Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
2000-03-02
2001-04-03
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C315S001000, C324S404000, C348S379000
Reexamination Certificate
active
06211629
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a cut-off level setting device, a method of setting a cut-off level, and a display apparatus with a cathode ray tube and a cut-off level setting device.
In a cathode ray tube, an electron gun comprises a cathode heated by a heater filament, a first grid, a second grid, and further grids for focussing an electron beam emitted by the cathode. In a color cathode ray tube, three electron guns may be present, each of which generates an electron beam which is deflected to hit the corresponding one of three phosphors which emit light of different colors. The beam current (the number of electrons in the electron beam, also referred to as cathode current) is largely dependent on a voltage difference between the cathode and the first grid. However, the second grid voltage also has an influence on the beam current, but this influence is substantially less pronounced. The beam current may be modulated by varying the cathode voltage in dependence on the video information to be displayed. At a voltage difference of zero volts between the cathode and the first grid, the maximal beam current flows. When the voltage on the cathode becomes more positive with respect to the first grid, the beam current decreases. At a certain value of the cathode voltage the beam current becomes exactly zero. The corresponding voltage difference between the cathode and the first grid is generally referred to as the cut-off voltage or black level voltage. The corresponding voltage level on the cathode is referred to as the cut-off drive voltage level. Varying the second grid voltage changes this cut-off voltage. Consequently, the second grid voltage also determines the maximum beam current which can be generated by the electron gun.
2. Description of the Related Art
International Patent Application WO-A-97/28525, corresponding to U.S. Pat. No. 5,892,337, discloses a black level or cut-off level setting method wherein two control voltage levels are supplied to a cathode in a predetermined ratio to generate two cathode currents. The two cathode currents are measured to compare each one of the cathode currents with an associated one of two reference values having a predetermined ratio. The ratio of the reference values has a predetermined relation to the ratio of the two control voltages.
0
Subtracting the associated reference value from each of the cathode currents generates two error voltages. One of the error voltages is integrated to obtain a black level setting value, the other error voltage is integrated to obtain a gain setting value. The black level setting value controls the DC-level of the control signal to be very near to the cut-off point of the cathode when a black video signal has to be displayed. The error voltages are controlled in a control loop in an iterative way to become zero after some time, such that the desired ratios are reached.
SUMMARY OF THE INVENTION
It is, inter alia, an object of the invention to provide a fast cut-off level setting. To this end, a first aspect of the invention provides a cut-off level setting device comprising drive means for successively supplying a first and a second video drive voltage level to a control electrode of a cathode ray tube to generate a first and a second beam current, respectively, and for supplying a first and a second measurement signal representing the first and second beam currents, respectively, and control means for supplying a control signal to the drive means to set a cut-off drive voltage level at which beam current is just about to flow in the cathode ray tube, characterized in that the control means comprises calculation means for calculating the cut-off drive voltage level from a mathematical formula defining the cut-off drive voltage level as a function of the first and second drive voltage levels and the first and second measurement signals. A second aspect of the invention provides a method of setting a cut-off level. A third aspect of the invention provides a display apparatus with a cathode ray tube and a cut-off level setting device.
The cut-off level setting device in accordance with the first aspect of the invention comprises a drive circuit and a control circuit. The drive circuit successively generates a first and a second drive voltage level (the control voltage level of the prior art) to obtain, respectively, a first and a second cathode current (further referred to as beam current) in the cathode ray tube. Moreover, the drive circuit measures the first and second beam currents. The invention is based on the insight that it possible to calculate the cut-off drive voltage level directly. The calculation circuit calculates the cut-off drive voltage level in one step from a mathematical formula defining the cut-off drive voltage level as a function of the first and second drive voltage levels and the first and second beam currents. Consequently, the circuit in accordance with the invention does not need to perform the time consuming iterative process of the prior art circuit, and the prior art integrator is superfluous.
In an embodiment of the invention, the cut-off voltage drive level is calculated using a formula which is a good representation of the reality in current cathode ray tubes. Further, in a more sophisticated embodiment it is possible to calculate the cut-off drive voltage from a mathematical formula defining the cut-off drive voltage level as a function of more than two drive voltage levels and associated measurement signals. Also other parameters may be included in the calculation, such as the value of the voltage on the second grid.
In another embodiment of the invention, it is possible to select a desired ratio of the first and the second measurement values. This is relevant when a highly accurate measurement has to be performed. For example, when an analog-to-digital converter is used, the values of the measurement signals should optimally fit within the dynamic range of the analog-to-digital converter.
In another embodiment of the invention, it is possible to obtain a desired ratio of the first and the second measurement values such that the term &ggr;{square root over (Im
2
/Im
1
)} is an integer and the calculation of the cut-off drive voltage level is simple.
In another embodiment of the invention, the measurement values are corrected for the off-set current to better represent the corresponding beam currents. The off-set current is caused by the leakage current of the electron gun and by the drive circuit.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 4370674 (1983-01-01), Johnson et al.
patent: 5872432 (1999-02-01), Beeteson
patent: 5892337 (1999-04-01), Van Den Broeke
patent: 5994841 (1999-11-01), Allen et al.
patent: 0840272A3 (1998-05-01), None
patent: 0840272A2 (1998-05-01), None
patent: 9728525 (1997-08-01), None
Moors John H. P.
Teuling Dirk J. A.
Clinger James
Goodman Edward W.
U.S. Philips Corporation
Wong Don
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