Patent GB-590066-A (GB)
Title: Improvements in or relating to multi-channel high-frequency signalling systems
IPC: H04J3/06
Filed: 1944-08-15
Abstract: 590,066. Pulse modulation. REID, D. G. Aug. 15, 1944, No. 15581. [Class 40 (v)] In a multi-channel pulse modulation communication system, the transmitter and receiver are maintained in synchronism by means of signals in the form of a group of pulses of form and pattern distinguishable from the pulses of the intelligence channels or any random combination thereof. Fig. 1a shows the arrangements at the transmitter of a four-channel system. The output from a sinusoidal oscillator 1 of the pulse repetition frequency is fed to a phase splitter 2 to provide five equally phase displaced waves 15-19. The additional wave 15 is fed to a unit 3 the output 20 of which comprises a synchronising signal in the form of three closely spaced pulses. The remaining waves 16-19 are fed to modulator units 4a-4d to produce outputs 21-24 in the form of pulses which are time modulated in accordance with signals applied at 5a-5d. The modulated pulses from the four channels are combined together with the synchronising pulses and applied to modulate a carrier wave at 6 for radiation at 7 or transmission over a wire line. Fig. 1b shows the receiving arrangements in which the output from the receiver 9 is in the form of unidirectional pulses comprising the triple synchronising pulses and the four sets of time-modulated channel pulses. The mixed output is fed to a synchronising signal separator 10 which responds only to three pulses having a predetermined spacing and which provides at 27 a set of single pulses corresponding to the last of each set of three synchronising pulses. These synchronising pulses are fed to channel units 11a-11d which provide time spaced square Waves occurring at the same time as the pulses in the corresponding channels. These waves are fed to units 12a-12d to open up the units only when pulses may occur in the respective channels. This effects selection of the respective trains of channel pulses as the mixed pulse input is fed from 9 to all the channels. The output from units 12a-d is in the form of width modulated pulses which are demodulated by low pass filters 13a-d for utilisation at 14a-d. Alternatively, the received synchronising pulses may be used to synchronise a local oscillator which is used to produce the channel selecting waveform. The unit 3, Fig. 1a, is in the form of a known device (Fig. 3, not shown) which converts the sine wave input into a square wave of duration slightly greater than that of a set of synchronising pulse. This square wave is applied to a multi-vibrator having a period equal to the desired separation of the pulses, thus producing three pulses during each positive square wave, the pulses being differentiated and one half selected for use as the triple synchronising pulses. Fig. 4 shows the construction of one of the pulse generator-modulators 4a-d, Fig. 1. Pentode 50, of a type in which the suppressor grid potential controls the anode current as much as does the control grid potential, has a condenser-resistance combination 51, 52 in the cathode lead so that when a sine wave, Fig. 4b, is applied to the control grid, the cathode potential is as in Fig. 4c. The anode current follows a curve as in Fig. 4d in which one slope of the triangle (shown dotted) is variable according to the potential of the suppressor grid. This is effected by the modulating signal applied at 54. This is applied to a squaring amplifier 59 to give an anode voltage curve Fig. 4e which is differentiated by the condenser resistance combination 58, 60 to give pulses, Fig. 4f, the negative ones of which applied to cathode of 62 produce anode voltage pulses, Fig. 4g, which are timemodulated in accordance with the signal. Output resistance 63 may be common to all the channels and to the synchronising signal. The synchronising signal separator 10, Fig. 16, comprises a pentode (Fig. 5, not shown) biassed so as only to conduct when a pulse arrives simultaneously at the control grid and suppressor grid. The triple pulses are fed directly to the suppressor, and, through a network having a delay equal to the pulse spacing, to the control grid. Thus only the second and third pulses of a set are passed. This action is repeated after phase reversal and only one pulse is passed, corresponding in time to the last of the set of triple synchronising pulses. All other differently spaced pulses are rejected. Fig. 6 shows the arrangement of units 11a-d, Fig. 1b, for producing a channel selecting square wave, Fig. 6e. from the synchronising pulses, Fig. 6b. The pulses are applied at 78 to the suppressor of a pentode 79 reciprocally coupled to a triode 82a to produce across cathode resistor 81 a wave form as in Fig. 6c, the duration T3 being determined by adjustable resistor 84. This wave form is differentiated over condenser 85 and resistor 86 and the negative pulse, Fig. 6d, is used to cut off triode 82b. The time T4 for which it is cut off depends on the combination 85, 86 and this gives a positive square wave across the anode resistor 87 as in Fig. 6e in which the spacing T3 depends on resistance 84, and the duration depends on resistance 86. Thus, using different values of 84 for the units 11a-d, Fig. 1, suitable channel selecting waves are produced. The units 12a-d (Fig. 7, not shown), each comprises a valve biassed so as to conduct only on receipt of a pulse on the control grid when the channel selecting wave is also applied to the screen. The valve is coupled to a triode so that it continues to conduct after cessation of the pulse until the end of the positive going half of the square channel selecting wave. This provides an output in the form of a pulse having a width-modulated leading edge. Specification 589.606 is referred to.