GB720699A - - Google Patents

Abstract

720,699. Radio signalling. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Feb. 12, 1953 [Feb. 15, 1952], No. 3981/53. Class 40 (5). Relates to systems having a main station which communicates with a plurality of substations and provides for the transmission of a modulation tone from the main transmitter, which disables all the substation receivers except those which have responded to a call, automatically on the receipt of the response. At the main station, Fig. 1, the output valve 10 of the receiver 7 ... 12 is blocked, when no carrier is received, by a circuit 13 in which an amplifier/rectifier 16, 18 is fed from the detector 9 with noise or other voltages outside the speech-frequency band, through filter 14, 15. The blocking voltage is the difference between the rectified voltage across resistance 19 and a fixed voltage across resistance 20. When a signal is received from one or more of the substations the noise voltage decreases and valve 10 is unblocked to feed the loud-speaker 12 and to operate a relay 11. The contact 62 is thereby closed to connect a tone generator 63, operating at, say, 12 Kc/s, to modulate the transmitter 1 ... 6. At the receivers which have not responded to a call, this tone fre. quency blocks the output valve. The substation receivers have a blocking circuit 37, Fig. 2, similar to circuit 13 at the main station, which disables the output valve 36 of the receiver 33 ... 36, 55. When the main station transmits a calling carrier, the valve 36 is unblocked and the loud-speaker 55 rendered operative. At those stations called, the operator lifts the microtelephone handset 28 which operates switch 54. Contacts 56, 57 are thereby closed to connect the telephone and microphone in circuit, contact 58 operates relay 60, 61 to start up the transmitter 27, 29 ... 32 and contact 53 shunts a resistance 52 across a tuned circuit 50, 51 in the anode circuit of the blocking valve 38, 40. In the receivers of those substations which have not responded to the call, circuit 50, 51, which is tuned to the frequency of the tone generator 63 at the main transmitter, responds to this tone transmitted from the main station and provides a blocking voltage for the output valve 36.