Image reading apparatus with adjustment of offset/gain of image signal

Abstract

An image reading apparatus includes an image sensor and an offset adjusting circuit for generating an offset-counterbalancing signal in different levels. The level is selected in accordance with an offset in an image signal. The offset is adjusted by adding an offset-counterbalancing signal to the image signal to output an offset-adjusted image signal. Sample clock signals are generated in different phases. A signal sampling circuit samples the offset-adjusted image signal to output a sampled image signal. A peak value detector detects peak black and peak white values in the sampled image signal upon reading reference black and reference white images. A memory stores the peak black and peak white values in association with the respective corresponding sample clock phases, controller instructs the offset adjusting circuit generate an appropriate level offset-counterbalancing signal, commands the reading of the reference black image to detect a peak black value to determine an offset-counterbalancing value such that a value made by subtracting the offset-counterbalancing value from the peak black value falls within a predetermined appropriate range for the peak black value. A second controller selects a phase at which the peak black and white values fall in the respective predetermined appropriate ranges.