| 摘要 |
<p>In an optical document reader a video signal detector is used to receive analog input signals (A) and to provide digital output signals (40, 70, F), said detector comprising a white level peak detector (C1, D3), a black level peak detector (C2, D1) and means (R2, R4, R8, R10) for combining the outputs from the white level and the black level peak detectors into a threshold signal (30, 60). Blocking means (D2) will prevent the black level peak detector to have an effect on the threshold signal when low contrast video signals are present. An output comparator (6) is used to compare a delayed video signal (E, 20, 50) with the threshold signal, whereby the delayed video signal is applied over a feedback resistor (R1) to the black peak detector. The combining means include control means (7, R10) for controlling the threshold signal level (a) and to compensate for displacement effects due to the optical components in the document reader.</p> |
