A deflection unit having a distortion correcting coil in a cathode ray tube apparatus

Abstract: A cathode ray tube apparatus is provided with correcting coils that correct distortion caused in an image on a fluorescent screen which is scanned by electron beams deflected by a deflection device. In this cathode ray tube apparatus, correcting coils (33, 34) are disposed near the deflection center in the deflection unit and are formed as low inductance of 1mH or less electrically independent from the deflection unit. Further, the correcting coils (33, 34) are connected to the distortion correcting current source provided independent from a main deflection current supply source that supplies an electric current to main deflection coils (30, 31) in the deflection unit, and generate the almost uniform magnetic field by the electric current supplied from the distortion correcting current supply source. Further, a first inductance element connected to the deflection coils is so magnetically coupled to a second inductance element connected to a correcting coils as to compensate induced electro magnetic force which is produced due to an interlinkage of the deflection magnetic fields.

Description: The present invention relates to a deflection unit having a distortion correcting coil in a cathode ray tube apparatus, and more particularly to the cathode ray tube apparatus such as a color cathode ray tube apparatus that is possible to correct arbitrary distortion of an image. Generally, in the color cathode ray tube apparatus, three electron beams emitted from an electron gun are deflected in a horizontal and vertical directions by the magnetic fields which are generated from the deflection unit and are directed through apertures of a shadow mask into a fluorescent screen. Thus, the fluorescent screen is scanned by the deflected electron beams in the horizontal and vertical directions so that an color image is displayed on the fluorescent screen. As such a color cathode ray tube apparatus, an in-line type color cathode ray tube apparatus is the main current, in which three electron beams arranged in the same plane. In such a color cathode ray tube apparatus, in order to obtain a good quality of the image displayed on the screen, it is important to improve an image distortion, for example, the distortion of raster. In a conventional tube apparatus, the image distortion is usually adjusted by distribution of the magnetic fields generated from the deflection unit. However, recently, in the color cathode ray tube apparatus, a deflection angle is widen and high quality is seriously requested. Because of the widen deflection angle and the high quality, it is difficult to make the image distortion less only by adjusting the distribution of the magnetic fields generated from the deflection unit and the request to the image distortion is severer. There is disclosed a method of improving the image distortion and purity and convergence characteristics in Japanese Utility Model Disclosure No. 60-32871, in which a correcting coil unit as shown in FIG. 1 is assembled in the tube apparatus. The correcting coil unit has a structure that six sets of coils 2 are wound on twelve protrusions provided inside an annular body 1, in an apparatus provided with the correcting coil unit, the image distortion, the purity and the convergence are corrected by energizing each coil 2. That is, two of six sets of coils cause two sets of NS magnetic poles as shown in FIGS. 2A and 2B by turning on the electricity and cause the magnetic field 4 shown by dot lines to displace the electron beams 5 in the direction shown by arrows 6 and to provide correcting effects of the image distortion and the purity to the electron beams. Other four sets of coils 2 cause four to six NS magnetic poles as shown in FIGS. 2C through 2F by turning on the electricity and cause the magnetic field 7 shown by dot lines to displace three electron beams 5 in the direction shown by arrows 8 and to provide the correcting effect of the convergence to the electron beams. A similar method is used by an apparatus disclosed in US-A-5 070 280 also providing a ring magnetic subcore having a plurality of protrusions with correcting coils. An advantage of the correcting coil is that it is possible to adjust arbitrarily correction of the image distortion and the purity, therefore, to possible to realize advance of the image distortion and the purity characteristic. However, when correcting the image distortion by using the correcting coil, a problem is caused that not only the image distortion but also the purity (the purity of color) are changed. This problem is described as follows. Usually the characteristics of the image distortion and the purity of the color cathode ray tube apparatus depend on a deflection angle of the electron beams by which the fluorescent screen is scanned. Here, there will be considered a correction of the distortion in the vertical direction, the distortion being applied to an image on the fluorescent screen, which is formed by the electron beams deflected by the magnetic field generated from the deflection unit. As shown in FIGS. 3A and 3B, if the correcting magnetic field 13 generated from a correcting coil unit 12 disposed behind the deflection unit 9 is the uniform magnetic field in the same direction as that of a vertical deflection magnetic field 10 generated from the deflection unit 9, the sum of amount of the magnetic field contributing to the vertical deflection is increased, the electron beams 5 that should be reached a point P will be reached a point P1 outside of the point P and it will be possible to correct image distortion. However, at the same time, as the correcting coil unit 12 is disposed behind the deflection device 9, the center of the deflection is moved from O to O1 backwardly, when the electron beams are reached the fluorescent screen 11, deflection angles of the electron beams 5 become smaller and incident angles into the shadow mask are changed to have an effect on the purity characteristic. That is, though it is possible to correct arbitrarily the image distortion at each point on the screen substantially by the correcting coil unit 12, the purity characteristic is changed at the same time and uniformity of the screen is damaged remarkably. Further, the correcting of the image distortion by the correcting coil unit 12 causes cost more expensive. As another method to correct the image distortion in the vertical direction, there is a method that adjusts finely amount of the deflection by the main deflection coil in the deflection unit itself. As in case of correcting the distortion by a high frequency signal, for example, correcting the image distortion formed in a part of a horizontal scanning region, it is demanded that a high frequency modulating signal about ten times of horizontal deflection frequency is supplied. However, in this method, because inductance of the vertical deflection coil (Usually the vertical deflection coil of the cathode ray tube apparatus for TV has the inductance larger than 1 mH) is too large, load is increased, as a result, a problem is caused that necessary high frequency correction voltage becomes larger as it is hard to make practice. As described above, as the method to improve the image distortion characteristic of the color cathode ray tube apparatus, a correcting coil unit has already known that six coils are wound on twelve protrusions provided inside of an annular body and the image distortion, the purity and convergence are corrected by energizing the coil. In this correcting coil unit, it is possible to correct the image distortion or the purity by energizing six sets of coil to form two magnetic poles. However, when the image distortion is corrected by using the correcting coil unit, the purity is varied accompanying it and uniformity of the image on the screen is damaged remarkably. Variation of the purity characteristic is a serious problem in the color cathode ray tube apparatus having the screen that each color fluorescent layer is formed as a dot type. Further, there is a problem to increase cost in this correcting coil unit. As another method to correct the image distortion, as mentioned above, there is a method to adjust finely amount of deflection by the deflection coil in the deflection unit itself without using a particular coil unit. However, as inductance of the deflection coil is too large in this method, there is a problem that the load is increased in case of correcting the high frequency distortion and the actually necessary voltage for the high frequency correction becomes larger impractically, therefore, it is hard to adopt this method. An object of the present invention is to provide a cathode ray tube apparatus possible to correct image distortion without having an effect on purity characteristic by using a correcting coil unit. According to the present invention, there is provided a color cathode ray tube apparatus comprising: a generating means for generating electron beams; deflection means for having a deflection center generating the magnetic field which deflects the electron beams the deflection means including a main deflection coil; a screen for emitting light rays when the screen is scanned by the deflected electron beams to display an image; a shadow mask provided between the generating means and the screen; a vacuum envelope in which the generating means is stored and having an inner surface on which the screen is formed; first supply means for supplying a main deflection electric current to the main deflection coil of the deflection means; correcting means for correcting distortion of the raster drawn on the screen, the correcting means including correcting coils electrically independent from the main deflection coil, each of the correcting coils having a deflection center corresponding to the deflection center of said deflecting means and having an inductance of 1 mH or less; second supply means provided independent from the first supply means and supplying a correcting electric current to the correcting coil of the correcting means, the second supply means causing the correction coil to generate a substantially uniform correcting magnetic field around the center of the deflection. As mentioned above, in the color cathode ray tube apparatus in which the correcting coil is provided electrically independent from the deflection unit and generates the almost uniform correcting magnetic field near the deflection center in the deflection unit by supplying current from the correcting current supply source, independent from the main deflection current supply source, to the correcting coil, it is possible to generate the desired correcting magnetic field without displacing the deflection center in the deflection unit. As a result, it is possible to avoid variation of the purity correction even if correcting the image distortion, i.e. raster distortion. Further, if the correcting coil has a low inductance of 1 mH or less, preferably, about 100 µH, the load applied to the correcting coil is reduced when a high frequency currentis applied to the correcting coil and good response in the high frequency of about 1 MHz is obtained. Further, degradation of convergence characteristic in correcting the distortion is prevented by generating the almost uniform correcting magnetic field. In case of the cathode ray tube apparatus used for a computer display, in the high frequency of about 1 MHz, almost of the load of the coil is a component of the inductance, when the high frequency correcting is performed in the usual current supply circuit, responsibility is degraded by a transient phenomenon. Therefore, considering following property, the correcting coil is preferably the impedance of 100 µH or less. Further, to obtain a good correction sensitivity, the correcting coil is preferably the impedance of 20 µH or more. It is possible to reduce the induced electro magnetic force that is not preferable for the deflection of the electron beams, which is caused by interlinkage flux between the main deflection coil and the correcting coil, when the main deflection coil in the deflection unit is connected to a first inductance element in series and the correcting coil is connected in series to a second inductance element which is magnetically coupled to the first inductance element. Further, it is possible to make the structure of the inductance element simple in which the first and second inductance elements are provided on the same magnetic material by winding several times or on the annular magnetic material forming a closed circuit by winding more than once. Further, it is possible to match the induced electro magnetic force generated in the inductance element with the induced electro magnetic force generated in the main deflection coil and is possible to improve the compensation effect of the electro magnetic force by connecting the first or second inductance element with a resistor or a capacitor that deforms a wave form of the induced electro magnetic force generated in the first and second inductance element. 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 cross section schematically showing a structure of a correcting unit incorporated into a conventional color cathode ray tube apparatus. FIGS. 2A though 2F are explanatory drawings of various magnetic fields generated from magnetic poles of conventional electron correcting coils as shown in FIG. 1. FIGS. 3A and 3B are drawings for disclosing problems in correcting effect of image distortion by the correcting coils as shown in FIG. 1. FIG. 4 is a partial cross section showing schematically a structure of a color cathode ray tube apparatus relating to one embodiment of the present invention. FIG. 5 is a cross section in a plane perpendicular to a tube axis showing a deflection unit and a correcting coil unit assembled in the color cathode ray tube apparatus as shown in FIG. 4. FIG. 6A is a circuit diagram showing relation between a horizontal main deflection coil in a deflection unit and the correcting coil that corrects the distortion in the horizontal direction of the correcting coil unit as shown in FIG. 5. FIG. 6B is a circuit diagram showing relation between a vertical main deflection coil and the correcting coil that corrects the distortion in the vertical direction of the correcting coil unit as shown in FIG. 5. FIG. 7 is an explanatory drawing of a producing method of the correcting coil unit as shown in FIG. 5. FIGS. 8A and 8B are explanatory drawings of relation between the magnetic field generated from the vertical main deflection coil and the correcting magnetic field generated from the correcting coil unit that corrects the distortion in the vertical direction and variation of landing positions of electron beams respectively. FIG. 9 is a drawing showing a structure of a correcting current supply circuit that supplies an electric current to the correcting coil. FIG. 10A is a drawing showing an voltage wave form generated between terminals of the vertical distortion correcting coil and the vertical main deflection coil without connecting first and second inductance elements. FIG. 10B is a drawing showing an voltage wave form generated between terminals of the vertical distortion correcting coil and the vertical main deflection coil in case of connecting the first and second inductance elements. FIG. 11 is a drawing showing a structure of a first inductance element connected to the vertical main deflection coil, and a second inductance element magnetically coupled with the first inductance element and connected to the vertical distortion correcting coil. FIG. 12 is a drawing showing another structure of the first inductance element connected to the vertical main deflection coil, and the second inductance element magnetically coupled with the first inductance element and connected to the vertical distortion correcting coil. One embodiment of a color cathode ray tube apparatus of the present invention will be described, with reference to the accompanying drawings. FIG. 4 is a schematic representation of the color cathode ray tube apparatus. As FIG. 4 shows, the apparatus comprises an envelope which is composed of a panel 20 and a funnel 21. The funnel 21 is formed integral with the panel 20 and has a neck 24 and a large part 26. The apparatus further comprises a fluorescent screen 22, a shadow mask 23, an electron gun assembly 25, and a deflection unit 27. The screen 22 consists of a number of layers provided on the inner surface of the panel 20, in the form of either stripes or dots. These phosphor layers emit light when excited. More precisely, some of them emit blue light, some others emit green light, and the others emit red light. The shadow mask 23 is provided in the panel 20 and located behind the fluorescent screen 22. Hence, the mask 23 faces the fluorescent screen 22. The electron gun assembly 25 is arranged in the neck 24 of the funnel 21, for emitting three electron beams aligned in a line in a horizontal plane. The deflection unit 27 is mounted on the funnel 21, at the junction between the neck 24 and the large part 26. The unit 27 is designed to deflect the electron beams emitted from the electron gun assembly 25. A correcting coil unit is attached to the deflection unit 27, for eliminating distortion of the image formed on the fluorescent screen 22 as will be described later. The deflection unit 27 and the correcting coil unit have structures as shown in FIGS. 5 and FIGS. 6A and 6B. FIG. 5 shows a cross section of the deflection unit 27, as viewed along the tube axis. FIG. 6A and 6B shows relationship between a circuit diagram of the main deflection coil of the deflection unit 27 and that of the correcting coil unit. As shown in FIG. 5, the deflection unit 27 consists of a pair of horizontal main deflection coils 30 that are disposed at upper and lower inner sides of an ellipsoid cylindrical mold 29, a pair of vertical main deflection coils 31 that are disposed at left and right outer sides of the mold 29, and a core 32 disposed outside the horizontal and vertical main deflection coils 30 and 31. The main deflection coils 30 and 31 are formed by saddle winding. The horizontal main coil 30 is divided into a pair of coils 30 HT and 30 HB, both arranged symmetrically to the X-axis and generates pin-cushion shape magnetic field. The vertical main deflection coil 31 is divided into a pair of coils 31 VL and 31 VR, both positioned symmetrically to the Y-axis and generates barrel shape magnetic field. The correcting coil unit has a horizontal correcting coil 33 and a vertical correcting coil 34. The coil 33 eliminates or reduces horizontal distortion of a raster, caused by the horizontal main deflection coil 30. The coil 34 eliminates or reduces vertical distortion of a raster, caused by the vertical main deflection coil 31. The horizontal correcting coil 33 consists of a pair of correcting coils 33 HT and 33 HB located symmetrically to the X-axis at almost the same positions as those of the coils 30 HT and 30 HB of the horizontal main deflection coil 30. The the vertical correcting coil 34 consists of a pair of correcting coils 34 VL and 34 VR positioned symmetrically to the Y-axis at almost the same positions as those of a pair of coils 31 VL and 31 VR of the vertical main deflection coil 31. Each of distortion correcting coils 33 HT, 33 HB, 34 VL and 34 VR is located near the deflection center of the deflection device. More precisely, the divided correcting coils 33 HT and 33 HB are have their deflection centers located near the deflection center of the horizontal main deflection coil 30, and the correcting coils 34 VL and 34 VR have their deflection centers located arranged near the deflection center of the vertical main deflection coil 31. The term "deflection center" means a point on the tube axis, at which a magnetic field has a peak intensity. The correcting coils 33 HT and 33 HB of the horizontal distortion correcting coil 33 are connected as shown in FIG. 6A. The correcting coils 34 VL and 34 VR of the vertical distortion correcting coil 34 are connected as shown in FIG. 6B. Inductance of the horizontal and vertical coils 33 and 34 is 1 mH or less to generate the almost uniform magnetic field to three electron beam. As shown in FIG. 6A, the coils 30 HT and 30 HB of the horizontal main coil 30 are connected in parallel. An inductance element 35 is connected in series to the divided coils 30 HT and 30 HB and also to a main deflection current supply source 60 by the inductance element 35. The source 60 supplies a main deflection current. The correcting coils 33 HT and 33 HB of the horizontal correcting coil 33 are connected in series. An inductance element 36 is connected in series with the correcting coils 33 HT and 33 HB and also to a correcting current supply source 62 that supplies an independent correcting current. Further, the inductance element 35 is magnetically coupled with the inductance element 36, thereby to compensate for the electron magnetic force induced between the horizontal deflection and correcting coils. As shown in FIG. 6B, a pair of coils 31 VL and 31 VR of the vertical main coil 31 of the deflection device 27 are connected in series. An inductance element 37 is connected in series to the coils 31 VL and 31 VR. The coils 31 VL and 31 VR are connected in series to a main deflection current supply source 60 by the inductance element 37. A resistor 38 or a capacitor is connected in parallel to the inductance element 37. Further, the correcting coils 34 VL and 34 VR of the vertical correcting coil 34 are connected in series. An inductance element 39 is connected in series to the divided correcting coils 34 VL and 34 VR. The correction current supply source 62 independent from the main deflection current supply source 60 is connected by the inductance element 39. The inductance element 37 is magnetically coupled with the inductance element 39, to compensate the electromagnetic force induced between the vertical deflection coil and the vertical distortion correcting coil. The resistor 38 or the capacitor connected to the inductance element 37 in parallel causes the waveform of the electromagnetic force generated in the inductance element 37 to match the waveform of the electromagnetic force generated in the vertical main deflection coil 31, so that the compensation may be achieved more effectively. The horizontal distortion correcting coil 33 and the vertical distortion correcting coil 34, described, above, may be manufactured by the same method. Therefore, it will be described how to manufacture the coil 34 only, with reference to FIG. 7. Assume that a jig is used to form the coils 30 HT and 30 HB of the horizontal main deflection coil 30 and the coils 31 VL and 31VR of the vertical main deflection coil 31. First, a wire 41 is connected at one end to a terminal 42A. Then, the wire 41 is wound around the jig, forming the vertical main deflection coil 31. Next, the wire 41 is connected at two parts to terminals 42B and 42C. Next, the wire 41 is wound around the jig, forming the vertical correcting coil 34. Then, the wire 41 is connected at the other end to a terminal 42D. Finally, the wire 41 is cut at the part extending between the terminals 42B and 42C. To the terminals there may be connected inductance elements or the like. In this method, the vertical correcting coil may be first formed and the vertical main deflection coil may then be formed. The method is advantageous particularly when the main deflection soil is made by using a slot-type winding jig which allows the wire to be wound at any desired position. In addition, the correcting coil can be manufactured at a low cost since it is formed simultaneously with with the main deflection coil. In color cathode ray tube apparatus described above, the distortion of the image can be eliminated or reduced without changing purity. In the conventional apparatus, the correcting coil is disposed behind the deflection unit (at the neck side), and the correcting magnetic field is generated at the back of the deflection center of the deflection device. Therefore, to eliminate or reduce the distortion of the image by the correcting magnetic field, the deflection center must be displaced backward, inevitably changing the purity. In the color cathode ray tube apparatus of this invention, the correcting coil is provided near the deflection center in the deflection unit 27, the center of the deflection is not displaced when generating the correcting magnetic field, and an incident angle to a shadow mask does not change at all. Therefore, the distortion of image can be eliminated or reduced without changing the purity. A magnetic field 45 almost uniform may be generated in the same direction as that of the vertical deflection magnetic field 44 generated from the deflection unit 27 as shown in FIGS. 8A and 8B. Then, three electron beams, which would be deflected to point P on the fluorescent screen 22 if the correcting magnetic field 45 are not not generated, are deflected to a point P1 outside of the vertical direction without displacing the deflection center O in the deflection unit 27. It is possible to eliminate or reduce the distortion of the image in the vertical direction, without changing the purity. Similarly, it is also possible to eliminate or reduce the distortion in the horizontal direction, without changing the purity, by supplying the correcting current to a pair of the divided coils 33 HT and 34 HB of the horizontal correcting coil 33. The coils 33HT and 33HB of the horizontal correcting coil 33 and the coils 34 VL and 34 VR of the vertical correcting coil 34 are connected to the distortion correcting current supplying source. The distortion correcting current supplying source is independent from the main deflection supply source that supplies the current to the horizontal and vertical main deflection coils 30 and 31 in the deflection unit 27. Therefore, it is possible to supply the arbitrary current so as to correct the desired distortion of the image. Furthermore, since the inductance of the horizontal and vertical correcting coils 33 and 34 of the correcting coil are 1 mH or less, good response to high frequency can be attained. The distortion of the image can be eliminated or reduced over a broad band, from a low frequency to a high frequency, by supplying the current to the horizontal and vertical correcting coils 33 and 34 of the correcting coil (shown about the vertical distortion correcting coil 34) by means a simple current supply circuit in which the resistors 48A and 48B are connected to an amplifier portion 47 for amplifying the correcting voltage as shown in FIG. 9. Moreover, as indicated above, the inductance element 35 connected in series to the divided coils 30 HT and 30 HB of the horizontal main deflection coil 30, the inductance element 36 connected in series to the divided coils 33 HT and 33 HB of the horizontal main distortion coil 33, the inductance element 37 connected in series to the divided coils 31 VL and 31 VR of the vertical main deflection coil 31, and the inductance element 39 connected in series to the divided coils 34 VL and 34 VR of the vertical distortion image correcting coil 31 are combined magnetically. Therefore, it is possible to eliminate the influence on the beam deflection, imposed by the electromagnetic force generated in the main deflection coils 30 and 31. Without the inductance elements 35, 36, 37 and 39, the pulse electromagnetic force 51 generated, as shown in FIG. 10A, between the terminals of the vertical main deflection coil 31, due to the correcting current 50. This is because the current 50 is a high frequency pulse wave that flows in the divided coils 34 VL and 34 VR of the vertical distortion correcting coil 34 since the interlinkage magnetic field that is generated between the vertical main deflection coil and the vertical correcting coil, imposing adverse influence on the vertical main deflection coil 31. In contrast, thanks to the inductance elements 35, 36, 37 and 39, the electromagnetic force 51 can be controlled, as illustrated in FIG. 10B corresponding to FIG. 10A. A method of eliminating or reducing the distortion of the image will be described in detail. The coils 34 VL and 34 VR of the vertical correcting coil 34 of the correcting coil are formed by winding wires in the same winding grooves as the coils 31VL and 31 VR of the vertical main deflection coil 31. The inductance element 37 connected in series to the coils 31VL and 31VR and the inductance element 39 connected in series to the coils 34 VL and 34 VR are wound by 150 and 25 times around a drum type ferrite core having a diameter of the core of 5 mm and a length of 13 mm. The drum type ferrite core (not shown) is provided outside the deflection unit, imposing no influence on the deflection of the electron beam. The total inductance of the coils 34 VL and 34 VR is 30 µH, the inductance of the inductance element 37 is 1.6 µH, the inductance of the inductance element 39 is 37 µH, and the inductance of the whole vertical correcting coil system (the coils 34VL and 34VR and inductance element 39) is 70 µH. The structure described above is sensitive enough to eliminate or reduce the vertical distortion of image larger than 1 mm, by applying a correcting current of 100 mA. Therefore, variation of the electron beams landing on the fluorescent screen can be decreased to less than 1 micron. Further, it is possible to eliminate or reduce the arbitrary distortion of image, up to the high frequency of about 1 MHz. In the embodiment shown in FIG. 11, the inductance element connected to the horizontal main deflection coil in the deflection unit, the inductance element magnetically coupled to this inductance element and connected to the horizontal correcting coil, the inductance element connected to the vertical main deflection coil, and the inductance coil magnetically coupled with this inductance and connected to the vertical distortion correcting coil can be made by connecting insulated electric wires to the horizontal main deflection coil, the horizontal correcting coil, the vertical main deflection coil and the vertical correcting coil, and by winding these wires around the drum type core 54 made of ferromagnetic material such as ferrite. (The inductance elements 35 and 36 are shown in FIG. 11.). As shown in FIG. 12, an inductance element can be formed by passing an insulated electric wire 57 connected to the main deflection coil (or the correcting coil) through the center of the annular magnetic material 55 forming a closed circuit, and by connecting an insulated electric wire 56 to the correcting coil (or the main deflection coil) and winding the wire around the annular magnetic material 55 more than once. If made by using the annular magnetic material 55, the inductance element is particularly simple in structure. The correcting coil and the main deflection coil are formed integral in the embodiment. Instead, this correcting coil may be made independent of the main deflection coil and may be installed near the deflection center of the main deflection coil. The correcting coil may be wound in the winding groove provided inside the deflection unit. The correcting coil may be composed of one or both of the horizontal and vertical distortion correcting coils. A desired correcting magnetic field can be generated without moving the deflection center in the deflection unit, by positioning the correcting coil near the deflection center (the correcting coil having low inductance of 1 mH or less and electrically independent of the deflection unit), and by forming the structure that generates an almost uniform magnetic field from the current supplied from the distortion correcting current supply source independent of the main deflection current supply source which supplies the deflection current to the main deflection coil. Even if the distortion of image is eliminated or reduced, variation of the purity characteristic can be avoided. In addition, it is possible to good response to the high frequency, by decreasing the inductance of the correcting coil to 1 mH or less. Moreover, it is possible to reduce the induced electromagnetic force which affects the deflection of the electron beams and which is generated by interlinkage magnetic flux between the main deflection coil and the correcting coil. Further, the inductance element can be made simple, by winding a first inductance element and a second inductance element around the same magnetic material, several times, or around the annular magnetic material of which the closed circuit consists more than once. The electromagnetic force can be effectively compensated for, by making the force generated in the inductance element agree with the electromagnetic force generated in the main deflection coil by connecting the first and the second inductance elements with the resistor or the capacitor that changes the waveform of the electromagnetic force generated in the first and second inductance elements. The present invention can be applied to both a stripe-type fluorescent screen and a dot-type fluorescent screen. It is particularly advantageous when applied to a dot-type fluorescent screen because a degree of additional coverage to displacement of landing of the electron beam is small.

Claims: A color cathode ray tube apparatus comprising: generating means (25) for generating electron beams; deflection means (27) having a deflection center, for generating the magnetic field which deflects the electron beams, the deflection means (27) including a main deflection coil (30, 31); a screen (22) for emitting light rays to form an image thereon, when the screen (22) is scanned by the deflected electron beams; a shadow mask (23) provided between the generating means (25) and the screen; a vacuum envelope (20, 21, 24) in which the generating means (25) is received, the envelope (20, 21, 24) having an inner surface on which the screen (22) is formed; first supply means (60) for supplying a main deflection electric current to the main deflection coil (30, 31) of the deflection means (27); correcting means (27) for correcting distortion of the raster drawn on the screen (22); characterized in that the correcting means (27) includes correcting coils (33, 34) electrically independent from the main deflection coil (30, 31), each of the correcting coils (33, 34) has a deflection center corresponding to the deflection center of said deflecting means and having an inductance of 1 mH or less; said color cathode ray tube apparatus further comprises second supply means (62) provided independent from the first supply means (60), for supplying a correcting electric current to the correcting coil (33, 34) of the correcting means (27), the second supply means (62) causing the correction coil (33, 34) to generate a substantially uniform distortion correcting magnetic field around the deflection center by the correcting current. The color cathode ray tube apparatus according to claim 1, characterized by further comprising: a first inductance element (35, 37) connected to the main deflection coil (30, 31) in series; a second inductance element (36, 39) connected to the correcting coil (33, 34) in series, the first and second inductance elements (33, 36, 37, 39) magnetically coupled in the direction that an induced electro magnetic force generated by the interlinkage magnetic flux between the main deflection and correcting coils (30, 31, 33, 34) is compensated. The color cathode ray tube apparatus according to claim 2, characterized in that the first and second inductance elements (33, 36, 37, 39) are wound on the same magnetic material (54, 55). The color cathode ray tube apparatus according to claim 2, characterized by comprising annular magnetic material (55) on which at least one of the first and second inductance elements (35, 36, 37, 39) is wound more than once to form a closed circuit. The color cathode ray tube apparatus according to claim 2, characterized by further comprising an electric element (38) connected to a least one of the first and second inductance elements (35, 36, 37, 39), for deforming a waveform of the induced electro magnetic force generated in one of the inductance elements (38), the electric element (38) including a resistor and/or a capacitor. The color cathode ray tube apparatus according to claim 1, characterized in that the main deflection coil (30, 31) has a horizontal main deflection coil (30) generating the magnetic field for deflecting the electron beams in the horizontal direction and a vertical main deflection coil (31) generating the magnetic field for deflecting the electric beams in the vertical direction, and the correcting coil (33, 34) has a horizontal distortion correcting coil (33) corresponding to the horizontal main deflection coil (30) and a vertical distortion correcting coil (34) corresponding to the vertical main deflection coil (31). The color cathode ray tube apparatus according to claim 6, wherein the horizontal main deflection coil (30) generates a pincushion shape magnetic field, and the vertical main deflection coil (31) generates a barrel shape magnetic field. The color cathode ray tube apparatus according to claim 1, wherein each of the main deflection coil (30, 31) and the correcting coil (33, 34) is formed by saddle winding.

Inventors: KOJIMA TADAHIRO, SANO YUICHI, YOKOTA MASAHIRO, KOJIMA, TADAHIRO, SANO, YUICHI, YOKOTA, MASAHIRO, Kojima, Tadahiro, c/o Intellectual Property Div., Sano, Yuichi, c/o Intellectual Property Div., YOKOTA, MASAHIRO, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1996
Main classification: H01J29
IPC codes: H01J29