GB523780A - Improvements in time base devices for cathode ray tubes in television reception - Google Patents

Abstract

523,780. Television; thermionic valve generating circuits. HIRSHMAN, C. L., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Dec. 15, 1938, No. 36611. [Classes 40 (iii) and 40 (v)] In a time base device for providing the scanning voltages for the cathode ray tube the discharge device that produces the frame sweep voltage has two control electrodes both of which can start the discharge whilst their effects are additive and means are provided for applying to these electrodes voltages of different wave shape or amplitude derived in different ways from the frame synchronizing signals. The incoming picture signal and the synchronizing pulses 1, Fig. 1, are applied to the terminals 2 and are separated by a diode D back-biassed by a battery V1 so that only the synchronizing pulses 3 are present across the diode load resistance R1. These are integrated across a series resistance R2 and a shunt condenser C2 and are applied to the first grid G1 of a double-grid gas-filled valve Th. The voltage across the resistance R1 is simultaneously differentiated across a condenser C3 and resistance R3 in series and applied to the second grid G2. The gas-filled valve Th is adjusted to generate a saw-tooth wave of lower frequency than that required by appropriate choice of the anode resistance R4 and shunt condenser C4. The valve is then tripped at the required frequency by positive impulses on the two grids G1, G2. During line operation the differentiated line pulses are unable to trip the valve Th because of a large bias applied to the grid G1 from a battery 6. During the framing pulses this bias is gradually removed by the increasing voltage across the condenser C2 so that a state is reached when one of the differentiated pulses is of sufficient amplitude to ignite the gas-filled valve Th. In a modification, Fig. 5, the backbiassing voltage of the diode D, Fig. 1, is replaced by a high time constant circuit comprising a resistance R6 and condenser C6 in parallel, connected in series between the anode of the diode D and the cathode circuit of the video output valve V1. Any other form of limiter-separator, such as a pentode or a screened grid valve, may be employed instead of a diode. In a further modification, Fig. 6, the gas-filled valve is replaced by a hard time base valve V2 of the " squegger " type. The differentiated voltage is applied to the second grid G2 by means of the condenser C3 and resistance R3. The integrated voltage is applied to the first grid G1 through a condenser C7 and a tapping point 7 on a resistance R4, R5 connected between the high-tension supply and the cathode of the valve V2 through one winding of a transformer 8. The other winding of this transformer is connected between the anode and a source of positive voltage. The frequency of the saw-tooth voltage generated across the condenser C4 is controlled by the time constant of the circuit comprising the condenser C4 and the resistance R4, R5. The control electrodes G1, G2 may be in the form of curved plates surrounding the hot cathode 9 between this and the cylindrical anode A, Fig. 7. Alternatively they may be arranged on either side of the cathode as shown in Fig. 8, two anodes Al, A2 being situated opposite the gaps between the plates G1, G2.