Optical black and offset correction in ccd signal processing

Abstract

An image processing apparatus for a charge coupled device including analog front end circuitry having optical black and offset correction, whereby the an offset and optical black correction circuit has a digitally programmable bandwidth is disclosed herein. The image processing apparatus includes a sampling circuit to sample the incoming optical black signal output from a CCD. The sampled signal is filtered through an analog-to-digital converter for processing by a digital detector circuit which detects the average optical black level of the sampled signal. The sum of the channel offset and optical black level present at the output of the digital detector circuit as a digital error signal is averaged for a given number of lines and optical black cells per line by a digital averager included within the digital detector circuit. Moreover, calibration logic digitally calibrates the channel to obtain a user programmed ADC output which corresponds to that average. Furthermore, the correction circuit includes a digital-to-analog converter that converts the difference into an analog signal to be applied as an adjustment to the analog image signal at the input of the image processing apparatus. Thus, the error signal is fed back to the analog channel through the DAC in order to modify the existing signal such that a desired optical black level at the output of the ADC exists.