Patent GB-1267652-A (GB)
Title: Improvements relating to radio communication systems
Inventor: COVILL DENNIS HENRY
IPC: H04B7/12
Filed: 1968-01-31
Abstract: 1,267,652. Radio signalling. E.M.I. ELECTRONICS CANADA Ltd. 28 April, 1969 [31 Jan., 1968], No. 4907/68. Heading H4L. In a radio communication system the transmission is interrupted at intervals to enable a search to be made automatically through a sequence of carrier frequencies to determine a satisfactory carrier frequency to be used under the prevailing conditions. At station A, Fig. 1, audio signals are supplied via a gate 8 and data signals via a gate 9 to a modulator 6 receiving a selected carrier frequency from a bank of oscillators 7. A clock 19 provides a pulse, say every five minutes, and if an audio signal is being transmitted switches Sl, S2 are closed and this pulse is supplied via OR gate 15 to inhibit the gate 8. If a data signal is being transmitted switches S1 S2 are open and a bi-stable 13 is set by clock 19 and cleared when a space in the signals is detected at 12 whereupon gate 9 is blocked. When either gate 8 or 9 is blocked a pulse is supplied from clock 19 or bi-stable 13 to a trigger 17 which causes block 18 to provide an output consisting of a " carriage return " character followed by the call sign of station A which is fed via OR gate 10 to the modulator 6. The output of the modulator is supplied via gate 5 and power amplifier 2 to the station B. A pulse from clock 19 or bistable 13 is also supplied via OR gate 15 and delays 14, 20, delaying the pulse for atime sufficient to allow transmission of the carriage return and call sign, to reset the tuning of station A transmitter and receiver through carrier select circuits 11 and 28 respectively. The pulse from delay 14 blocks gate 5 and opens gate 4 so that attenuator 3 causes the succeeding signals to be transmitted at reduced power. The pulse from delay 20 is fed via OR gate 21 to trigger 22 so that an output consisting of three characters of 21 bits generated at 23 is supplied via OR gate 10 to the modulator 6 to be transmitted at reduced power. At station B, Fig. 2, the carrier in use at station A is supplied via amplifier 49 to a frequency changer 50, the resulting signals being supplied to a detector 55. Reception of the carriage return character and call sign causes these signals to be printed and detector 55 provides a pulse which after a delay at 59 causes carrier select circuits 47, 52 to reset the tuning of the transmitter and receiver to the appropriate frequencies. When the three character 21 bits signal is detected at 55 gate 60 is opened so that this signal is fed to the modulator 45. Another pulse from detector 55 blocks gate 44 and opens gate 43 so that the 21 bits signal is retransmitted to station A at reduced power. At station A these signals are frequency-changed at 26 and supplied to a detector 31 which, if the signal-tonoise ratio is satisfactory, supplies a pulse to open gate 5 and block gate 4, open gates 8 and 9 and via trigger 34 cause two characters of 14 bits to be supplied via gate 10 to the modulator 6. These characters are transmitted at full power and on being detected at 55, station B, cause gates 43, 60 to be blocked and gate 44 to be opened to resume normal operation. If the signal-to-noise ratio of the signal at reduced power from station B is unsatisfactory as detected at 31, Station A, a pulse is supplied via output 33 to cause circuits 11, 28 to select the next pair of frequencies in the sequence and trigger 22 is operated to cause another signal of three characters of 21 bits to be transmitted at reduced power. At station B, detector 55 causes the signal to be retransmitted and also provides a pulse to circuits 47, 52 to select the appropriate carrier frequencies this process being continued until satisfactory communication is established. If the complete sequence of carrier frequencies is tested without obtaining satisfactory communication means are provided to signal a " communication out " condition. Each time a pulse from detector 31, Station A is supplied to gate 21 a bi-stable 36 is set and starts a counter 37 and similarly at Station B a pulse from detector 55 via delay 59 sets a bi-stable 61 to start a counter 62. When detector 31 indicates a satisfactory signal-to-noise-ratio, the bi-stable 36 is reset via output 32 and stops the counter 37 and when detector 55 receives the two character 14 bits signal the bi-stable 61 is reset and counter 62 stopped. If satisfactory signalling is not established within a predetermined timethe counters 37, 62 trigger alarms and also supply pulses to circuits 39, 64, respectively so that they sequentially select each carrier frequency pair in turn. At Station B this stepping occurs at twice the normal rate but at Station A it takes place at a much slower rate so that the transmitter and receiver dwell on each frequency long enough for Station B to step through all the available frequencies, thus dispensing with the need for synchronization. Reception of a satisfactory signal at Station A will cause the system to respond as described previously.