  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.
