Abstract

A non-invasive clinical instrument provides for in vivo detection of a restriction in an artery. A microphone is applied to the body over the artery and generates a complex electrical signal which is processed to remove noise and amplified. A digital bandpass filter circuit includes a plurality of individual bandpass filters, each of which covers 1/3 of an octave and covers adjacent octaves to cover the complete spectrum between about 2 to 2,000 hertz. Switched-capacitor bandpass filters are used. Each three bandpass filters separated by one-third of an octave are connected such that setting the center bandpass filter automatically resets the two adjacent bandpass filters to cover a single octave in steps of one-third. The sensed signal which is an AC analog signal is transmitted by an analog low pass filter to block undesirably higher frequency components from the digital filters. Digital low-pass filters connect the analog low-pass filter to the bandpass filters. A display unit includes a multiplexor for sequentially transmitting the several bandpass signals to the horizontal input in proper timed relation. Each bandpass filter includes an AC (RMS) to DC (average) converter for establishing an appropriate DC output signal, which is passed through a sample and hold circuit to freeze the display.

Other References

• "Evaluation . . . by Phonoangiography", Duncan et al., New Eng. J. Med., vol. 293, No. 22, pp. 1124-1128, 1975