Adaptive and programmed threshold level detection arrangement

Claim 1

In an optical character recognition machine of the type in which a beam of light is caused to scan across successive lines of characters and reflect onto an array of sequentially adjacent photosensitive elements, each of which drives an amplifier to provide an electrical signal at a reference level when a non-character portion of the line is reflected thereon and at a character level when a character portion of the line is reflected thereon, a threshold control circuit for varying a threshold level to which said character level is compared in a character recognition process, said threshold control circuit serving to compensate for variations in said reference level, said threshold control circuit comprising: a plurality of reference level storage circuits numbering less than half the number of said photosensitive elements, each reference level storage circuit comprising: capacitive storage means; a charge current path for said capacitive storage means having a relatively short time constant; a discharge current path for said capacitive storage means having a relatively long time constant; a diode in said charge current path connected and poled to conduct said electrical signal at said reference level from a respective one of said amplifiers as charging current to said capacitive storage means; wherein the amplifiers providing electrical signals to said reference level storage circuits are driven by photosensitive elements which are all spaced from one another in said array to enhance the probability that at all times at least one of the spaced photosensitive elements has a noncharacter portion of a scanned line reflected thereon; and averaging circuit means connected in the discharge current path of each of said reference level storage circuits for averaging the levels stored in all of said capacitive storage means and providing said threshold level as a function of that average.