Patent GB-408329-A (GB)
Title: Improvements in or relating to two-way telephone and like electrical systems
IPC: H04B3/20
Filed: 1932-09-03
Abstract: 408,329. Two - - way transmission. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Electra House, Victoria Embankment, London, and MURPHY, F. M. G., Orchard Lodge, London Road, Chelmsford, Essex. Sept. 3, 1932, No. 6796/34. Divided on 408,300. [Class 40 (iv).] A device for rendering operative one path in a two-way repeater and disabling the other path comprises a triode valve which is controlled differentially by voltages derived from two rectifiers which receive energy from different parts of the system. As shown in Fig. 2 the condition of the repeater is determined by the voltage across a resistance R3 in the anode circuit of a valve V3 the grid circuit of which includes resistances R1, R2 which are included in the circuits of rectifiers V1, V2 fed respectively from the transmitting and receiving paths. The voltage across R<3> thus depends on the difference between the voltages across R1 R2, so that if V2 is receiving noise V1 needs a correspondingly larger input to operate the direction controlling device. The time constants of the circuits formed by resistances R1, R2 and their associated condensers are preferably much smaller than that of the circuit formed by R3 and its condenser which thus determines the hang-over time. It will be seen that since the voltage across R3 can never reverse however strong the signals passed to the rectifier V1, the hang-over time is not affected by increases in signal strength above a given level. The connections from V3 to the resistances R1, R2 are preferably made by adjustable tapping points. The time constants of the circuits may be adjustable. Fig. 3 shows the invention embodied in a terminal repeater coupling the line L with a radio transmitter and receiver. Received energy (noise or speech) after passing a filter favouring frequencies between 600 and 2000 to discriminate against noise and a volume regulator GC1 is amplified at BB1 and rectified by a full wave rectifier V<1>2 to produce a voltage across resistance R<1>2. Energy from the line L and also unbalance energy from the receiving path after passing a similar filter and a volume regulator GC2 and after amplification at A Alsimilarly produces a voltage across resistance R<1>1. These voltages act in opposition on a valve arrangement CV3 which normally biasses the transmitting amplifier DA past its cut-off point and the receiving amplifier GE to its optimum point. When speech arrives from the line L the voltage across R<1>1 has a preponderating effect and the condition of the amplifiers GE, DA is reversed. The switching action is improved by the provision of condensers NC for balancing the internal capacities of the valves. The time constant of the resistance-condenser combination in the anode circuit of the first of the valves at CV3 determines the hang-over time and is adjustable. Further apparatus is provided at Z to disable the amplifier AA1 in the presence of speech (but not of noise) from the radio receiver so as to give the distant speaker a preponderating control. This apparatus comprises an amplifier feeding a rectifier which controls the bias of the first valve in the apparatus AA1. The amplifier is so arranged with the help of a further (bridge) rectifier coupled to its output circuit and providing bias for its input circuit that its output remains constant with increasing values of noise, the apparatus AA1 thus remaining unaffected, but at the level corresponding to normal speech rises suddenly to disable the apparatus A A1. The device is also adjusted so that when the energy it receives again falls to the noise level its output is suddenly reduced. An adjustable time control circuit is provided in the output circuit of the apparatus Z to give a suitable hang-over for weak syllables and echo effects. Specification 398,509 also is referred to.