Abstract: The charge coupled device (23) provides serialized signals from a line of a document (11) to an A/D converter (28). In order to compensate for factors such as non-uniformity of lighting, non-uniformity of CCD cell response and variations with time, a reference strip (29) is read before each reading of a document and the corresponding digital values are stored in a RAM (31). Thereafter the stored values and the values read from each line of the document are compared to provide compensated values to a half tone generator (42) which codes the output. In particular, the comparison is effected by means of an EPROM (32) addressed in part by the digital value derived from a pixel in the line of the document and in part by the digital value stored in the RAM (31) from the corresponding pixel in the reference strip. The values held in the EPROM (32) correspond to the ratios between the addressing values.

Abstract: The line of a document is focused onto a series of reading elements, such as the cells of a charge coupled device, from which a serial output signal is obtained and applied to an analogue-to-digital converter. In order to compensate for the fall-off in the transfer function of the objective and the said elements at high spatial frequencies, the signal is processed, before A/D conversion, by a multi-stage filter (27), comprising an adjustable band pass stage (36) for setting the zero level, an adjustable band pass stage (41) for setting the output level and an intermediate band pass stage (38) with unity gain, which eliminates sampling noise. Matching is provided by an input, low pass filter, emitter follower stage (33) and an output, high pass filter, emitter follower stage (43).