Patent GB-1350498-A (GB)
Title: Display systems
IPC: G06F7/66
Filed: 1971-09-27
Abstract: 1350498 Cathode-ray tube displays ELLIOTT BROS (LONDON) Ltd 27 Sept 1971 [25 Sept 1970] 44904/71 Heading H4T In a cathode-ray tube display system, e.g. for use in aircraft, cursive or symbolic information is displayed by deflecting the beam at constant speed along the lines to be displayed whilst pictorial information is displayed by intensity modulation of a conventional line scanned raster. Additionally, radar information may be displayed on a PPI scan. When cursive or symbolic information is to be displayed the system is operated in a " DDA " (digital differential analyser) mode in which the coordinates of the initial point of a line to be drawn are set up in X and Y registers (which comprise more significant 10 bit counter portions X2, Y2 connected via gates 23, 25 and carry lines to least significant 10 bit accumulator portions X1, Y1) by a computer 11 which then also loads increment registers DX1, DY1 with the required increments and then via timing pulses on line 21 and OR gate 22 successively increments the count in the X and Y registers the outputs of which, after conversion in D/A converters 24, 27, are supplied via gates 35 and 36 (enabled only in the DDA mode) to the X and Y deflection plates of the display tube 12. When pictorial information is to be displayed (" TV mode ") gates 39 and 40 are enabled and the resulting raster produced by line and frame scan generators 37, 38 is intensity modulated by input video signals on line 41. If desired, the DDA and TV modes may be combined by utilizing the frame flyback intervals of the TV mode for the tracing of cursive &c. information by the DDA mode so that the latter information appears superimposed on the TV display. In a further mode of operation (" TV mode with area shading ") concerned with producing a display consisting of a " horizon " line the tilt and vertical position of which indicate roll and pitch of an aircraft together with emphasizing means consisting of shading the display screen so that it is bright above and dark below the horizon line to simulate bright sky and dark ground, the horizon line itself is produced by DDA mode operation during the frame flyback intervals of the TV mode and the shading of the screen is produced during the TV mode operation by utilizing the DDA circuits to control the beams intensity during the TV raster scan in addition to any video signal control via line 41. In this mode the gates 23, 25 are not enabled and registers X1 and Y1 are notionally combined via gate 28 as a 20 bit X register and registers DX1 and DY1 are notionally combined as a 20 bit dx increment register the contents of the more significant half X1 of the X register being fed via gate 29 to the X2 register by means of incrementing line pulses fed to OR gate 22 from gate 49. The resulting output of converter D/A 24 then provides one input to a comparator 45 which compares this input with the line sawtooth from generator 37 and on equality, provides a control input to a flip-flop 46. The latter then provides, via gate 47, blanking pulses for the beam of tube 12 so that successive live scans are blanked corresponding to a dark ground. The position at which this blanking commences is determined by initially setting in register Y2 (not otherwise employed in this mode) the number of lines which is required to be unblanked at the top of the display (corresponding to a light sky) the register Y2, on being counted down to zero by line frequency pulses via gate 48, supplying an enabling input to gate 49 which then passes line pulses, as the incrementing pulses, to OR gate 22. Also, the effective starting point of the boundary between the light and dark areas may be arranged to lie between two successive scan lines by setting an initial number in the X register, which differs from the maximum or minimum contents by less than the increment dx. The Specification discusses the initial state which flip-flop 46 is required to have at the beginning of each field scan in accordance with the direction of slope of the boundary (i.e. the " horizon " line) and indicates that the required control can be achieved by conventional logic circuitry. The " TV mode with area shading " may utilize both fields of an interlaced raster scan by setting the initial contents of the X-register to differ by half the dx increment on successive frames (to take account of the relative shift between successive fields). Alternatively, only one field may be utilized and the beam spot defocused slightly to remove the " lininess " and the time normally used for the second frame employed for other purposes such as drawing symbols in the DDA mode. A solid-state display device including a matrix of individually energizable electro-optical elements may be used instead of a cathode-ray tube in which case the D/A converters 24, 27 are not required and the scan generators 37, 38 are formed as counters producing multi-bit digital outputs.