METHOD AND APPARATUS FOR GENERATING MULTI-COLOR DISPLAYS

Abstract: A method and apparatus for generating multiple new color shades for use in alpha-numeric, color graphic digital television displays, such as RGBI digital monitors (11A) driven by a digital computer (1), and ink-jet, laser or thermal printers. Using a standard display raster scan system, the phosphor excitation time or pixel printing time of an individual pixel may be reduced so as to darken the color shade compared with full beam intensity or printing density. Pulse width modulation is utilized on each color drive line (B, G, R) (12A) and the intensity control line (I) (12A) to provide the reduced excitation. A modulatory clock (Kx) is synchronized with the horizontal pixel sweep and set at a high frequency by a factor of K. When so set, it is shown that (2K+1)<4> color shades are logically possible with a three color system having intensity control.

Description: Speσification "Method And Apparatus For Generating Multi-Color Displays" BACKGROUND OF THE INVENTION Field of the Invention This invention relates generally to multi-color display systems, and more particularly to devices and methods for increasing the number of color shades and intensities ' available in raster scanned digital color displays. Although the method of improved color gen¬ eration described herein is more directly applicable to computer generated graphics, the concept can also be applied to other video display equipment. Since certain limitations in color generation and related improvements originated with television, a description of the prior art must include these T.V. roots. Description of the Prior Art Raster displays have been standarized for tele¬ vision within the U.S. by the National Television Systems Committee (NTSC) system. System standards for color transmission are constrained by monochrome T.V. compatability requirements, a 6 MHz station bandwidth and a 4.5 MHz FM sound carrier. Signal standards provide for a carrier which is amplitude modulated with color brightness, or luminance information, and a 3.579 MHz suppressed color subcarrier which is phase modulated with chrominance information. Hue and saturation data is encoded and transmitted as color difference signals generated by subtracting the luminance signal from the three primary color components, red (R) , green (G) , and blue (B) , from the scene. Chrominance information is contained within a 1.5 MHz bandwidth within the luminance bandwidth of 4.2 MHz. After detection at the receiver, chrominance elements are added to the luminance subcarrier to develop the three color components. These drive the grids of three electron guns within a cathode ray tube (CRT) , which are aligned so that their beams converge in an aperature of a metal shadow mask and diverge to impinge upon respective color phosphor dots on the screen. Beams are swept horizontally at 15,734 Hz and vertically at 60 Hz. The implications of the low color T.V. band- widths may be simply noted by considering that the color carrier frequency is only 227.5 times the horizontal sweep rate of the display. Color changes in a scan line therefore can be handled to something less than 200, resulting in a maximum of about 40 characters per line. Modulation methods exist in the prior color T.V. art which are associated with color generation. Szegho, for instance, in U.S. Patent No. 2,431,088, proposed an early non-moving part receiver in which separate colors would be produced by beam intensity modulation using a screen phospor mixture in which substance fluorescence selectively occurs at different current density levels. In U.S. Patent No. 3,371,153, W. Matzen dis¬ closes a color display system in which a CRT screen is also coated with phospors having different detection beam energy activation levels for each color. The accelerating high voltage of the tube is amplitude switched in accordance with the particular color signal which is intensity modulating the beam. Urich, in U.S. Patent No. 4,214,277 notes the "half tone" technique of representing a continuous tone image by use of only black and white levels so that the eye perceives a grey image. He achieves this by digitally subdividing analog picture elements into a matrix of black or white subelements. Kurahashi , et al, in U.S. Patent No. 4,383,256, controls the excita¬ tion period of individual LED displays in accordance with image signal amplitudes in order to produce half-tone images. Knight, et al, in U.S. Patent No. 4,340,889, provides dimming of * =.. complete display by duty factor and period variation. None of the above, however, has used a time modulation method on the three primary colors within a picture element (pixel) period for the purpose of increasing the quantity of available color shades on a digital color display device, as described herein. Computer generated images may escape the trans¬ mission limitations of low video bandwidths, low sweep rates, and slow serial data transfer rabes by direct display on a memory mapped device. In the most common systems, controlling circuitry is adapted to work both with ordinary television sets using standard vertical and horizontal sweep frequencies as well as a wideband red, green and blue (RGB) color monitor. Memory in the computer may be shared between the cen¬ tral processing unit (CPU) and the display controlling device which refreshes the display dot pattern. Such display controllers continuously produce separate horizontal and vertical sync pulses needed by the display while reading out the updated memory between sync pulses to produce synchronized video data. With a bit mapped display, an output is generated for each dot on the screen, said output consisting of separate red, green and blue signals. Two basic methods are used to send the three primary colors (red, green, blue) to a color monitor: 1. Analog: Each color input to the monitor can have an analog level which corrresponds to a shade, or intensity. By varying the input voltage, the shade of the color can be controlled. 2. Digital: In this relatively simple method, only 16 colors can normally be generated. Each primary color input can have one of two states, on or off. With two possible states per line, 8 color combinations can be generated. In addition, on most digital monitors another digital input can be used to cut the intensity of each color in half. This yields 16 colors. The effect of more than 16 colors on a digital input monitor can be produced by dithering. The inforamtion sent to two adjacent pixels are dithered, which produces a blending effect. Pixel 1 is one color and an adjacent pixel 2 is another color during one complete scan of the screen. On the next scan the colors sent to the two pixels are swapped. This continuous rapid changing of two pixels causes a blending affect which produces a new color at the expense of reduced resolution since two pixels, instead of one, are necessary to generate the color. The maximum number of colors is: (16 * 15)/2 + 16 There are 16 colors which can be blended with 15 other colors. Half of these are unique since blending red with blue is the same as blue with red. To these 120 colors is added the original 16 colors. Thus, 136 is the theoretical maximum limit that can be produced using dithering with two pixels. However, the resolution is cut in half since the number of pixels needed to produce a color other than one of the 16 is t o. In considering the drawbacks of dithering and analog control, it is noted that they both add costly circuitry. Analog control also introduces problems associated with drift and color bleeding. Dithering suffers further from lower resolution. It is clear therefore that a need exists for increasing the number of colors available with a digital RGB monitor without decreasing resolution. SUMMARY OF THE PRESENT INVENTION In a system in accordance with this invention, the time each individual primary color is illumiated is independently varied in order to produce a variety of new color shades. In general, in a digital color monitor, the intensity and color of a particular pixel is proportional to the energy imparted to it by the electron beam. This energy can be varied by controll¬ ing the amount of time the pixel is excited by the electron beam or beams. If the digital color monitor uses an intensity control line, the energy of the pixel can also be varied by controlling the amount of of time the intensity line (I) is exited for a given pixel. On a time basis, the luminance of a pixel is dependent upon the ratio of the amount of time the pixel is excited to the amount of time the pixel is not excited in each frame. If the ratio is varied by time modulation, control of the appar.ent color shade ay be obtained. Pulse width modulation, for example, may be utilized to reduce the time period during which a pixel is illuminated, and when integrated by the eye, a darker shade will be perceived. When so used, linear shade control may be obtained in approximate accordance with the relationship: where k = a constant x = modulated signal pulse width in sec t = time period for one pixel in sec y = full luminance of each color i = resultant lower intensity or shade. Apparatus to implement the above concept may be easily added to or integrated with existing controller circuitry. In general, additional coded color input data lines are required to control pulse width modulation (PWM) using a higher frequency clock or digital delays. The resulting coded PWM data is latched with a dot clock and decoded so as to form separate PWM data lines for each color output. These separate data lines drive PWM sequence generators which are clocked by the high frequency clock. The serial output of each sequence generator is the PWM gating signal for that color output. Logical combination of the PWM gating signals and the RGB signal outputs from the conventional circuits will produce the new digital PWM color output. These new signals will now drive a standard digital RGBI monitor without change. By pulse width modulating the inten¬ sity signal to the digital color monitor, different intensity levels of each unique color are obtained. They can also be used to drive other digital display devices such as single gun digital color mintors, multiple color input digital monitors, digital Red, Green, Yellow (RGY) monitors and digital television. It is therefore a primary object of the present invention to p'rovide a new method of generating additional colors in a raster-scanned graphics display device by means of independent time modulating the digital color control inputs and the intensity control line that controls the intensity of all the three primary color guns. It is also a primary object of this invention to proivde a new method of generating additional colors in any and all printing equipment (including hard- copy printers) that can use the technique of pulse- width odulatoin. Printers that can currently use this technique are ink-jet printers, laser printers and thermal printers. It is a further object of this invention to provide apparatus for practicing this new method which may be simply added to existing digital color display systems and/or to existing printing equipment by means of replacement card assemblies or piggyback boards. It is another object of this invention to provide apparatus for practicing this new method which may be simply integrated into existing module structures for incorporation in newly built systems. The integration may consist of a set of discrete, standard or semi-custom or custom integrated circuits/ hybrid circuits. The above, and other objects, features and advantages of the invention will be apparent in the following description of illustrative embodiments of the invention which are to be used in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a conventional arrangement of a digital color graphics display controller ; Fig. 2 presents tabular data showing color states available with the conventional arrangement of Fig. 1; Fig. 3 is a timing diagram showing relative signal states for a particular color state; Fig. 4 is an isometric section of a conventional shadow mask CRT showing the relationships between gun, mask and screen areas; Fig. 5a is a pictorial isometric close up of an electron beam traversing a single pixel in a conven¬ tional arrangement; Fig. 5b presents the beam signal state with time asociated with Fig. 5a; Fig. 6a is a pictorial isometric close up of an electron beam traversing a single pixel subject to the modulation conditions taught by this invention; Fig. 6b presents the beam signal state with time associated with Fig. 6a; Fig. 6c is a pictorial isometric close-up of an ink-jet traversing a single printing-pixel with the jet open for its normal -pixie time; Fig. 6d is a pictorial isometric close-up of an ink-jet traversing a single printing pixel, subject to the modulation conditions taught by this invention; Fig. 7 is a block diagram, of an embodiment of a digital color graphics display controller arranged in accordance with this invention; Fig. 8 is a block diagram of an embodiment of the sync circuit modifications associated with Fig. 7; Fig. 9a is a circuit diagram of a pissible implementation of the PWM circuit, using a high frequency crystal oscillator; Fig. 10 is a timing diagram showing relative signal states when multiple color intensities are produced in accordance with the concepts of this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, a block diagram of a conventional arrangement for a digital color graphics display controller system is presented for later reference comparison. Central Processing Unit 1, (CPU), under control of program memory 2 is shown in communication with display video controller 4 via system data bus 5. Both CPU 1 and controller 4 have shared access to dual port raster memory 3, which is used for screen refresh updating. Shared control is implemented by the multiplexer 6 on the address lines. The display controller 4 controls synchroniza ion of both data output from raster memory 3 to buffer 7 and data output from sync circuit 13 to the video display device 11. Reading out of screen data from random access memory 3 is performed in between initiation of sync pulses by the controller 4. When the CPU 1 desires to write new information into memory 3, the address multiplexers 6 and data buffers 7 are switched by CPU control signals. Data output from the buffer 7 is converted from parallel to serial at the converter 8 and is shifted out at a rate determined by the system dot clock 9. This clock defines the number of pixels that can be obtained as the beam makes a scan across the screen. During each cycle of the dot clock, a new color is defined by the four color controls, R, G, B, I. The total set of output data is presented to video display device 11 on a group of 6 output lines 12. The set includes 3 color lines for red, green, and blue, two sweep synchronization lines for horizontal and vertical raster control, and one line containing the screen intensity command. When this latter command goes high, color brightness or luminance is increased for each of the color components, resulting in a doubling of availble colors from 8 to 16. Fig. 2 presents a tabulation of the total colors available. Fig. 3 gives a timing diagram showing relative signal states for a chosen primary color. The diagram is scaled to associate the time period for scanning one pixel with one cycle of the dot clock. The red signal is arbitraily altered from low to high state for both corresponding conditions of the intensity bit. Maximum luminance occurs at periods T2 and T4 with both the color and intensity bits at a logical one-. With the intensity bit low and a red high, as at T5, a lower brightness, or darker shade is obtained. Fig. 4 is an isometric view of a typical shadow mask cathode ray tube, with a partial section of the mask and screen exploded for improved viewing . Electron beams 13 are generated from each of three individual color guns 12 in the neck of the tube. The shadow mask 14, a thin perforated metal electrode is placed close to the screen and registered with it such that each hole 17 in the mask coincides with a triad of three phosphor dots on the tube screen 15, one for each primary dot color 16. Gun alignments are such that the electron beams 13 converge to a point 18 at the center of a shadow mask aperature, then diverge so that each beam impingment is only upon one color phosphor dot. Beam sweeping is accomplished by means of an external deflection yoke 19, consisting of conventional line and field scanning coils. A pictorial isometric close up of a pixel window 16 being swept by the beam 22 is depicted in Fig. 5a. Associated with this sweep is the full-on condition 20 of the beam intensity while traversing the aperature 16 as shown in Fig. 5b. As the beam traverses the opening 17 in the mask 14 and illuminates the dot area 16, a dark cross hatched swath 23 indicates maximum phosphor excitation and luminance, or a bright color. Fig. 6a shows a similar view of a beam sweep but under conditions of change in beam intensity during the aperature exposure time. In Fig. 6B , pulse width modulation is indicated by the intensity going low during about two-thirds of the on-target time. In Fig. 6A, a light cross hatched area 24 indicates the result of excitation for less than the full available pixel time. This results in lower illuminance, or a darker shade. Fig. 6σ shows a view of a jet of printing-ink 25 issuing out of a nozzle 25 onto a printing media 27. The nozzle 25 is open for a pixel time frame causing a dot 28 of bright coor of the printing-ink to be printed on the printing media 27. Fig. 6d shows a similar view of an ink-jet printer under conditions of change in time that the nozzle 25 is open within a pixie time frame. This causes a dot 29 of lighter shade to be printed on the printing media 27. Hence, dot 29 has a less density filled spray of small dots than dot 28. Fig. 7 shows a block diagram of one embodiment of a digital color display controller arranged in accordance with the principles of this invention. A similar embodiment can be used to control ink-jet, laser and thermal printers in accordance with the principles of this invention. These principles can also be used in digital television sets. General similarity of the system elements with those of Fig. 1 will be noted. Hardware changes to accomplish the improvements are denoted by subscript A. Pulse width modulation changes are primarily interfaced with the sync circuits in module element 10A. This module receives additional data line inputs containing coded PWM data and a higher frequency clock input. The clock can be synchronized with and set at a multiple "K" times the system dot clock. The selec¬ tion of "K" can be made based upon the color combina¬ tions desired. Thus, the number of logically possible shades of color per digital color output line are (2K+1) . Fig. 8 presents the synchronization circuit changes associated with Fig. 7. The PWM circuit latches the coded PWM data from the raster screen memory into a PWM latch 13. The dot clock 9 is used as the clocking signal for this PWM latch. The latched data is decoded using a high speed data decoder 14 so as to form separate PWM data lines for each color output. These separate data lines in turn drive PWM sequence generators which are clocked by the high frequency clock 15. The serial output of each sequence generator is the PWM gating signal for that color output. A logical combination of the PWM gating signals and the RGBI serial outputs from the sync circuit is made at 16 to provide the digital PWM color outputs for the RGBI display device inputs. A logical combination of the PWM gating signal with the intensity-bit serial output from the sync circuit provides additional variation of the intensity of each resultant color obtained in accordance with the prin¬ ciples of this invention. Fig. 9a is a detailed circuit diagram of a possible implementation of the PWM circuit, using a high frequency oscillator. In this example, a four times Dot Clock has been chosen. The crystal oscillator 30 generates a high frequency, digital clock signal, which in this example is four times the Dot Clock. This signal is split into inverting and non-inverting clocks with invertor 33. These two clocks drive the 74AS164 shift registers. A finite state machine is formed with shift registers 31, 32 and the NAND gate 34, to produce the train of pulses of desired pulse widths. These pulses have a time period equal to that of the Dot Clock. The pulses have duty cycles of 25%, 37.5%, 50%, 67.5%, 75%. 87.5%, respectively. These pulses are fed to four 8 to 1 multiplexers 35, 36 ,37 and 38. The 100% duty cycle is obtained by connecting that line of the multiplexer to a logical high (Vcc) . The 0% duty cycle is obtained by connecting that line of the multiplexer to a logical low (Gnd) . The desired duty cycle pulse is selected' for each dot period by the data stored in the Video RAM. These data lines addresses the select lines of the 8 to 1 multi¬ plexers. The selected pulses from the multiplexers are then synchronized and driven by line drivers (74AS805) 39, 40, 41 and 42 to form the PWM signals for the R, G, B and I lines of the video display device. The above example uses commercial 74AS Series parts. However, the circuit can be implemented in any other available technology. The circuit could be imple ented as a custom or semi-custom chip or chip¬ set. It could also be implemented as a custom hybrid or by using Programmable Array Logic (PALs) . The high frequency crystal oscillator 30, could be replaced with a Dot Clock frequency oscillator and Digital Delays to generate RWM signals. One such implementation has been illustrated in Fig. 9b. The Dot Clock 43, drives the Digital Delay device 44. In this example, the Digital Delay device has 6 delay tappings. The outputs D0, Dl and D2 of the Digital Delay device are successive time delayed outputs of the video dot clock. Outputs /D0, /Dl and D2 of the complementary signals of D0, Dl and D2 respectively. These 5 outputs are logically combined with the Dot Clock with gates 45, 46, 47, 48 and 49 to produce the required train of PWM pulses. These pulses have the same time period as the Dot Clock and have duty cycles as in the previous example. These pulses are fed to 8 by 1 multiplexers 35, 36, 37 and 38 and synchronized with line drivers 39, 4ø, 41 and 42 to produce the PWM Color and Intensity output signals. In general, if a digital delay device has n taps, then: (n + 2) shades of color per color line and (n + 2) intensities of color can be obtained. Hence, if a digital monitor has "c" color input lines and one intensity line, then the total number of color combinations possible are: T = (n + 2) c * (n+ 2) , or, T = (n + 2) c+1 . In Fig. 10, a timing diagram is presented in which the intensity control is varied in steps of 25% increments of time corresponding to five states shown. The resultant color from the PWM combination of R, G and B lines is assumed to be constant for comparison. In this example, with K=2, 5 different intensities of color are available. Hence, in all, (5) = 625 total new colors are obtained. The number of colors generated may- be computed as follows: These are (2K+1) shades per line. For σ color lines: There are (2K+1) shades With PWM intensity control, the possible combinations increase to: (2K+1) 4 • = "A" colors total To encode these A combinations, X data bits are used such that 2 - A, or 2 X = (2K+1) 4 4 Log(2K+l) X - Log 2 , For K = 2, X £ 10 It will be noted from the above examples that a very large increase in color shades may be made avail¬ able at a modest increase in hardware cost. Although the signal command system described herein was associated with a computer generated set, it is not limited thereto. Command signals could come of course from other storage media, or result from real time transmission. While a particular preferred embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention in its broader aspects. The aim of the appended claims, therefore, is to cover all such changes and modifications as fall within the true spirit and scope of the invention,

Claims: CL IMS 1. A method for producing additional color shades and color intensities in a graphics display system having means for developing a plurality of individual color video signals at respective terminals in response to a time sequenσed pixel command signal set and output means adapted for processing said color video signals for producing a cor esponding time sequenced pixel set forming a colored image on a viewing device, comprising the steps of: generating a selectable set of variable time width pulses, each of which has time durations equal to or less than one pixel; and modulating each said color video signal in response to a selected reduced time width pulse so that an additional color shade is formed on said viewing device. 2. A method for producing additional color shades and intensities in a graphics display system as recited in claim 1, wherein said viewing device consists of a cathode ray tube. 3. A method for producing additional color shades and intensitites in a graphics display system as recited in claim 1, wherein said graphics display system further comprises a digital graphics display system. 4. A method for producing additional color shades and intensitites as recited in claim 2 wherein said plurality of color video signals include red, green, blue and intensity control. 5. A method for producing additional color shades and intensitites as recited in claim 3 wherein said time sequenced pixel command signal set is derived from a program storage memory. 6. A method for producing additional color shades and intensities as recited in claim 4 wherein said colored image on a viewing screen is formed by a plurality of individual color electron beam guns and the intensity control. 7. A method for producing additional color intensities as recited in claim 6 wherein said time sequenced pixel signal set is further controlled by the intensity control. 8. A method for producing additional color shades and intensities as recited in claim 5 wherein said program memory is a part of a digital computer. 9. A method for producing additional color shades and intensities as recited in claim 5 wherein said program memory is stored on a video disc. 10. In a graphics display system having means for developing a plurality of individual color video signals and the intensity control signal at respective terminals in response to a time sequenced pixel command signal set and output means adapted for processing said video signals for producing a corresponding t ime-sequenced pixel set forming a colored image on a viewing screen of a display device, the improvement comprising: modulation means for receiving a pixel color shade and intensity command signal and for modulating said plurality of individual color video signals with or without intensity control so as to produce a corresponding pixel color shade and intensity on said viewing screen. 11. A graphics display system according to claim 9 wherein said graphics display system is of a digital graphics display system. 12. A graphics display system according to claim 10 wherein said modulation means consists of pulse width modulation means. 13. A graphics display system according to claim 11 wherein said pulse width modulation means comprises: means for measurement of pixel excitation time and developing a first clock pulse train having a period proportional thereto; means for receiving a pixel color shade and intensity command and for developing a second pulse train sychronized with said first clock pulse train and having a reduced duty cycle corresponding thereto; and means for receiving said second pulse signal and for developing a corresponding reduced duty cycle on each said color and intensity line video signal so as to produce a corresponding pixel color shade. 14. A graphics display system according to claim 12 wherein said viewing screen consists of a cathode ray tube having a plurality of electron beam guns. 15. A graphics display system according to claim 12 wherein said time sequenced pixel command signal set is derived from a program memory. 16. A graphics display system according to claim 14 wherein said program memory is stored in a digital computer. 17. A graphics display system according to claim 14 wherein said program memory is stored on a video disc or tape. 18. A graphics display system according to claim 14 wherein said display system is a digital television monitor. 19. A graphics display system according to claim 14 wherein said display device is selected from the group consisting of an ink-jet printer, a laser printer or a thermal printer. 20. A graphics display system according to claim 14 wherein said display device is a digital gas discharge flat screen. 21. A graphics display system according to claim 14 wherein said display device is either a digi¬ tal Liquid Crystal Matrix Displ-ay or a Light Emitting Diode Matrix Display.

Inventors: MUMMAH PHILLIP E, MUNDKUR KIRAN R, MUMMAH, PHILLIP, E., MUNDKUR, KIRAN, R.
Applicants: MUMMAH PHILLIP E, MUNDKUR KIRAN R, MUMMAH, PHILLIP, E., MUNDKUR, KIRAN, R.
Application year: 1985
Main classification: G09G1
IPC codes: G06K15, G09G5, H04N1