GB1137448A - Improvements in or relating to audio frequency amplifier systems - Google Patents

Abstract

1,137,448. Microphone circuits; automatic volume control. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 24 Aug., 1967, No. 38994/67. Headings H4J and H4R. In an audio amplifier system of the fixed frequency pilot tone A.G.C. type the fixed frequency tone is of ultrasonic frequency and is injected acoustically. The system is for use with an ultrasonic pilot tone generator and transducer carried by a performer, e.g. on a lapel, spectacles, tie-pin or hung round the neck. By such arrangement movements of the performer, which cause variations in input volume, cause little variation in output volume. A loud-speaker S (Fig. 1) is fed by an amplifier A which amplifies both audio signals and ultrasonic pilot signals. A filter F separates the pilot signals which are utilized to provide A.G.C. (e.g. by rectification) to control the amplifier gain. A single microphone M receives both sound and ultrasonic waves. The sound source P of the performer has polar diagram p and the pilot transducer T has a polar diagram t. The transducer T is more directional than the performer but this does not matter at low ultrasonic frequencies (e.g. 20 to 25 kc/s.) if major movements are relied upon. With a higher ultrasonic frequency the polar diagram t may be widened, e.g. by using two or more transducers in parallel. Alternatively a microphone (M1), Fig. 2 (not shown) receives sound waves while a microphone (M2) receives the ultrasonic waves, the outputs from the microphones (M1, M2) being applied to the amplifier (A) by a mixer (C1). In another arrangement the ultrasonic microphone (M2), Fig. 3 (not shown) is coupled directly to the A.G.C. device which controls the amplifier (A), there being no feed-back type A.G.C. loop.