Title of Invention

A BEAM CURRENT CONTROL CIRCUIT, A METHOD OF CONTROLLING THE SAME AND A DISPLAY APPARATUS

Abstract

The present invention relates to a beam current control circuit for a cathode ray tube (CRT). The beam current control circuit comprises a comparing circuit (COM, CS) for comparing a signal (VB). Signal (VB) is related to a beam current generated in the cathode ray tube (CRT). In response to video drive signals (Ro, Go, Bo) with a reference value (VREF) to supply a control signal (II) for decreasing an amplitude of the video drive signals (Ro, Go, Bo). If the signal (VB) related to the beam current crosses the reference value (VREF), the beam current control circuit comprises a feedback circuit (FB). This feedback circuit (FB) modulates the reference value (VREF) to vary in response to the control signal, the reference value (VREF) being modulated for counteracting a decrease of a difference between the signal (VB) related to the beam current and the reference value (VREF).

Full Text

The invention relates to a beam current control circuit for a cathode ray tube as defined in the precharacterizing part of claim 1, and to a display apparatus comprising a cathode ray rube and such a beam current control circuit. The invention further relates to a method of controlling beam current as defined in the precharactexizing part of claim 4. processing circuit which receives an input video signal and supplies video drive signals to corresponding beam current control electrodes of the CRT. The beam current limiter comprises a control circuit responsive to a sensed signal representative of the combined mstantaneous magnitudes of the video drive signals. The control circuit provides a control signal to the video processing circuit for controlling the amplitudes of the video drive signals. The control circuit includes, in the order named, a first low pass filter, a first peak detector, a second low pass filter, and a second peak detector. The first low pass filter filters the sensed signal to reduce the sensitivity of the control circuit to narrow video signal peaks such as occurring in a cross-hatch partem. The second peak detector comprises a threshold detector which receives an input signal which is a processed sensed signal "When the input signal of the threshold detector crosses a reference voltage, the control signal decreases the gain of the video processing circuit to decrease the amplitude of the video drive signals. In this way, the beam current limiter is able to prevent warping or blistering of a shadow, mask of the CRT due to scenes having a high beam current in, for example, an area equal to or greater than five to ten percent of the display screen area of the CRT. is likely to occur the gain of the video processor circuit has to be lowered fast enough to prevent the mask to be heated for such a long time that the warping would become visible- It is a drawback of the prior art that it is difficult to design a control circuit which shows the desired performance and operates in a stable way without oscillatory behavior. It is, inter alia, an object of the invention to provide a fast beam current control circuit that is less sensitive to oscillations, To this end, a first aspect of the invention provides a beam current control circuit for a cathode ray tube as claimed in claim 1. A second aspect of the invention provides a display apparatus as claimed in claim 2. A third aspect of the invention provides a method of controlling beam current as claimed in claim 4. Advantageous embodiments are defined in the dependent claims. The beam current control circuit in accordance with the invention comprises a comparator (or a threshold detector) for comparing a signal related to the beam current with a reference level to generate a control signal. The control signal controls the gain of a video processing circuit which supplies the video drive signals to the CRT. The gain is lowered when the signal related to the beam current crosses the reference level, thereby indicating that warping of the mask of the CRT is likely to occur. The beam current control circuit in accordance with the invention further comprises a feedback circuit to modulate the reference level to vary in response to the control signal When the gain is lowered, the reference level has to be changed such that the indication that warping of the mask of the CRT is likely to occur is generated at a lower beam, current. Or in other words, the reference level is varied in a direction to counteract the decreasing difference betv,-een the signal related to the beam current and the reference level. For the sake of clarity, the operation of the beam current control circuit is elucidated with respect to the next example, When the signal related to the beam current increases with increasing beam current, and a decreasing control signal causes a decrease of the amplitude of the video drive signals, the reference level should be lowered when the control signal decreases. First, lei it be assumed that the reference level is not varied. when the signal related to the beam current crosses the reference level a fast reduction of the gain lowers the signals. Consequently, the level of the signal related :o the beam current decreases rapid!}- below the fixed reference level. Now, the beam current control circuit no longer detects a situation in which warping of the mask is likely to occur and the gain is increased. But, because the video signal is still the same, the larger gain causes the signal related to the beam current to cross the reference level again, and a fast reduction of the gain • follows, and so on. This oscillating behavior of the beam control circuit causes a repetitive modulation of the contrast of the displayed picture which is very annoying. Still with respect to the example, the reduction of the reference level in response to the control signal prevents that the decreasing level of the signal related to the beam current immediately drops below the reference level, and .consequently the gain is not increased too soon. A smooth convergence is obtained to a gain appropriate for the video signal applied. The signal related to the beam current may be obtained in many ways. For example, each one of the cathode currents may be measured as a collector current of a transistor arranged in series with the cathode, as is commonly known. It is also possible to use the RGB output signals supplied to the video output amplifiers, or the luminance signal. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. The sole drawing shows a circuit diagram of a display apparatus with a beam current control circuit in accordance with the invention. The display apparatus comprises a video processing circuit VP, to process an mput video signal Vi to obtain RGB output signals Ro, Go, Bo. The input video signal Vi may be a CVBS signal originating from a television tuner. This signal is processed in a signal processor SP to obtain input RGB signals Ri. Gi, Bi. Also, the input video signal Vi may be in the form of RGB signals as originating from a computer, in this case the signal processor SP is not required. The inner RGB signals R1. G1. Bi are sunnlied to resnective sain controllable Ro, Go, Bo to respective control electrodes of the cathode ray tube CRT via corresponding ourput amplifiers OAl, 0A2, 0A3. The video processing circuit VP comprises a control input CI which controls a gam of the controllable amplifiers Al, A2, A3. A capacitor Cl is connected lo this control input CI. The voltage on the control input CI may be controlled by The user to adjust the contrast of information displayed on the screen of the cathode ray tube CRT. Further an average and peak white beam current limiter may be connected to the control input CL The average beam current limiter limits the long term load of the cathode ray tube CRT to avoid an unacceptable shortening of the life time of the cathode ray tube CRT. The peak white current limiter limits the beam current during very small bright spots to avoid blooming of the spot. The beam current control circuit in accordance with the invention may act on this control input CI, also. The beam current control circuit in accordance with the invention comprises an adding circuit AS, a low pass filter LPl, a comparator COM, a control circuit CS and a feedback circuit FB. In a preferred embodiment, the adding circuit AS comprises a npn transistor Tl with a base receiving the output RGB signal Bo via a resistor Rl, a collector connected to a supply voltage Vb, and an emitter connected via a resistor R2 to ground and via a resistor R3 to a junction Pi. A npn transistor T2 has a base receiving the output RGB signal Go via a resistor R4, a collector connected to the supply voltage Vp and an emitter connected via a resistor R5 to ground and via a resistor R6 to the junction PL A npn transistor T3 has a base receiving the output RGB signal Ro via a resistor R7, a collector connected to the supply voltage Vp, and an emitter connected via a resistor R8 to ground and via a resistor R9 to the junction P1. VA denotes the voltage on the junction PI, The output RGB signals Ro, Go, Bo drive the cathodes of the cathode ray tube CRT via the output amplifiers OAl, 0.A2, 0A3 and thus determine the beam current in the cathode ray tube CRT. The output RGB signals Ro, Go, Bo are an accurate representation of the beam current if the display apparatus is provided with a known black current stabilization loop (not shown). Consequently, the adding circuit AS generates an output current which is a representation of the total beam current in the cathode ray tube CRT, The adding circuit AS is not essential to the invention, many other ways are known to obtain a current which is a representation of the beam current in the cathode ray tube CRT. For example, the current in the cathodes may be measured directly at the cathodes, or, less preferably, the input video signal Vi may be used. The low pass filter LPl low pass filters the output current of the adding circuit AS to obtain a voltage VB, The low pass filter LPl may comprise a resistor P,lOO and a capacitor C2 which are arranged in series between the junction PI and ground. The voltage VB occurs across the capacitor C2. The resistor PvlOO may be omitted, a, the resistors R3, R6 and R9, may as well determine the time constant of the low pass filter LPl. The time constant 01 thel ow pass filter LP 1 has to be selected such that high beam currents which have a short duration only (for example as occurring during a crcss-hatch pattern, or when characters of a sub-title or karaoke aredisplayed) are smoothed and will not be detected by the comparator circuit COM. ' It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the an will be able to design many alternative embodiments without departing from the scope of the appended claims. For example, the embodiment of the beam current control circuit as shown in Fig. 1 can easily be adapted by a skilled parson such that levels vary in other directions. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. 1. A beam current control circuit for a cathode ray tube (CRT)J the beam current control circuit comprises a comparing circuit (COM, CS) for comparing a signal (VB) related generating a control voltage across a capacitor CI, the feedback circuit comprising a diode arranged between said output and the capacitor (CI), A method of controlling a beam current of a cathode ray tube (CRT), the method comprising the steps of: supplying (VT) video drive signals (Ro, Go, Bo) causing a beam current in the cathode ray tube (CRT), and comparing (COM) a signal (VB) related to the beam current with a reference value (VREF) to supply (CS) a control signal (II) to the video drive circuit (VP) to decrease an amplined of the video drive signals (Ro, Go, Bo) if the signal (VB) related to the beam current crosses the reference value (VrEF), characterised in that the method further comprises a feedback step (FB) of modulating, in response to a signal related to the control signal (II), the reference value (vREF) in a direction to counteract the decreasing difference between on the one hand the signal (VB) related to the beam current and on the other hand the reference value (VReF). 5. A beam current control circuit, substantially as hereinabove described and illustrated with reference to the accompanying drawings.

Documents:

024-mas-2000-abstract.pdf

024-mas-2000-claims filed.pdf

024-mas-2000-claims grand.pdf

024-mas-2000-correspondnece-others.pdf

024-mas-2000-correspondnece-po.pdf

024-mas-2000-description(complete) filed.pdf

024-mas-2000-description(complete) grand.pdf

024-mas-2000-drawings.pdf

024-mas-2000-form 1.pdf

024-mas-2000-form 19.pdf

024-mas-2000-form 26.pdf

024-mas-2000-form 3.pdf

024-mas-2000-form 5.pdf

024-mas-2000-other document.pdf