GB1311763A - Circuit for controlling the brightness of the scanning spot in a cathode ray tube - Google Patents

Abstract

1311763 Function generators IWATSU ELECTRIC CO Ltd 22 May 1970 [16 June 1969 1 Aug 1969] 24878/70 Heading G4G [Also in Division H4] In an arrangement for controlling the intensity of the beam of a cathode-ray tube as a function of the scanning velocity in order to produce constant brightness independently of such velocity, voltages VLx and VLy (see Fig. 2) proportional respectively to the voltages developed across the X and Y deflection coils Lx, Ly are derived in circuits 2, 3 (each following the arrangement shown in Fig. 1), squared and added in operational circuit 4 to produce V<SP>2</SP>L and then modified in function generator 5 to produce the function-V L <SP>5</SP>/ 14 which is supplied to the beam intensity modulation grid via an amplifier 6 which is controlled by the beam blanking-unblanking signals. The description includes a mathematical analysis of the parameters of the circuit of Fig. 1 and shows that the output voltage EOαV L is independent of the voltage developed across the deflection coil L due to the inherent resistance R<SP>1</SP> of the coil. Also the considerations leading to the production of the control function V L <SP>5</SP>/ 14 are set out. Circuit details of the X portion of the operational circuit 4 and of the function generator 5 are given with reference to Fig. 3 and to Figs. 4A and 4B respectively (not shown). The arrangement of Fig. 1 may be utilized to control the production of beam unblanking pulses so that in the event of failure of scanning the beam remains blanked and the tube screen phosphor is not damaged. Fig. 5 shows the scheme in which circuits 9 and 10 produce constant amplitude pulses from the positively and the negatively going portions, respectively, of the output EO of the amplifier 1 (a similar arrangement being employed for the -Y deflection circuitry) such pulses being supplied via OR gate 11 to a holding circuit 12 from which after shaping they are supplied via gate 13 to the CRT intensity modulation amplifier as beam unblanking pulses. When the DC resistance of the deflection coils is negligible the operational amplifiers may be replaced by a simplier circuit arrangement (Fig. 7, not shown).