Dynamic focus circuit of electromagnetic focusing type for cathode ray tube of video image displaying apparatus

Abstract

A dynamic focus circuit of electromagnetic focusing type for a cathode ray tube of a video image displaying apparatus, is provided with a first oscillating circuit and a second oscillating circuit. The first oscillating circuit is provided with: an electric power source; a first switching device; and a capacitor, which are connected in series to form a loop, the first oscillating circuit being opened and closed by the first switching device. The second oscillating circuit is provided with: a second switching device; the capacitor, which is commonly used with the first oscillating circuit such that the first and second oscillating circuits are parallel with respect to the capacitor; and a focus coil for focusing an electron beam of the cathode ray tube, the second switching device, the capacitor and the focus coil being connected in series to form a loop, the second oscillating circuit being opened and closed by the second switching device. The first switching device opens and closes at a timing in correspondence with a blanking time period or a video image time period based on a horizontal scanning frequency of video image data of the video image displaying apparatus, to charge and discharge the capacitor. The second switching device opens and closes at a timing according to an inverse logic of the first switching device, to charge and discharge the capacitor.

Dependencies are inferred from the claim text.