GB779912A - Improvements in or relating to methods and devices for making screened pictures - Google Patents

Abstract

779,912. Copying by scanning. NAAMLOOZE VENNOOTSCHAP LETTERGIETERIJ AMSTERDAM VOORHEEN N. TETTERODE. Dec. 9, 1954 [Dec. 9, 1953], No. 35738/54. Class 40 (3). In an arrangement for producing a printing plate comprising a series of equally spaced printing dot elements engraved by means of an electromagnetic tool controlled by an electric signal consisting of a triangular wave the instantaneous amplitude of which is representative of the tone of an original picture being copied, the maximum (or minimum) values of the wave are arranged to occur at constant intervals of time and the occurrence of the minimum (or maximum) values are then determined by the amplitude of the signal which is representative of the tone values of the picture at a preceding instant. With this manner of operation the printing dots are uniformly spaced and have a constant rectangular shape resulting in a clearer picture with well-defined contours of the picture elements. As shown in Fig. 3, a saw-tooth, wave derived via a resistor/ condenser network 13, 14 respectively from rectangular pulses produced by a multivibrator 8 (the latter being controlled by a signal generated by an intermittently illuminated photo-electric cell 6) is developed with opposite phases at the anode and cathode respectively of a valve 15 and supplied to the anodes of respective valves 17 and 19. The control grids of the latter valves are supplied alternately with a signal representative of the tone value during a brief instant of the picture being copied, this being brought about by means of a switching arrangement 5 to the input 4 of which the tone value signal (generated in a photo-electric cell 1 by point-by-point scanning of the original picture) is supplied and the switch being operated by means of the positive and negative " spikes " of the differentiated rectangular wave output of multivibrator 8 to cause condensers 30 and 31 to charge to a potential representative of the successive tone values. In this manner the control grids of valves 17 and 19 are held at a constant potential representative of the instantaneous value of the tone in the intervals between the operation of the associated momentary switching actions. The waveforms of the anodes of these valves are then as shown and by means of diodes 20, 21 the most negative values of the potentials at the points 16, 18 pass to a phase-splitting valve 23 and thence to a push-pull amplifier 24, 25 which supplies the electro-magnetic engraving tool 36. A mechanical construction suitable for carrying out the present invention and including means for the photo-electric scanning of the original picture and for the engraving of the printing plate is described with reference to Figs. 4 and 5 (not shown).