GB1458663A - Microphone circuits - Google Patents

Abstract

1458663 Microphone circuits ARD ANSTALT 14 Dec 1973 [15 Dec 1972 9 Jan 1973] 58174/72 and 1071/73 Heading H4J Unwanted sound entering two microphones is substantially cancelled by processing the electrical outputs of the two microphones, each of which outputs contains a wanted component and an unwanted component. (Unwanted components correspond to those components of sound from a single source which differ in phase at the two microphones. Wanted components correspond to those components from a single source which have equal phase at the two microphones). From the outputs two signals are derived, a first signal representative of the wanted and unwanted components and a second signal representative of only the unwanted component. The phase and/or amplitude of the second signal relative to the unwanted component of a selected one of either of said outputs or of the first signal are/is adjusted so as to derive a third signal of equal amplitude and opposite phase to the unwanted signal. The third signal is then combined with the selected signal so as to derive a fourth signal representative substantially of only the wanted component. An application is loudspeaking telephone apparatus. The outputs of two microphones M (Fig. 1) are applied to an additive circuit A (e.g. a transformer or amplifier) and to a subtractive circuit S<SP>1</SP> (e.g. a transformer or differential amplifier). The additive and subtractive outputs are passed through respective phase and amplitude adjusters P and summed in an amplifier O. Its output is connected to an amplifier K and loudspeaker L through a hybrid H, if required, and through a line terminal T to like apparatus at a remote station. Outputs from the additive and subtractive circuits A and S<SP>1</SP> are also passed to an automatic gain control circuit G effective to control amplifiers O and K so as to eliminate any spurious changes arising from the signals being processed.