System for converting tonal images to line drawings

A device for electronically and instantaneously converting a tonal image to a line drawing comprising, scanning means to scan said image and produce a time varying video signal representative of the level of intensity of said tonal image as the image is scanned, a passive differentiator responsive to said video signal for producing substantially instantaneously a first intermediate signal which consists of a pulse for each change in level of said video signal, said pulses being representative of the approximate first time derivative of the time varying signal and being positive for any increase in level and negative for any decrease in level, a unipolar amplifier to amplify said first intermediate signal and produce a second intermediate signal, said second intermediate signal consisting of amplified pulses of the same polarity corresponding to the respective ones of positive and negative pulses of said first intermediate signal, an adjustable level detector responsive to all pulses of said second intermediate signal which are above a predetermined adjustable level for producing a third intermediate signal, a pulse generator responsive to said third intermediate signal for producing an output signal consisting of a pulse of constant magnitude and width for each pulse of said third intermediate signal, and a video receiver having video display means responsive to said output signal for displaying a line drawing, said video receiver being synchronized with said scanning means, whereby a line drawing of a tonal image may be instantaneously displayed by a video receiver and the selection of lines in the line drawing in accordance with its respective significance in the tonal image may be instantaneously made by manually adjusting the level detector.