GB1466282A - Method of coding for facsimile transmission - Google Patents

Abstract

1466282 Data transmission; facsimile systems; methods of coding HASLER AG 6 June 1974 [8 June 1973] 25284/74 Heading H4P In a method of coding for facsimile scanned lines, successive points of equal brightness are counted and code words of varying length are allocated to numbers of points such that longer words correspond to greater number of points and each word of at least one brightness consists of one or more groups each of constant length and comprising two parts, the first part indicating brightness and the combination of second parts being formed by adding 1 to the number of points, the sum being in binary code, and then omitting the most significant bit=1; the number of groups is chosen so that the second part of the group allocated to the most significant bit or bits contains a 1. The group length may differ for different brightness values. In Fig. 1 (not shown) the first bit c indicates black/white significance followed by 2 information bits. The number of groups and the information bits together form a combination which determines run length upon reception. Fig. 2 (also not shown) has only one information bit per black/white bit. In the limiting case each group consists of one bit = original bit; the code may be different for black/white. At the transmitter, a signal is passed to controller 5 at the end of each line of 0/1 bits from scanner 1 fed into shift register 2 causing serial read out to coder 20 which includes a run length counter 10, which is reset at each bit change with the count dumped into shift register 12. This corresponds to operation as described in copending application No. 25283/74. In the present arrangements, circuit 21 produces a code word from the counter, and assuming two bit groups, a colour indicating bit from flip-flop 17 and a count bit from register 12 are transmitted. Timing is controlled by a clock 22 and divider 23. As soon as the most significant bit reaches the last stage of register 12, decoder 14 through inverter 28 blocks gates 25, 27 and signals controller 5 that code word transmission is complete, i.e. the most significant bit is not transmitted. The receiver (Fig. 4, not shown) is approximately a "mirror image" of the transmitter plus a clock pulse extraction circuit. Received signals are passed into a register (34) connected to a read only memory (39) and down counter (40) through gates (not shown). Another counter (64) is reset at each brightness change thus serves to ascertain the number of groups in a code word. Before reset, counter (64) reading is transmitted to ROM (39) which determines the number of groups and associated number of points from the code word which are passed into counter (40) and thence to a register (45) and to a writing device (45). Advantage of the arrangement is stated to derive from the fact that the code word is approximately proportional to the logarithm of the run lengths.