| 摘要 |
PROBLEM TO BE SOLVED: To provide an image reader that automatically performs perfect ground elimination by eliminating ground elimination failures due to the ground of an original changing depending on a place. SOLUTION: A ground elimination circuit 6 in this image reader, provided with a circuit which follows the peak value of an image signal serially outputted from an analog signal processing circuit 3 and performs peak detection, outputs the detected peak value as a reference voltage Vref of an A/D conversion circuit 4 and eliminates a ground. A charging and discharging circuit is used for peak value tracking, and charging and discharging characteristic is changed by a control signal from a timing signal generation circuit 8. It is possible to eliminate ground elimination failures, etc., that take place in an original, where a void part exists at the front edge and a dark ground exists by setting a time constant which increases the charging and discharging speed at the original front edge and gradually reduces the charge and discharge speed toward the rear edge to the charge and discharge circuit.
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