal at a particular frequency is derived from frequencies present<br />

throughout the life <strong>of</strong> the signal or during one or a few selected periods. The TFA, with<br />

its ability to perform local time-frequency decomposition (as long as it satisfies the<br />

Heisenberg's Uncertainty Principle as mentioned in section 3.2), is able to measure the<br />

time dependent variations <strong>of</strong> the instantaneous frequency contents <strong>of</strong> a nonstationary<br />

signal.<br />

5.2.1 Time Frequency Analysis Applied to Sine Waves<br />

Before seeing how TFA perform on real data, the advantages and disadvantages <strong>of</strong> the<br />

four time frequency distributions (the Short Time Fourier Transform, Smoothed<br />

Pseudo-Wigner Ville, Choi-Williams and Born-Jordan-Cohen distributions) used in this<br />

dissertation are more clearly seen when applied to signals with known spectra. Since all<br />

time frequency analyses used for heart rate variability studies in this dissertation were in<br />

the frequency range between 0.04 Hz to 0.5 Hz, a test signal was constructed with three

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