Browse Prior Art Database

Thresholder for Optical Scanners

IP.com Disclosure Number: IPCOM000080692D
Original Publication Date: 1974-Jan-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 2 page(s) / 33K

Publishing Venue

IBM

Related People

Siverling, MM: AUTHOR

Abstract

Circuit 10 generates a threshold level for digitizing an analog video signal to a black/white binary output, according to whether the analog signal is above or below the threshold. To digitize signals from shaded and degraded printing, and to maintain the threshold between background noise and print-contrast minima when exiting an area containing a character, circuit 10 varies the threshold at differing rates for black and white video signals.

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Thresholder for Optical Scanners

Circuit 10 generates a threshold level for digitizing an analog video signal to a black/white binary output, according to whether the analog signal is above or below the threshold. To digitize signals from shaded and degraded printing, and to maintain the threshold between background noise and print-contrast minima when exiting an area containing a character, circuit 10 varies the threshold at differing rates for black and white video signals.

An input signal blacker (more positive) than the threshold stored in capacitor C1 back biases diode D1. Transistor Q5 is the active element of a saturable peak detector. When D1 cuts off, current through R1, D2 and R2 causes Q5 to saturate. The resulting voltage at the collector of transistor Q1 cuts off transistors Q2-Q4.

C1 then charges exponentially until either a maximum is attained, or the input signal decreases sufficiently. The charging circuit includes R1-R5, and an output reflection through R7. The output terminal is connected to a virtual ground, such as the input of an operational amplifier. Thus, this circuit seeks the maximum black/white contrast of its input, at a rate which is independent of the difference between the input and the threshold level stored in C1. Instead, the rate is a function of the threshold level; C1 charges faster from a low contrast than from a high contrast.

When the input signal decreases enough to forward bias D1, current flows through R1. Transistor Q5...