Patent GB-701607-A (GB)
Title: Improvements in or relating to two-way telecommunication systems
IPC: H04B3/20
Filed: 1951-05-29
Abstract: 701,607. Two-way transmission. RYALL, L. E. May 16, 1952 [May 29, 1951], No. 12598/51. Class 40 (4) In a voice-operated switching arrangement for controlling the effective direction of transmission over a two-way communication system, the voltage for controlling the switching is derived from a circuit coupled to both the incoming and outgoing signal paths, wherein a circuit element common to both input circuits and having a time constant less than the hang-over time of the switching arrangement produces a potential to suppress the effect of unwanted switching signals, e.g. hybrid leakage currents, tending to reverse the existing switching condition. In the schematic arrangement, Fig. 2, controlling potentials for controlling attenuator networks S<SP>1</SP>, S2 are generated across points AB and A<SP>1</SP>B of a balanced network comprising the anode circuits of valves V<SP>1</SP>, V<SP>2</SP> by changes in the anode current of the two valves in response to voice currents in either the transmit or receive path. Speech currents in the transmit path are fed over transformer T<SP>1</SP> and rectifier W<SP>1</SP> to apply a + ve bias to valve V<SP>1</SP> which bias increases the anode current in valve V<SP>1</SP>, and through common cathode resister UAB reduces the current through valve V<SP>2</SP> to make point A even more positive to point B. The positive voltage applied to the grid of V<SP>1</SP> is limited by the flow of grid current and in the circumstance when V<SP>2</SP> is used as a transmitter amplifier stage the negative bias does not vary outside of limits suitable for class A amplification. The series grid resistor R<SP>2</SP> is one half of the value of resistor R<SP>1</SP> and the change of voltage distribution arising from the flow of grid current reduces variation in discharge time arising from change of signal amplitude. The common network R<SP>1</SP>, C<SP>1</SP> serves to back-bias that rectifier of W<SP>1</SP>, W<SP>2</SP> - which is not applying the effective + ve bias to its controlling valve and any excessive negative bias which may result on the grid of the other valve is suppressed by the appropriate rectifier W<SP>7</SP>, W<SP>8</SP>. Although the network R<SP>1</SP>, C<SP>1</SP> is designed to suppress false switching signals such as leakage signals across from one path to the other, a frequency discriminating network UBD, UBB, UBJ, UBL is inserted in each input circuit to overcome the larger leakage signals due to hybrid unbalance at higher frequencies, and a condenser QC is connected across points AB to reduce inter-path leakage. A complete loudspeaker terminal incorporating both-way amplifiers voice controlled in accordance with the invention in which the switching valves serve also as speech amplifiers is described with reference to Fig. 3 (not shown). Additional features included are mains hum rejection, protection of the rectifiers in the attenuation networks, receiver volume control arrangements and an arrangement to counteract the reversal of the switching by random noise when the incoming signal is weak. Provision is also made to reduce the heaters to a condition of semienergization whilst the amplifiers are quiescent. Specifications 415,767; 416,372 and 430,567 are referred to.