njit-etd2003-081 - New Jersey Institute of Technology

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length of the IBI signal depends on the number of heartbeats. Now, in order to take an
8192-point FFT of a signal that has only 6,000 samples, a technique called zero padding
must be used. Essentially all samples from 6001 to 8192 are given the value of zero.
The only effect this has on the spectrum is that it increases the frequency resolution.
When the FFT is calculated, the spectrum is limited from 0 Hz up to 10 Hz. The
frequency resolution is now 20/8192=0.00244 Hz. Once the power spectrum is
obtained, it is smoothed twice by applying a modified Daniel Rectangular smoothing
algorithm. Figure 3.8 illustrates the power spectrum of the IIBI signal in Figure 3.7.
Figure 3.8 Power spectrum of the heart rate IIBI signal in Figure 3.7.
It should also be noted that when performing the FFT, the IIBI signal is
windowed by applying a split cosine bell taper. The taper begins at 20 percent from
each end of the time signal. The application of the split cosine bell taper is done before
the zero padding takes place so as to provide a smooth transition to zero rather than the
window's abrupt changes from one to zero.