GB1297980A - - Google Patents

Abstract

1297980 Engraving half-tone printing plates WERKSPOOR AMSTERDAM NV 16 April 1970 [17 April 1969] 18295/70 Heading H4F A half-tone printing plate which is seamless in a direction perpendicular to the direction of the line scans is produced from a master 2 having a seam 3 in that direction, the printing plate and master being mounted on rotating drums for example, by repeatedly engraving a number of points in the line direction so that the seam is entirely bridged. Preferably the points repeatedly engraved on a line are staggered in the line direction in relation to the points repeatedly engraved on the adjacent lines. As shown, the analogue video signal from photo-electric scanning element 4 is digitized, 17, and passed to the first memory cell 18 of a group of such cells 18-23 which form a shift register. These memory cells and the AND gates 25-30 are controlled by signals from screen frequency circuit 24, and signal transmitters 9 and 10 which comprises cylindrical parts 11 and 12 having one and six apertures respectively which rotate in synchronism with the drums between light sources 13, 14 and detectors 15, 16. The pulses from detectors 15 and 16 are fed to the memory cells 18-23 and to a register comprising bi-stables 33, 34 and 35, the output of which is connected to a decoding circuit 36. Each pulse from 16 increases by "1" the contents of register 33, 34, 35 and each pulse from 15 resets this register and the shift register 18-23. In operation when six points on a line have been scanned the entire register 18-23 is filled with corresponding information and when the seventh point is scanned the information relating to the first point (now in cell 23) is lost. This information up to now has been passed via gate 25 to the engraving element 6 but when the contents of register 33, 34, 35 are increased by "1" gate 26 is opened and gate 25 closed so that information relating to the point which is in the memory cell 19 is engraved, i.e. a point is engraved twice. Since six pulses are produced in one line scan (revolution) by transmitter 10 six spaced points in each line scan are engraved twice in succession, it being assumed that the width of the seam 3 is equal to six points. Circuit 43 may include a delay element with a variable time-lag so that corresponding points will not be engraved twice on adjacent lines, thus preventing the repeatedly engraved points from being disposed on a straight line.