n and Modified Huffman coding. frequent run lengths are represented by the shortest length codes. (See Table 60.2) Because of the different distributions of black run lengths and white G3 facsimile equipment 60/7 Table 60.3 G3 resolution and Modified Huffman Coding. Table 60.4 Extract from Modified Huffman code table (See Figure 60.2) (terminating codes) Make up code Terminating code White Code word Black Code word length of pels length length 01100100 01010011 00110101 0000110111 0000010111 000111 0110111 00010010 000011001000 000001100101 010111 0101011 00000010111 0101000 00000010111 00011 01100100 01000 10011 000101 10100 000100 End of line 000000000001 00111 0000100 Total bits 01000 0000101 001000 0000111 000011 00000100 An end of line code is added to provide synchronisation and allow 110100 00000111 the receiver to check that the decoded line is 1728 elements. Typi- 110101 000011000 cally a compression of about 7:1 can be obtained. 101010 0000010111 101011 0000011000 60.4.2.2 Modified READ 0100111 0000001000 CCITT allows two optional 2 dimensional coding schemes. These 0001100 00001100111 are both RElative ADdress or READ techniques where the new line 0001000 00001101000 to be coded is based on the previous line. The first CCITT option of the modified version of READ (MR) was standardised before ECM 0010111 00001101100 (error correction mode) was available. It had to operate in the 0000011 00000110111 presence of network errors and ensure that the remainder of the data on the page could be decoded. To provide for this only one two 0000100 00000101000 dimensional line was permitted to follow the one dimensional MH 0101000 00000010111 line in standard resolution, and 3 lines were permitted in fine resolution. Inclusion of this redundant data in an MH line allows 0101011 00000011000 decoder recovery. 0010011 000011001010 Figure 60.3 and Table 60.6 illustrate the Modified READ coding 0100100 000011001011 technique. A pair of points either between reference and coding lines or on one or other of the lines is re