Patent ID: 6034789
Filing Date: 2000-03-07
Classification: H04N

Abstract:
An image reading apparatus, comprising:image reading means for reading an image area by area and for generating an image signal in synchronism with clocks for timing pixels of image, said image being composed of a plurality of image areas each including a predetermined number of pixels, and said image including a reference black image, and a reference white image;offset adjusting means for generating an offset-counterbalancing signal in a plurality of different levels including a 0-level at which the offset-counterbalancing signal has no counterbalance effect, for selecting a level for the offset-counterbalancing signal from among the plurality of different levels in accordance with an offset included in the image signal for adjusting the offset of the image signal by adding the offset-counterbalancing signal at the selected level to the image signal, and for outputting an offset-adjusted image signal;sample clock generating means for generating sample clock signals in a plurality of different phases in synchronism and for selecting one after another the plurality of different phases for each of the sample clock signals;signal sampling means for sampling the offset-adjusted image signal in synchronism with the sample clock signals and for outputting a sampled image signal;peak value detecting means for detecting a peak black value in each of the image areas included in the sampled image signal when said image reading means reads the reference black image and a peak white value in each of the image areas included in the sampled image signal when said image reading means reads the reference white image;storing means for storing the peak black and peak white values, sampled by the sample clock signals at each of the plurality of different phases, in association with the respective corresponding phases;first controlling means for instructing said offset adjusting means to temporarily select and set the 0-level for the offset-counterbalancing signal from among the plurality of different levels and to generate the 0-level offset-counterbalancing signal, for commanding the reading of the reference black image and the detection of a peak black value in each of the image areas included in the sampled image signal, for determining 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, for selecting a level for the offset-counterbalancing signal from among the plurality of different levels in accordance with the determined offset-counterbalancing value, and for instructing said offset adjusting means to finally set the level for the offset-counterbalancing signal to the level corresponding to the determined offset-counterbalancing value; andsecond controlling means for selecting, from among the plurality of different phases for the sample clocks, a phase at which both the peak black and peak white values stored in said storing means fall in the respective predetermined appropriate ranges for the peak black and peak white values, and for instructing said sample clock generating means to fix the phase for the sample clock signals to sample the offset-adjusted signal at the selected phase.