Patent GB-958763-A (GB)
Title: Radio pulse communication systems
IPC: H04W99/00
Filed: 1963-02-21
Abstract: 958,763. Pulse code modulation systems; combined radio receiving and transmitting systems. MOTOROLA Inc. Feb. 21, 1963 [March 22, 1962], No. 7113/63. Headings H3Q and H4L. In a pulse transmission system in which the presence or absence of a pulse constitutes the information to be transmitted, each pulse is allotted a predetermined number of time slots and is transmitted as a plurality of short pulses of different frequencies appearing in succession in selected time slots. Each receiver is allotted its own combination of frequencies and time slots to represent the coded pulses and can only respond to signals of that combination, thus enabling a plurality of communications between different transmitters and receivers of the same type to take place simultaneously over a single wideband channel. The system is described in relation to portable transmitterreceivers using delta modulation. As shown in Fig. 2, signals from the microphone in handset 10 are supplied to a delta modulator 12 controlled by a clock generator 15 and the resulting coded pulses are supplied via a jack 13 (which enables the insertion of a secrecy device) to an " Address selector " 20 for selecting the combination of frequencies and their timing in accordance with the desired receiving station. The address system includes oscillators 16, 17, 18 which may be switched to different crystals so that each oscillator provides a choice of a plurality of different frequencies. A timing unit 25 is controlled by the clock 15 and controls gates 21, 22, 23 via address selector 20 so that they are opened when a pulse is applied simultaneously from the modulator 12, to allow a wave from the corresponding oscillator 21, 22 or 23 to be passed via amplifier 26 to the aerial. The timing unit establishes eight time slots for each pulse from the modulator 12 (as shown in Fig. 4), and the selector 20 selects three of these time slots so that, for example, the pulse group X consisting of frequencies F 1 , F 3 , F 5  is transmitted for each pulse. A, B and C shows the times at which gates 21, 22 and 23 are opened. The frequency components have a shorter duration than that of a time slot so that a spacing is provided between components in adjacent slots. Received signals are supplied via an R.F. amplifier 30 to a mixer 31. Oscillators 32, 33, 34 are coupled to the mixer 31 via respective gates 36, 37, 38 which are controlled by timing unit 25 to be operative at the appropriate time slots and supply the appropriate frequencies for that receiver so that the correct intermediate frequency is supplied via I.F. amplifier 40 to the detector 41. A counter 44 counts the detected pulses in each group received and if all three pulses are present supplies a pulse to operate a multivibrator which reconstitutes the original pulse train. If the count is less than three no pulse is produced and the counter is reset by clock 15. The pulse train from the multivibrator 46 is decoded at 49 and supplied via A.F. amplifier 50 to the receiver of the handset 10 or, alternatively to a loud-speaker 53, the loud-speaker being connected when the handset is on its rest so that a calling signal is provided. The clock 15 and aerial switch 27 are controlled by push-to-talk switch 54 via relay 55. This changes the clock frequency from say 320 kc/s. during standby to say 320.4 kc/s. during transmission. This difference permits the time slots provided by the receiver to slide into coincidence with the pulse groups received, and when this condition is reached the A.F.C. circuit 42 operates to increase the frequency of clock 15 to 320.4 kc/s. so that it is in phase with the pulse groups being received. An A.G.C. circuit 43 controls amplifiers 30 and 40, both the A.G.C. and A.F.C. circuits being controlled through a circuit 45 which responds only to the smallest pulse of a received group to avoid errors due to disturbances occurring at the time of a received pulse. Fig. 5 shows details of the A.G.C. and A.F.C. circuits, pulses from detector 41 being supplied via amplifier 55 to an amplifier 56 which is biased to produce an output only when the input exceeds a particular level. Thus the signal pulse 55b will be rejected and only interference pulses 55a will be passed. The signal from detector 41 is also supplied via a delay 57 to a gate 58 which is blocked by the pulses from the amplifier 56 so that only the signal pulse (the smallest pulse) will be passed and this is used to control the A.G.C. circuit, which also varies the reference bias on amplifier 56, and the A.F.C. circuit. The A.F.C. circuit includes a phase comparator 60 receiving the pulses from the gate 58. The clock generator includes oscillators 64, 65, whose difference frequency from a mixer 66 is also supplied to the phase comparator 60 for synchronization in known manner. The output of the mixer 66 is also supplied via a clipper 67 to operate multivibrators 67 to 70 to provide the timing pulses for timer 25. A switch 71 enables the pulse repetition frequency to be changed as required.