GB1353147A - Scanning arrangements - Google Patents

Abstract

1353147 Cathode-ray tube circuits EMI Ltd 28 May 1971 [18 April 1970] 18612/70 Heading H4T The effect of geometrical distortions on the output signals of a television pick-up tube is compensated by pre-setting a compensating distortion in the line and frame scanning generators. As shown in Fig. 1, a pick-up tube 1, " viewing " a bar-pattern test card 15, supplies corresponding video signals, but subject to the geometrical distortions, to a comparator 9 in which they are compared with the output signals from a " cross-hatch " generator 11, producing signals corresponding to a replica of the card 15, the output of the comparator which is indicative of the distortion introduced by the tube 1 and associated optical system 14, being fed to a store 5 and employed to modify the output of line and frame counters which via D/A converter 4 provide the input to the scanning coils 3. The store 5 includes an addressable digital storage section (Fig. 5, not shown) in which " + " or "-" error signals indicative of the relative time of occurrence of signals from generators 11 and from the tube 1 corresponding to each " bar '' of the pattern are stored and to allow a finer control to be effected store 5 includes means for interpolation between the bars. Also, since the required modification of the output of the line and frame counters may be in the " + " or "-" directions a fixed initial count for all address (including zero) is set up in the digital store the resulting D.C. shift which is superimposed on the scanning waveforms, being removed by known arrangements. A suitable cross-hatch generator is described with reference to Fig. 2 (not shown) and an arrangement for digitizing the error signals from comparator 9 is described with reference to Fig. 4 (not shown). In use the apparatus is operated, with switch 12 closed, for a number of frames after which the switch is opened and the tube together with store 5 and the D/A converters are then employed in conventional manner. The invention is applicable to colour television cameras, separate store being preferably provided for each tube. Also a tube together with its compensated store &c. may be employed to correct for geometrical distortions in a computer controlled display by programming the computer to display the bar pattern and then modifying the computer store in accordance with deviations between the video output of the camera (viewing the display) and the output of the "crosshatch " generator.