GB773170A - - Google Patents

Abstract

773,170. Radio signalling. PHILIPS ELECTRICAL INDUSTRIES, Ltd. June 10, 1955 [June 15, 1954], No. 16750/55. Class 40(5) In a F.M. radio communication system between a central transmitter/receiver and a number of mobile transmitter-receivers, all the mobile 'receivers are normally muted, but are rendered operative by a short burst of 2,900 c/s tone transmitted from the central station prior to announcement of a call for a particular mobile unit, then a short burst of 2,750 c/s tone from the central station again mutes all mobile receivers except the selected one, who, it is assumed, overides the muting circuit. At the central transmitter, Fig. 1a when a calling key S is closed, a system of relays A, B, C, D Fig. 2 (not shown) is so arranged that relay B connects the tone generator 25 to the modulator 2 of the transmitter over contacts b<SP>1</SP> for about 0-5 to 1 second, to transmit the 2,900 c/s release tone, then b<SP>1</SP> changes back to the position shown and so reconnects the hand set 11 to the transmitter. An announcement calling a particular mobile unit can now be made. Meanwhile relay D has closed contacts d' to change the frequency of the tone generator 25 to the 2,750 c/s muting tone. On releasing the calling key, relay B again connects the tone generator 25 for 0.5 to 1 second to transmit the muting tone. Finally the relays re-set to their starting position, with the handset 11 connected to the transmitter and the tone generator switched for the release tone. At each mobile unit, Fig. 1b, filters 34, 35 are tuned to the release tone and the muting tone respectively and their rectified outputs are applied in opposition to a normally cut-off triode 49 controlling the muting relay 51. Reception of the release tone applies a positive bias to triode 49 relay 51 pulls up and connects the loudspeaker 55 to the receiver over contacts 54<SP>1</SP> and the triode is held conducting over contacts 50<SP>1</SP>. The announcement from the central station is now received, and if appropriate, the hand set 58 is lifted and its hook-switch bypasses the muting circuit over contacts 57. Reception of the muting tone cut-off triode 49 and restores the muting circuit thereby again muting all mobile receivers except the one at which the hand set has been lifted.