GB619073A - Improvements in or relating to telephone systems - Google Patents

Abstract

619,073. Direct-call systems; anti-side-tone circuits. AUTOMATIC ELECTRIC LABORATORIES, Inc. Sept. 24, 1946, No. 28521. Convention date, Sept. 24, 1945. (Class 40 (iv)] In order to prevent feed-back between the receiver and transmitter of a substation, a two-station intercommunication system is provided with automatic means which alternately render effective the transmitter at one station simultaneously with the receiver at the other station, and the receiver at the first station simultaneously with the transmitter at the other. As shown, two thermionic tubes 4, 6 are connected in series with the transmitter 3 and receiver 5 respectively of a substation A. The control electrodes of these tubes are connected to opposite ends of the secondary of a transformer 9, to the primary of which the output of a low-frequency oscillator 23 is connected. The bias voltages thus applied to the tubes 4, 6 with a phase difference of 180 degrees cause their anode impedances to alternately increase and decrease about the mean value, thus rendering the transmitter 3 and receiver 5 alternately effective. A portion of the output of the oscillator 23 is transmitted over the line 1 through low-pass filters 11, 21 to the primary winding of transformer 19 at substation B where it is applied in a similar manner to the control electrodes of two further tubes to render alternately effective the transmitter and receiver of that substation. The connections are arranged so that the transmitter of substation A is disabled simultaneously with the receiver at substation B. The control oscillation used is of a frequency below the voice frequency range. If desired, a frequency above the voice frequency range may be used, an additional line being provided between the stations to carry the control voltage..