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njit-etd2003-081 - New Jersey Institute of Technology

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137<br />

4.2.7 Time-frequency Analysis<br />

The original time-frequency analysis program for STFT, SPWD, CWD and BJCD was<br />

written by Adib [45] in 1995 and was intended to analyze electromyogram (EMG) data<br />

in the frequency range from 20 Hz to 500 Hz. In order to use the same program to<br />

analyze HRV data in the frequency range <strong>of</strong> 0.05 — 0.4 Hz, modifications to the 3D<br />

mesh and contour graphic routine were made for this program to display the plots with<br />

enough frequency resolution even in the low frequency range between 0.05 — 0.4 Hz.<br />

The wavelet analysis programs used were taken from the MATLAB wavelet<br />

toolbox (The Math Works Inc., Natick, MA, 2000) also with the appropriate<br />

modification to fit the low frequency range required for HRV, systolic BP and RESP<br />

signals <strong>of</strong> the study in this dissertation. The main MATLAB m-file used was the cwt.m<br />

file. This program computed and plotted the wavelet coefficients for the physiological<br />

signals with positive scales, using the wavelets: morl (Morlet), meyr (Meyer), dB4<br />

(Daubechies 4), mexh (Mexican Hat) and Naar (Haar). The program also ran other<br />

wavelets whose names appeared as shown in Table 4.1.<br />

Basically, the process used in the cwt.m file was as follows:<br />

Let s(t) be the analyzed signal and k the wavelet. The wavelet coefficient <strong>of</strong> s at<br />

scale a and position b is defined by:<br />

Since s(t) is a discrete signal, one can use a piecewise constant interpolation <strong>of</strong><br />

the s(k) values, k=1 to length(s) where s is the total number <strong>of</strong> data points <strong>of</strong> the<br />

analyzed signal rounded up to the nearest power <strong>of</strong> two.

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