Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
2001-07-11
2004-02-03
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C313S421000
Reexamination Certificate
active
06686708
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a color display device comprising a cathode ray tube, means for generating at least one electron beam, a display screen, and a deflection unit for generating deflection fields for deflecting electron beam(s) across the display screen, and magnetic field-generating means at or near a display screen-facing end of the deflection unit to reduce raster distortions.
U.S. Pat. No. 4,746,837 discloses a color display device having a deflection unit, in which a number of pole shoes are arranged around the deflection unit and at the side of the deflection unit facing the display screen. A pincushion-shaped distortion of the deflection field is formed between the pole shoes. Said pincushion distortion provides for a raster correction.
Although the known devices and similar devices in which magnetic correction fields are provided substantially reduce raster errors especially in the corners of the display screen, remaining raster errors are still noticeable, especially at 0.5 N and 0.5 S of the display screen.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a display device having a deflection unit in which improved raster corrections are obtainable.
To this end, in accordance with an aspect of the invention, the display device according to the invention is characterized in that the magnetic field-generating means comprise correction coils and means for supplying said correction coils, in operation, with a correction current, the ratio between the correction current and the vertical deflection current being a function of the vertical deflection and being less for full vertical deflection than for half vertical deflection.
The invention is based on the recognition that, when a correction field, for which the strength is proportional to the strength of the vertical deflection field, is generated independently of the vertical direction (as is the case in the prior art because the poles divert the flux of the vertical deflection coils and thus the strength of the correction field is directly proportional to the vertical deflection field), it is possible to correct for raster distortions at the top and bottom (North and South) of the display screen, but raster corrections at positions between the bottom and top of the display screen and the East-West axis of the display screen are less than optimally performed. In the display device and deflection unit in accordance with the invention, the ratio of the current through the correction coils and the current through the vertical deflection coils is less for full vertical deflection than for half vertical deflection.
This enables the correction coils to better influence the inner pincushion errors (i.e. around 0.5 North and South vertical deflection) as well as the geometry along the edges. An improved raster correction is thereby obtainable.
Preferably, said ratio of the currents at half the vertical deflection is between 1.5 and 2.5 times larger than that at full vertical deflection.
Preferably, the delay time between the current through the correction coils and the current through the vertical deflection coils after fly-back is less than 400 &mgr;sec, preferably less than 300 &mgr;sec. A larger delay time introduces raster distortions in North or South.
REFERENCES:
patent: 4746837 (1988-05-01), Vink
patent: 5446339 (1995-08-01), Kinoshita et al.
patent: 6326742 (2001-12-01), Iwasaki et al.
patent: 6384546 (2002-05-01), Nakajima
patent: 2-220336 (1990-09-01), None
patent: 7-212773 (1995-08-01), None
patent: 7-212778 (1995-08-01), None
Copy of International Search Report dated Oct. 31, 2001 for corresponding PCT application, PCT/EP01/07405.
Koninklijke Philips Electronics , N.V.
Vu Jimmy T.
Wong Don
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