cognised and the appropriate code transmitted with the addition of an MH code for longer runs. MR coding is successful because of the high degree of correlation between facsimile lines in documents and typically a compression 01001011 000000101100 of about 10:1 can be obtained. 00110010 000001011010 00110011 000001100110 60.4.2.3 Modified Modified READ 00110100 000001100111 When the G3 ECM option is used the reference lines in MR are redundant and CCITT permit the Modified Modified READ (MMR) option. In this case the line before the first line of the page MMR is particularly suited to higher resolutions where the corre- is assumed white and all subsequent lines are referenced to their lation between lines increases. For twice the vertical resolution with previous line. This coding is the same as T.6 used in G4 and can give MH the number of bits to be transmitted doubles but with MMR the compressions of the order of 14:1. increase reduces to around 1.4:1. 60/8 Facsimile transmission Table 60.5 Extract from Modified Huffman code table 60.4.3 Modulation and demodulation (make up code) Digital signals from the data compression encoder are unsuitable for White run Code word Black run Code word direct transmission to the telephone network. For 9.6kbit/s the lengths lengths necessary transmission frequencies extend from d.c. to 4800Hz, too wide for the telephone networks nominal bandwidth of 300Hz to 11011 0000001111 3000Hz. For transmission the digital signals are transformed by the 10010 000011001000 modem to analogue voice band signals which can tolerate the 010111 000011001001, frequency shift, noise, amplitude and group delay variation with frequency, and also the echo and phase jitter of the international 0110111 000001011011 telephone network. G3 line signals are analogue. They vary in amplitude as a speech signal, but they are generated from digital signals and may be completely and accurately recovered providing line conditions permit. 010011010 0000001011011 60.4.3.1 G3 signal transmission 011000 0000001100100