Patent GB-860820-A (GB)
IPC: H04W84/02
Abstract: 860,820. Radio signalling. START DER NEDERLANDEN, TEN DEZE VERTEGENWOORDIGD DOOR DE DIRECTEUR-GENERAAL DER POSTERIJEN TELEGRAFIE EN TELEFONIE, DE. March 11,1958 [March 18, 1957], No. 7794/58. Class 40 (5). A secret and selective radio calling system including a main station and substations, in which each substation has a receiver normally blocked, and each receiver has filters selective to a release tone and a blocking tone respectively, the outputs from the filters controlling the blocking circuit so that normally reception of a release tone enables the reproducing circuit, and subsequently reception of the blocking tone disables said reproducing circuit, each receiver having means whereby a blocking tone from the main station disables all substation receivers except the one called, and means wherein the filters at the called substation resume their normal functions after a delay so that a further blocking tone from the main station also disables the said called substation. In a known embodiment, a circuit Fig. 1 (not shown), is connected by terminals p, q to the output of a receiver. L 1 , C 1  resonate at f 1  and L 2 , C 2  at f 2 . A tapping on L 1  goes to anode 1 of B 1  and a tapping on L 2  through C 3  to anode 2 of B 1 . On receipt of signal f 1 , L 1 , C 1  resonate, and part of the voltage across L 1  is rectified by the left-hand diode B 1 , whereby the cathodes become positive and the grid of B 2  also is positive. Relay TR is normally maintained unenergized by positive bias S. This bias is overcome and relay TR energizes, and is self-holding. The speaker (not shown) is now connected. The filter R 3 , C 5  prevents operation of the circuit by speech frequencies. After transmission of f 1  all subscriber stations are operational. The called subscriber then connects his loud-speaker directly to the receiver, switching-off the blocking device. The main station then transmits f 2  and L 2 , C 2  resonates, voltage across L 2  is applied to righthand diode of B 1 , and the potential at the junction of R 2 , R 3  is decreased, thus decreasing grid potential of B 2 , relay TR releases muting the receivers of those mobile stations who have not disconnected the blocking devices. In order to ensure secrecy, in the embodiment described in Fig. 3, the mobile stations forming part of a selective system do not control the blocking device themselves. The tap on L 1  goes via relay contact a 1  to anode 1 of B 1 , and the tap on L 2  via relay contact a 2  to anode 2 of B 1 . 1 Relay A is energized whenever the selective receiver operates. The selective receiver SO, Fig. 2, including relay A is connected to the mobile station M including TX antenna ZA, and tone-operated blocking device TS with blocking relay TR and telephone set BT. On reception of a selective call from the main station the called subscriber's receiver SO operates, operating relay A. After the selective call, the main station transmits a blocking tone f 2 , only the called subscriber station will be unblocked by operation of relay A, because the tapping on L 2  is connected via the front contact a 2  to the left-hand anode of B 1 . Relay A is slow releasing. Thus at the end of the call the main station operator can block the called subscriber by transmitting f 2 .