International digital facsimile coding standards

Abstract

Recently Study Group XIV of CCITT has drafted a new Recommendation (T.4) with the aim of achieving compatibility between digital facsimile apparatus connected to general switched telephone networks. A one-dimensional coding scheme is used in which run lengths are encoded using a modified Huffman code. This allows typical A4 size documents in the form of black and white images scanned at normal resolution (3.85 lines/mm, 1728 pels/line) to be transmitted in an average time of about a minute at a rate of 4800 bit/s. The Recommendation also includes a two-dimensional code, known as the modified relative element address designate (READ) code, which is in the form of an optional extension to the one-dimensional code. This extension allows typical documents scanned at high (twice normal) resolution (with every fourth line one dimensionally coded) to be transmitted in an average time of about 75 s at 4800 bit/s. This paper describes the coding schemes in detail and discusses the factors which led to their choice. In addition, this paper assesses the performance of the codes, particularly in relation to their compression efficiency and vulnerability to transmission errors, making use of 8 CCITT reference documents.