Measurement of data compression in advanced group 4 facsimile systems

The CCITT (Consultative Committee for International Telephone and Telegraph) has defined Group 4 facsimile apparatus as that which transmits an ISO A4 (210 × 297 mm) document over public data networks including Packet Switched Data Networks (PSDN) and Circuit Switched Data Networks (CSDN). Data compression is employed in Group 4 systems to minimize the number of bits required for transmission of a message. CCITT Recommendation T.6 defines the basic Group 4 coding scheme to be a variation of the modified READ code (MRC II) which is the Group 3 coding option. This paper measures the compression of the MRC II algorithm for three different documents at resolutions of 200, 240, 300, 400, and 480 picture elements per inch. The compression measurement was accomplished by means of computer simulation. It was determined that the bits per page measure increases linearly with resolution. Results are presented and conclusions drawn. The Group 4 CCITT Recommendations include a mode of operation known as Mixed Mode where a page is divided into two parts: character-coded information and facsimile-coded information. This paper describes four alternative mixed-mode algorithms and measures the compression of each coding technique for CCITT document number 1. The compression measurements were performed by means of computer simulation. Conclusions are drawn regarding the relative advantages of the four algorithms.