Patent GB-1460458-A (GB)
Title: Graphic data redundancy reduction
IPC: H04N1/17
Filed: 1973-12-20
Abstract: 1460458 Data transmission; facsimile systems LITTON INDUSTRIES Inc 20 Dec 1973 [5 Feb 1973] 59046/73 Headings H4P and H4F In a facsimile system, optical scans along (normally successive) strips of a document are converted into binary signals which are placed into a series of stores, one for each scan line, bits in the stores being read out in groups successively from adjacent groups of stores and a majority decision operation performed. In an embodiment described, bits are read in pairs alternately from adjacent pairs of stores but compression of data from three or more adjacent bits and stores i.e. 9 : 1, 16 : 1 &c. is envisaged in the Specification. In one mode, according to density of print, i.e. half tone, bits read out may be coded directly and transmitted; in a second mode for say type characters, the pair sets may be subjected to a majority decision operation before coding, marker signals being transmitted to indicate change of mode. Output from a photo-cell (42, Fig. 1, not shown) is amplified and applied through a threshold i.e. Schmidt trigger circuit 80 and shaper 82 to a series of memories 84 &c., one for each scan line, in turn. Content is read, two adjacent bits at a time from adjacent pairs of memories 84, 86, then 88, 90. As soon as 84, 86 are emptied they are refilled with new digits et seq. If the document scanned or part is half tone, i.e. high density, a high number of signal changes relative to “ clock input will be applied to a resolution detector 108 comprising an up/down counter, and bits read from memories will be coded directly, by-passing a majority decision arrangement through a delay circuit (126, Fig. 5, not shown). For low density, e.g. type characters, detector 108 will switch read "1" bits to a majority decision circuit (118) where 0-1 bits =0 and 3-4 bits= 1 which output is passed through a 1 bit store (120); 2 bits cause the previous value to be read out whether 0 or 1. The coder (Fig. 6, not shown) comprises run length counters connected to an output register which temporarily stores the counts as, say, 5 bit words each representing 27 bits. Character 11111 indicates change of mode and 00000 an idle period. The receiver operates in reverse order to the transmitter, signal outputs from memories controlling a glow tube (52) which works through a common optical system. Control of optical system (see Fig. 1, not shown).-A drum 12 holding a document to be transmitted is rotated at constant speed and a small area is illuminated by a source (24, 30) reflections from which area pass through lens (32), a fixed mirror (34) and rotatable mirror (36) mounted in a d'Arsonval galvanometer arrangement 44 focussing spots on to photo-cell 42. For each scan line a signal on line 103 applied to counter 102 through a digital/analogue converter 104 causes galvanometer mirror (36) to be displaced an amount allowing photo-cell (42) to read the next line. At count=16, an output through NAND 74 causes stepping motor 22 to operate at a rate determined by clock 66 hence travelling head 18 is displaced; signals from a transparent graduated scale (56) cause mirror (36) to be deflected reversely in correspondence, maintaining the optical path substantially on the same scan line. On reception, glow tube (52) is illuminated according to information received and galvanometer mirror is adjusted in a manner similar to transmission. The arrangement described has a scan of 1200 lines/inch hence each bit represents a square area of 1/1200 inch side.