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 .