SE320760B - - Google Patents

Abstract

1,008,027. Stethoscopes; microphones; radio signalling. MARCONI CO. Ltd. July 30, 1964 [Aug. 9, 1963], No. 31470/63. Headings H4J and H4L. In a cardiophone the output from a microphone is translated into a frequency modulated carrier signal and this signal is translated into a correspondingly frequency modulated sound wave having its mid-frequency in a sensitive part of the spectrum of human hearing. A capacitor microphone 3 (Fig. 1), designed to pick up foetal heart-beat sounds in the range 5 to 50 c/s. forms part of a frequency determining resonant circuit 2 which, together with a transistor 1, acts as a generator with a mid-frequency of between 800 and 1,200 c/s. Frequency modulated oscillations produced by variations of the amplitude of response of the microphone 3 are fed to a unit 4 including low power amplifying and radio transmitting apparatus. A capacity or loop aerials may be used. An apparatus 7 recovers the frequency modulated wave and applies it to a loud-speaker or headphones 8. The frequency modulated carrier signal and the frequency modulated sound wave may be of the same or different frequency. The mid-frequency of the sound wave may be adjusted by a heterodyne circuit. The radio link may be replaced by a flexible connector. Alternatively, the microphone, generator and sound reproducer may be a single unit adapted to be placed on a patient. Where the apparatus is in two parts the transmitting link may be a supersonic sound wave channel or an audible sound wave channel. Risk of " howling " is reduced by the arrangement and may be further reduced by filtering. Microphone.-An inertia controlled capacitor microphone is used and includes a fixed electrode plate 32 (Fig. 2) and a movable electrode 36. The electrode 36 is secured on a heavy cylinder 37 supported in a cylindrical casing 31 by springs 38. The resonance frequency of the cylinder 37 with its springs 38 is a few cycles per second and is critically damped by air flow friction between the cylinder 37 and casing 31. Alternatively, electrical or mechanical damping may be used. Eddy current damping may be obtained by making the cylinder 37, or part of of it, in the form of a magnet the field of which penetrates the outer cylinder 31, A stem 33, insulation 34 and a collar 35 form a socket to receive a plug (not shown).