Patent GB-1297071-A (GB)
IPC: H04W76/02
Filed: 1970-02-19
Abstract: 1297071 Automatic exchange equipment CIT-COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS 19 Feb 1970 [19 Feb 1969] 8105/70 Heading H4K A telecommunications system includes a telephone exchange network and a radio telephone network operating in duplex connection with a set of common carrier frequencies and the radio telephone network has at least two transmitter/receivers which broadcast a general call code continuously on a periodically changed single free channel, whereby a radio telephone subscriber can locate a free channel when it wishes to originate a call to the telephone exchange network and where a call code individual to one radio telephone can be broadcast on a free channel by the transmitter/receiver in response to call originating in the telephone exchange network. The channels used are spaced apart by 50 incremental frequency steps and both channels have to be free before a connection can be made. Transmitter/receiver (Fig. 2).-Each channel in the transmitter/receiver is provided with a general call coder 23, individual call coder 24 and engaged signal generator 25 gated to modulator 21a through AND gates 26, 27, 28. The frequency channel selector 30 supplies the respective transmitting frequency f 2  to frequency changer 21c so that a composite signal is passed to multiplexer 8 and aerial 5. A connection to the telephone exchange is made through two line pairs 37, 38. Signals are received on aerial 6, de-multiplexed and passed to receiver 41a and diluted on frequency f 1  at 41b before passing to voice frequency detector 42, carrier frequency detector 43 and engaged tone detector 44. Under standby conditions a positive voltage is applied to terminal 16 of an idle channel causing relay 29 to be energized through gate 22 and the general call code and engaged tone f 0  to be transmitted. After a predetermined period selector 30 is stepped to another free channel and the general call code and engaged tone transmitted on this channel for the predetermined period. When calling a given radio telephone from the exchange, relay 35 is energized which causes the battery supply to be transferred from terminal 16 of this channel to terminal 16 of the next free channel and relay 29 to drop so that the individual calling code is transmitted. On receipt of its own code a radio telephone set transmits an engaged tone f 0  on its frequency channel f 1  which is detected at 44 and energizes relay 34 to cut off the clock pulse supply to frequency channel selector 30 by contacts q. The centre point B of transformer 38 is earthed to obtain service from the telephone exchange and gate 28 energized so that engaged tone f 0  is still transmitted to keep the channel connected. Radio Telephone (Fig. 3).-The radio telephone employs a frequency channel selector providing frequencies f 1 , f 2  and has a free channel detector 71 and an individual call detector similar to that shown in Specification 1,185,789. An additional general call code detector 73 used when the radio telephone is originating a call is provided and is connected to receiver 62 through relay contacts 82. When a call is received the outputs of decoders 71, 72 close gate 80 to energize relay 83 and stop switch 64. The handset can be lifted operating switch 86 which charges capacitor 87 so that on return of the handset it resets bi-stable 84 de-energizing relay 83. To call a subscriber, switch 88 is operated switching on bi-stable 85 to energize relay 82 and thus detect the general call code signal causing relay 83 to operate stopping switch 64 and transmitting the engaged tone f 0  through transmitter 61.