An optical character reader providing output signals proportional to the lightness or blackness of the document being scanned generates a clipping level voltage which is not a fixed percentage of the peak black minus peak white signals. The clipping level percentage varies with the density, or blackness, of the character. The black and white peak signals are applied to a differentially connected operational amplifier having an output proportional to a first multiple times the black peak minus a second multiple times the white peak, where the first and second multiples are unequal. The output of the operational amplifier is added to a signal proportional to white background thereby providing a clipping level which is a variable percentage of the difference between a black and white peak signal.