This invention relates to synthetically generated displays for aircraft flight instrumentation, and more particularly to hybrid cathode ray tube displays using digitally generated rasters and stroke vector displays.

Stroke written cathode ray tube (CRT) displays trace the shape of figures to be presented by deflecting the electron beam in a manner which connects a successive sequence of strokes, which may be straight or curved. In a raster system the beam is caused to trace a repetitive pattern of parallel scan lines and the information is presented by intensity modulating the electron beam at the appropriate points along each line.

A hybrid display system includes a conventional stroke vector generator and a conventional raster symbol generator which supply sequentially a single CRT with a picture that includes both raster and stroke information. This composite display permits rapid update of the character symbology and a coloured background with minimal requirements for memory and processing time. In hybrid CRT display systems used in applications such as aircraft instrumentation, real time high speed updates of raster and stroke symbology are required. It is also desirable to produce complex and dynamic raster symbology in addition to stroke symbology, which has not heretofore been attainable with conventional raster displays. The raster symbology must be produced in an efficient manner with respect to computation time, quantity of circuitry, and power dissipation.

Digital raster display generators are known in the prior.art which utilise permanently wired dedicated rasters-symbol generation circuitry for generating video signals during the time intervals defined by the digital circuitry generating the raster. See, for example, Display Systems Engineering, Luxenberg & Kuehn, McGraw-Hill, Inc., 1968, pp 267-269. Such.systems generally utilise a unique permanently wired symbol generator for each raster symbol or pattern to be displayed. Such systems have the disadvantage of lack of flexibility, since they are not programmable, and they require large amounts of permanently wired circuitry. Consequently, this approach also requires significantly increased volume and power for the electronics. These desiderata are particularly significant in the field of airborne systems.

Other prior art digital display generators use software intensive program techniques. Software intensive techniques have a primary disadvantage of using large amounts of valuable computer time in a real-time system, where processing time is critical.

Two basic methods have been used by prior art software systems. In one approach, as outlined in High Resolution Graphics System, William Burden, Jr., Popular Computing, July 1982, pp 116-120, a full-field memory or bit-mapped technique is used, where each resolution element of the display is defined by a group of memory bits in accordance with the individual picture elements on the display screen. The picture is loaded into memory from a computer and the entire memory is read out in synchronism with the digital circuitry generating the raster. An image is produced by specifically setting, for each picture element, the colour and intensity desired by writing the appropriate data into the full-field memory. The serial digital memory output words are converted to analogue form and are transmitted to the display for each frame refresh. From a hardware stand-point, this approach is unattractive because of the size of the required memory and support circuitry. For example, for displays of nominal size utilising an adequate colour and contrast range, memory capacities of up to one million bits are required. Further, for dynamic symbology, the changing data for each memory element must be repetitively calculated and specifically programmed and stored in memory. This results in a prohibitively high use of processor time and a resulting image whose update rate is unacceptably slow. It will be further appreciated that because of the necessity for rapid readout of the large memory required, a high speed memory system would of necessity be utilised, which tends to be complex, expensive and critical in operation.

A second approach to raster symbol generation is disclosed in Applicants U.S. Patent Specification No. 4,070,662. In this approach, the face of the display device is divided into a matrix of cells. A symbol library contains a number of symbols or bit patterns which may be placed into each cell as desired. While reducing the storage requirements for pattern generation, the memory required, along with the support hardware necessary to access and control the memory, makes the cell approach a moderately expensive implementation. Further, the cell approach has not proved to be well suited for the dynamically changing symbology commonly found in flight display applications. Although such movement can be accomplished, it can be done only to a limited extent in practice and may require a significant amount of processor time to calculate the appropriate cell and symbol definitions.

The invention is defined in the appended claims and provides apparatus for superposing a raster symbol display and a vector symbol display. The apparatus comprises clock means for providing timing signals for synchronising the raster symbol display and the vector symbol display. A programmable vector generating means is responsive to the clock means and provides signals representing a stroke vector of predetermined length, origin, and slope, the vector defining regions of predetermined colours. A programmable raster generating means, responsive to the clock means and the vector generating means, provides the raster symbol display sequential to the vector symbol display, the raster display providing at least one region of predetermined colour defined by the stroke vector, and comprised of a plurality of raster lines, at least a portion of which are sequentially disposed.

In a preferred embodiment, the programmable raster generating means includes control logic means for receiving positional data provided by the vector generating means. The control logic also is responsive to signals corresponding to ones of a plurality of sequential raster lines and synchronisation energisation signals for the vector symbol display and the raster symbol display, and selectively provides positional data and sequential raster line signals to an addressing means. Memory means are coupled to the addressing means for storing positional data corresponding to a pixel position on a raster line. A comparator compares the stored positional data in the memory means and sequential signals corresponding to a plurality of sequential picture elements along one of the plurality of sequential raster lines, thereby providing a signal to a digital switch means when the signal corresponding to one of the plurality of sequential picture elements equals the signal from the memory means. The digital switch means is synchronised with the raster lines to provide a raster colour command signal for each line in the raster symbol display.

A hybrid display system in accordance with the present invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:-

• Figure 1 is a schematic illustration of the display face of a cathode ray tube of the system in accordance with the invention comprising a hybrid stroke vector and raster scan display,
• Figure 2 is a schematic representation of the raster scan showing details of the scan line and pixel structure,
• Figure 3 is a schematic illustration of the display face implemented in accordance with the invention,
• Figure 4 is a schematic block diagram of the display system in accordance with the invention,
• Figure 5 is a schematic block diagram of a raster symbol generator used in the system of Figure 4, and
• Figure 6 is a schematic block diagram of a display interface as utilised in the system of Figure 4.

In general terms, the face of a display apparatus, such as a cathode ray tube, is sequentially scanned by a stroke vector display and a raster scan display. The stroke vector generator provides positional data and colour video data for the raster scan during the stroke period of the refresh cycle. The positional data and colour data are stored in a digital memory in a raster symbol generator, defining the colour of each raster scan line and the picture element at which a colour transition occurs. During the raster scan period of the refresh cycle, the data in the memory is fetched in synchronism with the raster scan along the X and Y coordinates. By triggering colour control logic at the point of intersection of a stroke vector with a raster scan line, zones of colour, filled in by the raster scan, are defined by the stroke vector with minimal requirements for data storage and process time while permitting dynamically rotating colour symbology. Typically, a raster containing 256 scan lines with 512 picture elements along each line requires a random access memory size of 2304 bits, while a prior art bit-mapped display of the same resolution requires 16,384 bits for a two-colour display.

Referring to Figure 1, a pictorial representation of a display screen generally denoted by reference numeral 10 comprises a display face 12 for displaying thereon a multicolour symbology, which may for example be a sky-ground representation, the line 14 denoting the horizon, colour zone 16 denoting the sky and colour zone 18 denoting the earth. The display face 12 may be, for example, the face of a conventional CRT display but it is appreciated that the invention is applicable to other types of displays as well, such as gas plasma displays, liquid crystal displays or other electrically actuated displays.

A conventional raster generator provides a raster on the display face comprising raster lines 20 made up of individual pixels 22. For clarity of description, the preferred embodiment will be described in terms of a simple non-interlaced raster. It should be understood that these raster lines may be generated sequentially, each raster line containing a number of sequentially generated pixels. A typical display might consist of 256 raster lines, each containing 512 pixels. Greater resolution may be had by increasing the number of lines or the number of pixels per line in a given display.

It will be appreciated that the precepts of the present invention are also applicable to a system having a conventional interlaced raster with the odd raster lines written in one frame and the even raster lines written in the following frame.

Further, while the present invention is described in terms of a rectangular coordinate (X,Y) raster scan display system, it will be appreciated that the principles of the invention could also be applied to displays having other scanning systems. For example, addresses may be specified by polar coordinates for a circular scan, and still other displays use a spiral raster system. In any case, the position of the electron beam is known or can be derived so that stored positional information can be fetched and applied to the beam control circuits in accordance with the principles of this invention.

For purposes of describing the invention it will be assumed that the raster generator generates a raster beginning at origin 0 in the lower left hand corner of the display face 12 and then draws a raster line 23 vertically by holding the X deflection constant while ramping the Y deflection signal through successive pixels 22 of the scanned line. At the end of the first line, the X deflection is incremented to the second raster line 24, and the Y deflection is initialised or returned to the baseline 26. The second raster line 24 is then drawn vertically by holding the X deflection constant while ramping the Y deflection signal. The screen may be blanked during the retrace portion of the cycle. In this manner, the entire raster pattern is generated. It will be appreciated that the starting point or origin 0 in the lower left hand corner is chosen for convenience and is not to be construed as a limitation of the invention.

With continued reference to Figure 1, two colour zones or intensity shadings, 16 and 18, respectively, are seen as already mentioned. The desired hue and intensities are provided by control of the colour video signal applied to the cathode ray tube. Because of the digital nature of the raster scan, this colour choice may be determined at any of the pixels 22 in a raster line. Further, the colour choice may be changed at any succeeding raster line.

Line 14 represents a stroke vector as may be drawn by a stroke vector generator. In the present embodiment, the stroke vector 14 delineates the zones of differentiated colours and intensity. It may be seen that "filled in" raster figures may be produced by defining specifically only the outlines of the desired figure and allowing the raster scan to fill the outlined areas with the predetermined colour. The digital nature of the vector generator to be described permits defining the figure outline as digital bits or pixels along each raster line. This technique allows the use of the vector generator hardware and software for both stroke and raster symbol generation, in a manner to be described.

Figure 2 is an enlarged and exaggerated view of Figure 1, illustrating the generation of a raster scan pattern which would result in two areas of different colours defined by a stroke vector 30. As in the previous example, the raster lines are shown drawn from screen bottom to screen top, and sequenced from the origin 0,0 on the left to the right of the screen. In a preferred embodiment of the invention, the composite display is refreshed at a 40 Hz rate, with alternating vector stroke and raster scan displays at an 80 Hz rate. For the interlaced scan, the stroke display is thus refreshed at an 80 Hz rate, and individual fields of the raster scan are also displayed at an 80 Hz rate.

By defining the point of intersection between the stroke vector 30 and each raster line 32 where the colour is to change, shown here as a dot 34, and predetermining the colour at the screen bottom at the start of the line, say line 14, the repetitive raster scan may then be used to fill the areas in the designated colour zones. The result is an area of the start colour in the region below the line 30 and an area of a different colour in the region above the line 30. For example, in generating artificial sky-ground shading symbology in electronic attitude displays, typically the bottom area will be brown (representing ground) and the top area will be blue (representing sky).

Figure 3 shows the resulting raster scan pattern with a start colour shown as thin lines 40 and a second colour as bold lines 42. The "stair stepping" nature of the demarcation between the colour zones, shown by bits 44 and 46, is characteristic of raster displays of a digital nature.

Referring now to Figure 4, a schematic block diagram of the display system implemented in accordance with the present invention is illustrated. The apparatus includes a clock means or timing module 50, a conventional programmable vector generator 51, a programmable raster symbol generator 52, which is described below, a display interface 53 for converting the digital colour and beam position data to analogue form, also described below, and a CRT 54 the face 10 of which is illustrated in Figure 1. The timing module 50 provides horizontal and vertical synchronisation pulses for energising the X (horizontal) and Y (vertical) sweeps for the raster scan on the CRT 54. The timing module 50 also produces a command signal on line 56 to the vector generator 51 to initiate generation of the display format. A mode control signal on line 55 is provided to the raster symbol generator 52 and to display interface block 53 to initiate appropriate display functions during the respective stroke vector portion and raster scan portion of the refresh cycle. Further, the timing module 50 also provides a raster X count on bus 57, representing the sequence of raster lines being generated, a raster Y count on bus 58, representing the sequence of pixels corresponding to a raster scan line, and a reset line 59 for establishing the appropriate colour conditions at the beginning of a raster scan line. For each one of the plurality of raster scan lines, a complete pixel count will be generated.

The timing module 50 is provided with a clock oscillator 60 for generating regular clock pulses. In the preferred embodiment, this clock preferably operates at 13.1 MHz. However, other clock rates suitable for the required display updating and ancillary circuitry are also suitable. The frequency of the clock oscillator 60 is determined by the resolution of the required X and Y counts, a higher frequency being required for higher resolution systems. The clock pulses are sent to a pixel counter 61 and a control PROM (Programmable Read Only Memory) and latch 62 to effect a controller function. Pixel counter 61 is initialised by a signal from PROM 62 on lead 63 at the beginning of each raster scan and counts in synchronism with the pixels being generated to provide the raster Y digital timing signal. A line counter 64, also initialised by the signal on the line 63, is driven by a binary count sequence from control PROM and latch 62 and thereby counts in synchronism with the raster lines being generated to provide a second digital timing signal for the raster X count. Thus, taking pixel counter 61 and line counter 64 together, the timing module generates a pixel number and line number corresponding to the pixel address currently being generated by the conventional raster generator. In terms of the X, Y Cartesian plane, the pixel counter 61 generates the Y position and the line counter 64 the X position. The control PROM and latch 62 is further programmed such that as the count sequence progresses, control signals to the stroke vector generator 51, raster symbol generator 52, and display interface 53 are generated in the appropriate order and time. A control signal on the line 56 is sent to the stroke vector generator 51 to indicate it is to begin generation of the stroke vector display format. When the stroke vector generator has completed its portion of the refresh cycle, it returns a signal on line 66 from a stroke control logic block 65 to control the PROM 62 which initiates the raster scan portion of the display cycle.

The control PROM and latch 62 is organised to command video modulation at a rate of 80 fields per second where a field is comprised of 128 raster lines with a resolution of 512 pixels per line. A rate of 80 fields per second is required in order to obtain a flicker-free presentation on the CRT face. Preferably, interlaced fields of 128 lines alternate every 12.5 milliseconds to form a complete display format on the face of the CRT at a frame rate of 40 Hz. The timing module 50 determines the field rate by utilising approximately 6.25 milliseconds to sweep the cathode ray beam across the CRT face during generation of the stroke vector display in each frame, followed by another interval of 6.25 milliseconds to generate a field of 128 raster lines. These time frames include the time for vertical retrace of the beam, and are followed by a second frame of 6.25 milliseconds for the stroke vector display and 6.25 milliseconds to generate the alternate field of 128 raster lines. When used, the even-odd fields are generated conventionally whereby the first line of the raster is started at the lower left corner of the screen and is initiated at the start of the raster refresh cycle. The odd field, for example, will end at the extreme right end of the next-to-last raster line ℓ- 1 of Figure 2. On the next raster refresh cycle the even field will start at the lower left extremity of raster line 1 and end at line ℓ at the upper right edge of the screen. An interlace signal, not shown, controls the raster starting position in a conventional manner.

With continued reference to Figure 4, the block 51 shows a conventional vector generator which produces horizontal (X position) deflection waveforms and vertical (Y position) deflection waveforms and video (colour) control as commanded by instructions stored in a random access memory 70. A conventional computer (not shown) applies digital instruction signals to an address bus 71 and data bus 72 in accordance with the display presentation to be generated on the display face 10 of the cathode ray tube 54. During the raster display interval a multiplexer 73 accepts address data on the bus 71 and applies the address data to the stroke instruction memory 70 via a bus 74. At the beginning of the stroke display interval, the data bus 72 from the computer interface applies data to a buffer 75 which is written in to the storage locations in the memory 70 via buses 76 and 77 at the address provided on the address bus 71. The instructions are stored sequentially and completely define the picture to be presented. Preferably, the last instruction in the memory will indicate that the display is complete.

When the signal on the line 56 from the timing module 50 initiates an appropriate command to the stroke control logic block 65, a control signal is provided to the multiplexer 73 through a bus 84 to the stroke instruction memory 70. By means of a control bus 78 to a stroke vector generator 80, the stroke control logic 65 loads instructions through the instruction bus 76 from the memory 70. The vector generator 80 uses these instructions to generate the necessary deflection signals on buses 81 and 82 and video bus 83 to present the desired stroke display. Upon completion of the instruction loading, a command is sent on line 66 to the control PROM 62 indicating that the stroke vector generator 80 has completed the display update.

Stroke vector generator 80 further comprises a conventional X accumulator, Y accumulator, and video latch, not shown. The vector generator 80 after initialisation causes the initial X axis and Y axis positional data and the initial video to be stored in their respective accumulators or latch, respectively. The X-position accumulator updates the stroke vector positional component along the X axis after each digital increment of position information, and provides the current X coordinate CRT of beam position on the bus 81 to the raster symbol generator 52 and display interface 53. Similarly, the accumulator for the Y-position updates the stroke vector Y-position component and thereby provides the current Y-component of position on the bus 82 to the raster symbol generator 52 and display interface 53. In a like manner, the V.G. video bus 83 provides current video information data to blocks 52 and 53.

Referring now to Figure 5, a schematic block diagram is shown of the raster symbol generator 52. A memory 101 may conveniently be instrumented as a random access memory (RAM) with read-write capability. The RAM 101 is organised such that there are adequate addressed locations to represent all raster lines and wide enough data fields to provide the desired resolution along each line. For example, in the preferred embodiment, a raster containing 256 scan lines with 512 picture elements along each line requires a RAM size of 256 words x nine bits/word. This provides a location for each of the 256 raster lines and sufficient resolution in each data word (nine bits) to identify any one of the 512 pixels. Thus, each of the 256 storage locations is associated with a particular raster line, and each location permits designation of the particular pixel at which a colour change is desired. In a manner to be explained, an X-counter and Y-counter of the timing ciurcuitry which synchronise the sweep of the beam across the display face in raster fashion address the storage locations of the memory 101 so as to provide a real time association between the words of the memory 101 and the X and Y positions of the CRT beam. Note that colour information per se is not stored in RAM but is determined by associated circuitry to be described.

During the stroke vector portion of the refresh cycle mode, the control line 55 (Figure 4) establishes the RAM 101 in the write mode and permits an address 102 from the vector generator X-position bus 81, which identifies the raster line number through a multiplexer 106, and data from vector generator Y-position bus 82 through a buffer 108 and buses 103, 112 and 111, to write data into the memory 101 which identifies the pixel at which the colour is to change. Thus at each increment or change of vector generator X-position data on bus 81 or vector generator Y-position data on bus 82, the data 103 is written into the memory 101 at address 102. Thus, for example, a programmed stroke vector corresponding to the solid diagonal line 14 in Figure 1 would result in point data represented by the dots 34 in Figure 2 along the raster lines O - 2. While the preferred embodiment for clarity is illustrated with only a single colour change, if more than one colour change is required along a given raster line, additional RAM circuitry may be provided.

The readout of the memory 101 is controlled by the mode control line 55 so that whenever the raster line scan commands the beam to a predetermined line position, the pixel number at which a colour change is desired is read. Thus, when the first vertical raster line 0 is being scanned, then the pixel number desired for a colour change on line 0 is available for comparison with the pixel count 0 to P, which corresponds to the vertical position of the beam as the raster line is scanned. When the second vertical raster line 1 is scanned, the colour change pixel number for line 1 is read and compared to the actual pixel count 0 to P, and so on through the memory until the final vertical raster line £ is scanned.

In this manner, following completion of the stroke vector display, the mode control signal on the line 55 changes state and causes the memory 101 to be operated in the "read" mode. The buffer 108 is disabled, thus preventing any transmission of a signal on buses 82 and 111. The raster X count on bus 57, which identifies the raster scan line which is to be displayed, will change only at the end of a line's display interval. The signal on bus 57 is transmitted through the multiplexer 106 to provide an address to the memory 101. The memory 101 then provides the data located at the selected address on the bus 103 to a compare block or comparator 110 which provides the pixel location at which a desired colour change was programmed by the stroke generator signal. A second input to the comparator 110 is provided by the raster Y count on the bus 58, which identifies the pixel number being displayed. The signal on the bus 58 is reset and increments as each of the raster lines is scanned and when it is equal to the pixel count data on the bus 103, the comparator 110 provides a control signal to a toggle logic switch 130.

Thus, the comparator 110 tests whether the pixel currently being generated on the bus 58 is equal to the pixel stored at the corresponding raster scan line in the memory 101 where a colour change is desired. The comparator 110 compares the binary values corresponding to the two pixel positions and when they are numerically equal will output a logical high or logical zero to the logic switch 130, as determined by the reset line 59, whose function is described below. The logic switch 103 receives the output of the comparator 110 and being a single signal of digital (1 or 0) nature, can define two colour states. It will be clear to one skilled in the art that by partially duplicating the memory and control functions described above and processing their raster video outputs in parallel to provide additional output channels, additional colour combinations may be realised.

Figure 5 also shows a reset signal 59 which is applied to the logic switch 130. During the blanking interval between the end of one raster scan line and the beginning of the next line, the reset signal 59 returns the state of the output of the logic switch 130 to that desired for the start of the next raster line. The origin of the reset signal is the timing module 50. While a reset would typically result in a zero output at the logic switch 130, some situations may require a ONE. For example, when generating sky-ground shading with a stroke vector representing the horizon, a typical reset value would be zero for a brown start colour for normal attitudes, representing ground. However, if the aircraft is flying in an inverted attitude, the reset value could be ONE for a blue start colour, representing sky. The value of the start colour and the state of the reset signal may be determined by conventional attitude sensor and control logic circuitry, not shown.

Referring again to Figure 4, the apparatus includes a conventional cathode ray display tube 54.with an electron beam whose position is controlled by X deflection and Y deflection signals applied to corresponding electrodes. A video signal applied to suitable control electrodes determines the hue and intensity of the displayed output. Details of the display interface block 53, which transposes the digital input data to analogue values suitable for driving the CRT are shown in Figure 6, as described below.

Referring now to Figure 6, the X (horizontal) and Y (vertical) sweeps for the raster of the cathode ray tube 54 are provided by conventional sweep generators 140 and 141 respectively. The X generator 140 feeds an X-deflection amplifier 142 through a multiplexer 143 and the Y generator 141 feeds a Y-deflection amplifier 144 through a multiplexer 145. The sweep generators 140 and 141 may be comprised of the usual sawtooth waveform X and Y sweep generators for providing the conventional linear raster. During the interval between raster displays, the stroke vector display will be energised.

The raster is synchronised by horizontal and vertical synchronisation pulses X SYNC and Y SYNC derived from the X counter and Y counter of the timing module 50. The synchronisation pulses respectively turn on the sweep circuits to scan each raster line in sequence. Such synchronising circuits are well known in television and display units employing a raster scan. Since the generation of the X and Y raster sweeps from the sweep generators 140 and 141 are synchronised via the horizontal and vertical SYNC pulses from the control PROM and latch 62, which also controls the raster X count and raster Y count, the digital outputs from the counters respectively correspond to the X-Y position of the beam of the cathode ray tube 54. As discussed previously with respect to Figure 1, the face 10 of the display screen is considered as comprised of a 256 x 512 matrix of resolution elements. Thus, the instantaneous binary numbers in the raster X and raster Y count provide X and Y coordinates of the resolution element of the display screen on which the beam is about to impinge.

The outputs of the vector generator 51 representing X-position 81 and Y-position 82 are supplied to the CRT 54 through a digital-to-analogue converter 146 for the X axis and convertor 147 for the Y axis via the respective multiplexers 143 and 145. Selection of either vector generator position output or raster scan output is determined by the mode control signal 55 from the control PROM 62. Thus, during the stroke interval, the vector generator position data will be selected, and during the raster scan interval, the sweep generator output will be selected.

In addition to the deflection circuitry, Figure 6 also shows the circuitry for selecting vector generator or raster video information. Video information from the vector generator 51 and raster symbol generator 52 is supplied on lines 83 and 151, respectively, to a multiplexer 152. During the stroke portion of the refresh cycle, the mode control signal 55 directs vector generator video 83 through the multiplexer 152 to a digital-to-analogue converter 154 and an amplifier 155. On the alternate portion of the refresh cycle, when the raster scan is being displayed, raster video on the line 151 is selected by the mode control signal 55 and transmitted through the multiplexer 152 and digital-to-analogue converter 154 to the amplifier 155. Thus, video from the two sources is sequentially provided to the cathode ray tube 54 in synchronism with the corresponding stroke vector and raster scan sweep of the electron beam.

The digital memories used in the preferred embodiment can be a commercially available RAM integrated storage chip such as is used in small or microdigital data processors. The various control functions including storing, fetching and applying digital values as described above can be implemented conveniently by processor or other control logic included in or associated with the CRT display. Such control facilities are well known in digital displays for effecting various operations in synchronism with the display raster, e.g. character generation, cursor location, and stroke vector generation.

The digital-to-analogue convertors 147,154 may be of any suitable kind which combine binary voltages or currents to produce resultant outputs according to the inputs shown. The amplifiers may be conventional analogue amplifiers, such as may be formed by hybrid and integrated circuit techniques.

In operation, the apparatus of Figure 4 may be applied for providing moving displays of the type that are utilised, for example, in aircraft. On initiation of the stroke display interval by the timing module 50, the stroke control logic 65 is commanded to execute a sequence of stroke instructions which have been stored in the stroke instruction memory 70 by means of the computer address bus 71 and computer data bus 72. The instructions are loaded into the stroke vector generator 80 through the buses 76 and 77 and result in the production of digital outputs representing the X-position, Y-position and video (colour) for each position of the electron beam of the cathode ray tube 54. The digital X position values on the bus 81 and digital Y-position values on the bus 82'are converted to corresponding X and Y deflection voltages by the display interface 53 and applied with the converted video information on the bus 83 to drive the CRT 54. Simultaneously, X-position data on bus 81 and Y-position data on bus 82 are directed to the raster symbol generator 52 to provide the respective address and data for entry into a random access memory in block 101, as shown in Figure 5. At each increment of change in position of X and Y or change in colour of video, the vector generator output is updated. Since the vector output is synchronised with the raster generator, new values are entered into the memory corresponding to each raster line. Thus, during the stroke display interval, the random access memory 101 is loaded with the complete picture information for presentation of the raster display as well as the stroke display. Beneficially, it is seen that vector generator 80 and its associated functions (including software) are used for both stroke and raster symbol generation, thereby resulting in substantial economy in circuitry, space, weight, and power consumption.

On completion of the stroke vector portion of the refresh cycle, the timing module 50 initiates the raster scan pattern. During the raster display interval, the timing module 50 generates a count sequence to identify sequential raster scan line numbers which are provided on the bus 57 and another count sequence to identify the pixel numbers corresponding to the selected raster line on the bus 58. These two counts are then entered into the raster symbol generator 52.

Referring again to Figure 5, the memory 101 in the symbol generator 52 is now operated in the read mode and provides an output corresponding to an address entered by the raster X count (scan line number), thereby enabling retrieval of the corresponding pixel data previously entered therein. At the same time, the output of RAM is compared with the raster Y count which is sequencing through the range of 512 pixels per scan line. When the pixel numbers read from the memory 101 and raster Y count bus 58 are equal, a raster video command is produced by the comparator 11 energising the logic switch 130. The raster video output is produced by the raster symbol generator 52 in synchronism with X-deflection and Y-deflection raster sweep waveforms generated by the display interface 53.

The digital raster video is then converted to analogue video in the display interface 53 for determining the colour of the corresponding raster scan lines during the respective phases of the scan interval. Thus, both the stroke vector and raster scan video may be independently predetermined.