Abstract: An encoder/decoder for video signals is especially designed for truer conversion of half-tone signals. An image scanner produces analog signals which are converted to digital signals and buffer stored for eliminating timing differences. Then, the signals are preprocessed to increase the run-length of the digital video signals. The signals are monitored both before and after the preprocessing. A comparison of these monitored signals leads to a selection of the one signal before or after preprocessing which appears to be the best. That selected signal has a tag inserted to demark it from the next succeeding signal and to indicate whether the preprocessing was or was not included in the transmitted signal. The tag signal is used to include or exclude a post processing in order to undo the encoding.

Abstract: A coder/decoder system develops signals for printing half tone images. The invention uses a selected one of many dither matrices to provide a selected resolution of half tone images. Each dither matrix has n-by-n capacity. The coder and decoder includes a cascaded number of register and EX-OR gate combinations corresponding to the largest dither matrix. A number of steering gates block off parts of the cascade corresponding to the size of the dither matrix that is actually selected for the transmission of any given image.

Abstract: In order to increase decoding speed of two-dimensionally encoded facsimile data, changing pixels a.sub.0, a.sub.1, a.sub.2 and b.sub.1 (defined by CCITT) on a decoding and reference lines are detected, and the address of b.sub.1 is stored into a reference line address memory. The address of b.sub.1 is compared with the address of a.sub.1 during the vertical mode, and if a.sub.1 >b.sub.1, the address of a.sub.1 is stored into the reference line address memory. During the horizontal mode, the addresses of b.sub.1 and a.sub.2 are compared. If a.sub.2 >b.sub.1, the address of a.sub.2 is stored into the reference line address memory.