| 摘要 |
PURPOSE:To suppress the output voltage less than the breakdown strength of a switching circuit, by connecting a diode between a power supply and the output of a switching circuit which projects a character signal and a TV image superposed on each other and at the same time connecting a constant voltage diode between input terminals of another circuit. CONSTITUTION:A control is performed to lower the video contrast when a character signal and a TV image are superposed on each other to be projected with reproduction on a cathode-ray tube. In this case, the cathode of a constant voltage diode 10 of 8V connected between a transistor 8 and an output terminal 4 for control of contrast is set at 10V with the anode at the side of a terminal 3 of an integration circuit 1 set at 5.7V respectively when the terminal 3 of the circuit 1 which processes the received character signal is receiving a video signal and while the transistor 8 is cut off. Therefore the diode 10 does not conduct, and the terminal 3 does not exceed the breakdown strength of the circuit 1 since it is set at 5.7V. When the transistor 8 coducts and an image is superposed on characters for projection, the diode 10 conducts. At the same time, the voltage of the terminal 4 is dropped down to 8V to lower the contrast. |
