AUTOMATIC LUMINANCE LIMITING CIRCUIT

PURPOSE:To constitute so that a digital picture is not influenced by brightness of a broadcasting picture by applying a feedback only to a video amplifying circuit for a broadcasting picture, applying no feedback to an amplifying circuit for a digital signal picture, and controlling it by itself, when a beam current exceeding a prescribed value is detected. CONSTITUTION:When an average beam current of a cathode-ray tube 6 increases exceeding a prescribed value, it is detected by an automatic luminance limiting and detecting circuit 8, and basing on this detecting circuit, a luminance controlling circuit is controlled. In this case, the gain of a video amplifying circuit 3 drops and luminance on the cathode-ray tube picture, especially, luminance of the background picture is suppressed. However, luminance of a character and a pattern displayed on the picture by a digital signal is always unchanged, and it is varied only by adjusting a luminance controlling circuit 11.