Patent GB-1276941-A (GB)
Title: Radiotelephonic system
IPC: H04B7/155
Filed: 1970-03-24
Abstract: 1276941 Automatic exchange systems. INTERNATIONAL STANDARD ELECTRIC CORP 24 March 1970 [2 April 1969] 14112/70 Heading H4K A radiotelephone system has a central relay station which operates automatically and includes n complete transmitter/receiver units operating on different frequencies which can be used simultaneously and when one of said channels is not busy it transmits a low power signal comprising a low frequency modulation of its carrier frequency so that the subscriber sets which can scan the range of frequencies will lock on to this available channel. Relay station (Fig. 1).-This comprises a plurality of transmitter/receiver units each operating at a different carrier frequency and arranged so that when a first unit is available other units are disabled. Under standby conditions one channel radiates an availability signal controlled by a relay (4a) whose contacts insert a resistance into the transmitter circuitry to reduce the radiated power and inhibit operation of the adjacent channel (b). All the subscriber sets will be locked to the frequency Fa due to the presence of the low frequency modulating signal Fi and call for service on a first come first served basis by radiating a modulated carrier signal Fd for 500 ms. When signal Fd is detected in channel (a) a second relay (8a) operates to disable relay (4a) to restore the transmission to full power and to enable adjacent channel (b) which now transmits the availability signal Fi on its carrier frequency Fb. Relay (8a) also connects the transmitter directly to the receiver and is held until a break of 30 secs. in conversation allows a timer to discharge. When relay (8a) drops the channel (a) will return to the standby condition ready to radiate the availability signal when enabled by the contacts of relay (4) in an adjacent channel. Subscriber set (Fig. 2).-This consists of a transmitter (25) and receiver (23) which are connected alternately to a single antenna (29) and are tuned to the frequencies of the relay station channels by a ring counter (30) controlled by a clock pulse generator (33) which is operative until the availability frequency Fi is detected when the sets locks on to that frequency and the indicator lamp (75) is extinguished. Each set is provided with a plurality of push-buttons (50) providing a sequence of frequencies representing digits 1-10 and an additional key for radiating the frequency Fd. To make a call the hand-set (54) is lifted and the keyswitch Fd operated for at least 500 ms. to acquire the available relay station channel. The decimal keyswitches are operated corresponding to the number of the required subscriber and a relay (35) operates to put loudspeaker (39) in circuit. In the called set the frequency Fd is detected followed by the number code frequencies which if detected as correct cause a bi-stable circuit (59) to be set and energize relays which operate a calling signal oscillator (79) whose output is applied to the loud-speaker (39). If no response is obtained, i.e. the microphone (54) is not lifted, the bistable circuit will reset after a time delay causing a second bi-stable circuit (60) to be set, thus starting the clock pulse generator (33) to search for the next free channel frequency. Whenever the free channel is occupied the bi-stable circuit (60) will cause the search for a new channel to resume. Reference has been directed by the Comptroller to Specification 1,010,299.