A Phase Filter Applied to Spectral Phonocardiography

Spectral phonocardiography is the name applied to the process of analyzing heart sounds to provide a frequency-amplitude-time display. A segment of heart sound is recorded on a magnetic medium and then repeatedly analyzed by a heterodyne filter system. Because the bandwidth of a conventional filter determines both the frequency and the time resolution, a difficulty arises in that the selection of a wideband filter to obtain adequate time resolution leads to a poor frequency resolution and vice-versa. By using the fact that the phase-frequency characteristic of a filter is changing at resonance whereas its amplitude response is not, it is possible to obtain both good time and frequency resolution at the expense of amplitude information. This paper describes the application of this principle to the design of a phase filter and shows that it produces an analysis having some of the features of the sense of hearing. The design, construction, and operation of the phase filter for this application are described. Included are several sets of sonograms comparing results obtained with the conventional filter and with the phase filter. Suggested changes that could be incorporated in subsequent models are discussed.