Patent GB-1405884-A (GB)
Title: Raster-scanned display devices
IPC: G09G1/16
Filed: 1971-10-27
Abstract: 1405884 Cathode-ray tube displays GECELLIOTT AUTOMATION Ltd 27 Oct 1972 [27 Oet 1971] 50024/71 Heading H4T Vectors are displayed on a raster scanned cathode-ray tube by intensity modulation of the beam under the control of video signals derived from input digital signals and quantized into a plurality of discrete brightness levels. These are then utilized to produce a progressive increase and decrease of brightness level in each raster line and results in each vector trace (e.g. forming part of a graphical symbol) having a substantially unstepped appearance, especially for near-horizontal lines. Fig. 1, shows the overall scheme which has provision for the display of vectors and of circles, the latter, however, being produced at constant brightness level. The vectors can only be positioned on a restricted area of the screen each vector being represented by digital data specifying; the starting point, (measured from the start of the area) Xo; the number of the line scan (counted from the top of the area) in which the starting point occurs, Yo; the number of the line scan in which the end point occurs, Y1 and the tangent of the angle which the vector makes with the vertical, tan #. This data is loaded into a random access memory M1 (Fig. 3a), which can store data for a maximum of sixteen vector, during field retrace periods and the parameters of any selected vector may be modified by addressing the appropriate word (16 words each of 59 bits) and writing in the modified parameters. Figs. 3a and 3b together form the vector generator which includes a further random access memory M2 having a capacity of 512 words each of six bits B1 to B6, 512 being the notional number of horizontal video elements in the restricted area of the screen. In the general operation of the system the portion of the vector generator shown in Fig. 3a processes the input data in order of decreasing values of tan 0 and supplies an eight-bit binary output "T" which is equal to tan # when the integral part of the latter is greater than unity and equal to 1 (unity) when the integral part is less than unity. The "T" output together with X co-ordinate information is supplied to the portion shown in Fig. 3b which then operates to store in the memory (video store) M2 information specifying a complete line scan in terms of the start or end points of three discrete brightness unit (BU) levels which may occur in each video element. These levels are 1/3 BU for ¢ tan #, 2/3 BU for ¢ tan 0, full BU for tan 0, 2/3 BU for ¢ tan 0 and 1/3BU for ¢ tan 0 and such levels may be either "ON" or "OFF''. These levels which occur in sequence in each raster line and apply to vectors which make an angle 0 greater than 45 degrees with the vertical are produced in terms of the count in counters 30, 31, 32 and are converted to video (bright-up) signals in circuit 33 and employed for intensity control of the beam of the display tube. When, however, # is less than 45 degrees some of the video elements may overlap and where this occurs it is arranged that the higher brightness level overrides the lower one. Thus, in Fig. 2 the sequence of brightness levels follows that set out above for a vector such as P (# greater than 45 degrees) whereas in the case of vector Q (0 less than 45 degrees) the brightness level for line scan B has the sequence, 2/3 BU, full BU and 1/3 BU and for line scan C the sequence 1/3BU, full BU and 2/3 BU. Fig. 4a shows, schematically, a portion of the memory M2 storing the discrete brightness levels for the line scan portion A-A of Fig. 2 and Fig. 4b shows the corresponding video signal. Although data for sixteen vectors may be stored in M1 data for only eight vectors can be processed in the time available and so only eight vectors can be displayed in any given line scan. The store M2 may comprise two halves each containing 256 6 bit words the two halves being addressed alternately when writing into or reading out of the store. Alternatively, the store M2 may comprise two separate stores each capable of storing a complete line scan, the contents of one being read-out whilst the processing of data to update the other is being performed, the roles of the two stores being interchanged during alternate line scans. In a further arrangement the store M2 may comprise an associated memory divided into six stores two for each level of bright-up, (Fig. 5, not shown). Schematic circuit digrams of parts of Figs. 3a and 3b are described with reference to Figs. 6 and 7 (not shown) and flow diagrams representing the operating logic of Fig. 7 are shown in Figs. 8a, 8b and 8c (not shown).