| 摘要 |
PURPOSE:To output a video signal resistant to external noise even in a case where no signal exists on a master picture and the cycle of whose synchronizing signal coincides with the cycle of an ordinary television signal, by using the synchronizing signal of a slave picture side by detecting the fact of the no signal of the master picture when it occurs, in the case that the slave picture is inserted in the master picture. CONSTITUTION:From inputted master picture and slave picture, the synchronizing signals are extracted by synchronization separating circuits 12 and 15, and are inputted to a synchronizing signal switching circuit 14. The synchronizing signal switching circuit 14 switches the synchronizing signal to that of the slave picture only when no signal exists on the master picture by a master picture no signal detection circuit 13. A signal made into the slave picture by a slave picture conversion circuit 16 is synthesized with the master picture generally at a picture synthesizing circuit 11, and when no signal exists on the master picture, only a slave picture forming signal is outputted. Next, a synchronizing signal detector 21 detects the synchronizing signal from the master picture. The signal is stored in a capacitor, and increases the base potential of a transistor 22, and turns ON the transistor. As a matter of course, the transistor is burned OFF at the time of no signal on the master picture. By turning OFF the transistor, a vertical synchronizing signal VD is switched from a master VD to a slave VD by a synchronizing signal change-over switch 23.
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