AU2001100380A4 - Entirely-D.S.P.-based correction for design-based distortion and outer pin-cushion mis-alignment in direct-view C.R.T's. - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR AN INNOVATION PATENT APPLICANT: NANKIN, Paul ADDRESS: Flat 7, 3 Somers Street, Noble Park Victoria, 3174, Australia ACTUAL INVENTOR: NANKIN, Paul ADDRESS FOR SERVICE: Paul Nankin Flat 7, 3 Somers Street, Noble Park, Victoria, 3174, Australia INVENTION TITLE: ENTIRELY D.S.P. BASED CORRECTION FOR DESIGN-BASED DISTORTION AND OUTER PIN-CUSHION MISALIGNMENT IN DIRECT-VIEW C.R.Ts.

The following statement is a full description of this invention, including the best method of performing it known to __0 TITLE: Entirely D.S.P. Based Correction For Design-Based Distortion And Outer Pin-Cushion Misalignment In Direct-View C.R.Ts.

TECHNICAL FIELD This invention relates to methods and apparatus for correcting distortion, that is inherent in at least Philips Cybertube cathode ray tubes (CRTs, also known as Braun tubes) and similar rasterized image-display devices used for television (TV) receivers, computer monitors, video displays, radar displays and the like. However, the invention is applicable to other vacuum-related displays, such as plasma displays.

In this specification, a rasterised image refers to one which is composed of a plurality of horizontal lines each comprising a row of image elements, often referred to as 'dots', 'pels' (picture elements) or 'pixels'. Unless otherwise indicated, an image element is taken to comprise a cluster of three sub-dots, one of each of the primary colours employed in the screen.

Inherent distortion in CRT devices arises from imperfections or inherent limitations in beam deflection produced by the electron gun or guns and their deflector plates and/or coils, by the shadow mask, by the horizontal and vertical deflection drive circuits and, importantly, by the angular scanning of an electron beam across a tube face which is non-spherical. However, is not confined to such CRT devices as it is also applicable to photo-cathode displays such as that disclosed in US patent 5,726,524 and to electro-florescent displays such as disclosed in US patent 5,859,508. This invention is not concerned with computer graphics software for the manipulation of rasterised images for the purpose, inter alia, of correcting image distortion (eg, US patent 3,976,982 to IBM). Nor is it concerned with optical devices external to the CRT adapted to mitigate or correct inherent distortion; such as the lens and/or mirror systems sometimes placed in front of the tube face in projection television systems. Nor, again, is this invention concerned with the use of deflector magnets and other passive means of modifying the deflection of the electron beams in selected areas of the screen. Indeed, the present invention may well make such devices of the prior art redundant.

It is not, however, for the correction of inner pin-cushion distortion (as in US patent 6,002,454 to Toshiba), although it would have to be employed in order to make preferred embodiments of the suggested invention feasible.

Nor is it automatic alignment of a CRT video display in local magnetic fields (eg, US patent 6,130,505 to Display Laboratories Inc), although it maywell be employed in the preferred embodiments of the suggested invention.

Neither is it vertical pin distortion correction apparatus and method for a multi-scan display (eg, US patent 6,2008,320 to Sony Corporation), although it too could be in preferred embodiments of the suggested invention.

Nor does it constitute digital focusing (integral to Philips Cybertyube), although it too would be used in technologically viable preferred embodiments.

BACKGROUND TO THE INVENTION Since the advent of TV and computers, there has been a growing demand for larger and flatter screens. This has led to a constant struggle to reduce the volume and weight of the tube without causing unacceptable distortion due to beam angulation and the need for ultra precision in gun and tube manufacture. The result has been that the cost of a computer monitor or TV display having a given resolution and flatness increases disproportionately rapidly with area of the screen and that high resolution CRT-based monitors are still deeper than they are wide. Consequently, CRT technology is steadily losing market share to flat-panel LCD (liquid crystal display) and plasma technologies which, though substantially more expensive for a given screen area and image resolution, are not subject to angulation distortion and are very much thinner and lighter than CRTs. These limitations are also present in large plasma screens, particularly in multi-metre sizes.

US patent 4,618,801 to Kakino teaches the use of a CRT having a grid of electron guns arranged in a plane that is parallel that the flat screen of the tube, each gun addressing its own small area of the screen in such a way that adjacent areas abut one another in a seamless fashion. Such a CRT is clearly much more complex and expensive to make than one with a single gun and its electronic circuitry will be much more complex to effect the subdivision of the image and the seamless juxtaposition of each sub-image with its neighbours. US patent 4,784,449 to US Philips Corp discloses the use of an array of electron multipliers between the screen which can be controlled to provide successive colour scanning lines for each scanning line of a received colour video signal, again with the aim of allowing the tube to have a flatter screen than otherwise (for a given angulation distortion). Not only does this arrangement introduce the expense of the additional scannable array of electron multipliers, but it also requires a trebling of the horizontal scanning frequency and its consequent additional expense. A plasma screen, in a waffle-iron assembly might be as big as a front-projection screen.

Finally, the development of pruning algorithm-type correction (eg, US patent 6,040,926 to Hewlett Packard) and that of both the technical and cost-viability of the variation of DSP technology required (such as that of gallium arsenide, at present) means that, for the first time, since its inception, manipulation of the magnetic flux to correct distortion can become redundant. That is, instead of 'stuffing up' a print to match the CRT, one is now 'stuffing up' a CRT to match the print! OBJECTIVES OF THE INVENTION It is an object of the present invention to provide a method and apparatus for the dynamic correction of inherent distortion in CRTs and similar devices for displaying images in rasterized format, whereby such design-based distortion is constituted by differences in curvature between the sides of such a screen or tube.

OUTLINE OF INVENTION From one aspect, the present invention comprises storing, in a register or registers, the horizontal and/or vertical deflection voltages needed to effect the desired energisation or targeting of each pixel in a rectilinear array of pixels of a CRT, employing the horizontal and/or vertical deflection signals generated by the

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corresponding CRT video circuits to address said register or registers to cause the corresponding deflection voltage or voltages to be output thereby, and using said output voltage or voltages to effect the targeting of the desired pixel. Preferably, the register(s) is addressed by using the vertical and/or horizontal synchronising pulses and a timer or counter, rather than by the vertical and/or horizontal deflection voltages generated by the video driver circuits.

The total horizontal and vertical voltages needed to effect the correct (or desired) targeting of pixels in the display may be stored in the register. Similarly, while the total or supplementary voltages for every pixel in the display may be stored in the register(s). The voltages stored in the registers will entirely digital (with the possible exception of electronics used to maximize the electron guns' effective size or scope.

From another aspect, the invention comprises a CRT image display system wherein video signals are converted into a rasterised image that is displayed upon a screen, the system being characterised in that it includes a register for storing the correct or the desired correcting voltages for each of a plurality of pixels in the screen, said register is connected to be addressed by said video signals, and wherein the output of the register comprises a corrected video signal whereby the correct screen pixel can be addressed despite inherent distortion in the deflection circuits and/or in beam angulation resulting from the use of a screen which is substantially flatter than spherical. The addressing of the register is preferably effected by the use of a counter circuit(s) which is reset by the corresponding vertical and/or horizontal synch pulse and which has a counting rate corresponding to the rate at which pixels in the screen are scanned.

From a more general aspect, the invention may be said to be characterised by the employment of register means to store correct or correcting deflection signals for the targeting of desired screen pixels in a CRT display, and (ii) the use of the video signals (or only the synch pulse thereof) to effect the addressing of the register(s).

It will be appreciated that, while the deflector coils of conventional CRTs are currentdriven devices (in contrast to the electrostatic deflector plates in an electron gun), the desired current for a given beam deflection can be obtained by the application of the appropriate voltage from a source of suitable impedance including amplifiers or the like. Reference to correct or correcting voltages should therefore be understood in this light.

DESCRIPTION OF EXAMPLE: Process of Digital Convergence (in Chip) Having broadly portrayed the nature of the present invention, one particular example will now be described by way of illustration only. In the following description, reference will be made to the accompanying drawings in which: Figure 1 is a block diagram of a TV receiver which includes a Philips Cybertube flat planar CRT fitted with distortion correction means in accordance with the principles of the present invention.

This example relates to a relatively cheap commercial TV set comprising a receiver circuit 12 and a CRT display having a large planar flat screen 14, the inside curved surface of which is coated with an array of phosphor dots (not shown) that form the image pixels as is well known in the art. While known techniques of distortion correction may be incorporated in the receiver circuit 10, within and/or on the CRT 12, it is possible here that they will be of minimal cost and that they will be particularly effective, even given the use of a large flat screen 14 utilizing DSP technology.

In this example, the correct vertical location (in digital format) of every pixel of screen 14 is stored in a vertical-position register (or V-register) 16, while the correct horizontal position of every pixel (in digital format) is stored in a horizontal-position register (or H-register) 18; register 18 having one sub-register or bank 18a for each line of screen 14 of tube 12. The stored data at any location in a register appears at its output as that location is scanned (in a manner to be explained). The digital outputs from V-register 16 and H-register 18 are, respectively, converted to analogue signals (voltages) by digital to analogue circuits 20 and 22, amplified by amplifiers 24 and 26 and fed to the appropriate deflection coils and plates (not shown) of the electron gun 28 of tube 12. Thus, the tube may be of any conventional single-gun design, but is one which preferably has a large flat face. If a three-gun tube is employed (eg, a the Sony Trinitron), the system shown may be duplicated for each additional gun.

The usual vertical synchronising (V-synch) and horizontal synchronising (H-synch) pulses are obtained from receiver 10 on output lines 30 and 32 (respectively). The normal chrominance and brightness signals generated by receiver 10 are fed on output 34 to gun 28. The V-synch and H-synch pulses on lines 30 and 32 are not fed directly to the gun, as would be conventional, but are intercepted by the system of the invention.

A vertical oscillator 36 running at the nominal vertical (line) frequency drives a digital counter 38 which is reset to zero each time it receives a vertical synch pulse from the receiver on line 30 (indicating the switch to a new horizontal line of the raster).

Similarly, a horizontal oscillator 40 running at approximately the horizontal (pixel) frequency drives a digital counter 42 which is reset to zero each time it receives a horizontal synch pulse on line 32 (indicating the end of a horizontal line of the raster).

The output of V-counter 38 drives a scanner or V-position selector 44 which selects a position in V-register 16 which corresponds to the horizontal line number set by Vcounter 38. However, this line number may not be correct (from the standpoint of a TV viewer) because of distortion caused by manufacturing errors in the gun, the shadow mask or the screen, and because of the inherent geometrical distortion caused by the radial scanning of a large flat tube which is only planar-flat externally.

Thus, throughout the screen, the nominal line number (vertical position) should be changed vertically by one or more pixels (lines). The correct line number is stored in V-register 16 corresponding to the nominal line number and is read out as register 16 is scanned in accordance with the output of counter 38.

As already mentioned, the digital data read from V-register 38 is then converted in to an analogue signal in D-A circuit 20, amplified by amplifier 24 and fed to the vertical deflection coils (not shown) of electron gun 28, ensuring that the position of the electron beam is corrected to yield a display which has no vertical distortion.

The digital output of the V-register drives a V-position scanner or 'set' circuit 46 which selects the bank (sub-register 18a) of the H-position register 18 to be read. As indicated above, H-counter 42 drives a scanner or H-position selector circuit 48 which selects a position in the selected bank 18a of H-register 18 that corresponds with the pixel number of the selected (corrected) display line set by V-counter 38. However, the horizontal position of the selected pixel may not be correct (from the standpoint of a TV viewer) because of distortion caused by manufacturing errors in the gun, the shadow mask or the screen, and because of the inherent geometrical distortion caused by the radial scanning of a large flat tube face. Thus, across the whole screen, the nominal horizontal position of a pixel on the selected line should be changed horizontally by one or more pixels. Thus the correct horizontal pixel position is stored in H-register 18 (as a digitally encoded horizontal deflection voltage) corresponding to every nominal pixel position designated by the receiver output. As a raster line is scanned, the selected H-sub-register 18a is scanned in accordance with the output of H-counter 42 and H-position selector 48. The digital data read from the H-register 18 is then converted in to an analogue signal by D-A converter 22, amplified by amplifier 26 and fed to the horizontal deflector plates (not shown) of electron gun 28, ensuring that the position of the electron beam is corrected to yield a display which has no vertical or horizontal distortion.

This example is especially relevant to planar-flat direct-view art, where there is curvature only on the phosphor side of the screen.

It is to be noted that, in this example: Display Labs' calibration method is not shown, Toshiba's digital focus is not shown Sony's art is not shown, The curvature differential art, Non-linear convergence (outer pin-cushion similar to US patent 6,040,926) is not shown, Finally to electron gun.

It will be appreciated by those skilled in the art that the method and apparatus for distortion correction of the chosen example operates at a hardware or firmware level and is independent of any software that might be used in a digital TV or in a computer. It allows the raster of a CRT display to be corrected (within the accuracy of the grid of phosphor dots applied to the CRT screen) whatever the cause of the inherent distortion. This new method of distortion correction facilitates the automated and individual adjustment of a CRT display to correct for manufacturing variations where there is also design-based inherent distortion (such as in the recent Philips Cybertube CRTs. The use of vertical and horizontal counter circuits (which are reset using the corresponding synch pulses) to, in effect, select the pixel to be addressed by the chrominance and brightness signal, further reduces lack of accuracy and distortion caused by the cheaper horizontal and vertical driver circuits. The invention is especially applicable to displays with high aspect ratios and to projection systems in which the projection optics would otherwise result in distortion.

It will also be appreciated, however, that many variations and alterations may be made to the example without departing from the scope of the present invention. That is to say, where the curvature differential between the inner and outer surfaces of the screen or tube face is especially great.

Finally, and as already noted, though the methods of the invention will have especially advantageous application to CRT screens (as explained) they may also find useful application in other forms of rasterised displays (such as flat panel CRTs) and it is intended that the invention should extend in scope thereto. Also, by using the method of generating the horizontal and vertical deflection signals disclosed herein, it is possible to simply adjust a standard driver circuit to suit planar-flat, direct-view TV and monitor CRTs of any dot pitch in a cheap and effective manner. The invention also embraces such methods and applications.

It will be clear to those skilled in the art that over the years, there has been: a) an enormous increase in both the quality and quantity of glass used inCRTs especially, or at least, in the componentry required around planar-flat CRTs such as the Sony Wega tube; b) greatly reduced cost of signal processing technology, c) improvements in electron-guns, and d) advances in mathematical analysis of distortion caused by curvature differentials in screens, hence, combined with the teachings of the present invention, huge reductions in the costs of CRTs have been made possible by the present invention. In fact, over 90% of the cost of conventional CRTs can be saved in planar-flat, direct-view HDTV screens.

Indeed, the present invention allows plastics to replace glass.

It will be appreciated by those versed in the art, that there are many ways of utilizing the invention without departing from its spirit, especially in relation to cinematic aspect ratios and making viable direct-view CRTs which are effectively flat-panel displays in fact, over 99.9% of the implementation.

Paul Nankin September 2001