Television – Cathode-ray tube display beam current control
Reexamination Certificate
1998-12-30
2001-09-04
Eisenzopf, Reinhard J. (Department: 2614)
Television
Cathode-ray tube display beam current control
C348S379000, C348S377000, C315S382100, C315S383000
Reexamination Certificate
active
06285401
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to television display systems and particularly to an apparatus for suppressing overshoots of kinescope be am current measurement pulses.
BACKGROUND OF THE INVENTION
Display systems for digital television receivers, including high definition television receivers, and computer or multi-media monitors may have one or more modes with horizontal scanning rates higher than (e.g., 2.14 times) the conventional 1H scanning rate in accordance with the conventional NTSC, PAL and SECAM video transmission standards. In such higher than normal scanning rate monitor/receiver display systems, an automatic kinescope bias (AKB) arrangement has a shorter cathode cut-off current measurement time period compared with that of display systems operating in accordance with the conventional standards. It is herein recognized that prior AKB arrangements may not operate properly in higher than normal scanning rate display systems for the following reasons.
AKB systems employ feedback to regulate the beam cut-off level of kinescopes. Typically, an AKB system generates beam cut-off test pulses that are applied to a kinescope cathode. One horizontal line (typically a line of the vertical interval) is used to measure the beam cut-off current of each gun of the color kinescope (or CRT). A cathode current sensor detects the actual beam cut-off current produced in response to the test pulse and produces a voltage pulse across a load resistance. The measurement voltage pulse across the load resistance is compared to a reference level during the measurement time period. The bias voltage for each CRT gun is then adjusted by the system to correct for errors in cut-off bias voltages.
The process of turning on and turning off each gun causes the kinescope capacitance to be charged and discharged, The result is that the red, green and blue AKB measurement pulses may exhibit overshoots on each transition that may interfere with accurate measurement of the pulse amplitudes and therefore interfere with the adjustment of the cut-off bias voltages.
The presence of such overshoots for normal 1H operation (i.e., standard horizontal scan rates) is not a significant problem because the AKB pulse measurement period can be relatively long compared to the width of the overshoots and because a relatively large capacitor may be placed across the load resistance because of the slower scanning rate. However, the AKB measurement period at higher than normal 1H scanning rates has a relatively limited duration. It has been found that using relatively large capacitor across the load resistance will tend to reduce the amplitude of the shoots but will also cause undesirable tilt or slope on each pulse. It is herein recognized that such pulse distortions (tilt or slope) can result in AKB pulse measurements errors in such higher than normal scan rate systems. More specifically, a non-constant amplitude of the pulse, as it is being compared to a reference, can cause the AKB system to change bias during the AKB period and to not adjust (or “servo”) to a consistent level. Additionally, there may be color temperature errors or inconsistencies between the various scanning modes.
SUMMARY OF THE INVENTION
The present invention resides, in part, in the recognition of the heretofore unrecognized problem (discussed above) regarding the effect of high speed scanning on kinescope cathode current measurement pulses.
The present invention is directed to meeting the need for pulse correction apparatus in a display system for removing overshoots of kinescope beam current measurement pulses without significantly affecting the overall pulse amplitude.
Advantageously, the measurement pulse correction circuit of the present invention removes excessive cathode current measurement pulse overshoots (in either direction) without significantly affecting pulse amplitude. In a specific example of the invention herein described, a cathode current measurement pulse correction circuit has been added to an exemplary AKB arrangement. The correction circuit senses the overshoot and switches in a capacitor to suppress the shoot during the shoot time. The capacitor is switched out during the remainder of the pulse measurement time period. The AKB measurement pulses are not significantly modified except during the shoot time. It will be noted that a conventional level sensitive clipping circuit can not be used for this purpose because the level of each pulse can change, both with component tolerances and with color temperature adjustment. That is, the correct level of each pulse can not be predicted.
In addition to providing current measurement pulse correction in AKB systems, the invention also has use in other applications. As discussed later, for example, the pulse correction circuit of the present invention may be used in conjunction with an automatic white level or gain (drive) arrangement since automatic gain adjustment arrangements also measure cathode currents which are generated in response to test voltage pulses during an operation similar to that of AKB operation.
Pulse correction apparatus in accordance with the invention comprises cathode current sensing means (
18
) coupled to a cathode (K
1
) of a kinescope (
24
) and having an output (
28
) for providing a beam current measurement pulse (RP) during a beam current measurement interval and a load circuit (
200
) responsive to the measurement pulse for producing an output voltage pulse (RP) which may tend to exhibit an overshoot. Such overshoots are corrected by a pulse correction circuit (
300
) comprising a capacitor (Ca) coupled from the load circuit (
200
) to a source of reference potential (Gr) via a switch (Q
300
); and a control circuit (Ca, Ra, Rb), responsive to the presence of the overshoot, for closing the switch for a predetermined length of time and for opening the switch otherwise.
Advantageously, in exemplary applications of the invention herein described, the switch comprises a transistor of either the bipolar or field effect type providing the dual functions of both pulse suppression and providing threshold detection for the control circuit.
In a first illustrative embodiment of the invention, the switch comprises a bipolar transistor (Q
300
) having an emitter (E) coupled to the source of reference potential (Gr) and having a collector (C) AC coupled to the load circuit (
200
) via said first capacitor (Ca); and the control circuit comprises a second capacitor for AC coupling the load circuit to a base (B) of the bipolar transistor and a resistor (Ra) for DC coupling the base (B) to the source of reference potential (Gr).
In a further illustrative embodiment of the invention, the switch comprises a field effect transistor (Q
300
A) having a source (S) coupled to the source of reference potential (Gr) and having a drain (D) AC coupled to the load circuit (
200
) via the first capacitor (Ca); and the control circuit comprises a second capacitor (Cb) for AC coupling the load circuit to a gate of the field effect transistor (
300
A), a resistor (Ra) for DC coupling the gate (G) to the source of reference potential (Gr), and a threshold device (D
1
) coupled between the gate (G) and the drain (D) of the field effect transistor.
REFERENCES:
patent: 4017681 (1977-04-01), Smeulers et al.
patent: 4536800 (1985-08-01), Parker
patent: 4549203 (1985-10-01), Shanley, II
patent: 4591912 (1986-05-01), Belotserkovsky et al.
patent: 4694350 (1987-09-01), Hinn
patent: 5177413 (1993-01-01), Wilber
patent: 5680173 (1997-10-01), White et al.
Eisenzopf Reinhard J.
Kiel Paul P.
Thomson Licensing S.A.
Tran Trang U.
Tripoli Joseph S.
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