Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
1999-06-28
2001-09-11
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C315S408000, C315S291000, C348S379000
Reexamination Certificate
active
06288503
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the driving of picture tubes, in particular of colour picture tubes used in television receivers or monitors.
The colour picture tubes used in contemporary colour television sets are predominantly controlled via the cathode of the respective beam current system, the maximum possible luminous efficiency being determined by the respectively available modulation range. The control electrodes designed as a Wehnelt electrode are usually connected to reference-earth potential (earth). In order to obtain a high luminous efficiency, variables which limit the modulation range, such as e.g. maximum supply voltage of the RGB output stages, saturation of the RGB output stages, cut-off regulating range, must be carefully taken into account during the circuit design.
For the colourless reproduction of black and white pictures, it is customary in contemporary colour television sets to provide a cut-off regulation for each colour channel. In this case, a so-called cut-off measurement line is respectively keyed in during a vertical blanking interval, to which line a reference cathode current I
k1
of e.g. 10 &mgr;A corresponds. Cut-off regulation of this type is known e.g. from the video processor circuit TDA4580 from Valvo.
Tubes go through various phases during their ageing process. At the beginning, roughening and, as it were, blooming of the cathode surfaces frequently occur.
These reduce the effective spacing between cathode and Wehnelt electrode. If the operating voltages are kept constant, the field strength between the electrodes therefore increases and, consequenty, the cathode current decreases. With increasing age, more and more cathode material is eroded, with the result that the effect is reversed. The cut-off regulation reacts to the rise in the field strength described first in a manner such that the reverse voltage acting between cathode and Wehnelt electrode is correspondingly reduced in order to be able to drive the same reference cathode current I
k1
. However, the reaction of the cut-off regulation must be taken into account for the circuit design since this results in a reduction in the modulation range, which, in turn, has an adverse effect on the maximum luminous efficiency that can be achieved.
SUMMARY OF THE INVENTION
The object of the invention, therefore, is to eliminate in particular such ageing-related impairments of a picture tube and also other changes which might reduce the modulation range by way of the cut-off regulation.
This object is achieved by means of the invention specified in claim
1
.
The invention is based on the idea of modifying the cut-off regulation in such a way that an ageing-related change in the field strength between the cathode and the Wehnelt electrode of the respective beam system of a colour picture tube is automatically compensated for. According to the invention, the decreasing of the cathodes below a defined potential is prevented by shifting the potential of the Wehnelt electrodes oppositely with regard to the cathodes by regulation by means of a DC voltage offset. Consequently, it is ensured that the quasi cut-off—that is the voltage potential of the measurement line at the “highest” cathode, which potential is calibrated with the aid of the screen grid voltage calibration to a voltage defined during the circuit design—which is usually calibrated in the apparatus manufacturing process is not undershot by any of the three beam systems present in a colour picture tube. It is essential that differences between the systems are still corrected by means of the cathodes.
The same principle according to the invention can be applied in order to prevent the cathodes from rising above a defined potential.
Since ageing-related tolerances are no longer corrected exclusively by means of the picture tube cathodes, which are driven by amplifier stages having a limited modulation range (between reference-earth potential and positive operating voltage V
cc
), the invention has the advantage that the respective available modulation range for the useful signal becomes larger or is maintained and can thus be better utilized.
Furthermore, the invention has the advantage that, in the event of a repair which might influence the cut-off loop (e.g. picture tube, video processor, &mgr;p or memory), the apparatus can be calibrated in a conventional manner by the Wehnelt electrodes simply being earthed and, after correct calibration, the measurement loop can be reinitialized.
REFERENCES:
patent: 3555348 (1971-01-01), Van Clifton Martin
patent: 4160995 (1979-07-01), Jensen
patent: 4438366 (1984-03-01), Kamata
patent: 5130615 (1992-07-01), George
patent: 5894327 (1999-04-01), Griepentrog
patent: 2702817A1 (1977-09-01), None
patent: 1575604 (1980-09-01), None
Patent Abstracts of Japan, Pub. No. 01-103091 [JP1103091], pub. Apr. 20, 1989.
Patent Abstracts of Japan, Pub. No. 56-112184 [JP 56112184 A], pub. Sep. 4, 1981.
Patent Abstracts of Japan, Pub. No. 06-292223 [JP 6292223 A], pub. Oct. 18, 1994.
Hartmut Harlos, “Integrated TV circuits including cut-off point control”, Funk-Technik 38, 1983, Issue.1, pp. 25-29. (translation included).
*corresponds to DE2702827A1, cited under reference AM.
Deutsche Thomas-Brandt GmbH
Dinh Trinh Do
Kiel Paul P.
Tripoli Joseph S.
Wong Don
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