Optical density representing input signal thresholder.

摘要

A scanner converts light reflected from a narrow band of a page bearing an image into a series of analog signals representing the optical density of elements of the image. The analog signals are input on a line (18) to a thresholder to determine whether to treat the element as black or white and to output a digital signal in accordance with the determination. The input signal is complemented (21) and added (22) to a first reference amount from a buffer (23). A non-linear bias derived from a table look-up ROM (24) is given to the difference which is then combined (25) with the first reference amount and produces an updated first reference amount in the buffer (23). This amount is complemented (32) and added to a second reference amount selected from a shift register (30). A non-linear bias derived from a table look-up ROM (34) is given to the difference which is combined with the selected second reference amount and produces an updated selected second reference amount in a buffer (36). This amount replaces the selected second reference amount in the shift register (30) and is applied to a comparator (40).
<??>The input signal is also applied to a multi-stage shift register (20) to introduce a delay and a non-linear bias is applied by a table look-up ROM (40) prior to its application to the comparator (40) whose output on line (42) is a digital signal representing black or white.