VIDEO SIGNAL CONTROL CIRCUIT INCLUDING AUTOMATIC BRIGHTNESS AND CONTRAST CONTROL RESPONSIVE TO EXCESS CRT BEAM CURRENT

摘要

A video signal control circuit having a video amplifier and a signal transmission channel coupled to the video amplifier for supplying a video signal to a cathode ray tube includes a brightness control circuit responsive to the manual operation thereof to vary the DC potential applied to the signal transmission channel and a contrast control circuit responsive to the manual operation thereof to vary the gain of the video amplifier. A first voltage supply is provided to supply a DC voltage to the video amplifier and to the signal transmission channel. A second voltage supply is provided to supply a gain determining voltage to a gain control circuit coupled to the video amplifier. Level control means are coupled to the signal transmission channel and responsive to the voltage supplied by the second voltage supply for compensating variations in the DC potential on the signal transmission channel when the contrast control means causes a variation in the gain determining voltage, to thereby minimize variations in the brightness of an image displayed on the viewing screen of the cathode ray tube when the contrast thereof is varied. A brightness limiting circuit is coupled to the first and second voltage supplies and is responsive to a cathode ray tube beam current in excess of a predetermined value for varying the voltage supplied by both said first and second voltage supplies to thereby simultaneously vary the DC potential applied to the signal transmission channel and the gain of the video amplifier, whereby the brightness of an image displayed by the cathode ray tube is limited and the contrast of said image is reduced.