Patent GB-519594-A (GB)
Title: Improvements in or relating to television image transmission systems
IPC: H04N3/04
Filed: 1938-09-27
Abstract: 519,594. Television ; valve generating circuits. ELECTRICAL RESEARCH PRODUCTS Inc. Sept. 27, 1938, No. 28098. Convention date, Oct. 1, 1937. [Classes 10 (iii) and 40 (v)] [Also in Group XXXV] A television transmission system comprises a photo-electric device, such as a light-sensitive electron-multiplier, in which the current output is momentarily reduced at the end of each complete scanning, by reducing the impedance of a valve 330, Fig. 1 connected across two of the secondary electrodes of the multiplier 44, thus reducing the amplification of the multiplier. The valve 330 may be controlled by frame synchronizing impulses initiated by a commutator 160 on the shaft 13 of the scanning disc 10. In the film transmitter described, a continuously moving film is scanned by a circle of lenses 11 on the disc 10, the modulated light being carried to an electron multiplier 44 by a light-conducting tube 40 of glass, quartz, or " pontalite." The electron multiplier is of the type described in Specification 500,447, and develops picture signals, including d.c. components, across resistance 319, the output being connected directly to a modulator M, Fig. 3, for transmission by a carrier, so that low-frequency components are transmitted to the receiver. The scanning disc 10 is enclosed in a casing 14 to reduce wind resistance. A lever 32 serves to adjust the film in the gate. Line synchronizing signals are developed by each lens, some time before scanning, traversing an image of an aperture 62 over a slot 65, a light-conducting tube 66, carrying the light to an electronmultiplier 60. The line synchronizing signals developed across resistance 73 may vary in amplitude due to inequalities in transparency &c. of the different lenses and they are therefore limited in limiting amplifier 75. The synchronizing impulses of constant amplitude are then applied to gas-filled valve 82 to discharge condenser 88 charged from battery 86, thus developing a saw-tooth oscillation which is amplified in parallel-connected valves 91, 92, 93 each feeding a transformer 104, 105, 106. The output from transformer 104 is applied to a filter 110 to develop a sinusoidal oscillation which is phase-shifted at 114 and fed to transformer 122. Transformer 122 has two secondary windings 123, 124, each feeding one of the valves 125, 126 so that one valve is conducting when the other valve is non-conducting thus developing a square-topped oscillation across resistance 128. This is applied to the grid of valve 133 so that when its anode current decreases, the grid of valve 132 is made more positive and valve 132 then conducts to rapidly discharge condenser 134 which is normally charged uniformly over valve 131. The coupling between valves 132, 133 makes this discharging more rapid. The saw-tooth oscillation developed across condenser 134 is amplified at 271, the output being applied to valve 276. The proportion of the output applied to valve 276 is inversely proportional to the amplification of valve 276 so that the valve acts as a phase reverser, the total output across resistance 283, 284, thus being a saw-tooth potential in push-pull. This is applied to the deflecting plates 54 of the monitor tube 50. If desired, the output from transformer 105 may be applied over delay network 151 and transformer 153 to contact 150 of switch 138 to control the saw-tooth generator instead of using the output from 104. Frame synchronizing signals are developed by a commutator 161 on the driving shaft of the scanning disc. They are amplified in valve 174 and applied to render valve 177 conducting, thus charging condenser 166 which is then slowly discharged over valve 168. The back coupling between valves 177 and 174 make the charging more rapid. The saw-tooth voltage developed across condenser 166 is amplified at 181, 187 and applied to the deflecting plates 55. Frame synchronizing signals are also applied to valve 330 to reduce picture signals during frame flyback. Controlling speed of scanning disc.-Line synchronizing pulses dependent on scanning disc speed are applied to transformer 106 and thence to valves 207 ... 210 in phase. A voltage of constant line frequency from 205 is applied to valves 207 ... 210 over a bridge network 211 so that valves 207, 208 and 209, 210 are each energized in phase opposition, the phase difference between the two pairs being 90‹. A four-phase motor 215 is fed by the difference frequency currents and operates the slider 233 of a resistance 234 in series with the shunt field winding 203 of the motor 12 driving the scanning disc to vary its speed so that the frequencies at 106 and 206 tend to equality. Alternatively, the motor 215 may operate a contact 232 to insert resistance in series with the shunt field winding.