Amplitude Modulation Effects in Cardiac Signals

TL;DR

This paper discusses how amplitude modulation affects the spectral analysis of cardiac signals and proposes rectification or Teager-Kaiser operator application before FFT to preserve important frequency information.

Contribution

It introduces a simple method to prevent amplitude modulation effects in spectral analysis of cardiac signals using rectification or Teager-Kaiser operator before FFT.

Findings

Amplitude modulation can obscure heart beat signals in spectral analysis.

Rectification before FFT restores lost spectral information.

Teager-Kaiser operator provides an alternative to rectification for spectral clarity.

Abstract

A subject's heart beat can be nearly invisible in a spectrum, when that spectrum is generated using conventional methods of Fourier analysis. The phenomenon has been observed in records of both electrocardiography type and seismocardiography type. The mechanisms of nonlinear physics responsible for these complexities involve the phenomenon of amplitude modulation. Fortunately, there is a simple remedy to prevent loss of valuable frequency domain information. Instead of operating on the raw signal, one simply rectifies that signal before performing the fast Fourier transform (FFT) calculation. Alternatively, nearly equivalent spectra can be obtained by operating on the signal with the Teager-Kaiser operator before doing the FFT.