Patent GB-542259-A (GB)
Title: System and methods for television reception
IPC: H04N5/08
Filed: 1940-10-24
Abstract: 542,259. Television ; relaxation oscillators. STANDARD TELEPHONES & CABLES, Ltd. Oct. 24, 1940, No. 15654. Convention date, Dec. 22, 1939. [Classes 40 (iii) and 40 (v)] A television receiver is adapted to separate the components of the synchronizing signal generated by the transmitter described in Specification 542,258, which signal comprises usual unidirectional pulses at line frequency and trains of high frequency waves at frame frequency. This signal with the video signal, after rectification from the carrier wave, is supplied to a filter 20 comprising a condenser in series with an inductance tuned to the high frequency of the frame trains, so that this frequency is passed by a line 14a to an amplifier 46 while the line pulses with the video signal is passed by a line 10a to the valve 37 which is biased to separate the line pulses from the video signal by amplitude differentiation. The input and output circuits of the amplifier 46 are both tuned to the high frequency of the frame trains, and the output is used to trigger a gas-discharge tube 17, which produces saw-tooth voltage across the resistance 26 for supply to the frame scanning electrodes 19 of the cathode ray tube 28 through a paraphase amplifier and a centralizing circuit. A highly exhausted vacuum tube could be used instead of the gas-discharge tube 17 ; but the gas-discharge tube is preferred, being independent of pulse amplitude. The line pulses separated by the valve 37 are passed to a similar gas-discharge tube 27 for producing saw-tooth voltage at line frequency ; and this is amplified by a valve circuit having a feed-back connection 43, 44 to straighten the charging part of the cycle and to improve horizontal linearity. The output is supplied to the line scanning electrodes 42 of the cathoderay tube through an autotransformer 68. The circuit of the gas-discharge tube 27 includes resistors 33, 60 for adjusting the maximum horizontal deflection and for prolonging the life of the tube by limiting peak currents. The circuit of this tube also includes resistor 40 and condenser 41 whereby cathode potential is kept above ground potential for a given time rendering the tube 27 less susceptible to random disturbances occurring between the line pulses. The sweep voltages are supplied to the control electrodes through circuits 65, 66 which serve to centre the image on the screen 29, the second anode 56 of the cathode-ray tube being connected to the point 57 on a voltage divider formed by resistors 58, 59 across the centring controls. A circuit 47 is provided to deflect the scanning spot off the screen if the line synchronizing channel is inoperative. In normal operation, diode 48 fed with the saw-tooth voltage develops negative voltage on resistors 49, 50 holding triode 51 beyond cut-off. If the saw tooth voltage fails, the triode 51 draws plate current through resistors 52, 53 from potential source at 64 thereby causing a voltage drop across resistor 53 to unbalance the potential applied to the scanning electrodes 42, so that the scanning spot is deflected off the screen 29. Resistor 54 and condenser 55 are provided to balance the resistive and capacitative load of the triode 51 and resistor 52.