Reduced-Time Facsimile Transmission by Digital Coding

The growing importance of digital communication links has resulted in a family of coding methods designed to achieve reduced-time digital facsimile transmission. This paper reports on a study of the effectiveness and feasibility of one of these methods, Run Length Coding (RLC). RLC is a well-known digital method in which the area and shade of the copy are quantized to form a mosaic pattern. Neighboring elements of the mosaic are successively scanned, and redundancy is removed by coding and transmitting only lengths between shade changes (run lengths) rather than individual area elements. Measurements of the statistical distribution of run lengths of various copy were made to determine the possible transmission time reduction. Digital equipment and a drum scanner were used to measure the statistics of weather maps, topographic maps, text, diagrams, and continuous-tone pictures (quantized to four shade levels). The maximum possible time reduction was calculated for each type of copy, and practical codes were then designed and evaluated. The average time reduction factor for black and white copy was four, compared to a theoretical optimum of 4.7. Since the maximum bit rate of the transmission line should be continuously utilized, and since time reduction factors for particular runs depend on the lengths of the runs, a practical RLC facsimile system must possess either a variable speed scan, a large and elastic buffer storage, or some combination of the two. A system is proposed whose encoding principle permits a feasible compromise between these two extremes, and whose implementation involves a flying-spot scanner that is converted into a digital device by means of optical reference gratings.