Purity adjustment device for video display appliance

Abstract

A purity adjustment device for a video display appliance which automatically compensates for a purity of a picture displayed on a CRT screen that is varied due to the change of the earth magnetic field by electromagnetic deflection using a purity coil. The picture adjustment device includes an initial value determining section for determining a first voltage corresponding to an earth magnetic field of a specified place where a purity of a picture displayed on the CRT presents white, an earth magnetic field detecting coil for detecting the earth magnetic field of a present place where the video display appliance is located, and producing a second voltage corresponding to the detected earth magnetic field, a comparison-amplifying section for comparison-amplifying a third voltage, in which the first and second voltages are superimposed together, with reference to an input reference voltage, an amplifying section for amplifying an output of the comparison-amplifying section to produce a purity compensation voltage, and a purity coil for producing a magnetic field for controlling a position of electron beams scanned on a screen of the CRT in accordance with the purity compensation voltage.

Dependencies are inferred from the claim text.