Image signal processing LSI for facsimile

The basic function of the facsimile reader is to correct the shading distortion (low‐frequency distortion due to low peripheral intensity) produced by the optical system in the analog signal from the one‐dimensional image sensor and to produce the correct black‐and‐white binary signal. Based on quantitative evaluation of the shading distortion, the authors developed LSI for signal processing, realizing a high‐precision binary signal transformation. The LSI first memorizes the shading distortion in the RAM in digitized form and then the information read from the RAM goes to the DA converter in synchronization with the input image signal. Comparing the reproduced analog distortion signal and the input image signal, highly precise distortion correction is realized. In this scheme the analog distortion is proportional to the input signal level and the operation can follow the change of source intensity and the change of manuscript background brightness. This function is realized by division between the reference voltage and the input voltage of the AD converter. The binary slice level is set with an accuracy within ±3%. The proposed LSI was fabricated by 5‐μm N–MOS technique and it was verified that the realized LSI satisfies the requirement for facsimile.