Patent GB-571776-A (GB)
Title: Sweep synchronising and beam blanking device
IPC: H04N3/24
Filed: 1943-12-10
Abstract: 571,776. Television. STANDARD TELEPHONES & CABLES, Ltd. Dec. 10, 1943, No. 20679. Convention date, Jan. 21. [Class 40 (iii)] [Also in Group XXXVI] An arrangement for generating deflecting voltages and blanking signals for application to a cathode-ray tube comprises a saw-tooth wave generator including a grid-controlled gas discharge tube adapted to be periodically fired for generating the waves, and means for applying the potential pulses, generated by the ionization of the gas discharge tube on a grid electrode thereof, to the cathode-ray tube for suppressing the cathode beam during the period of the ionization. A synchronizing signal from a source S, Fig. 1, is passed through an impedance transformer triode 3, the cathode 4 of which is earthed through a resistor 5. The valve 3 is coupled through a condenser 6 and resistor 7 to to the grid 2 of a gas triode or saw-tooth oscillator 1, a grid leak 7<1> being provided. At the same time the signal S is applied through an attenuator 10 to the grid 8 of a valve 9, the cathode 11 of which is connected to the cathode 12 of a valve 13. The cathodes 11, 12 are earthed through a resistor 14. The grid 15 of the valve 13 is connected to the grid 2 of the valve 1. The output 18 from the valve 13 is in the form of blanking pulses which are applied to the grid G of a cathode-ray tube T for blanking the beam during the return trace of the time base oscillation. A resistor 24 is connected from the positive supply to the cathode 25 of the valve 1, which has a bias resistor 27 connected to earth and byepassed by a condenser 28. A variable condenser 29 is charged through a resistor 21 and discharged by the valve 1 to produce a saw-tooth wave form 20 which is applied to the deflection plates of the cathoderay tube T. The synchronizing signal in the form of a sine wave 31 is applied to the grid of the valve 3, the amplitude of the output of which is controlled by a variable contactor on the resistor 5. At the same time the wave 31 is applied to the grid 8 of the valve 9 and thence to the cathode 12 of the valve 13 so that this signal and the sine wave component of the signal from the cathode 4 are in phase and are regulated in amplitude by the potentiometers 5, 10 so that they are balanced out by the grid 15 and cathode 12. No plate signal therefore is generated in the valve 13 by the signal S.. The gas triode oscillator 1 is fired at desired frequencies by the sine wave voltage from the valve 3, increased potential being applied to the grid 2 during firing periods as shown by the wave 32. This wave is applied to the grid 15 of the valve 13, the sine wave component is balanced out, and the rectangular component appears with reversed polarity as shown at 18. In a modification, Fig. 2, the synchronizing voltage is applied in the cathode circuit of the valve 33 through a condenser 41, leaving the grid' 42 free of synchronizing voltage. Blanking signals are generated by the grid 42 assuming a potential due to space charge during the gaseous ionization cycle of the valve 33. This grid impulse 18<1> has a duration equal to that of the gaseous discharge in the valve 33 and is applied to the grid amplifier of the cathode-ray tube. In a modification, the gas'discharge valve has two grids, of which one is employed to generate the blanking pulses and the other receives the synchronizing signals.