Patent GB-309509-A (GB)
IPC: H04J3/17
Filed: 1929-04-11
Abstract: 309,509. Marconi's Wireless Telegraph Co., Ltd., (Assignees of Ranger, R. H.). April 11, 1928, [Convention date]. Multiplex signalling; copying and writing telegraphs.-Relates to a commutating device employing thermionic valves for cyclically opening and closing a number of signalling channels, particularly applicable for wireless transmission of pictures and facsimiles, but also adapted for multiplex signalling in general and for operating' flash signs. Fig. 1 shows a system with four incoming signal channels A - - D each feeding one grid of a four-electrode valve, 1, 6, 11, 16. The other grid of the valve 1 is connected to the grids of valves 2 - - 5 the valves 2, 3, 4 being provided for the purpose of blocking the channels B, C, D whilst the channel A is operative; and the valve 5 serving to hand on the incoming signal to modulate an oscillator 49, which energizes a transmitting aerial or line 106. Assuming that the five connected grids of channel A have just become positive, each of the valves 2, 3, 4 passes plate current, which in traversing resistances 32, 31, 30 in the grid circuits of valves 6, 11, 16 swings each of these grids negative and so blocks the corresponding channels. A condenser 21 which is connected across the plate and filament of the valve 1 and has previously been charged by the plate battery 33 through a high resistance 25, discharges through the valve 1 and causes its plate voltage to fall. The plate is connected through a bias battery 39 and resistance 38 to the grid of the valve 4, which becomes relatively negative and decreases the plate current. This reduction of plate current through the resistance 30 throws the grid 62 of valve 6, and with it the grids of valves 7 - - 10, more positive and opens the channel B. As the plate current rises through the valves 7 - - 9 the channel A is completely shut, and the channels C, D remain closed. The same cycle of operations is now performed in the channel B, and so on, the last channel acting to open the channel A as it shuts its own channel. The operative period for each channel is determined by the time of discharge of the condensers 21 - - 24. With similar conditions prevailing in each channel, their operative times would normally be equal. In certain cases, however, the channels may have unequal operative periods as for instance when one or more channels are not carrying signals. If facsimile transmissions are being effected, only a small fraction of which comprises the 'black' or signal-transmitting portion, any channel transmitting a 'white' portion may be closed as quickly as possible. This is effected by arranging that the grids 53 - - 56 are swung strongly positive during 'white' signals, which enables the condensers 21 - - 24 to discharge immediately; whereas for 'black' signals the grids 53 - - 56 are less positive and the discharge of the condensers is retarded. In this way when only one channel is operative, it may occupy practically the whole time available. The signals are used to modulate the output of oscillators 49 - - 52, which operate at different frequencies and feed a common aerial or line 106. Neon lamps 108 - - 111 or other glow indicators serve to indicate the operative periods of the several channels. In a modified arrangement (Fig. 2, not shown), ordinary triode valves replace the four-electrode valves 1, 6, 11, 16, and in each channel the signal line is connected to the grid of a further triode which has its plate connected to the plate of the first-named valve. The common plate circuits are fed through neon lamps, which determine the operative period for each channel.