MXPA00002015A - Digital raster correction - Google Patents

Abstract

A projection display uses a matrix of digital correction factors defining deflection correction factors between which linear interpolation is used for intermediate points on the display. A circuit for generating a correction signal to correct image distortion on a display screen, comprises a memory storing displacement values corresponding to spaced points on the screen in a grid of rows and columns. The displacement values of the grid generally define correction curves for correcting the image distortion without introducing significant banded regions on the display screen. An interpolator is coupled to the memory for interpolating intermediate values between adjacent ones of the stored displacement values, and a digital to analog converter is coupled to the interpolator for providing the correction signal.

Description

DIG ITAL FRAME CORRECTION This invention relates to the field of generation of offset correction waveform and, in particular, to the digital generation of convergence digital correction signal waveforms. BACKGROUND OF THE I NVENTION Digital convergence in a projection television receiver can be achieved by using a two-dimensional matrix of adjustable factors that can be applied to points distributed at regular intervals through the visible screen area. The degree of correction of the deviation can be finely adjusted at each of these points independently. In the horizontal direction, the correction of the deviation is determined by a numerical digital factor applicable to the points of the matrix, which is converted to an analog signal to activate a convergence correction coil. At the intermediate points between the points in the matrix, the correction factor is determined by averaging with an analog filter. In the voltage direction it is necessary to calculate the values for the intermediate scan lines that correspond to the points in the correction matrix. In a lower cost deployment system, the correction for the lines between the points that are defined numerically is determined by taking a difference between the correction values for the closest set point above and below the point in question, dividing by the number of lines between the set points, and weighting the correction value for the particular line that is being determined by linear interpolation. Thus, the waveform that is generated follows a straight line between the two numerical points. To facilitate interleaved scanning, a shift value can be added to the correction data for alternate fields. Digital correction can affect the geometry of the image as well as converge the corresponding points in the three color frames. Green is commonly selected to be centered on the optical axis of the projection system. In this position, the image on the face of the green tube undergoes less geometric distortion. The red and blue displays are positioned vertically on the axis axis but are commonly located outside the optical axis horizontally. As a result, the red and blue frames are further distorted and require trapezoidal shaping to compensate for this off-axis projection location. Since the optical distortion is minimal for the green image, it is selected as the geometric reference. The green plot is measured and formed by correction waveforms to minimize geometric distortion. Then, the red and green frames are reconciled to precisely align them with the green image. The uncorrected green pattern suffers a large vertical pincushion distortion. For optimal geometry, the correction waveform along each column has a different S shape that has sinusoidal and parabolic components, and for the off-axis blue and red images, an additional linear component is required. For example, the correction waveform can be adjustable along a factor matrix that defines 13 rows and 16 columns. For each point in the matrix, the numerical factors define the associated displacement of the red, green and blue frames that are going to be carried out at that point to achieve an alignment and precise image geometry of the red and blue frames. Since the number of correction points or nodes for each column is relatively small, for example twelve vertically separated points in the visible area of the screen, the S-shape correction waveform can have abrupt changes in slope when crossing each node. These slope changes in each adjustment line cause blocks of scan lines to appear with a difference in brightness due to non-uniform separation of the horizontal lines in adjacent areas where the convergence is adjusted in accordance with different matrix factors. . If a video signal that has a constant level or a "flat" field is displayed, the screen will exhibit a series of different lines or bands of different bright resulting from the correction waveform discontinuities. BRIEF DESCRIPTION OF THE INVENTION In a configuration of the invention, a circuit generates a correction signal to correct the image distortion in a display screen. A memory stores offset values that correspond to separate points on the screen in a grid of rows and columns. The offset values of the grid generally define correction curves to correct the image distortion without introducing significant regions of bands on the display screen. An interpolator is coupled to the memory to interpolate intermediate values between the adjacent values of the saved offset values. A digital-to-analog converter is coupled to the interpolator to provide the correction signal. In a further configuration of the invention, a video apparatus has a cathray tube that displays an image subject to image distortion. A deflection coil is placed in the cathray tube and is activated by an activation amplifier. A digital-to-analog converter generates an output signal that is coupled to the activation amplifier. A memory has interpolated offset values that correspond to separate points in a grid of rows and columns. An interpolator is coupled to the memory and responds to the stored interpolated offset values, the interpolator interpolates values adjacent to the stored interpolated offset values. The digital-to-analog converter is coupled to the interpolator and generates a correction signal to activate the deflection coil to correct the image distortion. BRIEF DESCRIPTION OF THE DIAMETERS Figure 1 is a block diagram showing the elements of a digital correction circuit. Figure 2 is a block diagram illustrating a projection display having digital plot correction means. Figure 3 illustrates a nominal weft pattern with ideal geometry, the visible portion has double hatching and a matrix of set points shown as circles or dots. Figure 4 shows a portion of the pattern of Figure 3, with points interpolated between the set points shown as crossovers or Xs. Figure 5 is a plane of geometry convergence correction waveforms ideal for columns of a central axis plot (for example, the green screen of a projection television), except that it has straight lines connecting the dots of adjustment as a characteristic of linear interpolation. Figure 6 shows a flat-field video frame with brightness bands that result from steep slope changes in a column correction waveform due to linear interpolation between discrete matrix fit points shown in Figure 5 Figure 7 is a plane of waveforms of linear correction instead of S-shaped to eliminate the formation of bands as in Figure 6. Figure 8 shows the distortion of internal pincushion produced by linear column correction as shown in Figure 7.

Figure 9 is a plane of column corrections in accordance with an aspect of the invention. Figure 10 is a table of equations for determining column values in accordance with one aspect of the invention as shown in Figure 9. Figures 1 1 A and 1 1 B are five-by-five adjustment matrices. illustrating the simplified linearization value interpolation and S-shaped correction of selected linearized matrix column values, Figure 12 is a simplified flow chart illustrating a convenient addition of the S-shaped curvature to a matrix of linearized correction of 13 by 16. D ESCR I PTION D I NVE N ETALLA NTION A digital frame correction system generally consists of a pattern generator, an amplifier to activate the convergence yokes, a memory to store constants of correction, and an interface to a controller or microprocessor. A system of this type is shown with a single cathode ray tube and a deflection configuration in Figure 1. The digital frame correction system includes an internal controller 1 02. A microprocessor 30A can be used together with an external video circuit 20 coupled to a convergence pattern generator 120 for independent adjustments and to observe convergence or geometric correction. An external fixing microprocessor, shown with a dotted line, is used to process the initial set point values that are written to the electronically erasable and erasable read only memory 1 03. This setting is activated by the microprocessor. External can be achieved using automatic means to record and analyze convergence test pattern displays, and to generate initial or updated correction factors. Adjustments to convergence or geometry correction may be required, for example, to correct an erroneous convergence resulting from changes in the orientation of the display relative to the Earth's magnetic field or as a result of component failures, and these changes to the correction values saved in the programmable read-only memory and erasable electronically 1 03 can be made by the microprocessor 30A. Figure 1 illustrates some digital portions and Figure 2 illustrates the activation circuits and cathode ray bosses of an exemplary projection deployment system with which the apparatus and the digital plot correction method can be conveniently employed. the invention . The digital correction circuit 1 00 in Figure 1 includes the electronically erasable programmable read-only memory 1 03, which provides non-volatile storage of adjustment or correction factors, the internal random access memory 104 for volatile storage, and a digital-to-analog converter DAC 105, each coupled to the controller 1 02. The convergence or geometric adjustments are made by defining a matrix of adjustment points for each of the three colors. The on-screen video pattern generator 120 can provide location references for adjustments at particular locations in the display corresponding to the array of data points stored in the electronically erasable programmable read-only memory 103. Correction data or electronically erasable programmable read-only memory setting 103 can be read in the internal random access memory 104 at startup, and then used to generate additional factors such as interpolated factors for use between adjacent reference locations. The internal random access memory 104 stores 1248 data words representing an array of 13 by 16 points, with X alignment points horizontally by Y alignment points vertically, by 2 representing vertical and horizontal corrections, by 3 representing the three deployment devices that form the R, V, and A images. The active scanning area of a deployment medium, for example, is divided into an array of rows and columns, for the purposes of corrective control. The border lines of the rows and columns can be considered to define a double-striped grid in which the active or visible area is defined by a shaded rectangle. For purposes of illustration, the number of vertical grid points or samples is 13, and the number of horizontal samples is 16. These numbers are selected only by way of example. The number of rows and columns, and the resulting sections, are selected depending on the desired degree of correction in the resulting frame. Vertical and horizontal corrections, positive or negative can be used for each of the three colors at each measurement point, as well as in the horizontal scanning lines intermediate between the measurement points. With reference to Figure 1, the controller 102 reads the signals and is coupled to the digital-to-analog converter 105, whose output is smoothed by the low-pass filter 106 and amplified by the activation amplifier 107. The output of the filter is a current signal representing the current required in the correction coil 108 to correctly converge, or minimize the geometric distortion of the frame generated in the cathode ray tube 10. The amplifier 107 may be a feedback amplifier which measures the coil current instantaneous and matches the current in the correction coil 108 as a function of the output of the digital-to-analog converter 105 as the adjustment factors are read during the frame scan. The correction current flow through the coil 108 produces the scanned image of the screen formed in the cathode ray tube 10 which appears with convergence correction or minimized geometric distortion. In Figure 4, the six points circulated C20 to E40 represent a subset of point locations for which the correction data is defined. The correction values at grid points marked X, C2. to C2, for example, the values used between C20 and C30, are calculated by interpolation between the values of C20 and C30. As a result, if the correction values are formed in planes for the columns, they are categorized by linear straight lines between the circles, with discontinuities, or abrupt changes of slope, occurring in the points circulated. In Figure 2, the separate red, green and blue amplifiers 1 10, 210 and 310 correspond to amplifier 107 in the more general diagram of Figure 1. The amplifiers 1 10, 210 and 310 activate the correction coils RVC, VVC, AVC with signals provided by the digital convergence generator 30, shown generally in Figure 2. The corresponding amplifiers (not shown) are provided for the coils of horizontal correction RHC, VAC and AHC. The signal activation correction amplifiers 110, 210 and 310 and their respective coils are read synchronously with the horizontal and vertical speeds provided by a synchronization separator 20. The horizontal and vertical speeds are also coupled to a generator in the form of wave controlled by bus 21, which generates horizontal and vertical waveforms. The vertical sawtooth signal is adjusted in the generator 21 to provide S-shaped, trapezoidal and pincushion correction and this pre-distorted or formed waveform is coupled to a vertical deflection amplifier 80 and the deflection yokes VV, RV, AV of the three cathode ray tubes. In Figure 2, the amplifier and vertical deflection generator 80 is shown with a dotted box to indicate that the generation and formation of vertical waveform occurs in the waveform generator 21. Figure 3, represents a green double-stripe reference pattern with perfect geometry, as would be achieved by the correction curves shown in Figure 5. The Y axis of Figure 5 shows the numerical correction values that activate the digital converter to vertical green analog and represent correction current amplitude in the VVC correction coil. Each of the sixteen curves is a line plane where the discrete values at adjacent points separated along the 16 vertical columns of the double line are connected by lines. The C0 -O0 series are the columns that are visible on the screen. Series A, B and P occur during the horizontal retrace and can be used for fine adjustment of the vertical position of the ends of the horizontal lines of the double line. There are more horizontal lines in the display than in vertical points in a matrix of the convergence correction factors as suggested by Figure 4. To proceed smoothly from one matrix value to the next, the controller 102 linearly interpolates the value for each vertically separated horizontal successive scan line. With reference to Figure 4, the correction in the intermediate lines "X" between the points of matrix C20 and C30 progresses linearly in equal increments. Subpoint C2. = C20 + (C30 - C20) / 5, and sub-point C22 = C20 + 2 * (C30 -C20) / 5, etc. The correction values for the circulated matrix points are stored in the electronically erasable programmable read only memory 103 and read at the start of the display. The values for the intermediate points or "X" can be calculated at the start and stored in the internal random access memory 104, or calculated "in the air" during each deployment period from the stored circulated matrix point values using an interpolation integrated circuit such as the Thomson STV4020 SGS Digital Convergence IC. Alternatively, the incremental difference (ie, (C3-C2) / 5) can be calculated at the start and saved, and added incrementally to C20 or the previous convergence correction factor used. Linear interpolation is relatively simple but produces sharp inflection points where the slope of the S-shaped correction waveform nominally changes. Silos values shown in Figure 5 are used, the lower and upper part of an unmodulated or "flat field" plot will display band regions of different intensity, actually row-to-row differences in vertical placement of scan lines horizontal The result of such correction waveform discontinuities is shown in Figure 6, which for exaggerative purposes is exaggerated and ignores the effects of optical projection brilliance. The band regions, shown in Figure 6, result from abrupt changes in the slope near the ends of some of the planes. Since the green cathode ray tube (image) contributes approximately 60% of the luminance of a white field, said band formation of brightness is more noticeable and unacceptable. If the correction values along each column had no slope changes, as shown in Figure 7, band formation is eliminated. The formation of bands can be minimized or eliminated by altering the correction factors used, to smooth the changes in slope. For example, the data can be adjusted so that the second derivative of the correction waveform along each column is continuous or almost continuous. Band formation could be completely eliminated by making the correction waveforms linear along each column, for example, making the second derivative zero. Band formation can be eliminated by taking correction values measured for the adjustment lines or horizontal matrix 2 and 12 of each column, and calculating a straight line between these values, that is, linearly interpolating the entire waveform for each column. However, this linear column correction produces an imperfect vertical distortion correction because the geometrically dictated correction waveform is S-shaped, rather than a linear function, and has an amplitude that varies with the position of the column relative to the central vertical axis of the screen. This linearized correction waveform, instead of S-shaped, produces a double-stripe display with upper and lower horizontal straight lines (2 and 12) and a pincushion curvature inward from the top and bottom reducing distortion zero on the center line (7) on all internal horizontal lines. The resulting geometric distortion is shown in Figure 8. The horizontal matrix lines 1 and 13 are not visible because they occur beyond the unfolded area but if they were visible, they would curve slightly outwards. The pincushion curvature of this type is unacceptable because it geometrically distorts the image as a position function on the screen. For example, distortion produces curved edges of on-screen image elements that an observer knows to be straight. For example, it is expected and required that the blocks and lines such as the edges of menus and strips of the stock market be straight. An analog vertical velocity convergence signal with an S-shape can be employed to correct the internal pincushion distortion described above, and to make fine adjustments to consider the remaining distortion with signals derived from digital correction factors. Since the analog correction is continuous and a digital part of the correction can be relatively small, problems with pincushion distortion and band formation are reduced. However, it is difficult to maintain the consistency of this analog correction signal due to temperature variations and component tolerance. This resulting compensation for such analog signal instability can result in a complexity, cost and energy dissipation of undesirable aggregate circuits. It is possible to calculate and save correction values for each frame line. This calculation may include forcing a second zero or continuous derivative along each correction column. However, this method is relatively expensive and complex, and may suffer the disadvantage between the minimization of band formation and pincushion distortion, as previously mentioned. In a deployment, band formation can be minimized to be substantially imperceptible in the interest of accurate displayed image geometry. It was experimentally found in equipment in accordance with this example, that the formation of bands in a flat-field video image was substantially imperceptible if the slope changes in the linearly interpolated data that were allowed but remained within a minimized range. For example, the curve for a vertical column can be adjusted so that it bends no more than plus or minus five steps of a digital-to-analog converter per double-line adjustment segment. A maximum curvature of three steps per segment of adjustment was selected to facilitate the S-shaped correction while maintaining the formation of bands at a substantially imperceptible level. In accordance with one aspect of the invention, a modified linearization technique is used to make the band formation substantially imperceptible and to significantly reduce the distortion resulting from the linearization of correction value. With reference to the matrix of Figure 3, for example, a vertical sawtooth waveform is generated by the bus-controlled waveform generator 21 and coupled to activate the three vertical deflection yokes VV, RV, AV. The vertical saw fits on the generator 21 to provide S-shaped correction to minimize the geometric distortion of the double line along columns F and N. These columns then have linear convergence correction in the vertical direction. Thus, the S-shaped correction in the vertical deflection coil minimizes the magnitude of the S-shaped curvature correction signal applied to the convergence coil, which changes polarity on opposite sides of the axial center. The other columns have an S-correction of correction that is limited to 3 LSB steps of linear value divergence. Columns A-D, H-L and P have 3-step curvature and columns E, G, M and O have 2-step curvature. The result is a simultaneous minimization of band formation and pincushion distortion, so that a significantly improved image can be achieved consistently with digital convergence correction using linear interpolation between successive correction matrix points. The column curves obtained from the equations are shown in Figure 9 and the equations are set forth in Figure 10. The double-stripe display produced with these equations is substantially like the ideal double-stripe shown in Figure 3. The devices that provide Digital convergence correction with linear interpolation can be corrected for band formation by forcing the second derivative of the correction waveform to be continuous at the end points of adjacent interpolation intervals. The invention described herein maintains the second derivative to substantially zero by linearization in the necessary areas, thus removing band formation. This is carried out using linear interpolation with adjustments in accordance with the equations shown. In accordance with one aspect of the invention, the specific horizontal lines located along the correction node points of a double-ray pattern are measured and corrected. In accordance with additional aspect, only vertical green corrections are preferably subject to a band-forming linearization algorithm so that said band-forming in the green display image is imperceptible. The green vertical screen component is corrected because the green image contributes approximately 60% of the perceived brightness of display and therefore degradation of the visible image occurs if band formation is present. As red and blue images contribute smaller amounts to total brightness, approximately 30% and 10% respectively, their contributions to the visibility of band formation can be ignored. The red and blue screen corrections can be calculated to place them exactly on top of the green image without the linearization requirement to suppress band formation. However, the use of the band-forming linearization algorithm is not excluded from its use for red and blue screen corrections. Since vertical corrections are somewhat inactive, the entire alignment process is iterative by making line location measurements and calculating and applying corrections until lines 2, 7 and 12 (in Figure 3) are in absolute and geometrically locations correct When the slope or first derivative of the correction waveform, a column is constant when linearized, the second derivative is zero. The mathematical requirements are met and the formation of bands are eliminated. In a simplified example matrix shown in Figure 11A, measurements are made to determine the exact location for only lines 1 and 5. The locations of intermediate lines 2 - 4 are determined by calculating the difference between correction values 1 and correction 5 and dividing by the number of intermediate nodes minus 1. This correction differential is added to the previous value to establish the correction value at node N-1, and represents the correction increase below each column between lines L1 and L5.

A feature of this approach is that the intermediate node points may not be in the desired locations. In fact, the locations of these lines tend to produce internal pincushion distortion. In Figure 11A, lines 2 and 4 would have some degree of curvature, which could be objectionable. The amount of distortion depends on the optics and the amplification of the projected image. Although band formation can be eliminated by reducing the second derivative to zero by linearization, it was recognized that band formation can be made substantially undetectable even if the second derivative is not zero but is allowed to have a small limited variation. In a configuration of the invention, the geometric distortion, for example internal pincushion, can be corrected in addition to an S-shaped component to specific correction columns. The small variation or difference between the correction values provided by S-shaped correction depends on several system considerations, for example, the number of vertical measurement nodes, the number of D / A steps used, the sensitivity of the deviation and Similar. In this exemplary system the variation corresponds to approximately two to three less significant bits (LSBs) in the written corrections to the nodes. This permitted variation allows a small degree of S-shaped correction to be added to certain columns of correction values. Due to the symmetry in the center, the magnitude of the required S-shape correction increases as the area of interest moves out of the center and then decreases when approaching the edge. Fig. 12 is a simplified flow diagram showing a method for derivation and application of corrective values of Fig. 10, which makes web formation undetectable and provides S-shaped correction to a vertical frame component. Figure 11B illustrates internal pincushion distortion. The simplified matrix of Figure 11A demonstrates the effect of adding S-shaped correction, in an example that assumes that columns have only five matrix points representing the top, bottom, midpoint, and quarter points. In this example, the correction range is 0 to 1023, which allows correction variation in the order of 6 to 12 correction steps. Assuming for the five vertical matrix points of column 1 that the nodes Ci and C5 have measured correction values of 100 and 500 respectively, the removal of the formation of bands by linearization provides the values of: C. = 100 C2 = 200 C3 = 300 C4 = 400 C5 = 500 If the maximum S-correction is 12 correction steps, the result is: Ci = 100 C3 = 300 C4 = 388 C5 = 500 The S-shaped correction summed has the effect to move from location C2 downwards while moving location C4 upwards. In this simplified model, the second column Col2, uses half of the correction of the CoM column with the center column, Col3, does not require correction in the form of S. An additional refinement is useful in production situations where the variability of optical and electronic assemblies requires additional adjustments. As applied to the previous example, this refinement consists of the optimization of the magnitude of the S-shaped correction, in the limitations of 2-3 LSB, to achieve the best positioning (pincushion minimization) of lines 2 and 4 below of each of the columns. This approach, although it has an empirical and iterative nature, is much more effective as the number of nodes under the column increases. The application of the methods described above makes band-forming artifacts, common to digital convergence correction devices, substantially undetectable while minimizing geometric image distortion.

Claims (11)

1. CLAIMS 1. A deviation correction circuit, comprising: a memory having in it the displacement values applicable to separate points in a grid of rows and columns, the displacement values are selected to avoid significant training effects while generally define S-shaped correction curves; an interpolator that generates intermediate values between adjacent values of such saved offset values; a digital-to-analog converter coupled to said interpolator to receive said intermediate values and generate therefrom a correction signal to activate a deflection correction coil.
2. 2. The circuit of claim 1, wherein said columns applicable to offset values generally define S-shaped correction curves that are maximized to at least two such columns corresponding to areas of a display screen that are separate from each other. a central axis and one of two opposite edges of said deployment screen, and are minimized adjacent to the central axis and the opposite edges.
3. The circuit of claim 1, wherein the S-shaped correction is added in successive steps proceeding from said areas which are spaced apart from the central axis, towards said central axis and toward said edges, respectively.
4. 4. The circuit of claim 1, wherein said offset values represent interpolated values applicable to said grid.
5. 5. A video display apparatus comprising: a cathode ray tube (10) for displaying an image, having a deflection correction coil (VVC) mounted thereon and coupled to the activation amplifier (107); a digital-to-analog converter (105) with an output coupled to said activation amplifier (107); a memory (104) containing displacement values applicable to separate points in a grid of rows (1-13) and columns (AP), said offset values for said columns (AP) generally define S-shaped correction curves having a maximum value in two areas of a display screen located substantially between a central axis (7) and one of the upper and lower edges (2, 13) of the display, said S-shaped correction curves have a value substantially zero in areas adjacent to such a central axis (7) and such edges (2, 13); and, interpolation means (102) for interpolating intermediate values adjacent to the displacement values and having an output coupled to said digital-to-analog converter (105) to generate a corrective signal (12corV) to activate said correction coil. deflection (VVC) to locally adjust a position of such an image so that the aforementioned band formation and pincushion distortion are controlled.
6. 6. The video display of claim 5, wherein the S-shaped correction is added in successive steps from said areas located substantially between said central axis (7) and those mentioned of the upper and lower edges (2, 13). ) towards said central axis (7) and towards said edges (2, 13), respectively.
7. The video display of claim 5, wherein said digital words stored in said memory (104) represent derived values during the alignment of such video display.
8. The video display of claim 5, wherein said digital words that define offset values stored in said memory represent values formed by interpolation of offset values apiicable to said grid.
9. 9. The video display of claim 5, wherein said linear interpolation means generates said intermediate values adjacent to the offset values during a deployment period.
10. 10. A method to digitally correct the geometric distortion of an image on a display screen, which comprises the steps of: defining a matrix of separate adjustment points (A1-P13) on the display screen, in horizontally separated vertical columns (AP) of values for local displacement of the image at the adjustment points on the screen For deployment, values for these columns define S-shaped vertical correction waveforms that have variable slope between adjacent values; linearize the values for at least two areas of the matrix corresponding to a central axis (7) and upper and lower edges (2, 13) and apply progressively more S-shaped correction coming from said central axis and the mentioned edges, to areas of the display screen positioned substantially between said central axis (7) and said edges (2, 13); save the matrix values in a memory (104); read said saved matrix values; and, locally shifting said image as a function of such stored matrix values for corresponding adjustment points to correct the image on the display screen so as to control band formation and pincushion distortion. The method of claim 10, further comprising linearly interpolating between the adjacent values of the matrix values in such vertical columns (AP) to define correction values for scan lines between the set points, and moving locally the image between the adjustment points as a function of the linearly interpolated correction values.