Threshold detector for optical character recognition system

Abstract

A threshold detector for use with a video signal including a first level representative of a white surface, a second level representative of a true black mark on the white surface, and a third level intermediate the first and second levels representative of a false black mark, the threshold detector comprising a DC restoring circuit for clamping the video signal such that the first level is clamped to a base level, a peak detector circuit responsive to the restored signal and operative to provide a peak detected signal having a magnitude that corresponds generally to the peaks of the restored second level, a voltage divider circuit for reducing the magnitude of the peak detected signal by a predetermined amount, a comparator responsive to the restored signal and the reduced peak detected signal and operative to produce a first bistate signal having a high level when the restored signal exceeds the reduced peak detected signal, a pulse generator for generating a sequence of pulses, one of which occurs when the first bistate signal has a high level, and a flip-flop responsive to the first bistate signal and the sequence of pulses and operative to provide a second bistate signal having a first state and a second state, and having a transition from the first state to the second state when there is time coincidence between the high level and one of the pulses, the second state corresponding to the occurrence of a true black mark.