Patent GB-1184830-A (GB)
Title: Radio Telephone System
IPC: H04W84/08
Filed: 1967-12-27
Abstract: 1,184,830. Radio signalling. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 27 Dec., 1967 [30 Dec., 1966], No. 58664/67. Heading H4L. A radio telephone system operates on a common channel used by a number of organizations in a working region, each organization having a control apparatus in a central office and a plurality of mobile stations, the communication between the office and the mobile stations of all organizations in a working region being established through a common base station, and means being provided to ensure secrecy between the various organizations. In Fig. 1, two organizations with offices A and E respectively are shown, each having a plurality of mobile stations such as A1, E1, the exchange of information taking place via a base station VP. If, for example station A1 wishes to communicate with the associated office A the call key 1 is pressed and generator OG supplies a call signal of frequency fa of the particular organization, which is modulated on the carrier wave of the transmitter and transmitted to the base station VP where it is received by a receiver TO1 and passed to a receiver TO in the office A. This lights a lamp OL and operates a buzzer ZO. When the microphone MIK at the office A is lifted, a switch 6 is closed and a short clearing signal of frequency f s  is generated at STG in station VP which is followed by an opening signal of frequency f a  from generator TG1. The clearing signal closes a busy contact 2 to light a lamp BE in each mobile station, the call key 1 being blocked by means not shown, and a busy lamp BL is lit in all the offices. The control apparatus BT in all the offices except office A also is cut off. The loudspeaker LU and the speech modulation circuit of the transmitter Z are normally cut off by the open contacts 3 and 5 respectively. The opening signal closes contacts 3 and 5 and opens contacts 2 of all the stations A1, the loudspeakers, transmitters and call keys in the mobile stations of the other organizations via microphone MM remaining cut off. Station A1 can then transmit to the office A by closing speech contact 4, all the stations of organization A being able to hear and if desired, take part in the conversation. To interrupt the conversation a long clearing signal of frequency f s  is transmitted from station VP which cuts off the loudspeakers and transmitters of stations A1 and releases the call keys, and in the remaining mobile stations extinguishes the busy lamp and releases the call keys. In the offices of the other organizations the busy lamp is extinguished and the control apparatus BT released. Different calling and clearing frequencies are used for each working region. Timing circuits may be included to determine the maximum communication period and also to determine a maximum interval in a communication or at the end of a conversation after which the channel is released automatically. A third timing circuit may be provided which comes into operation after the long clearing signal has been transmitted so that the clearing signal is repeated after a predetermined interval to ensure that all the mobile stations are blocked. To improve secrecy a gate circuit may be provided in the mobile stations which allows an opening signal to pass only when it is received within a given period after the short clearing signal.