Abstract: A threshold level is automatically varied to follow variations in an input signal reference level. In the disclosed embodiment the reference level is that produced by photodiode-amplifiers in an OCR machine when viewing white or non-character portions of a page, the reference level varying as a function of the change in the length of the optical path during a scan across the page. The threshold level is compared to the input signal level to determine whether a character segment is viewed by the photodiode. Variation of the threshold level is achieved by low pass filtering individual output signals from some of the photodiode-amplifiers and then averaging the filtered signals to provide the adjusted threshold.