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Optical Scanner Video Circuit

IP.com Disclosure Number: IPCOM000086336D
Original Publication Date: 1976-Aug-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 45K

Publishing Venue

IBM

Related People

Siverling, MM: AUTHOR [+2]

Abstract

Video channel 10, Fig. 1, amplifies and normalizes video data from a single-column photodetector array, or from one column of a rectangular array. It processes small signals impressed on large and variable DC levels; e.g., a 4mV signal on a 1.0 volt background level. Moreover, the circuit bandwidth varies with gain, so that full bandwidth is available at low gains, but the bandwidth decreases at higher gains to avoid instability.

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Optical Scanner Video Circuit

Video channel 10, Fig. 1, amplifies and normalizes video data from a single- column photodetector array, or from one column of a rectangular array. It processes small signals impressed on large and variable DC levels; e.g., a 4mV signal on a 1.0 volt background level. Moreover, the circuit bandwidth varies with gain, so that full bandwidth is available at low gains, but the bandwidth decreases at higher gains to avoid instability.

Video input voltage 11 is first converted to a current by conventional converter 12. Amplifier 13 charges capacitor 14, so that the peak (i.e., blackest) voltage 15 is fed back to a differential input of converter 12. Reset gate 16 periodically discharges capacitor 14 to the peak black level, under control of an external signal.

Current 17, which increases toward white video, is transformed by logarithmic converter 18 to a voltage 19. The static conversion constant is set by reference voltage 20.

The absolute DC value of signal 19 is determined by "white" (document background) video. The remainder of circuit 10 provides a white follower to reference the maximum white video signal 21 to a fixed voltage, independently of the maximum current at 19. Linear amplifier 22, limited by clamp 23, provides an input to comparator 24. This unit discharges storage capacitor 25 at a constant rate whenever output 21 falls below reference voltage 26.

Output 21 is also compared with the reference voltage 27 in bleach comparator 28. Th...