Comparison of Redundancy Reducing Codes for Facsimile Transmission of Documents

A comparison and a valuation of different redundancy reducing coding techniques for the digital transmission of facsimile documents on telephone lines is presented. Especially taken into account are those codes which have been submitted to the International Telegraph and Telephone Consultative Committee (CCITT) for standardization of digital group 3 facsimile machines. The reduction factor and the sensitivity of channel errors of these one- and two- dimensional codes have been investigated by computer simulations using the CCITT test documents and burst error patterns of real telephone lines. For a resolution of 1728 picture elements per line and 3.85 lines per mm one-dimensional run length coding proves to be the most economical solution. Using a higher vertical resolution of 7.7 lines per mm the effects of transmission errors are less visible in the received document and two-dimensional codes become more efficient. To achieve a transmission time of 1 min or less for a size A4 document a transmission bit rate of 4800 bits/s is required. For the higher vertical resolution a transmission time of 1 min cannot be guaranteed for all types of documents with this bit rate even if two-dimensional coding is used.