GB721041A - Improvements in and relating to apparatus for use in frequency responsive electricalremote control systems - Google Patents

Abstract

721,041. Exchange systems. BRITISH TELECOMMUNICATIONS RESEARCH, Ltd., HOLLOWAY, D. G., and FELIX, M. O. July 29, 1949 [Aug. 5, 1948], No. 20737/48. Class 40 (4). In a transmission system in which signalling is effected by substituting a frequency f2 for a normally present frequency f1, a receiver comprises a limiter 10 feeding circuits 12 and 22 tuned to f1 and f2 respectively whose outputs are rectified at 14 and 23 respectively to produce oppositely poled outputs. These outputs feed a circuit 15, 16, 28 which thereupon provides the valve 20 with a voltage substantially equal to the peak-to-peak value of the rectified voltages, the valve being biased so as to respond only to voltages substantially in excess of that produced by the one rectifier 14 acting alone. Conduction of the valve 20 operates the signalling relay 21. Discrimination against speech currents is effected through (a) the action of the limiter whose tuned circuits cannot produce a sufficiently large output unless the tones received are substantially pure, (b) the improbability of the frequency f2 occurring immediately after f1, and (c) the fact that the frequency f1 must be of sufficient duration to charge condenser 28.