GB1305359A - - Google Patents

Abstract

1305359 Preventing acoustic reaction SZM GABR 20 Jan 1970 32411/72 Divided out of 1303792 Headings H4J and H4R In a loud-speaking substation including transmit and receive transducers and at least one amplifier, part of the signal in one channel (transmit or receive) is fed back through a selectively variable modifying network and combined with the signals in the other channel between the transducer and an associated amplifier to render the net signal in that channel independent of the signal in the first channel. As described, part of the output of the microphone amplifier is delayed in phase (e.g. inverted) and combined with the loud-speaker amplifier output to cancel signals reaching this amplifier from the microphone via a hybrid or via the transmission path. It is stated that cancellation could alternatively be effected at the microphone. In one form as shown, the loud-speaker has two coils 12 and 14. The coil 12 receives incoming signals from the line 19 via hybrid 18 and amplifier 16. The microphone 10 is coupled to the line via an amplifier 15 and the hybrid. The hybrid inevitably partially couples the microphone output to the loud-speaker and additional coupling may be effected via the line. The effect of this is eliminated by providing a second path via a variable attenuator 21 and a variable phase shifter located at 22, 22a or 22b to the second loud-speaker coil 14 to provide a second input in opposition to the undesired signal. In a second embodiment the loudspeaker has a single coil coupled to the secondary of a transformer having two primaries functioning like coils 12 and 14.