| 摘要 |
<p>PURPOSE:To set a proper white level at either of white and black parts. CONSTITUTION:A sample holding circuit 31 is set in a sampling state where a white level signal is outputted in accordance with the change of a picture signal when an analog switch 42 is closed. When the switch 42 is opened, the circuit 31 is set in a holding state where the level of the picture signal inputted at the end of a period of the immediately preceding sampling state. When the position of a reading subject is changed to a white part from a black part, the output of a differentiation circuit 33 has the large positive value. Thus a comparator 34 sends out a set signal S to set a flip-flop 35. As a result, the switch 42 conducts. Meanwhile the output of the circuit 33 has the large negative value when the position of the reading subject is changed to a black part from a white part. Thus the comparator 34 sends out a reset signal R to reset the flip-flop 35. As a result, the switch 42 is cut off.</p> |
