Browse Prior Art Database

Contrast Amplifier

IP.com Disclosure Number: IPCOM000075244D
Original Publication Date: 1971-Aug-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 43K

Publishing Venue

IBM

Related People

Swart, DP: AUTHOR

Abstract

A contrast video amplifier is responsive to a signal from a photo-FET scanner, not shown, to provide an output voltage proportional to contrast between characters scanned and document background.

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Contrast Amplifier

A contrast video amplifier is responsive to a signal from a photo-FET scanner, not shown, to provide an output voltage proportional to contrast between characters scanned and document background.

The scanner operates in a pulsed mode whereby it is 70% ON and 30% OFF. During the time it is OFF, the scanner output is at a peak black voltage, VPB. When the scanner is ON and is scanning only the document background, the scanner output is at a peak white voltage, VPW. As a character is being scanned, the output of the scanner is an instantaneous signal voltage, VS. Contrast voltage is equal to: VPW-VS over VPW-VPB.

The output of the scanner is applied to terminal 10 which leads via resistor 11 to the negative input of amplifier 12. Amplifier 12 is part of black level correction circuit 13, which functions to take out the DC component of the input signal and provide amplification. Black peak detector 14 is referenced to ground potential and whenever the output of amplifier 12 goes negative with respect to ground potential, it charges capacitor 15. The positive voltage on capacitor 15 is applied to the positive input of amplifier 12 to adjust the DC output voltage thereof to ground potential. This represents peak black or 100% contrast. The black level corrected signal is VS, or the amplified instantaneous output voltage of the scanner. The voltage signal VS is applied to the negative input of inverting amplifier 20, via resistor 21, and to the negati...