GB527936A - Improvements in or relating to electric signal transmission systems - Google Patents

Abstract

527,936. Repeating arrangements ; automatic volume control. ELECTRICAL RESEARCH PRODUCTS, Inc. April 24, 1939, No. 12225. Convention date, April 28, 1938. [Class 40 (iv)] In a system having a four-wire section provided at either end with a volume compressor in the transmitting channel and a volume expander in the receiving channel, each comprising a voltage-operated impedance network, the control circuits are interconnected so that an increase of gain in the receiving channel is accompanied by a reduction of gain in the transmitting channel in order to avoid singing. As shown, the compressor 19 comprises a rectifier bridge 33, while the expander 22 comprises series rectifiers 46, 48 shunted by resistances. The compressor is subject to a backward control from the output of an amplifier 40 through a rectifier bridge 21, and the expander is subject to a forward control through 'a rectifier bridge 24. Condensers 43, 41 in association with resistances provide a delay in the controls. In Fig. 2, the outputs of the bridges 21, 24 are directly connected together, so that current in either control circuit reduces the gain of the compressor and increases that of the expander, and there is no increased tendency for the system to sing. In this arrangement, when the transmitting channel is in use, the noise which reaches the talker over the receiving channel is increased. This effect is obviated in the modification in Fig. 3 by the provision of a rectifier 55 in the connection between the bridges 21, 24 to prevent the control of the expander from the transmitting channel. In addition a battery 60 is inserted to provide a bias which must be overcome before the compressor can be affected by current from the receiving channel. This is advantageous when the noise level is high.