GB649470A - Improvements in or relating to signalling systems - Google Patents

Abstract

649,470. Electric selective signalling. WESTERN ELECTRIC CO., Inc. Aug. 6, 1948, No. 20806. Convention date, Aug. 7, 1947. [Class 40 (i)] A calling signal for a radio system comprises a plurality of distinctive audio frequencies transmitted simultaneously, so that the called station responds only to that signal which includes the pre-determined frequencies. The invention is described with reference to a plurality of mobile stations selectively called from a central office. The operator at the central office calls a particular mobile station by closing appropriate keys K1 ... K20 to put selected frequencies from generators G1 ... G20 into modulator M1 to modulate a carrier transmitted from A1 and received by all the mobile stations. At the called station, the modulating frequencies are detected at D1 and applied to an array of vibrating reeds S, each tuned to one of the selected frequencies so that when the reed contacts are coincidentally closed, valve 7 is energized to ring bell I. The removal of the handset H from its hooks opens contacts 4, 6 to put the array S out of circuit, and closes contacts 4, 5 to put the handset in circuit. An answer transmitted over M2, T2, A3 is received at the central office over A4, R2, D2, hybrid coil C, and earphones 3, the operator then' opening keys K1 ... K20 and either communicating over microphone 1 or connecting a caller on lines L1, L2 over jack C1 for communication with the mobile station. In order to reduce the time taken to the condition of simultaneous closing of the reed contacts of the called station, condensers 29 ... 43, Fig. 2, may be connected to be charged by the geographically preceding reed so that valve 44 conducts before all the reeds are in synchronism. The reeds may be timing forks, Fig. 3, 4 (not shown); they may be connected in series instead of in parallel ; they may be set into vibration by a common winding, Figs. 4, 5 (not shown) ; they may call by opening instead of closing contacts, Fig. 5 (not shown) the valve 7 may be replaced by a gas-filled diode, Figs. 3, 5 (not shown ) ; the valve 7 may be replaced by a locking relay, Fig. 4 (not shown).