| 摘要 |
PURPOSE:To improve video sensitivity by changing resistance of a pre-contrast variable resistance automatically according to strength of electric field. CONSTITUTION:When the field strength is relatively large, base potential of a transistor (TR) Q2 is lower than base potential of TRQ3. A TRQ1 is in the state of cut off and TRQ4, Q5 and a switching diode D1 becomes off state, and adjustment of contrast of a picture is made by operation of a contrast variable resistance VR2. When the field strength is small, TRQ2 of a voltage comparator conducts, and base potential of TRQ4 drops and TRQ4 conducts. Base potential becomes higher than emitter of TRQ5, and TRQ5 conducts, and emitter current flows in TRQ5 and a part is applied to the diode D1 to conduct it. Accordingly, a resistance R9 is connected in parallel to the pre-contrast variable resistance VR1. Emitter potential of TRQ1 is lowered, video output voltage becomes large and contrast of the picture is raised. |
