1. Field of the Invention
The present invention relates to a color cathode ray tube apparatus and, more particularly, to a general high-image quality color cathode ray tube apparatus for an EDTV or HDTV.
2. Description of the Related Art
A general color cathode ray tube apparatus having high image quality comprises a tube provided with a panel, a funnel contiguous with the panel, and a cylindrical neck connected to the funnel. A shadow mask is arranged inside the panel, and a phosphor screen surface comprising a tri-color light emitting layer is formed on the inner surface of the panel to oppose the shadow mask. A large number of apertures are formed in the shadow mask. The shadow mask has a frame on its periphery, and is supported on the panel through the frame. An internal magnetic shield is mounted on the frame. An internal conductive film is coated from the inner wall of the funnel to a portion of the neck. An external conductive film is coated on the outer wall of the funnel, and an anode electrode is provided to a portion of the funnel. An electron gun for outputting three electron beams is accommodated in the neck. A deflection device is arranged outside a boundary portion between a cone portion of the funnel and the neck so as to deflect three electron beams emerging from the electron gun in horizontal and vertical directions. In addition, a driver for applying an appropriate voltage to the electron gun and the anode electrode and supplying a voltage to the deflection device is arranged.
Red, green, and blue phosphor stripes or dots are distributed and coated on the phosphor screen surface. Three electron beams Br, Bg, and Bb emerging from the electron gun toward the phosphor screen surface are deflected by the deflection device. The electron beams Br, Bg, and Bb are selected by the shadow mask, and then become incident on the phosphor screen. Thus, the corresponding phosphors emit light to form an image. In an electron gun having an in-line arrangement, three parallel electron beams are generated. This electron gun has an electron beam forming unit GE for generating, controlling, and accelerating three electron beams, and a main electron lens unit ML for focusing and converging these electron beams.
A deflection yoke as the deflection device has horizontal and vertical deflection coils for deflecting the three electron beams in the horizontal and vertical directions. In the deflection yoke for deflecting in-line aligned electron beams, in order to precisely converge electron beams, a horizontal deflection magnetic field is formed into a pin-cushion pattern, and a vertical deflection magnetic field is formed into a barrel pattern, thus constituting a so-called convergence free system.
In order to correct a coma in convergence, a deflection magnetic field control element formed of a ferromagnetic material is arranged on a portion of the electron gun near the phosphor screen. In a color cathode ray tube apparatus, along with improvements in stress analysis in the tube, and in manufacturing techniques of large-size tubes and explosion proof bands, ultra-large-size tubes having screen diagonal diameters of 30" to 40" have been increasingly popular. Since an ultra-large-size tube inevitably has a large depth, a deflection angle of electron beams is set to be 100.degree. to 110.degree. to shorten the depth as much as possible. In this situation, high-image quality television broadcast systems have been developed. For example, an EDTV (clear vision) or an HDTV (high-vision or high-definition television) requires a high-image quality color cathode ray tube apparatus. That is, the following improvements in quality are required:
(1) a thermal countermeasure against heat generation of a deflection device since a deflection frequency is increased;
(2) an improvement in distortion of a beam spot on a peripheral portion of a screen; and
(3) an improvement in convergence characteristics of three electron beams on a peripheral portion of a screen.
Heat generation of a deflection device used in a conventional broadcast system reaches at most 30.degree. C. or less, and no problem is posed. In an EDTV or HDTV, however, since a horizontal deflection frequency is as high as 30 kHz or higher, losses caused by an eddy current due to a horizontal deflection magnetic field are increased. For this reason, a heat generation amount of the deflection device is considerably increased. Since an anode voltage is increased from 25 to 28 kV (conventional device) to 29 to 34 kV to attain a high luminance, this leads to an increase in deflection current for deflecting electron beams. Therefore the number of heat generation factors of the deflection device is increased. For example, when an anode voltage of 32 kV and a horizontal deflection frequency of 33.8 kHz are applied to a conventional deflection device to perform 110% scan, the temperature of the deflection device is increased beyond 60.degree. C., and the device is burnt.
Since this is a fatal drawback of a color cathode ray tube apparatus, some countermeasures against heat generation of the deflection devices are taken as follows:
(1) to improve a wire material (e.g., use a litz wire);
(2) to reduce a core heat generation amount using a core material having a small loss;
(3) to attain a high heat resistance of a deflection yoke material;
(4) to increase a deflection sensitivity to decrease a deflection current; and
(5) to increase a deflection coil in size to improve heat radiation efficiency.
Although a conventional deflection device has already employed a large deflection coil proposed in item (5), when the size of the deflection coil is increased too much, an average coil diameter of the deflection coil is increased, and a deflection sensitivity is decreased. This cannot attain any improvement. In order to increase the size of the deflection device without increasing the average coil diameter, the deflection coil must be extended toward an electron gun. In a color cathode ray tube apparatus having such an arrangement, electron beams are undesirably deflected by the deflection device to land on the neck. For this reason, a neck shadow phenomenon occurs, i.e., the electron beams cannot reach the phosphor screen. That is, it is difficult to increase the size of the deflection yoke without decreasing its sensitivity. Countermeasures against heat generation of the deflection device can be taken by employing a litz wire, a small-loss core material, and a high-heat resistance material. However, these materials inevitably result in an increase in cost, and such a product is too expensive to be used as a home color cathode ray tube apparatus. Even if these countermeasures are taken, since electron beams are deflected through 110.degree. at a horizontal deflection frequency of 40 kHz or higher in a European HDTV or in a high-definition TV system for computer graphics, a heat generation amount of the deflection device is considerably increased. As described above, heat generation of the deflection device poses an important problem in a color cathode ray tube apparatus for an EDTV or HDTV.
The next problem in a color cathode ray tube apparatus for a high-definition TV system such as an EDTV or HDTV is a decrease in resolution on a peripheral portion of the screen.
This problem is caused by an influence of a deflection magnetic field, and an influence of a difference between distances of electron beam paths on the central and peripheral portions of the screen. Under these influences, the resolution is decreased due to a so-called deflection defocus (i.e., a distorted beam spot) and a convergence offset of three electron beams on the peripheral portion of the screen. Such a decrease in resolution becomes conspicuous with an increase in size or deflection angle of a tube and a decrease in profile of a panel. When such a color cathode ray tube apparatus is applied to a high-definition TV system such as an EDTV or HDTV, the above-mentioned problems become worse.
As a countermeasure against the deflection defocus, a method of improving an electron gun and a method of improving a deflection device are known. Conventionally, an improvement in an electron gun is more effective than an improvement in a deflection device, and e.g., a dynamic focus method is available. In this method, a power of an electron lens of an electron gun is changed in synchronism with a deflection state of electron beams to correct a distortion of a beam spot. With this method, the distortion of the beam spot on the peripheral portion of the screen is remarkably improved. However, another problem is posed. That is, since the power of the electron lens is changed in synchronism with a deflection state of the electron beams, a voltage which is changed over about 1 kV or more in synchronism with the deflection state must be supplied. For this reason, cost of the color cathode ray tube apparatus is considerably increased. In order to correct a spot distortion of electron beams without increasing cost, not the electron gun but the deflection device is improved. In this improvement, a deflection magnetic field is changed to correct the distortion of the beam spot.
A method of controlling a deflection magnetic field will be described below. A beam spot is distorted since a horizontal deflection magnetic field is generated in a pin-cushion pattern and components of the horizontal deflection magnetic field are generated in a direction of the tube axis. The horizontal deflection magnetic field is generated in the pin-cushion pattern to realize a convergence free system of three electron beams. However, tube-axis direction components are generated by the horizontal deflection magnetic field, and they distort the beam spot. Therefore, if the tube-axis direction components of the horizontal deflection magnetic field can be eliminated, a distortion of the beam spot can be eliminated.
A method of controlling the tube-axis components of the deflection magnetic field is disclosed in Published Japanese Patent Application Nos. 59-173934, 60-146432, 61-188841, 61-288353, 63-207035, and the like. These references describe a method of reducing a tube-axis direction magnetic field by specific shapes of a deflection yoke core and coil, and a method of generating a tube-axis direction magnetic field in the opposite direction by an auxiliary coil. The method of reducing a tube-axis direction magnetic field by specific shapes of a deflection yoke core and coil has a small distortion reduction effect of a beam spot, and the method of generating a tube-axis direction magnetic field in the opposite direction by an auxiliary coil poses problems of a decrease in deflection sensitivity and an increase in cost.
When a convergence offset of three electron beams occurs, color mis-registration occurs, and this also causes deteriorated resolution. For this reason, convergence characteristics are considerably improved by optimal design of a deflection magnetic field distribution.
However, although design of the deflection device is optimized, four quadrants of the screen have different convergence characteristics due to variations of deflection coils, tubes of cathode ray tubes, and electron guns in the manufacture. For example, convergence offsets in the same direction may occur in two quadrants of the screen, and convergence offsets in the opposite direction may occur in the remaining two quadrants. In this state, these convergence offsets cannot be corrected by only adjusting the position of the deflection device with reference to the tube, and must be corrected by mounting a ferromagnetic member such as a ferrite member. With this correction method, however, only offsets on the extreme peripheral portion of the screen can be corrected. Since this correction increases a magnetic flux density, some offsets cannot often be corrected depending on a direction of convergence offset.
In a conventional color cathode ray tube apparatus, a deflection magnetic field control element formed of a ferromagnetic member is arranged on a portion of the electron gun near the screen to correct a coma in convergence, and controls so that deflection magnetic fields having different strengths are applied to a central beam and side beams. Thus, the coma in convergence is corrected, and a vertical deflection magnetic field distribution can be simplified to some extent. However, when the horizontal deflection frequency exceeds 30 kHz, the influence of a residual magnetic flux density of the magnetic field control element is increased, and asymmetrical convergence offsets occur on the right and left portions of the screen, resulting in a degraded image.
As can be seen from the above description, in a home color cathode ray tube apparatus applied to a high-definition TV system such as an EDTV or HDTV, the problems in heat generation of a deflection device, deflection defocus characteristics, variation characteristics of convergence, convergence offsets caused by a deflection magnetic field control element, and the like remain unsolved.