High resolution spectra of the EKG grid

To see an introduction to acquiring a 5x5 grid of EKG signals, please see the previous EKG grid page.

To see an introduction to the topic of high-res line spectra of EKG signals, please see the previous high-res spectrum of the average V(3,3) signal page.

The following image shows the results of applying the high-res spectrum technique to all 25 signals in the EKG grid. To create this image, each 1 second signal was duplicated 8 times to create an "absolute periodic" signal of 8 seconds duration. Then, a discrete Fourier transform of 2049 pts was performed on the composite signal. 2049 points were used so that each 1 Hz frequency interval would fall on an integer point of the spectrum (which is not the case for a 2048 pt spectrum). A frequency range of 0-32 Hz is shown, and a Blackman data window was used to perform the DFT.

(Click for larger image)

The spectrum of the average V(3,3) signal (at the center of the grid) is typical of the results shown. The 2049 pt line spectrum of the periodic signal is somewhat different than the 256 pt continuous spectrum of a single waveform, although they do have similar overall shapes:

Of particular interest in this, and other grid spectra, is the "gap" that occurs at about 5 Hz. Although the location (and number) of gaps varies from spectrum to spectrum, it does appear to be a stable feature of most EKG signals. A possible explanation for this gap, and for the difference in spectral envelopes below and above the gap frequency, is suggested in a subsequent page of this presentation.

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