Patent GB-1401022-A (GB)
Title: Display apparatus
IPC: G09G5/20
Filed: 1972-03-10
Abstract: 1401022 Cathode-ray tube displays ELLIOTT BROS (LONDON) Ltd 2 March 1973 [10 March 1972] 11275/72 Heading H4T In a symbol display system in which the symbols comprise one or more lines produced by selective bright-up of selected portions of raster lines traced on a cathode-ray display tube the basic parameters defining each symbol line, such as 1 (Fig. 1), comprising the central co-ordinates (Xc, Yc), the length (L), the width (W) and the angle the line makes with the vertical (#) are utilized by a digital computer 10 (Fig. 2) together with a quantity (h) representative of the spacing between the raster lines to produce outputs representative of Y 1 ; Y 2 ; Xst = (X start) and Xop = (X stop) successive valves of Xst and Xop being incremented by a quantity h tan 6 between Y 1  and Y 2 . The computer output is supplied to an output circuit 11 comprising, as shown in Fig. 3, respective resistors 16, 17 for the Xst and Xop quantities, feeding respective comparators 18, 19 which also receive inputs from a cyclic counter 12 synchronized with the line scan and producing an output at 15 MHz (thus notionally dividing each line scan into 780 sections), the comparators producing, on equality of the two inputs, outputs which are fed via gate 20, which develops a signal during the period when the output from counter 12 is #Xst and #Xop, to the intensity control electrode of the display tube 13 either through a delay circuit 14 and a pulse shaping circuit 15 or through the circuit 15 alone depending on the value of 0. The tube is scanned with a 625 interlaced line raster of which 585 lines are visible so that the Y-resolution is 1 in 585 and with a 4/3 aspect ratio a comparable X-resolution requires each raster line to be divided into 780 sections which therefore determines the frequency of 15 MHz for the cyclic counter 12. Ten bit words define Xst and Xop and ten bit words are also employed to define Y 1  and Y 2 , nine bits being sufficient if the line starts in an even field and finishes in an odd field and the tenth being added to indicate whether or not the first or last (or both) bright-up pulses should be suppressed. Since the increment h varies with the angle # fractional parts must be added for values of 0 between 0 and 45 degrees and this is done by splitting each word into ten bit words specifying respectively, the increment and the increment faction. When # = 45 degrees the X and Y increments are equal and the definition in the X direction is equal to that in the Y direction. However for lines near the vertical (#, small) a large number of Y increments occur for each X increment and for lines near the horizontal (0 large) a large number of X increments occur for each Y increment and in each case, the line has a stepped appearance. To compensate for the former (0 small) the X-definition is increased by a factor of 4 by the use of delay circuitry 14 comprising three 15ns delay circuits and a 4/1 multiplexer (Fig. 4, not shown) and to compensate for the latter (# large) the bright-up pulses are so shaped that each brightened-up portion varies in brightness along its length and blends with the other brightened-up portions to give the illusion of a continuous line (Figs. 5a, 5b and 6, none shown). The delay technique employed to increase the X-definition may be extended such the cyclic counter may operate on a lower frequency, e.g. 7À5 MHz to produce the same definition and when this is done since the width W must remain constant as # varies, the length of the bright-up pulses must vary and to ensure, in this case, that the lengths are correct, the arrangement of Fig. 4 (not shown) is modified by the introduction of a differential delay factor. Since the first part of the initial brightened-up portion and the last part of the final brightenedup portion are produced during a single field only, (Fig. 1) flicker occurs at the beginning and end of a line and to overcome this the length of the line is extended at both ends and the "excess" length blanked so that the flickering regions lie in the blanked off parts.