Patent GB-1409364-A (GB)
Title: Dual-line data compression method and system for compressing  transmitting and reproducing facsimile data
IPC: H04N1/415
Filed: 1973-02-07
Abstract: 1409364 Facsimile DACOM Inc 7 Feb 1973 [17 Feb 1972] 6014/73 Heading H4F In a redundancy reduction facsimile system, the original document to be transmitted is notionally divided into a grid of elemental areas arranged in lines (letters, Fig. 1) and columns (numbers, Fig. 1). The lines are sequentially scanned and processed in consecutive pairs-e.g. A and B, C and D, vertically aligned elements in the two lines being grouped as elemental area pairs (EAP) and each such pair used as the unit in a form of differential pulse code modulation. In the pair of lines shown in Fig. 1a, the first EAP is a black (mode) data pair (B), the second is a white (mode) data pair (W), the third is a first transition (mode) data pair (T 1 ) and the fourth is a second transition (mode) data pair (T 2 ). The lines A, B of Fig. 1 are reproduced in Fig. 2 in their mode format and are binary encoded according to the rules diagram of Fig. 3. Thus at the end of the first EAP (T 1 ) a T 1  to T 1  transition takes place and according to rule line 40 Fig. 3 a digital bit PC 2  having the same state O as the second line element of the first EAP is encoded. The same encoding PC 2  = 0 occurs at the end of the second EAP, but at the end of the third EAP a T 1  to W transition takes place. According to rule line 42, four digital bits are encoded-PC 2 , PC 1 , NC 1 , NC 2 , i.e. second line element, EAP 3, first line element EAP 3, first line element EAP 4, second line element EAP 4, or 0,1,00. A white (W) run of five EAP's now occurs and no encoding occurs until the transition to T 1  at the end of EAP 8. Rule line 44 indicates (RL W ) a digital representation of the number of EAP's in the white run, followed by a digit P = 1 indicating that the transition is to T 1  (or T 2 ). An attempt is first made to represent five EAP's by two digits. However the maximum number so representable is 1,1 or four, so a second attempt, using three digits is made, to give 0,0,1 or one. Therefore, followed by P = 1, the complete encoding at the end of EAP 8 is 1,1,0,0,1,1. If the run number cannot be represented by two digits plus three digits, then a four digit attempt on the remainder will be made and so on. The encoding at the end of the EAP's 9, 10 and 11 repeats that used at the end of EAP's 1, 2 and 3. At the end of the two white run at EAP 13, the RL w (2) digital representation is attempted using an initial attempt of three bits based on an assumption that the run is off the same length as the earlier white run. Thus 0,0,1 is produced, followed by P=1. The encoding process is continued until 512 bits of encoded data are produced. This is then stored in a buffer 300, Fig. 5 together with header data on various parameters of the encoded data and then combined in assembly and error control unit 400 with status, sync. and polycheck codes to give the frame of Fig. 4.  The scanner Fig. 6 comprises a line of 1726 light guide ends 113 viewing a document 104 stepped along in the frame direction by motor 130 under the control of document presence detecting microswitch 106. The other ends of the light guide are formed into a circle and scanned by a detector 118 rotated on a crank arm by motor 120. A shaft encoder 124 provides the line scan and element clock pulses. The printer, Fig. 22, comprises a line of 1726 electrostatic styli 906 printing on the stepped paper 914. The encoded data in the transmitted, compressed form, is converted to form of Fig. 2 and further decoded at 908 to provide energizing signal for the styli. Reference has been directed by the Comptroller to Specification 1,318,114.