Patent GB-1127841-A (GB)
Title: Improvements in data communication systems
IPC: H04B14/02
Filed: 1967-01-03
Abstract: 1,127,841. Multiplex pulse signalling. GENERAL ELECTRIC CO. 3 Jan., 1967 [3 Jan., 1966], No. 381/67. Heading H4L. In a pulse communication system, amplitude samples of an analogue signal are converted into corresponding frequencies and combined with sliding tone (chirp) signals so that the centre frequency of the chirp is varied in proportion to the appropriate amplitude sample. In a time division multiplex system the chirps overlap in time the receiver including pulse compressing means by which the individual signals are recovered. Transmitter, Fig. 1.-In a two-channel system, pulses generated at 10 are passed through a band-pass filter 11 to a distributer 12 supplying dispersive delay lines 13, 14 alternately. The delay in lines 13, 14 is greater for higher than for lower frequencies and the chirps thus produced are supplied to mixers 15, 16 respectively. The amplitudes of the dispersed frequencies are arranged to vary in such manner that when the two unmodulated chirps are subsequently combined in the transmitter a substantially constant amplitude signal is provided. Audio signals 22, 23 are amplitude sampled at 20, 21 and control the frequencies of oscillators 17, 18 respectively. The output of oscillator 17 is applied to mixer 15 and the upper sideband selected, and the upper sideband is similarly derived from mixer 16 which is supplied with the output of oscillator 18. These upper sidebands are combined at 19 and modulate radio transmitter 25, 26, 27. For channel identification a subsonic frequency from oscillator 24 is combined with the signal from source 22. Each source 22, 23 may comprise a plurality of subchannels. Receiver, Fig. 2.-Chirp signals detected by the receiver 45, 46, 47 are compressed into narrow pulses by a dispersive delay line 48 and by this means are converted into position modulated pulses which are passed through a filter 49 to a detector 50 removing the high frequency components. The output of detector 50 is supplied to a threshold detector 51 controlling a pulse generator 52 providing uniform rectangular pulses which are applied to a sync. circuit 53 to derive pulses at a repetition rate identical to that of pulse generator 10 at the transmitter. The pulses from generator 52 are supplied via a narrow bandpass filter 54 in the sync. circuit amplifier 55, detector 63 and threshold device 56 to maintain a sample and hold circuit 57 in a conductive condition so that a continuously varying input is supplied from integrator 58 to control the frequency of an oscillator 60. In the absence of an input the oscillator 60 supplies an output to drive a sync. pulse generator 62 at the centre frequency of filter 54 to control a sawtooth generator 66. When modulated pulses are received there may be temporarily no output from filter 54 but integrator 58 maintains a substantially constant output voltage due to its long time constant. If the pulse repetition rate of generator 10 should change slightly, a phase comparator 59 corrects the oscillator 60 accordingly. The output of sawtooth generator 66 is applied to a summing network 67 which through a threshold device 68 drives a pulse generator 69 to reset a bi-stable 70 which is set by the output of pulse generator 52. When in the set condition the bi-stable 70 operates a constant current generator 71 to charge a storage circuit 72 having a leakage resistance connected in parallel thereto. The voltage on circuit 72 via amplifier 73 provides the second input to summing network 67. By adjusting resistor 74 the time at which bi-stable 70 is reset may be varied with respect to the start of the sawtooth voltage from generator 66, the adjustment being such that when unmodulated pulses are being received the charge and discharge rates of circuit 72 are equal and no voltage is supplied to amplifier 73. This ensures that generator 69 produces pulses midway in time between adjacent sync. pulses. In this way bi-stable 70 produces duration modulated pulses from the incoming position modulated pulses which, if desired, may be demultiplexed and converted to respective audio signals for each channel. As described, output pulses from generator 69 are supplied to control a bi-stable 76 and a sawtooth generator 75. The two outputs of bi-stable 76 are supplied to AND gates 78, 80 respectively which receive the position modulated pulses from generator 52 as their second input, the output of gate 78 (channel 1) being compared at 82 with the sawtooth wave from generator 75 and the output of gate 80 (channel 2) being likewise compared at 84 with the sawtooth wave to produce corresponding amplitude modulated pulses across capacitors 81, 83 respectively. When bi-stable 76 switches to its channel 1 position a channel 2 amplitude modulated pulse is passed via gated amplifier 88, controlled by pulse generator 79, low pass filter 93, which reconstitutes the audio signal, and amplifier 94 to the channel 2 output circuit. Similarly, gated amplifier 87 is controlled by the channel 2 output of bi-stable 76 to pass the channel 1 signal to its output circuit. To identify the channels a bandpass filter 95 supplies the subsonic tone carried by channel 1 to the INHIBIT input of a gate 97 the remaining two inputs being provided by the channel 1 output of bistable 76 and the output of sync. pulse generator 62. If the subsonic tone should be absent in the output of amplifier 92 a sync. pulse is passed by gate 97 and supplied to the bistable 76 to alter the channel sequence. In a system containing a plurality of channels hunting will take place until synchronization is achieved. Modifications.-In a receiver for a four channel system, Fig. 7 (not shown), a ring counter is substituted for the bi-stable 76 and interconnected flip-flops precede the respective pulse demodulating circuits. This prevents spurious pulses from affecting the demodulated output signals.