Patent GB-627998-A (GB)
Title: Multiplex pulse systems of radio communication
IPC: H04B1/40
Filed: 1946-11-26
Abstract: 627,998. Radio signalling; multiplex pulse signalling. WESTERN ELECTRIC CO., Inc. Nov. 26, 1946, No. 35034. Convention date, Dec. 11, 1945. [Class 40 (v)] In a multiplex pulse radio telephony system providing two-way communication between a master station and a number of mobile subsidiary stations, the subsidiary transmitters are automatically controlled so that all their signals remain in the correct phase sequence to form one multichannel signal at the master station receiver. The master station, Fig. 1, transmits position modulated pulses of frequency f<1> representing, in sequence, eight channels and a wider marker pulse. A master oscillator 12 provides reference oscillations to control both the marker pulse generator and the modulated pulse generator 10 whose signal input is from channels such as 15 through band-pass filters as at 16. The receiver 21 passes signals of frequency f<2> from all the subsidiary stations to an amplifier, and to a channel selector from which signals in each particular channel pass to convertors such as 23, only channel 1 being shown, where they are mixed with gating pulses derived from a pulse generator 24, the resulting width modulated pulses being passed to a band-pass filter 27 and audio output 28. The gating pulse generators are controlled by the master oscillator 12 as the signals from the subsidiary stations contain no marker pulses. Part of the receiver output is passed via one of two filter networks 30, 31 which filter out the signal frequencies and supply a low-frequency modulating voltage to the transmitter dependent upon the average deviation of each pulse of the received signal. At each subsidiary station, Fig. 2, signals in receiver 40 pass to the converter 44 where those of the appropriate channel are selected by gating pulses which produce width modulated pulses which are demodulated by a band-pass filter 45. The timing of the gating pulses is controlled by the marker pulse selector 42. The transmitter comprises a reference oscillator 59 and, as at the master station, a pulse generator and modulator 62, amplifier 63 and transmitter 64. In order to keep the signals from each subsidiary station in the correct time phase after contact with the master station has been established, despite the differing signal transmit times as the stations move, the position of the pulse train is controlled accordingly. The master station transmits as a modulation of the mean position of the channel pulse, a pilot signal that is proportional to the error in the mean position of the signal being received in that channel. This pilot signal is the low-frequency voltage obtained from filter network 30. At the subsidiary station the pilot signal is passed by network 47 to a reactance valve 49 which varies the frequency of the reference oscillator 59 until the necessary phase shift of the outgoing signals has been effected. Alternatively, correct phasing can be achieved by passing the marker pulses from the master station through a variable delay network, the output of which controls the generation of the outgoing pulse from the subsidiary station. To establish contact, the oscillator 59 at the subsidiary station is swept rapidly through all the channels by a sawtooth generator acting on the reactance tube 49. At the master station the switch 26 will be in the " out of communication " position and the network 31 which has a short time constant and a band-pass width of about 100 c.p.s. is connected in circuit. The corresponding channel therefore responds to the rapid sweep and a pulse is transmitted from the master station to cut-off the saw-tooth sweep generator at the incoming subsidiary station. During this operation channels already in communication experience only a short burst of noise since once contact has been established the corresponding networks 31 are switched out of circuit. In a modification, Fig. 7 (not shown), switch 26 is replaced by a triode operated relay which, in the absence of a signal, places the short time constant network in circuit, and on receipt of a signal connects in the long time constant network. Instead of establishing initial contact automatically, it may be effected manually by the subsidiary station operator acting on instructions given by the master station operator who has the channel pulses displayed on a cathode-ray oscillograph. For details of the transmitters and receivers, Specification 607,610 is referred to.