Patent GB-912394-A (GB)
Title: Improvements relating to radio telephony systems
IPC: H03G3/20
Filed: 1960-11-25
Abstract: 912,394. Radio signalling. FERRANTI Ltd. Nov. 25, 1960 [Nov. 28, 1959], No. 40521/60. Class 40 (5). [Also in Group XXXIX] Relates to a radio telephone system wherein, to reduce the effects of fading, the signal is chopped before transmission into a series of increments and predetermined increments are given a time delay of a predetermined magnitude, the delayed and undelayed signals being interlaced to form a continuous signal. At the receiver the increments not previously delayed are given a corresponding delay whereby the original signal is reconstituted. As shown, the audio input 1 is connected to the movable contact 2 of a chopper switch 3 of the transmitter at a station, the switch 3 being operated at 25 c/s. via an amplifier 9, divider 8, and crystal oscillator 7. The increments of twenty milliseconds duration supplied to the terminal 5 of the chopper switch are supplied via amplifier 10 to a write head 11 co-operating with a continuous magnetic tape 13 and one second later are read by the read head 15 and supplied via an amplifier 16 to an adder 17. The increments supplied to the terminal 6 of the switch 3 are supplied direct to the adder 17 where they are interlaced with the delayed signals for application to the transmitter. The signals received by the receiver at a station are supplied to a chopper switch 19 which is controlled by the oscillator 7 and adjusted by a variable delay 24 so that non-delayed increments are supplied to the terminal 21 and via an amplifier 33 to a write head 34. After a one-second delay the signals supplied to the head 34 are read by the head 35 and supplied via an amplifier 36 to an adder 37 where they are combined with the transmitter delayed increments to reconstitute the original signal. The output of the adder 37 is supplied via amplifier 38 and contacts of relay 40 to the audio output 42. During fade conditions when no useful signals are received from the remote transmitter, provided tho fade is of less duration than the delay given to the audio signals, an intelligible audio signal may be provided by the signals recorded via the head 34. During a fade the receiver A.G.C. operates a trigger 54 which via an amplifier 53 energizes the coil 50 of relay 40 to change over contact 41. The output of the amplifier 38 is supplied to the terminal 27 of chopper switch 25 and after a twenty milliseconds delay provided by a unit 48, to the terminal 28, whereby a continuous audio signal is formed. If the fade lasts for, say, three-quarters of a second, normal operation will be possible for the next quarter of a second but after this period, for a period equal to the duration of the fade, there will be no signals from the tape 13 since these signals will have been lost in the fade. During this period a signal is provided by the increments which were delayed at the transmitter. To measure the duration of a fade the trigger 54 starts a 5 kc/s. oscillator 55 supplying a write head 56, the oscillator being stopped when the fade ceases. Thus a quarter of a second after the fade the 5 kc/s. signal is supplied via the head 57, amplifier 58, rectifier 59, and amplifier 60 to the coil 51 of relay 40 and 52 of relay 45. Relays 40 and 45 change over to connect the audio output 42 to the movable contact 30 of the chopper switch 29. The output of amplifier 38 is supplied to contact 32 of switch 29, and the output delayed by twenty milliseconds is supplied to contact 31, thus providing a continuous audio output signal. When the 5 kc/s. signal from the head 57 ceases the relays release and the receiver operates normally.