SHADOWMASKLESS TRACKING CATHODE RAY TUBE CONTROL CIRCUIT HAVING INVERSE BEAM CURRENT COMPENSATION

Abstract: A shadowmaskless tracking cathode ray tube control circuit is described, comprising a control loop, which is coupled to a deflection unit of a shadowmaskless tracking cathode ray tube. The shadowmaskless tracking cathode ray tube control circuit further comprises a loop gain control means for compensating the loop gain of the control loop in dependence on the inverse value of the beam current in the shadowmaskless tracking cathode ray tube. The effect is that a constant loop gain is created, which is independent of the beam current.

Description: Shadowmaskless tracking cathode ray tube control circuit having inverse beam current compensation The present invention relates to a shadowmaskless tracking cathode ray tube control circuit comprising a control loop having a loop gain control means, which control loop is coupled to the shadowmaskless tracking cathode ray tube Such a shadowmaskless tracking cathode ray tube control circuit is known from GB-A-1 403 061 Disclosed therein is a shadowmaskless tracking cathode ray tube (CRT, also referred to as index tube) wherein a feedback control loop controls the exact landing spot of electron beams on a front plate thereof The position of the spot is measured by using a conducting track structure or a sensor positioned on the inner side of the front plate When a spot is scanned along the tracks, a tracking signal which depends on the spot position is produced, which tracking signal can be used to generate, in a loop, a spot position output signal for a deflection unit on the shadowmaskless tracking CRT, which keeps the spot on track It has been found that the accuracy of the measurement is proportional to the loop gain and that its stability is inversely proportional to the loop gain The loop gain depends on the beam current This means that the loop gain has to be chosen in such a way that the stability will be sufficient in light areas on the screen This is, however, disadvantageous for the accuracy in darker areas on the screen Therefore, is an object of the piesent invention to provide a contiol loop circuit having an enhanced loop stability/ ccuracy balance under vaπous light and dai circumstances on the CRT screen To this end, the shadowmaskless tiack g cathode ray tube control circuit according to the invention is characteπzed in that the loop gain control means is arranged to compensate the loop gain of the control loop in dependence on the inverse value of a beam current in the cathode ray tube A compensation as thus proposed in the shadowmaskless tracking CRT circuit according to the present invention advantageously leveals a constant loop gain, which is independent of the beam current. The loop gain may at wish be optimized for improved stability and/or improved accuracy of the shadowmaskless tracking cathode ray tube control circuit according to the invention. An embodiment of the shadowmaskless tracking CRT circuit according to the invention, which is easy to implement is characterized in that the loop gain control means comprises an arithmetic means. A further embodiment of the CRT circuit according to the invention is characterized in that the arithmetic means comprises a multiplier. The multiplier thus multiplies the tracking signal by the inverse value of the beam modulation signal, which beam modulation signal is the video signal. A still further embodiment of the CRT circuit according to the invention is characterized in that the arithmetic means comprises a normalizer unit. Contrary to the previously mentioned further embodiment, this still further embodiment does not use the video signal, which comes from outside the loop, inside the loop. Instead, it arithmetically manipulates its signals, in particular current signals in order to calculate an inverse beam modulation signal to reveal the beam modulation independent loop gain. Advantageously, the arithmetic calculations do not involve time and frequency- dependent features of said signals. Another embodiment of the shadowmaskless tracking CRT control circuit is characterized in that the normalizer unit comprises an adder, a subtracter and/or a divider. These are components, which are easy to implement. The present invention also relates to an image device, such as a monitor or television device comprising a shadowmaskless tracking cathode ray tube control circuit as defined in claims 1-5, in which the shadowmaskless tracking cathode ray tube control circuit comprises a control loop having a loop gain control means, which control loop is coupled to a shadowmaskless tracking cathode ray tube, said device being characterized in that the loop gain control means is arranged to compensate the loop gain of the control loop in dependence on the inverse value of a beam current in the shadowmaskless tracking cathode ray tube. In addition, th i present invention relates to a method of controlling the spot position in a shadowmaskles tracking cathode ray tube, which method is characteπzed in that the spot position is controlled in such a way that the control takes the inverse value of the beam current in the shadowmaskless tracking cathode ray tube into account. The shadowmaskless tracking CRT control circuit, the image device, such as a monitor or television device, and the method according to the invention will be elucidated, while reference is made to the appended drawing, wherein similar components are denoted by the same reference numerals. In the drawing: Fig. 1 shows schematically a part of an image device provided with a shadowmaskless tracking cathode ray tube, and a shadowmaskless tracking cathode ray tube control circuit; and Fig. 2 shows schematically a shadowmaskless tracking cathode ray tube control diagram for implementing the method according to the present invention for use in the image device of Fig. 1. Fig. 1 shows a diagram of a part of an image device D provided with a shadowmaskless trackmg cathode ray tube, hereinafter referred to as CRT 1. The device D is provided with a coupling circuit 2 and a shadowmaskless tracking CRT control circuit 3 connected to the coupling circuit 2. The coupling circuit 2, which will not be further elucidated, convers a tracking signal T from the CRT 1 to the CRT control circuit 3. The CRT 1 is of a type which has no shadow mask, wherein the position of electron beams is measured by means of a conducting tracking or sensor structure S inside the CR 1 When a spot is scanned along the tracks of the structure S, the tracking signal is generated and used in the shadowmaskless tracking CRT control circuit 3 of Fig. 2 for spot position correction purposes. Fig. 2 shows a schematic control diagram of the shadowmaskless tracking CRT control circuit 3 as is usual in the control theory for modelling the control process. Such a CRT control circuit 3 comprises a comparator 4 having a positive input 5 for an external desired spot position signal DV, a negative input 6 for a measured spot position signal and an error output 7 for generating an error signal ε which equals the desired signal minus the measured or actual signal. The error signal is led to a deflection driver 8 for dπving the deflection coils 9 around the neck of the CRT 1 so as to control the spot position on the CRT 1. The tracking sensor signal T which is denved from the tracking structure S compπses spot position information. However, the spot position information is also modulated by the beam current BC, which modulatior is schematically shown in the control diagram of Fig. 2 by means of a modulator symbol M. As a consequence of this modulation, the loop gain for dark areas on the screen of the CRT 1 is lower than for brighter areas. Generally, a high loop gain is desired because of a corresponding high accuracy, viz. small error signal ε of the control loop. However, the control circuit will have a poor accuracy at low beam currents. Therefore, the loop gain control means 10 are included in the shadowmaskless tracking CRT control circuit 3 and are coupled between the negative input 6 and the tracking sensor S. In this way, the measured spot position signal T is led through the means 10, which compensates the loop gain of the control loop in dependence on the inverse value of the beam current in the shadowmaskless tracking cathode ray tube 1. The beam current in fact forms the video information in the scene on the CRT 1. The favourable total effect is that the loop gain no longer depends on the beam current. Thus, an improved loop stability and/or loop accuracy balance can be chosen now. The loop gain control means 10 may be embodied as an arithmetic means such as a multiplier or a normalizer. The tracking or sensor signal T can be represented in differential form by two currents: i , = 1^ (1/ 2 + ^) i 2 = I beπm (l / 2 - δ) (1) where Ibeam is a measure of the beam current and δ is a measure of the spot position (δ = 0 means the spot is exactly on track). To construct the error signal ε, only δ is of interest. It can easily be derived that: / ' , - i 2 = 2 I beam δ (2) and i , - i , = (i, + 2 )2δ (3) Both of the above formulas (2) and (3) yield the same result namely 2δ after dividing I beam and (ii + i 2 ) respectively. (2) can be realized by the multiplier, which multiplies ii - i 2 by the inverse value of I bea m- (3) can be realized by an analog adder for ij + i 2 , a subtracter and a divider for dividing ii - i by ii + i 2 . It is an advantage that the arithmetic calculations do not take time and frequency dependencies into account. Calculations based on formula (3) are normalizing calculations and only use tracking currents and no signal from another place in the image device, such as monitor or television device D.

Claims: CLAIMS: 1. A shadowmaskless tracking cathode ray tube control circuit (3) compnsmg a control loop having a loop gain control means (10), which control loop is coupled to the shadowmaskless tracking cathode ray tube (1), characteπzed in that the loop gain control means (10) is arranged to compensate a loop gain of the control loop in dependence on the inverse value of a beam current (I beam ) in the shadowmaskless tracking cathode ray tube (1). 2. A shadowmaskless tracking cathode ray tube control circuit (3) as claimed in claim 1, characteπzed in that the loop gain control means (10) compπses an aπthmetic means (10). 3. A shadowmaskless tracking cathode ray tube control circuit (3) as claimed in claim 2, characteπzed in that the anthmetic means (10) compπses a multiplier (10). 4. A shadowmaskless tracking cathode ray tube control circuit (3) as claimed in claim 2, characteπzed in that the aπthmetic means (10) compπses a normalizer unit (10) 5. A shadowmaskless tracking cathode ray tube control circuit (3) as claimed in claim 4, characteπzed in that the normalizer unit (10) compπses an adder, a subtracter and/or a divider. 6. An image device (D), such as a monitoi or television device, compnsmg a shadowmaskless tracking cathode ray tube control circuit (3), in which the shadowmaskless tracking cathode ray tube control circuit (3) compπses a control loop having a loop gain control means (10), which control loop is coupled to a shadowmaskless tracking cathode ray tube (1), characteπzed in that the loop gam control means (10) is arranged to compensate the loop gain of the control loop in dependence on the inverse value of a beam current (I be am) m the shadowmaskless tracking cathode ray tube (1) 7. A method of controlling the spot position in a shadowmaskless tracking cathode ray tube (1), characterized in that the spot position is controlled in such a way that the control takes the inverse value of the beam current (I beam ) in the shadowmaskless tracking cathode ray tube (1) into account.

Inventors: VAN LIESHOUT PETRUS J G, VAN LIESHOUT, PETRUS, J., G.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H04N9
IPC codes: G09G1, H01J31, H04N9