Color cathode ray tube apparatus and method for driving the same

Abstract: The electron gun assembly (64) of a color cathode ray tube apparatus (50) comprises a cathode, and a group of electrodes which jointly form at least one electron lens. The power of this electron lens changes in synchronism with the deflection of electron beams at different change rates during one scanning period.

Description: The present invention relates to a color cathode ray tube apparatus which is capable of displaying a high-quality image at every portion of the entire screen thereof. The present invention also relates to a method for driving the color cathode ray tube. A high-quality color cathode ray tube apparatus which is in general use is made up of a panel, a funnel continuous with the panel, and a cylindrical neck connected to the funnel. A phosphor screen having three kinds of light-emitting layers which correspond to red, blue and green colors, respectively, is formed on the inner side of the panel, and a shadow mask is arranged in such a manner as to face the phosphor screen. Provided around the shadow mask is a frame, by means of which the shadow mask is supported by the panel. An internal magnetic shield is attached to the frame. An external conductive layer is formed on the outer wall of the funnel, and an anode is provided on the selected portion of the funnel. An electron gun which emits three kinds of electron beams B R , B G and B B is received in the neck. To deflect the three electron beams in the horizontal and vertical directions, a deflecting device is provided around that envelope portion which is between the neck and the conical part of the funnel. A driving device is provided to apply a predetermined voltage between the electron gun and the terminal of the anode. The phosphor screen has a large number of light-emitting phosphor portions, namely, red light-emitting portions, green light-emitting portions, and blue light-emitting portions. These light-emitting phosphor portions are coated on the screen in either a stripe pattern or a dot-distributed pattern. The three kinds of electron beams B R , B G and B B emitted by the electron gun are deflected by the deflecting device, selectively pass through the shadow mask, and then fall on the phosphor screen. Of the light-emitting phosphor portions of the phosphor screen, those on which the electron beams fall emit light of their respective colors, whereby an image is displayed on the screen. The electron gun is of an in-line type, and three electron beams are emitted therefrom in a line (i.e., in parallel to one another). The electron gun includes: an electron beam-generating section which generates, controls and accelerates the electron beams; and a main electron lens section which converges and focuses the electron beams. The deflecting device is a deflection yoke which is made up of: a horizontal-deflection coil for deflecting the electron beams in the horizontal direction; and a vertical-deflection coil for deflecting them in the vertical direction. In order to accurately converge the electron beams emitted in a line, the horizontal-deflection coil generates a horizontal-deflection magnetic field having a pin-cushion shape, while the vertical-deflection coil generates a vertical-deflection magnetic field having a barrel shape (the generation of such magnetic fields is generally referred to as a convergence-free system). A color cathode ray tube apparatus is required to display a satisfactory image at every portion on the entire screen. In general, however, the conventional apparatus does not satisfy this requirement. That is, the image portion displayed at a peripheral portion of the screen is poorer in quality than that displayed in the center. This problem is partly attributable to the fact that the electron beams falling on the peripheral portions of the screen are liable to be distorted more than the electron beams falling on the central portion, due to the deflection magnetic fields having the pin cushion and barrel shapes. The problem can be solved to a certain extent by improving either the electron gun or the deflection device. In the conventional apparatuses, the improvement of the electron gun has been more effective in solving the problem than the improvement of the deflection device. For instance, a dynamic focus method has been used in the conventional apparatuses. In the dynamic focus method, the distortion of a beam spot is corrected by changing-the power of the electron lens of the electron gun in synchronism with the deflection of the electron beams. If this method is used, the distortion of a beam spot can be corrected in the peripheral portions of the screen. The dynamic focus method is achieved in two ways. One way is to increase the focal length of the rotation-symmetric electron lens, and the other way is to provide electron beams with distortion which could compensate for the distortion caused by deflection. The dynamic focus method based on the second way is disclosed in JP-A-63-943, for example. In either method, it is a generally-adopted technique to apply the converging electrode with a voltage whose level changes parabolically in the manner shown in Fig. 1, just like a curve of the second degree. In recent years color cathode ray tube apparatuses having a screen as large as 76,2 cm (30 inches) or so have come into general use. The deflection angle of such large-sized color cathode ray tube apparatuses is 110° as against 90° of the small-sized apparatuses developed before. Further, it is desired that the surface of the screen be as flat as possible. If the surface of the screen is flat, the distortion of the electron beam spot becomes more marked. This being so, it is hard to obtain an image of satisfactory quality at every portion of he screen merely applying the parabolically-changing voltage shown in Fig. 1 to the converging electrode, shown in Fig. 2. A cathode ray tube apparatus adapted for use in a high-quality TV system, such as an EDTV and an HDTV, is designed such that beam spots formed therein be as small as possible. Since such small beam spots are liable to be greatly distorted, it is difficult to obtain a satisfactory image at every portion of the entire screen by merely applying the parabolically-changing voltage to the converging electrode. Prior art document US-A-4 877 998 discloses color display system wherein an additional horizontal modulation rate voltage signal rises slowly from a center position of a horizontal axis of a display screen towards the border of this screen. This slowly rising voltage is applied in order to obtain an optimum focus condition in the peripheral region of the screen. It is an object of the present invention to provide a color cathode ray tube apparatus which is suitable for use in a high-quality TV system, is capable of producing a satisfactory image, with the deflection distortion being suppressed at every portion of the entire screen, and is very useful in practical application. Another object of the present invention is to provide a method for driving the color cathode ray tube apparatus mentioned above. To solve this object the present invention provides a color ray tube apparatus and a method used for driving a color cathode ray tube apparatus as specified in claims 1 and 3, respectively. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a graph showing a focusing voltage which is applied to the electron gun assembly of a conventional color cathode ray tube apparatus; Fig. 2 is an enlarged view of the beam spots which are formed by electron beams landing on the screen when the focusing voltage shown in Fig. 1 is applied; Fig. 3 is a sectional view of a color cathode ray tube apparatus according to an embodiment of the present invention; Fig. 4 is a sectional view of the electron gun assembly employed in the apparatus of the embodiment; Fig. 5 is a graph showing a focusing voltage which is applied to the electron gun assembly of the apparatus of the embodiment; Fig. 6 is a graph in which the focusing voltage shown in Fig. 5 is shown in more detail while being compared with the focusing electrode shown in Fig. 1; Fig. 7 is an enlarged view of the beam spots which are formed by electron beams landing on the screen when the focusing voltage shown in Fig. 5 is applied; Fig. 8 is a graph in which the size of the electron beam spots formed in the apparatus of the embodiment is compared with the size of the electron beam spots formed in the conventional apparatus; Fig. 9 is a graph showing a modification of the focusing voltage used in the apparatus of the embodiment; Figs. 10A and 10B illustrate how to provide a focusing voltage with the waveform indicated in Fig. 5; and Fig. 11 is a sectional view of another kind of electron gun assembly. An embodiment of the present invention will now be described, with reference to the accompanying drawings. Fig. 3 shows a color cathode ray tube apparatus according to the embodiment of the present invention. The color cathode ray tube apparatus 50 of the embodiment comprises an envelope 62 which is made up of: a panel section 56 including a substantially rectangular faceplate 52, and a skirt 54 extending from the side edges of the faceplate 52; a funnel section 58 coupled to the panel section 56; and a neck section 60 continuous to the funnel section 58. The interior of the tube defined by the panel section 56, the funnel section 58 and the neck section 60 is substantially maintained in a vacuum state. An internal conductive layer (not shown) is formed on the inner wall of the funnel section 58 and on a selected portion of the inner wall of the neck section 60. On the outer wall of the funnel section 58, an external conductive layer (not shown) is formed and an anode terminal (not shown) is arranged. An electron gun assembly 64, which emits three kinds of electron beams B R , B G and B B , is arranged inside the neck section 60. A deflection device 66 is provided on the outer wall of that envelope portion which is located between the funnel section 58 and the neck section 60. The deflection device 66 is made up of: a horizontal-deflection coil which generates a magnetic field used for deflecting the electron beams B R , B G and B B in the horizontal direction; and a vertical-deflection coil which generates a magnetic field used for deflecting the electron beams B R , B G and B B in the vertical direction. The electron gun assembly 64 is driven by a driving device 68 which applies an appropriate voltage to the anode terminal and the stem pin STP (which is connected to the electron gun assembly 64). A phosphor screen 74 is formed on the inner side of the faceplate 52 of the panel section 56. A substantially rectangular shadow mask 76 is arranged inside the tube such that it faces the phosphor screen 74, with a predetermined distance maintained. The shadow mask 76 is a thin metal plate and has a large number of slits 78. The shadow mask 76 is fitted within a frame 80, and this frame 80 is supported by the panel section 56 by means of elastic support members (not shown). An internal magnetic shield (not shown) is provided for the frame 80. The electron gun assembly 64 received in the neck section 60 will be described, with reference to Fig. 4. The electron gun assembly 64 is connected to a heater (not shown), and is made up of: a cathode K for generating electron beams; first and second grids G1 and G2 for shaping the electron beams; third to fifth grids G3-G5, intermediate electrodes 82 and 84, a sixth grid G6 and a convergence cup 86, all of which are used for converging the electron beams; an insulating supporter (not shown) for supporting the structural elements mentioned; and a bulb spacer (not shown). A resistor 90 is provided for the electron gun assembly 64. The resistor 90 has a first end 92 electrically connected to the sixth grid G6, a first intermediate portion 94 electrically connected to the intermediate electrode 82, a second intermediate portion electrically connected to the intermediate electrode 84, and a second end 98 which is the opposing end of the first end 92. An operating voltage-applying device 100 is electrically connected to the first end 92 of the resistor 90, and a variable resistor 102 is connected to the second end 98 of the resistor 90. Each of the first and second grids G1 and G2 is made by a thin plate and has three small holes for allowing electron beams to pass therethrough. Each of the third and fourth grids G3 and G4 is formed by putting two cup-shaped electrodes on each other. The fifth grid G5 is formed by putting a number of cup-shaped electrodes on one another and has three large holes for allowing electron beams to pass therethrough. The intermediate electrode 82 is made by a thick plate and has three large holes for allowing electron beams to pass therethrough. The sixth grid G6 is formed by putting two cup-shaped electrodes on each other and has three holes for allowing electron beams to pass therethrough. All these holes formed in the electrodes are circular. A d.c. voltage of about 150V is applied to the cathode K, and a modulated signal corresponding to an image is supplied to the cathode K. The first grid G1 serves as a grounding grid, the second grid G2 is applied with a voltage of about 600V, the third and fifth grids G3 and G5 is applied with a voltage of about 7 KV, the fourth grid G4 is connected to the second grid G2, and the sixth grid G6 is applied with an anode voltage which is as high as 25 to 30 KV. About 40% of the anode voltage is applied to the intermediate electrode 82 through the resistor 90, and about 65% of the anode voltage is applied to the intermediate electrode 84 through the resistor 90. The voltages to be applied to the intermediate electrodes 82 and 84 can be varied by use of the variable resistor 102. The fifth grid G5, the intermediate electrodes 82 and 84, and the sixth grid G6 jointly form a main lens. This main lens has a long focal length, and is generally referred to as an extended electric field lens because its lens region is expanded by the intermediate electrodes 82 and 84. In the electron gun assembly 64 mentioned above, the side walls of the fifth grid G5 have a certain effect on the convergent electric field generated inside the grid. That is, the equipotential lines of the electric field are made to have a smaller curvature in the vertical direction than in the horizontal direction. This being so, the convergent electric field has a stronger converging effect in the vertical direction than in the horizontal direction. In other words, the main lens of the electron gun assembly 64 has two 4-pole components having the opposite polarities, so that the deflection distortion of electron beams can be compensated for, with the focusing voltage being changed properly. The focusing voltage is applied to the electron gun assembly 64 by a dynamic focus circuit 103 by way of a diode element 104. The dynamic focus circuit 103 generates a voltage having a parabolic waveform. The diode element 104 is located between the dynamic focus circuit 103 and the third and fifth grids G3 and G5, and serves to clamp the parabolic waveform of the voltage generated by the dynamic focus circuit 103. The waveform of the voltage applied to the electron beam assembly 64 is illustrated in Fig. 5. As is illustrated, the voltage is constant with respect to the screen region which is within 1/3 of the screen size from the screen center, but increases substantially with the square of deflection with respect to the other screen regions. In other words, the focusing voltage applied to the electron gun assembly 64 is linear (i.e., constant) with respect to the electron beams which are deflected to the screen region which is within 1/3 of the screen size from the screen center, and increases substantially with the square of deflection with respect to the electron beams which are deflected to the other screen regions. Next, a description will be given of an optimal focusing voltage in relation to positions on the screen. In the graph shown in Fig. 6, a focusing voltage used in the present invention is compared with a focusing voltage used in a conventional apparatus, the former being indicated by the solid line B and the latter being indicated by the broken line A. As is shown, the focusing voltage used in the conventional apparatus merely increases, just like a function of the second degree. In contrast, the focusing voltage used in the present invention describes both a straight line and a curve of the second degree. In the case where the focusing voltage indicated by the broken line A is applied, beam spots are formed in the manner shown in Fig. 2. As is shown in this Figure, the beam spots formed in the center and nearly periphery of the screen are satisfactory in both size and shape. However, the beam spots formed in the mid region between the center and the periphery are larger than it is without dynamic focusing, since the related focusing voltage is so high that the focusing force of the main lens becomes too weak. In addition, the beam spots formed at the periphery of the screen are distorted since the focusing force is too strong and the deflection distortion is not sufficiently eliminated. In the case where the focusing voltage indicated by the solid line B is applied, beam spots are formed in the manner shown in Fig. 7. As is shown in this Figure, the beam spots are comparatively satisfactory at each portion of the entire screen. More specifically, the beam spots formed in the screen region between the screen center and the screen periphery are not very large since the focusing voltage VD corresponding to that screen region is not increased very much. Further, the beam spots formed at the screen periphery are neither large nor distorted since the focusing voltage VD corresponding to the screen periphery is increased. Fig. 8 is agraph showing the size of the electron beam spots formed in a 81 cm (32-inch) color cathode ray tube apparatus. More specifically, the graph in Fig. 8 shows how the diameter of an electron beam varies during a scanning operation from the screen center to the screen periphery, with the diameter of the beam spot formed in the screen center being represented by 1.0. The broken line A in the graph indicates relative values corresponding to the diameter of a beam spot formed when the focusing voltage used in the conventional apparatus is applied, i.e., when the focusing voltage indicated by the broken line A in Fig. 7 is applied. On the other hand, the solid line B in the graph indicates relative values corresponding to the diameter of a beam spot formed when the focusing voltage used in the present invention is applied. As is shown in Fig. 8, the value indicated by the broken line A is comparatively large with respect to the screen region between the center and the periphery. This means that the electron beams corresponding to that screen region are not sufficiently converged. In contrast, the value-indicated by the solid line B does not much increase with respect to the screen region; it is about 20% smaller than the value indicated by the broken line A. This means that the electron beams corresponding to that screen region are sufficiently converged. Moreover, as is indicated by the solid line B, the beam diameter gradually increases from the screen center to the screen periphery, so that the image is not unnaturally displayed on the screen. It should be noted that, according to the present invention, the power of the main lens of the electron gun assembly changes in accordance with the position at which an electron beam lands on the screen, because of the application of the focusing voltage shown in Fig. 5. When an electron beam moves from the screen periphery to the screen center during one scan, the level of the focusing voltage forst decreases gradually while describing a curve of the second degree, and then becomes constant. When the electron beam moves from the screen center to the screen periphery, the level of the focusing voltage is constant first and then increases gradually while describing a curve of the second degree. As may be understood from this, the focusing voltage changes at two change rates: namely, the change rate expressed by a function of the 0th degree (a constant), and the change rate expressed by a function of the second degree. (when the focusing voltage is constant, its change rate is 0 and can therefore be regarded as being expressed by a function of the 0th degree). In short, the change rate of the focusing voltage is expressed by two functions having different degrees. Having been described so far is an embodiment wherein the present invention is applied to the horizontal-deflection. Needless to say, the present invention is also applicable to the vertical-deflection. In the above, the focusing voltage was described as being changeable at the-two different change rates which are defined by a function of the 0th degree and a function of the second degree. However, the present invention is in no way limited to this. For example, the change rates of the focusing voltage are in no way limited to two, and , as is shown in Fig. 9, the focusing voltage may describe a linear line in the neighborhood of the screen center, describe a curve of the 2.5th degree between the screen center and the screen periphery, and describe a curve of the fourth degree in the neighborhood of the screen periphery. As may be understood from this example, the focusing voltage may be changed at two or more change rates. With the focusing voltage being changed at a number of change rates, when the beam spot has more difficult deflection characteristic, the shape of the beam spot formed by an electron beam can be controlled in an optimal manner. A level-changeable voltage, such as the focusing voltage mentioned above, can be obtained by use of rectangular pulses, such as those shown in Fig. 10A. The rectangular pulses shown in Fig. 10A can be easily generated by superimposing rectangular pulses A, B and C shown in Fig. 10B. In the embodiment, the present invention is applied to horizontal-deflection, Needless to say, the present invention is also applicable to vertical-deflection. In the embodiment, the electron gun assembly need not be a resistor-containing composite electron gun assembly. In the electron gun assembly shown in Fig. 11 (which electron gun assembly is disclosed in JP-A-61-74246), the voltage of grid G'4 serving as an auxiliary electron lens changes in accordance with the position to which an electron beam is deflected. A constant voltage is applied to the main electron lens formed by grides G'5 and G'6, without reference to the position to which the electron beam is deflected. That is, the power of the auxiliary electron lens is changed at different change rates in synchronism with the deflection. Needless to say, this type of electron gun assembly is applicable to a bipotential type, a unipotential type, etc. In the electron gun assembly disclosed in JP-A-59-53656, the second electrode serving as an accelerating electrode is made up of three electrode elements. A voltage which changes in synchronism with deflection is applied to the central one of the three electrode elements. The invention of the Japanese reference may be similar to the present invention, in that a voltage which is synchronous with deflection is changed at a plurality of change rates in such a manner as to eliminate deflection distortion at the screen periphery. In the above embodiment of the present invention, a voltage which changes at different change rates is applied to the main electron lens, so as to eliminate deflection distortion at the periphery of the screen. The embodiment employs an electron gun assembly of an in-line type. Needless to say, however, this electron gun assembly may replaced with an electron gun assembly of a delta type, if so desired. It is possible to display an image of satisfactory quality not only in the screen center but also in the other screen portions: namely the screen periphery and the screen portion between the center and the periphery. Therefore, a color cathode ray tube apparatus embodying the present invention is of high quality, and is of great industrial value.

Claims: A color cathode ray tube apparatus, comprising: an envelope having a tube axis and made up of a panel (56), a funnel (58) and a neck (60); a phosphor screen (74) formed on an inner side of the panel; an electron gun assembly (64), received in the neck (60), for emitting electron beams of three kinds, and including a cathode (K) and a group of electrodes (82, 84, 86, G1-G6), deflecting means (66), provided around an outer wall of the envelope portion which is located between the neck (60) and the funnel (58), for horizontally and vertically deflecting the electron beams emitted from the electron gun assembly (64); and an operating voltage-applying device (100) for applying a predetermined voltage to the electron gun assembly (64), characterized by means for applying focus voltage to at least one of the electrodes (G5, 82, 84, G6) which form a main electron lens, said focus voltage being substantially constant when the electron beams are deflected, during each scanning period, to a predetermined region which is within at least 1/3 of the screen size from the center of the phosphor screen (74), and said focus voltage rapidly rising when the electron beams are deflected to peripheral regions of the phosphor screen (74). A color cathode ray tube according to claim 1, characterized in that the focus voltage increases substantially with the square of deflection when the electron beams are deflected to the peripheral regions of the phosphor screen (74). A method used for driving a color cathode ray tube apparatus which comprises: an envelope having a tube axis and made up of a panel (56), a funnel (58) and a neck (60); a phosphor screen (74) formed on an inner side of the panel; an electron gun assembly (64), received in the neck, for emitting electron beams of three kinds, said electron gun assembly including a cathode (k), and a plurality of electrodes (82, 84, 86, G1-G6) which jointly form at least a main electron lens; a deflection means (66), provided around an outer wall of that envelope portion which is located between the neck and the funnel, for horizontally and vertically deflecting the electron eams emitted from the electron gun assembly; and an operating voltage-applying device (100) for applying a predetermined voltage to the electron gun, said method being characterized by comprising the steps of: applying a focus voltage to at least one of the electrodes (G5, 82, 84, G6) which form said main electrode lens, and selecting said focus voltage to be substantially constant when the electron beams are deflected, during each scanning period, to a predetermined region which is within at least 1/3 of the screen size from the center of the phosphor screen (74), and to rise rapidly when the electron beams are deflected to peripheral regions of the phosphor screen (74). A method according to claim 3, characterized in that the focus voltage increases substantially with the square of deflection when the electron beams are deflected to the peripheral regions of the phosphor screen (74).

Inventors: FUJIWARA TAKESHI C O INTELLECT, HASEGAWA TAKAHIRO C O INTELLEC, KOSHIGOE SHINPEI C O INTELLECT, TOKITA KIYOSHI C O INTELLECTUA, FUJIWARA, TAKESHI, C/O INTELLECTUAL PROPERTY DIV., HASEGAWA, TAKAHIRO, C/O INTELLECTUAL PROPERTY DIV., KOSHIGOE, SHINPEI, C/O INTELLECTUAL PROPERTY DIV., TOKITA, KIYOSHI, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: H01J29
IPC codes: H01J29