Television – Video display – Cathode-ray tube
Reexamination Certificate
1999-07-02
2004-01-20
Lee, Michael H. (Department: 2613)
Television
Video display
Cathode-ray tube
C348S806000, C315S370000, C315S008000
Reexamination Certificate
active
06680757
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique for compensating for the effects of an environmental magnetic field, particularly geomagnetism, in which a device is placed. More particularly, the invention relates to a geomagnetism compensating technique suitable for a high-resolution CRT display device for use as a computer display device.
2. Description of the Background Art
High-resolution CRT display devices for use as, e.g., computer display devices with 17- to 21-inch diagonal screens and with 1280-dot by 1024-line resolution have become predominant. Further, such devices with 22- to 24-inch diagonal screens have been increasingly required to provide a 1600-dot by 1200-line resolution. To meet the requirement for such a resolution, the currently prevailing pitch of phosphors is 0.28 mm. However, there is also a need for high-definition CRTs with a phosphor pitch as fine as 0.25 mm.
Unfortunately, such high-resolution computer CRT display devices are in some cases affected by geomagnetism, e.g. the vertical component thereof, to generate variations in horizontal image position, convergence and beam landing, causing degradation in image quality of the devices. In particular, the above-mentioned high-definition CRTs with fine phosphor pitch exhibit more profound effects of the same amount of horizontal image position variation, the same amount of misconvergence and the same amount of beam mislanding upon the degradation in device image quality than do CRTs with larger phosphor pitches, to show the adverse effects of geomagnetism, e.g. the vertical component thereof, which appear more significantly.
The following techniques have been proposed to prevent variations in horizontal image position, convergence and beam landing under the influence of such magnetism, e.g. the vertical component thereof:
(1) To prepare exposure designs separately for CRTs for use in the Northern Hemisphere and CRTs for use in the Southern Hemisphere since geomagnetism, e.g. the vertical component thereof, differs greatly between the Northern Hemisphere and the Southern Hemisphere on Earth.
(2) To enhance magnetic shielding to reduce the effects of geomagnetism, e.g. the vertical component thereof.
(3) To provide means for correcting the variations in horizontal image position, convergence and beam landing under the influence of geomagnetism, e.g. the vertical component thereof.
The technique (1) which prepares the separate CRT exposure designs increases costs and is therefore impractical. The technique (2) which merely enhances the magnetic shielding is insufficient to solve the problem. Hence, the technique (3) for correcting the horizontal image position variation, misconvergence and beam mislanding by using the correcting means has been examined.
FIG. 25
is a perspective view illustrating the correcting means of a CRT
16
. The CRT
16
comprises a deflection yoke
13
serving as a fundamental part, and a convergence correction coil
14
and a beam landing correction coil
15
which are mounted around the neck thereof. The convergence correction coil
14
and the beam landing correction coil
15
are provided to correct misconvergence and beam mislanding, respectively. Respective correction currents are supplied to the deflection yoke
13
and the correction coils
14
and
15
.
FIG. 26
is a block diagram of a circuit for supplying the correction currents. The circuit of
FIG. 26
comprises a first individual adjustment means
17
which sets standard adjustment values for correction of the horizontal image position, misconvergence and beam mislanding, for example, during the manufacture of a CRT display device, and a second individual adjustment means
18
for correcting the variations in horizontal image position, convergence and beam landing under the influence of geomagnetism, e.g. the vertical component thereof, at the position of installation, for example, when the CRT display device is installed.
Each of the first and second individual adjustment means
17
and
18
supplies three correction current adjustment values to adder-subtracter circuits
19
to
21
, respectively. The adder-subtracter circuits
19
to
21
perform addition and subtraction upon the adjustment values from the first and second individual adjustment means
17
and
18
. The results of addition and subtraction from the adder-subtracter circuits
19
to
21
are supplied to drive circuits
22
to
24
for driving the deflection yoke
13
, the convergence correction coil
14
and the beam landing correction coil
15
, respectively. Thus, the correction currents in accordance with the adjustment values are supplied to the deflection yoke
13
, the convergence correction coil
14
and the beam landing correction coil
15
, respectively.
The geomagnetism-related correction using the circuit shown in
FIG. 26
is required to adjust the second individual adjustment means
18
when the CRT
16
is installed, moved or changed in orientation thereof. However, such an adjustment requires special measuring equipment and expert knowledge for adjusting the horizontal image position, convergence and beam landing while making a measurement on a display screen of the CRT
16
. Therefore, the technique (3) is disadvantageous in that a user that makes the adjustment by himself or herself finds difficulty in accomplishing a successful result and in that the installation or movement of the CRT display device requires a complicated procedure such as the visit of a serviceman having expert knowledge. Another disadvantage is that the above-mentioned correction circuit constructed in hardware form has a very complicated and large-scale circuit configuration.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, an environmental magnetism compensating device comprises: a magnetism sensor for detecting a vertical component of a magnetic environment in which a cathode-ray tube including a deflection yoke, a convergence correction coil and a beam landing correction coil is placed to output a detection signal; an arithmetic unit for determining first to third parameters based on the detection signal; and a driver for supplying current having values set based on the first to third parameters, respectively, to the deflection yoke, the convergence correction coil and the beam landing correction coil.
Preferably, according to a third aspect of the present invention, in the environmental magnetism compensating device of the first aspect, the current supplied to the convergence correction coil varies at two different rates of change for a time period corresponding to one frame in synchronism with a vertical deflection signal for the cathode-ray tube.
Preferably, according to a third aspect of the present invention, in the environmental magnetism compensating device of the first aspect, the current supplied to the convergence correction coil varies at two difference rates of change for a time period corresponding to one frame in synchronism with a vertical deflection signal for the cathode-ray tube.
Preferably, according to a fourth aspect of the present invention, in the environmental magnetism compensating device of the first aspect, the current supplied to the beam landing correction coil is in synchronism with a vertical deflection signal for the cathode-ray tube and has a waveform symmetrical with respect to a midpoint of a time period corresponding to one frame.
Preferably, according to a fifth aspect of the present invention, in the environmental magnetism compensating device of the first aspect, the current supplied to the beam landing correction coil is in synchronism with a vertical deflection signal for the cathode-ray tube and has a waveform asymmetrical with respect to a midpoint of a time period corresponding to one frame.
Preferably, according to a sixth aspect of the present invention, in the environmental magnetism compensating device of the fourth or fifth aspect, the current supplied to the beam landing correction coil has a variable DC level.
According to a s
Lee Michael H.
NEC-Mitsubishi Electric Visual Systems Corporation
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