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.