WO2005029840A1

WO2005029840A1 - Compensating fluctuations of an anode voltage

Info

Publication number: WO2005029840A1
Application number: PCT/IB2004/051712
Authority: WO
Inventors: Leendert A. D. Van Den Broeke; Gerard A. De Wit; Robert H. De Nie
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-09-22
Filing date: 2004-09-08
Publication date: 2005-03-31

Abstract

A cathode ray tube (3) receives an anode voltage (Va) that has a predetermined non-linear relation (F I) with a beam current (Ia) flowing in the cathode ray tube (3). A compensation circuit comprises a sensing circuit (8) that senses the beam current (Ia) to obtain a sensed beam signal (Vb). A waveform generator (7) supplies a compensation signal (VAC) that is a predetermined non-linear function (F2) of the sensed beam current (Vb). This predetermined non-linear function (F2) is selected to correct for the fact that the sensed beam current (Vb) is not a one-to-one representation of the actual anode voltage level (Va). In this manner, a compensation signal (VAC) is obtained which is substantially representative for the anode voltage (Va).

Description

Compensating fluctuations of an anode voltage

FIELD OF THE INVENTION The invention relates to a compensation circuit for generating a compensation signal depending on fluctuations of the anode voltage of cathode ray tube, a display apparatus comprising such a compensation circuit, and a method of compensating for fluctuations of the anode voltage.

BACKGROUND OF THE INVENTION US-A-5,920,157 discloses a picture stabilizing circuit for a semi wide-screen television receiver in which bending of vertical lines is prevented by detecting and compensating for a high voltage variation caused by a picture brightness variation. A high voltage circuit generates a high voltage in a secondary winding of a flyback transformer. This high voltage is supplied to the anode of a cathode ray tube. A horizontal output device receives a supply voltage via the primary winding of the flyback transformer to generate a horizontal deflection current. The horizontal deflection current is corrected with an east/west signal. The picture stabilizing circuit comprises a detecting device which detects a variation of the high voltage in the secondary winding of the flyback transformer and which outputs a signal corresponding to the variation of the high voltage. The picture stabilizing circuit further comprises an adder that adds the output signal of the detecting device to the east/west signal. In an embodiment of this prior art, the beam current flowing through the secondary winding is sensed and integrated and then added to the east-west signal. This prior art has the drawback that the compensation of the variations of the high voltage is not optimal.

SUMMARY OF THE INVENTION It is an object of the invention to provide a compensation circuit that provides an improved compensation of the variations of the high voltage. A first aspect of the invention provides a compensation circuit as claimed in claim 1. A second aspect of the invention provides a display apparatus as claimed in claim 5. A third aspect of the invention provides a method of compensation as claimed in claim 8. Advantageous embodiments are defined in the dependent claims. The compensation circuit generates a compensation signal that is representative for the anode voltage (the high voltage of the prior art) of a cathode ray tube. Thus, fluctuations of the anode voltage are also present in the compensation signal such that the compensation signal can be used to compensate for these fluctuations. The anode voltage is the voltage that accelerates the electrons generated by the electron guns of the cathode ray tube towards the screen of the cathode ray tube. The compensation signal may be used to compensate for the influence of the anode voltage on the vertical deflection, on the horizontal deflection or on other geometry corrections, or on the dynamic focus. The compensation circuit comprises a sensing circuit that senses a beam current flowing in the cathode ray tube to obtain a sensed beam signal. The anode voltage has a predetermined non-linear relation with the beam current. Usually, due to the non-linear behavior of the output impedance of the anode voltage generator, the anode voltage decreases more steeply at low beam currents than at high beam currents. A waveform generator which receives the sensed beam signal supplies the compensation signal which is a predetermined non-linear function of the beam current signal. In an embodiment, this predetermined non-linear function is selected to correct for the fact that the sensed beam signal is not a one-to-one representation of the actual anode voltage. The sensed beam signal does not represent the non-linearity of the output impedance of the anode voltage generator. This non-linearity is introduced by the waveform generator. In this manner, a compensation signal is obtained which is substantially representing the anode voltage and thus also the fluctuations of the anode voltage. The relatively large variation of the anode voltage at small beam currents will be represented by a relatively large variation of the compensation signal at small beam currents. In the prior art US-A-5,920,157, the sensed beam current is only integrated. It is not disclosed to perform a predetermined non-linear operation on the sensed beam current to obtain a compensation signal that is substantially representing the anode voltage to • improve the compensation of the fluctuations of the anode voltage. In an embodiment in accordance with the invention as defined in claim 2, the predetermined non-linear function has the same shape as the function that defines the beam current as function of the anode voltage. If required, the function that defines the beam current as function of the anode voltage is corrected for the transfer function of the sense circuit. Usually, the sense circuit is a resistor divider. Such a resistor divider has substantially no influence on the shape of the waveform sensed. Consequently, the predetermined nonlinear function is substantial identical to the inverse of the non-linear relation between the anode voltage and the beam current only corrected by a scaling factor, if required. In an embodiment in accordance with the invention as defined in claim 3, the waveform generator is programmable to obtain a programmable non- linear operation. This has the advantage that the same waveform generator can be used in different applications wherein the sensed beam currents have different predetermined non-linear relations with the anode voltage. In an embodiment in accordance with the invention as defined in claim 4, the waveform generator approximates the non-linear function with a piece wise linear waveform. This has the advantage that only a small set of parameters is required to define the non- linear function. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Fig. 1 shows a block diagram of a display apparatus in accordance with an embodiment of the invention, Fig. 2 shows a waveform representing the predetermined non-linear relation between the anode voltage and the beam current in accordance with an embodiment of the invention, Fig. 3 shows waveforms representing the sensed beam current signal and the desired compensation signal as function of the beam current, Fig. 4 shows a waveform representing the predetermined non-linear function to obtain a compensation signal which substantially represents the fluctuations of the anode voltage in accordance with an embodiment of the invention, and Fig. 5 shows a piece wise linear waveform that approximates the predetermined non-linear function in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows a block diagram of a display apparatus in accordance with an embodiment of the invention. The video processor 1 receives an input video signal Vi to supply video signals VCS, a line synchronization signal HS, and a frame synchronization signal VS. The input video signal Vi may be a composite video signal comprising the video information and synchronization information. Usually, the video signals VCS are base-band component video signals indicated by RGB (Red, Green, Blue components) and are supplied to the cathode drive circuit 2 which amplifies the video signals VCS to obtain the video drive signals VDS which have a level suitable to drive the cathode ray tube 3. Usually, the video drive signals VDS are supplied to cathodes of the cathode ray tube 3 to control the number of electrons emitted by the cathodes. The anode voltage Va which is connected to an anode terminal AN of the cathode ray tube 3 accelerates the electrons emitted by the cathodes towards the screen of the cathode ray tube 3. These electrons are withdrawn from the cathode ray tube 3 via the anode terminal AN. The electron beams leaving the cathodes thus cause a beam current or anode current la flowing into the anode terminal AN. In practice not all emitted electrons leave the cathode ray tube 3 via the anode terminal AN because the cathode ray tube comprises grids, such as the focus grid, which have to receive a predetermined voltage level. Some of the electrons will leave the cathode ray tube 3 via these grids. But, anyhow, a substantial majority of the electrons flow through the anode terminal AN as the beam current la. This beam current la depends on the image to be displayed. If a completely black image is displayed, the beam current la is substantially zero. If a completely white image with a high brightness is displayed, the beam current la has a relatively high value. For example, for a cathode ray tube 3 in a television receiver the beam current la may change between zero and 1.8 milli-amperes. The anode voltage generator FB usually comprises a flyback transformer with a primary winding PW and a secondary winding SW. The secondary winding SW has a terminal which is coupled to the anode terminal AN via a high voltage diode D. In a practical implementation, the secondary winding S W may comprise several sections of series arranged diodes and windings. The primary winding PW receives a power supply voltage Vba and is coupled to the line deflection coil(s) LH and the line deflection circuit 5 as a choke coil. The line drive circuit 4 which usually comprises a synchronization processor receives the horizontal synchronization signal HS and the vertical synchronization signal VS of the video signal Vi to supply a line drive signal HD to the line deflection circuit 5, a frame drive signal VDS to the frame deflection circuit 9 and an east/west drive signal E/W to the east/west drive circuit 6. The frame deflection circuit 9 generates a frame deflection current through a frame deflection coil LV. The line deflection circuit 5 usually comprises a high voltage transistor connected between the node Nl and ground to obtain a periodical line deflection current ID through the line deflection coil(s) LH. Usually, the line deflection current ID has a shape of an S-corrected saw-tooth. The east/west drive circuit 6 supplies an east/west correction signal EWC to a suitable input of the line deflection. In the embodiment shown in Fig.l, the east/west correction signal EWC is supplied to a junction of a capacitor C and a coil L. The capacitor C and the coil L are arranged in series between a node N2 and ground. The line deflection coil(s) LH are arranged between the nodes Nl and N2. The operation of the east/west correction and the line deflection is not further elucidated because the skilled person knows this and it is not relevant to the invention. Alternatively, the line deflection may be of the well-known diode-modulator type. The diode-modulator based line deflection has also an input to receive the east/west correction signal EWC. The east/west correction primarily corrects the amplitude of the line deflection in the frame direction to obtain substantially straight vertical lines. Without the east/west correction, due to the higher deflection sensitivity towards the edges of the screen, the amplitude of the line scan would be larger towards the edges of the screen in the frame direction than in the center of the screen. Usually, the line deflection is performed in the horizontal direction and the frame deflection is performed in the vertical direction to obtain horizontal lines that succeed each other in the vertical direction. Alternatively, in transposed scanned systems, the line deflection occurs in the vertical direction and the frame deflection occurs in the horizontal direction to obtain vertical lines that succeed each other in the horizontal direction. Because the deflection sensitivity also depends on the level of the anode voltage Va, usually, both the vertical deflection and the horizontal deflection are corrected with a correction signal VAC which either is directly obtained by tapping in the anode voltage Va or by sensing the anode current la. GB-A-2325601 discloses that the anode voltage Va is tapped in by using a further secondary transformer winding on the flyback transformer. This tapped in anode voltage Va is of course a very good representation of the actual anode voltage Va, but requires an expensive extra winding and complex circuitry to process the voltage supplied by the winding. US-A-5,920,157 discloses the use of the anode current la (also referred to as beam current la). The beam current is sensed by connecting a resistor between the high voltage winding and a reference voltage. The beam current that flows through this resistor causes a voltage that changes proportional with the beam current. This voltage is integrated and used to correct the east/west drive signal. However, the beam current is not a one-to-one representation of the anode voltage because the output impedance of the flyback transformer is not linear. At a same variation of the beam current at a low value of the beam current, the anode voltage changes much more than at the same variation at a high value of the beam current. This effect is not taken into account because the beam current is used for the correction of the east/west drive signal. Thus, although this prior art discloses a simple and cost-effective solution, the quality of the compensation will not be optimal. In accordance with an embodiment of the invention, the sensing of the beam current la may be identical to that shown in US-A-5,920,157. A series arrangement of the resistors R2 and R3 is arranged between the power supply voltage Vba, which is also used for the line deflection, and ground. A resistor Rl is coupled between the junction of the resistors R2 and R3 and the terminal of the secondary winding SW that is not connected to the diode D. The beam current la which flows through the resistor Rl will cause a sensed beam signal Vb, which in this embodiment is a voltage, and which is proportional to the beam current la. An increasing beam current la causes a decreasing sensed beam current signal Vb. Of course, the skilled person will recognize that many alternatives are possible to obtain a sensed beam signal Vb by using the beam current la. For example, it is possible to use another power supply voltage than the power supply voltage Vba which is also used for the line deflection. It might than not be required to tap in the power supply voltage, which allows to directly connect the resistor Rl to this power supply voltage without requiring the resistors R2 and R3. In accordance with the invention, this sensed beam signal Vb is supplied to a waveform generator 7. This waveform generator 7 adapts the sensed beam signal Vb in accordance with a non-linear function F2 to obtain the compensation signal VAC. The compensation signal VAC compensates for the fluctuations of the anode voltage Va. In Fig. 1, the compensation signal VAC is supplied to the frame deflection circuit 9 to obtain a frame deflection amplitude which is less dependent on the value of the anode voltage Va, and to the east/west drive circuit 6 to obtain a line deflection amplitude which is less dependent on the value of the anode voltage Va. The non- linear function F2 has a shape to obtain a compensation signal VAC which has substantially the same shape as a non-linear function which provides the relation between the beam current la and the anode voltage Va. This nonlinear function is in fact the inverse of the non-linear function FI. The non linear function FI, which is shown in Fig. 2, is the anode voltage Va as function of the beam current la. The non-linear function F2, which is shown in Fig. 4, and which has to be generated by the waveform generator 7 has the same shape as the curve showing the beam current la as function of the anode voltage Va. Consequently, the compensation signal VAC represents the anode voltage Va much better than the beam current la does. At a particular change of the beam current la, the compensation signal VAC will change more at small values of the beam current la than at large values of the beam current la, substantially exactly the same as the anode voltage Va depends on the beam current la. The waveform generator 7 may generate the non- linear function F2 taking the transfer of the sensing circuit 8 into account. This is especially relevant if the transfer function of the sensing circuit 8 is not linear. The shape of the waveform generated by the waveform generator 7 is defined by parameters or coefficients PR. For example, the waveform generator may generate a spline waveform as disclosed in WO-A-97/41680. The sensed beam signal Vb may also be used by the video processor 1 to provide beam current limiting operations.

Fig. 2 shows a waveform representing the predetennined non-linear relation between the anode voltage and the beam current in accordance with an embodiment of the invention. The waveform shown in Fig. 2 defines an example of a predetermined non-linear relation FI between the anode voltage Va and the beam current la. For relatively low beam currents la, up to the beam current value lal the anode voltage Va steeply decreases from the maximum value Vao to the value Val when the beam current la increases. The maximum value Vao occurs at zero beam current la. From the beam current value lal onwards to the maximum beam current value lam the output impedance of the anode voltage generator FB is substantially constant: the anode voltage Va substantially linearly decreases from the value Val to its minimum value Vam when the beam current la increases from the value lal to the value lam.

Fig. 3 shows waveforms representing the sensed beam signal and the desired compensation signal as function of the beam current. The sensed beam current signal Vb is generated by the beam current la which flows through resistors, and thus is a linear function of the beam current la. This straight line starts at the value Vbl at zero beam current and drops to the value Vb2 at the maximum value lam of the beam current la. The values Vbl and Vb2 depend on the level of the power supply voltage Vba and the values of the resistors Rl, R2 and R3. The curve indicated by VAC shows a desired dependency of the compensation signal VAC on the beam current la. It is desired that the curve VAC has the same shape as the curve FI shown in Fig. 2. As with the curve FI, the curve VAC steeply decreases from the value VAC1 to the value VAC2 when the beam current la increases from zero to the value lal . The curve VAC decreases substantially linearly from the value VAC2 to VAC 3 when the current la changes from the value lal to the maximum value lam. Only if the shape of the compensation signal VAC is a one-to-one copy of the shape of the curve FI it is possible to obtain a perfect compensation of the fluctuations of the anode voltage Va. What counts is that any curve that has a shape substantially the same as the shape as the optimal curve indicated by VAC will provide a compensation of the fluctuations of the anode voltage Va, which is improved over the prior art wherein the curve indicated by Vb is used.

Fig. 4 shows a waveform representing the predetermined non-linear function to obtain a compensation signal that substantially represents the fluctuations of the anode voltage in accordance with an embodiment of the invention. The predetermined non-linear function F2 converts the sensed beam current information Vb into the compensation signal VAC. In the example shown in Fig. 4, the shape of the function F2 is selected to obtain the desired shape of the curve VAC of Fig. 3. The compensation signal VAC has the values VAC1, VAC2, VAC3 for the values Vbl, Vb2, Vb3, respectively, of the sensed beam current information Vb.

Fig. 5 shows a piece wise linear waveform that approximates the predetermined non-linear function in accordance with an embodiment of the invention. The non- linear function F3 shown in Fig. 5 is a piece wise linear approximation of the curve F2 shown in Fig. 4. In this example, five segments segO to seg4 are defined which approximate the curve F2. This linear approximation requires a few parameters PR only and requires only a low computational effort.

To conclude, in a preferred embodiment, the cathode ray tube 3 receives an anode voltage Va that has a predetermined non-linear relation FI with a beam current la flowing in the cathode ray tube 3. A compensation circuit comprises a sensing circuit 8 which senses the beam current la to obtain a sensed beam signal Vb. A waveform generator 7 supplies a compensation signal VAC that is a predetermined non-linear function F2 of the sensed beam signal Vb. This predetennined non-linear function F2 is selected to correct for the fact that the sensed beam signal Vb is not a one-to-one representation of the actual anode voltage level Va. In this manner, a compensation signal VAC is obtained which is substantially representative for the anode voltage Va. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may 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 may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A compensation circuit for generating a compensation signal (VAC) and comprising a sensing circuit (8) for sensing a cathode ray tube beam current (la) to obtain a sensed beam signal (Vb), and a waveform generator (7) for receiving the sensed beam signal (Vb) to supply the compensation signal (VAC) being a predetermined non-linear function (F2) of the sensed beam current (Vb), the predetermined non-linear function (F2) being selected to obtain the compensation signal (VAC) being representative for a cathode ray tube anode voltage (Va).

2. A compensation circuit as claimed in claim 1, wherein the predetermined nonlinear function (F2) has a same shape as a function (FI) defining the cathode ray tube beam current (la) as a function of the cathode ray tube anode voltage (Va).

3. A compensation circuit as claimed in claim 1, wherein the waveform generator (7) comprises an input for receiving parameters (PR) to supply the predetermined non-linear function (F2) being programmable.

4. A compensation circuit as claimed in claim 1, wherein the waveform generator (7) comprises a piece wise linear waveform synthesizer (7) for generating a piece wise linear waveform (F3) approximating the predetermined non-linear function (F2).

5. A display apparatus comprising an anode voltage generator (FB, D) for generating a cathode ray tube anode voltage (Va), a cathode ray tube (3) coupled to receive the cathode ray tube anode voltage

(Va), and a compensation circuit (7, 8) for generating a compensation signal (VAC) as claimed in claim 1.

6. A display apparatus as claimed in claim 5, further comprising a frame deflection circuit (9) for deflecting electron beams in the cathode ray tube (3) in a frame direction, and having an input for receiving the compensation signal (VAC) for compensating the deflecting in the frame direction for fluctuations of the anode voltage (Va).

7. A display apparatus as claimed in claim 5, further comprising a line deflection circuit (5) for deflecting electron beams in the cathode ray tube (3) in a line direction, and having an input for receiving an east-west correction signal (EWC) to correct the deflecting in the line direction for east/west errors, and an east- west correction circuit (6) for receiving the compensation signal

(VAC) to supply the east- west correction signal (EWC).

8. A method of compensating for fluctuations of an anode voltage (Va) of a cathode ray tube (3), the method comprising sensing (8) a beam current (la) flowing in the cathode ray tube (3) to obtain a sensed beam signal (Vb), and receiving (7) the sensed beam current (Vb) to supply the compensation signal (VAC) being a predetermined non-linear function (F2) of the sensed beam signal (Vb), the predetermined non-linear function (F2) being selected to obtain the compensation signal (VAC) representing a cathode ray tube anode voltage (Va).

9. A method as claimed in claim 8, wherein the cathode ray tube anode voltage (Va) has a predetermined non-linear relation (FI) with the beam current (la), and the predetermined non-linear function (F2) is related to the non-linear relation (FI).