FR2579370A1 - Device for measuring defects of geometry of the scanning of the screen of a cathode ray tube - Google Patents

Abstract

The device according to the invention comprises an array of vertical optical fibres f1 to f7 arranged on the front face of the screen E of a cathode ray tube. Each fibre is divided into stretches, the centres of the breaks being arranged along horizontal rows H1 to H5 parallel to the scan lines. The ends of the fibres are coupled optically to photodetectors PD which convert into electrical signals the light energy picked up as the scanning spot passes close to a break. Circuits CA for processing these signals determine the positions of the centres of the breaks and, on the basis of a chronometer measurement, formulate error-correction signals superimposed on the scan control signals. Application in particular to high-definition monitors.

Description

DEVICE FOR MEASURING GEOMETRY DEFECTS
SCAN OF THE CATHODE TUBE SCREEN
The present invention relates to a device for measuring faults in screen scanning geometry in a cathode ray tube, in particular of a tube used in a high definition monitor.

The cathode-ray tubes used in television or in visualization, in monochrome operation, present many defects among which
- geometrical deformations in cushion or barrel due to the fact that the scanning spot of the screen does not move on a sphere compared to the source point, ie the electron-emitting gun, but on a surface as flat as possible, perpendicular to the axis of the tube;
- errors due to the imperfection of the scanning circuits and the deflectors;
- and focusing faults due to imperfections in the convergence devices.

 Finally, the color cathode-ray tubes have, in addition, superposition defects by divergence of the three basic beams.

 To reduce these defects, the current technique consists in injecting into the scanning or convergence circuits error signals obtained experimentally, for example during the final adjustment at the time of the manufacture of an apparatus comprising a cathode-ray screen. .

 The magnetic fields of scanning or focusing can also be deformed using permanent magnets judiciously distributed around the neck of the cathode ray tube.

 All of these systems take a long time to implement and do not offer sufficient stability over time.

 For certain applications, involving a high definition standard, for example when it is a question of graphic consoles where a definition of the order of 1500 by 1500 pixels is necessary, the corrections of geometry and convergence become imperative.

 According to a first variant of Known Art, the necessary corrections are obtained in the equipment manually, using a small drawer where one can access a large number of potentiometers, typically from 30 to 60 , the adjustment is made on sight using a test pattern whose modifications are observed. This adjustment requires the intervention of very experienced personnel and requires approximately one hour. This process is of the analog type.

 An improvement consists in carrying out, in the factory, a manual adjustment, in digitizing it, then in recording it in a memory of the type "RAM" according to the English terminology. In normal operation, the memory is read cyclically. It makes it possible to obtain, using a digital-analog converter, error signals necessary for the correction. However, this correction is only approximate, since the calibration was carried out in the factory on new equipment which changes over time.

In addition, the calibration has ignored local phenomena which also influence the quality of the settings. For example, large tubes are sensitive to the orientation of the Earth's magnetic field. It is also necessary to use an operator who performs fine adjustment, also using a test pattern. In this case, it has a keyboard for modifying the data recorded in RAM.

 A device has also been proposed in the French patent application published under No. 2,480,032 for automating to a certain extent the acquisition of data necessary for the correction of convergence defects of a three-color tube. essentially comprises an opto-electronic sensor which is placed on the front face of the cathode-ray tube, a sensor associated with electronic signal processing circuits. However, even if the acquisition of data is, at least partly automated, there required the availability of equipment outside the system comprising the cathode ray tube and also required the intervention of an operator.

 More recently, so-called self-converging systems have appeared based on the analysis of optical feedback enabling the geometry of each electron beam to be known for a wide color tube. As with the digital method, a complex signal is stored in random access memory as a digital value. By correcting these values which then pass through a digital-analog converter, a processor adjusts the geometry of the beams so that they converge within the chosen limits, typically less than or equal to 0.2 mm.

 Tubes of this type have certain so-called index phosphor patterns covering the back of the mask. When the electron beams sweep across the screen, these patterns return a flow of light energy into the tube. A photodetector captures the radiation emitted by a window arranged in the wall of the cathode-ray tube, converts the captured radiation into electrical signals and transmits these signals to a processor. The phosphor patterns do not block the holes in the mask and do not change the energy of the beam attacking the phosphor on the screen. A very thin layer of aluminum is placed between the mask and the screen and returns to the observer the light emitted by the phosphorus, thus preventing any stray light striking the screen from reaching the photodetector. The phosphor patterns are arranged on the mask at strategic points, corresponding to critical thresholds of convergence on the screen. The processor corrects the geometry of the beams at each of these points, thereby automatically respecting the convergence characteristics over the entire viewing surface. Such arrangements not only spare the user any manipulation, but guarantee a more precise convergence than the manual methods previously described.

 Such devices are described, for example, in an article published in the review "Measures" of May 2, 1983, pages 7 to 14, or in two articles published in the review "IEEE Transactions an consumer electronics"; volume CE-27, no. 3, August 1981, pages 433 to 452.

 These devices, although marking a definite improvement over manual adjustment devices, are not without drawbacks. In particular, they require a special cathode ray tube comprising a window for the cell. In addition, it is also necessary to deposit phosphorus inside the tube, which is not always easy to achieve. The difficulties of realization are moreover admitted in the articles published in the aforementioned second review and it is, in practice, necessary to adopt additional provisions such as an improved electronic optics or a critical choice of phosphors.

 The present invention proposes to overcome the drawbacks of known art. According to the invention, there are also provided fixed geometric references, linked to the tube and determining its geometry. However, these references are not deposited inside of it, but on the front part, between the front face of the balloon of the tube and the slab attached to it. The measuring device according to the invention therefore does not require significant modifications to the structure of the cathode ray tube. As has just been recalled, the tube itself can be quite conventional. The process of manufacturing the complete tube includes an additional step compared to the stages of manufacturing a tube according to the known art consisting in the arrangement of the geometric marks on the front face of the balloon before bringing back the slab. This operation is simpler to realize that the operation of depositing marks on a mask. The marks according to the invention are carried by a vertical network of optical fibers cut into sections.

 The subject of the invention is therefore a device for measuring geometry faults in the scanning of the screen of a cathode ray tube by a light spot, characterized in that it comprises: - a network of optical fibers comprising cracks so defining a series of distinct sections separated by determined spaces and arranged on the screen according to a pre-established configuration; - At least one photodetector member coupled to one of the ends of the optical fibers converting the light energy captured by one of the end faces of the sections during the passage of said spot near one of the breaks into electrical signals; - And circuits for detecting the maximum values of the amplitude of these electrical signals, for generating reference clock signals, for determining from these signals the positions of the centers of said breaks, for generating signals for correcting said faults of geometry and of superimposition of these signals on control signals of said scanning of the screen.

The invention will be better understood and other characteristics and advantages will appear from the following description with reference to the appended figures and among which:
- Figure 1 schematically illustrates a network of fixed geometric references linked to the cathode ray tube constituted by optical fibers, cut into sections according to the invention
- Figure 2 is a partial view of this network, illustrating the operation of the tube according to the invention;
- Figures 3 to 5 are diagrams of signals obtained using a device according to the invention
- Figure 6 illustrates an additional variant of the device according to the invention
- Figure 7 illustrates more fully the electronic signal processing circuits associated with optoelectronic devices for converting the light picked up by the fiber optic network of the device according to the invention.

 FIG. 1 schematically illustrates a device for measuring the geometry faults of the scanning of a cathode-ray tube according to the invention.

According to one of the most important provisions of the invention, the device comprises a vertical network of optical fibers, cut into sections. By vertical, in the following is meant directions orthogonal to the scan lines of the cathode ray tube screen. The optical fibers, represented seven in number, fl to f7, are arranged on the front face E of the cathode-ray tube. In a preferred variant, these fibers are all parallel to one another and equidistant from one another. The optical axes of these fibers A 1 to Q7 therefore determine seven perfectly defined vertical axes. According to another important arrangement, each of these fibers is cut into sections. In FIG. 1, six sections have been shown, thus defining five breaks. The midpoints of these breaks are aligned and thus define five horizontal lines H1 to H5. The set of intersections of the horizontal and vertical lines defines a network of reference coordinates.

 To find out the defects in scanning of the screen by one or more electronic beams, it suffices to determine the positions of the impact of this or these beams on the screen when the scanning members associated with the tube are subjected to signals of ordered.

 When an electron beam hits the screen, light is usually produced. When this emission takes place near one of the breaks, part of the light will be captured by a terminal face of the facing sections of optical fiber, at the break. When certain conditions of incidence on these end faces, conditions which will be explained in what follows, are fulfilled, the light thus captured will be transmitted in guided mode by the sections of optical fiber. At least one of the ends of the optical fibers fl to f7 is optically coupled to a photodetector PD. With the loss of less, the fraction of light captured and transmitted by the optical fibers will be detected by this photodetector PD and converted into an electrical signal available on an SPD output. The photo-detector device PD can be constituted by separate photo-detectors, a strip of photo-detectors or a single photo-detector.

 In fact, if the electron beam scans the screen sequentially, only the light emitted near one of the breaks will be significantly captured. There is therefore no reason to discriminate the origin of the converted light, it suffices to make a correlation between the scanning of the screen E and the signals collected on the output of the photo-detector PD.

 The electrical signals from the photoelectric conversion are transmitted to an input of AC data acquisition and processing circuits. These circuits will also be explained in the following in relation to FIG. 7.

 With reference to FIG. 2, which shows an enlarged view of a fraction of the network of fiber optic sections, the manner in which an electron beam can be derived from error signals line by line scanning, will now be explained.

In Figure 2 are shown four fiber sections belonging respectively to the fibers fl and f2. The optical axes ål and A2 of these optical fibers are parallel to a vertical axis referenced V. The centers of the breaks are aligned on a horizontal axis OH perpendicular to the axis OV and have the abscissa h1 and h2. The interval between two sections of optical fiber is equal to a value e. FIG. 2 also shows two scanning lines L1 and L2 of the electron beam on the screen. These scan lines are parallel to the OH axis. On the left side of the diagram, it is illustrated more particularly the effect of the displacement of a light spot due to scanning along the vertical axis OV.
We isolate two light spots P1 and P2 belonging to the respective scanning lines L1 and L2, although the light emitted by these light spots has the same spatial distribution, the fraction of light captured by the fiber sections, in particular the section upper of the fiber fl in Figure 2, will be different.

 Indeed, the light captured by the end of a section of optical fiber is reduced to a cone of light determined by the apparent surface of the end face of the section of optical fiber seen by the light spot. The mean axes of these light cones form respective angles a e and a m with the axis OV. The scanning lines L1 and L2 were chosen so that they correspond to extreme cases.

The light energy captured becomes significant from an angle a e and increases to an angle a m determined by the digital aperture of the optical fiber. If we call SH the amplitude of the light intensity sensed by the end face of a section of the optical fiber f1, the amplitude of this light intensity changes as illustrated by the curve on the left part of the diagram in Figure 3. It starts from a minimum, increases regularly to a maximum marked by the ordinate M1 of the vertical axis OV and decreases suddenly. The amplitude of the maximum is determined by the digital aperture of the optical fiber.

 In addition, there is not a single curve, but an infinity of curves. Indeed, the amplitude of the maximum also depends on the spacing between the end faces of the sections of optical fiber, that is to say the length e of the break. In FIG. 3, three values of this spacing e have been shown: 100, 200 and 300 μm.

 This figure is naturally symmetrical. Indeed, if one places oneself at points P'1 and P'2, symmetrical with respect to the axis OH of points P1 and P2, the evolution of the light intensity, picked up by the lower section of the optical fiber fl , is identical to what we have just described, in particular the light intensity passes through a maximum, of ordinate M2 on the axis OV symmetrical of the ordinate M1 with respect to point O.

 The light intensity picked up also depends on the distance of the light spot from the axis Aî, that is to say the distance dh.

 Indeed, if we consider one of the scanning lines, for example the scanning line L2, located at a distance dv from the axis OH, we see that a section, the upper section of the fiber f2 of the right part of FIG. 2 for example, begins to capture the light for a position of the light spot P3 as far as a position P4 of this same spot. These positions, symmetrical with respect to the axis A2, are determined by the numerical aperture of the fiber f2. They correspond to cones of light inclined at an angle e with respect to the scanning direction, direction parallel to the axis OH. If SV is called the optical intensity captured by the entry face of the upper section of the fiber f2, the variations of this intensity are described by the curve represented on the diagram of FIG. 4. On either side of two points of abscissa M'1 and M'2, points symmetrical with respect to the abscissa h2 on the OH axis, the light intensity decreases suddenly. Between these two points M'1 and M'2, the amplitude of the light intensity increases slowly to pass through a maximum for the abscissa h2 and then decrease again.

 In reality, these two elementary responses do not exist in isolation. There is a combination of vertical movement due to the gradual displacement of the scan lines and a horizontal movement of a spot along the scan lines around the break. It is possible to establish, as illustrated in FIG. 5, a two-dimensional map, along the axes OH and OV, of the amplitude of the light intensity picked up by the visa-vis sections. Then we draw two sequent lines intersecting at the center of the break, symmetrical with respect to the optical axes of the fibers, for example the axis A2 for the optical fiber f and forming with an horizontal axis an angle equal to the angle e in value If the scanning spot is located in the lateral zones, that is to say to the right and to the left in FIG. 5, the light energy captured by one or the other of the opposite sections is practically nothing. On the other hand, if the scanning spot is located inside the upper and lower zones, one of the two sections captures light energy. If we move on a straight line also passing through the center h2, of the break and located inside the upper and lower zones, the variation in the amplitude of the light intensity captured is described by a curve similar to that shown in Figure 3, this is true whatever the line. We note the existence of two places of maxima, which are represented by the curves A and B in Figure 5, delimited respectively by the points C and D and C 'and D'.

 Knowledge of this cartography, in particular the two maximum locations, makes it possible to know precisely the relative positions with respect to two orthonormal axes of a spot and the centers of the breaks.

This is true for the different areas of the screen since the measuring device according to the invention comprises a network of sections of optical fibers distributed over the entire screen.

 Referring again to Figure 1, although it was shown for reasons of simplicity seven optical fibers and five break lines, this is of course not limitative of the invention. In reality, the number of optical fibers is greater as well as the number of breaks. In a typical embodiment, thirteen vertical fibers are used, cut into fourteen sections along thirteen equidistant horizontal lines. 169 measurement areas are thus defined, distributed regularly on the screen. Although an orthonormal matrix distribution has been described so far, one can naturally adopt any other topology if the distribution law is well known.

 To find out the horizontal position of a break in one of the measurement zones, i.e. the position of the axis A of the associated optical fiber, a simple method consists in locating the position of the steep sides of the signals due to a horizontal scan, in particular if one places oneself on the place of one of the maxima, for example the curve A. It suffices therefore to determine in this case the position of the points C and D. The calculation of the average distance between two rising edges of the light response gives the desired position.

 For the determination of the vertical position, an analogous method can be implemented, it suffices to determine the position of the midpoint of the maxima on either side of the break at a given abscissa.

 We will now describe circuits making it possible to determine the position of the centers of the breaks and to generate correction signals superimposed on the normal scanning signals.

In reality, we do not use a single photo-detector PD as shown in FIG. 1, but two photo-detector PD1 and PD2 each arranged at one of the ends of the optical fibers. Such a configuration is shown diagrammatically in FIG. 6. We find the network of optical fibers fl to fn. There is also shown an arbitrary scan line Lx
In practical terms, the only information that we will collect is the output voltage signals from the photo-detectors PD1 and PD2, voltages that can be summed in a single voltage. This voltage is the image of the light amplitude captured, it is therefore necessary to locate the places A and
B amplitude maxima located symmetrically with respect to the center of the breaks. To do this, according to the invention, the detected amplitudes are sampled line by line, and the maximum is determined.

 Figure 7 illustrates the process. On each scan line, for example at the arbitrary scan line n, the signal SpD passes through successive maxima near the different breaks. We measure these maxima and compare each line to the previous one. A majority decision is taken which eliminates unnecessary information: if the maxima detected on line n + 1 are greater than those detected on line n, only the maximum corresponding to line n + 1 is kept. In the diagram in Figure 7, the maxima are those corresponding to the line n + l. The detected signal then decreases, as illustrated for the line n + 2.

 The location of the maxima A being thus found, we search in the same way for the locations of the maxima B, and we deduce the center of the break.

 These break centers have very precise physical positions on the screen. To have a correct image, the scanning must pass at precise instants on these centers. If there is a vertical geometry defect, the center of a break is not reached at line n but at line n- x, x representing a distance. .

 Likewise, horizontally, the scan passes through the center of the break, not at a given time t but at a time tt At.

 The errors x and At are extracted by chronometry, that is to say by comparison with reference clock signals, and converted into electrical signals for example into voltages; these being sent to the scanning circuit and superimposed on the normal scanning signals as a correction parameter.

 A circuit capable of carrying out the operations which have just been recalled is shown diagrammatically in FIG. 8.

In this figure, the optical fiber network comprising breaks is represented, for reasons of simplicity, by a single fiber fx comprising two sections, placed in front of the screen E of a cathode ray tube
TC. The two ends of this fiber are optically coupled to photodetectors PD1 and PD2. These photo-detectors produce signals transmitted to AMP interface and amplification circuits generating a single SpD signal, itself transmitted to an analog-to-digital ADC converter. The digital signals resulting from the conversion are stored, from one scanning line to the other, in two buffer memories MEM1 and
MEM2. The switch is made using an electronic switch K controlled by SCO control signals. A comparator COMP selects the maxima and transmits them in the form of a digital signal
SM, to CME error measurement circuits. In addition, this circuit elaborates and transmits to the switch K the control signals SCO to admit into one or the other of the buffer memories, MEM1 or MEM2, the converted signals, according to the result of the comparison. These circuits receive clock signals HC serving as a reference. The frequency of these signals is directly related to the number of optical fibers. The occurrence of the detection of the maxima is compared with the timing of the clock signals HC. The time differences are converted into error correction signals transmitted to a digital-analog converter DAC which generates on its output an error correction signal SCE superimposed, in an SOM summator, on the normal scanning signals SB generated by the circuits. CB scan. The latter receive scan clock signals HB generated by the circuits CHR. The timing signals are generated by CHR circuits comprising conventional time bases. At the output of the summation circuits SOM, the composite scanning signals thus obtained SPC are transmitted via the usual interface and amplification circuits AMPB to the deflection circuits BD.

 Convergence is performed by iteration as a function of the loop gain specific to the circuit.

Claims (9)

 1. Device for measuring geometry faults in the scanning of the screen (E) of a cathode-ray tube (TC) by a light spot, characterized in that it comprises: - a network of optical fibers (fl to fn) comprising breaks so as to define a series of distinct sections separated by determined spaces (e) and arranged on the screen (E) according to a preset configuration; - at least one photodetector member (PD) coupled to one of its ends of the optical fibers (fi to fn) converting the light energy captured by one of the end faces of the sections during the passage of said spot near the one of the breaks in electrical signals; - and circuits for detecting (COMP) the maximum values (SM) of Amplitude of these electrical signals, for generating (CHR) reference clock signals (Hc), for determining (CME) from these signals of the positions of the centers (hl, h) of said breaks, generation (DAC) of correction signals (SCE) of said geometry faults and superimposition (SOM) of these signals to control signals (SB) of said screen scan (E) .  2. Device according to claim 1, characterized in that the optical fiber network (fl to fn) has a matrix organization with orthogonal lines (H1 to H5) and columns (Q1 to A7), the optical fibers defining without the columns and the centers (hl, h2) of the breaks being aligned on the lines, the scanning also taking place along lines (L1, L2) parallel to the lines of the matrix.  3. Device according to any one of claims 1 to 2, characterized in that it comprises two photodetector members (PD1, PD2), a first photodetector member (PD i> being coupled to the first end of all the optical fibers and the second photodetector member (PD2) being coupled to the second end of these optical fibers.  4. Device according to claim 3, characterized in that said network comprises thirteen vertical fibers (fl and +) cut into fourteen sections, the centers of the breaks thus defining thirteen horizontal horizon lines, equidistant, parallel to the scanning lines (lux)  5. Device according to any one of claims 1 to 4, characterized in that the cathode ray tube (TC) comprising a bulb and a glass slab attached to its front face forming a screen, the network of optical fibers (fl to fn) is placed between this bulb and this glass slab.  6. Device according to claim 5, characterized in that each optical fiber (fx) of said network is constituted by a relief engraving produced in a surface region of the slab in contact with the bulb.  7. Device according to claim 1, characterized in that the signal processing circuits include an analog-digital converter (ADC) receiving the electrical signals converted by each photo-detector member (PD1, PD2), a first (MEM1) and a second memory (MEM2) receiving the signals from the analog-digital conversion corresponding to a line of said scan, an electronic switch (K) routing the signals from the conversion to one or the other of said memories and a comparator ( COMP) selecting among the signals belonging to two consecutive scanning lines those having the greatest amplitude, controlling said switch (K) so as to keep, memorized, the signals of greatest amplitude and to route the signals of the following scanning line to the other memory and to transmit to error measurement circuits (ME) the signals having the maximum amplitude among all the detected signals.  8. Device according to claim 7, characterized in that it further comprises circuits (CHR) generating a reference clock signal (HR) at a frequency multiple of the frequency of the line scan, the multiplying coefficient being directly proportional the number of optical fibers, the error measurement circuits (CME) determining the time shift between these reference clock signals and the occurrence of the detection of said maxima, and in that these measurement circuits errors (CME) generate, as a function of the detected offsets, correction signals for said geometry faults.  9. Device according to claim 8, characterized in that it comprises a digital-analog converter (DAC) receiving said error correction signals and converting them into analog signals (SCE) and, the device comprising the circuits (CB) scanning signal generation (SB), it further comprises a summing circuit (SOM) superimposing on these scanning signals (SB), the analog signals (SCE) converted from error correction so as to form a signal (SBC ) scanning composite, this signal being transmitted to the circuits (BD), scanning the cathode ray tube (TC).