| 摘要 |
PURPOSE:To prevent luminance/contrast of a slave picture plane from fluctuating due to an APL (average luminance level) change in a master picture plane by providing a switch circuit for switching actions of an ABL/contrast circuit, by controlling the switch circuit by a pulse denoting a position where a slave picture plane is synthesized and eliminating such a control that luminance/contrast of a slave picture plane will not be controlled by a beam current passing through a CRT (cathode ray tube) for a period when a slave picture plane is synthesized. CONSTITUTION:A synthesized video signal A where master and slave picture planes are synthesized is inputted to a contrast control circuit 1. A switch circuit 10 is connected to a slave picture plane synthesization position specifying pulse generator circuit 15 through a line l. In said circuit 15 pulses A and B are formed from a horizontal synchronizing signal H and a vertical synchronizing pulse V. An AND output of the pulses A and B is outputted to the line l, for the outputting period the switch circuit 10 is controlled so that a C1 and a C3 will be connected in its interior, and for the remaining period said circuit 10 is controlled so that the C1 and a C2 will be connected. By this action, a ABL circuit is switched only for a period inserting a slave picture plane; therefore a luminance change in a slave picture plane due to an APL change in a master picture plane can be eliminated.
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