WO1995021500A2

WO1995021500A2 - Picture display device comprising a deflection correction circuit and method of correcting deflection errors

Info

Publication number: WO1995021500A2
Application number: PCT/IB1995/000045
Authority: WO
Inventors: Jacob Jan Hekker
Original assignee: Philips Electronics N.V.; Philips Norden Ab
Priority date: 1994-01-28
Filing date: 1995-01-19
Publication date: 1995-08-10
Also published as: TW276347B; WO1995021500A3

Abstract

Picture display device comprising a magnetic field compensation circuit for generating a compensation current through a winding surrounding the display tube. A rotational error caused by external magnetic fields is eliminated thereby. For correcting deflection errors (rotational and trapezium errors) which are caused by the display tube or by the deflection coil, or by the combination of both, the picture display device also comprises a correction circuit for generating adjustable correction currents which flow through the same compensation winding.

Description

Picture display device comprising a deflection correction circuit and method of correcting deflection errors.

The invention relates to a picture display device comprising a display tube, a deflection circuit for generating deflection currents through a deflection coil for deflecting one or more electron beams generated in the display tube in the horizontal (line) direction and the vertical (field) direction, and a compensation circuit for generating a direct current through a compensation winding surrounding the display tube for correcting a deflection error caused by an external magnetic field.

A picture display device of this type is known from United States Patent 5,021,712. In this known device the user can adjust the direction as well as the intensity of the current flowing through the compensation winding so as to generate a magnetic field which eliminates a rotation, caused by an external magnetic field, of the picture displayed on the screen of the display tube. Such a compensation winding is a rather costly element.

It is, inter alia an object of the invention to provide a more economical picture display device and correction method. To this end a picture display device according to the invention is characterized in that it also comprises a correction circuit for generating a correction current through the compensation winding so as to correct a deflection error caused by the display tube and/or the deflection coil. Advantageous embodiments are described in the sub-claims.

The invention is based on the recognition that in using the compensation winding not only the deflection errors caused by external magnetic fields and thus being errors coming from the exterior can be corrected, but also deflection errors which have other causes, viz. errors which originate from the interior and are caused by the construction of the picture display device itself. In this case the compensation winding, which is a costly component which is present anyway in the picture display device, is used for other applications. Different correction currents flowing through the winding can be generated and adjusted separately for correcting different deflection errors. A considerable economy is thus obtained in a simple manner, while the device has a great flexibility and a substantially faultless picture, at least as far as the deflection errors are concerned, is displayed on the display screen. The invention also relates to a method of correcting deflection errors in a picture display device comprising a display tube, a deflection circuit for generating deflection currents through a deflection coil for deflecting one or more electron beams generated in the display tube in the horizontal (line) direction and the vertical (field) direction, and a compensation circuit for generating a direct current through a compensation winding surrounding the display tube for correcting a deflection error caused by an external magnetic field, which method according to the invention is characterized in that a correction current flowing through the compensation winding is generated for correcting a deflection error caused by the display tube and/or the deflection coil.

Such a method, in which the picture display device is oriented in such a way that the influence of the external magnetic field is zero and in which the compensation direct current is subsequently set at zero, is advantageously characterized in that the intensity and the direction of the correction direct current are adjusted in such a way that the upper lines are displayed horizontally, whereafter a field-frequency sawtooth-shaped correction current is adjusted in such a way that the other lines are displayed horizontally, and in which finally the picture display device is directed in such a way that the influence of the external magnetic field is maximal, while the sensitivity of an amplifier amplifying a signal dependent on the magnetic field strength is adjusted in such a way that the upper lines are displayed horizontally in the same manner as in the situation in which the device is directed in the first mentioned way.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

In the drawings Fig. 1 shows a compensation circuit for compensating a deflection error caused by external magnetic fields,

Fig. 2 shows a first part of a correction circuit for correcting another deflection error,

Fig. 3 shows a waveform which is generated by the partial circuit of Fig. 2, and

Fig. 4 shows a second part of the correction circuit for correcting yet another deflection error, and the output stage for the compensation winding.

As is known, external magnetic fields influence the picture displayed by a display tube, not only as regards colour purity but also as regards the deflection of the electron beam(s) generated therein. The latter effect is particularly visible in display tubes having a wide display screen, for example display tubes having a 16:9 format. Even the comparatively weak earth's magnetic field has some influence. This is caused by the fact that the deflection angles at the edges of the screen are fairly large. If the 36" display tube of the Philips type W36E-WS001X is taken as an example and if an external magnetic field is, for example approximately 30 A/m (average value of the earth's magnetic field in Europe), it can be computed that the maximum, vertical deviation of the electron beam at a vertical edge of the screen due to the earth's magnetic field has a value of approximately 6.4 mm. This has also been measured in practice. Since the horizontal velocity of the electrons parallel to the screen at one edge of the screen is equal in intensity and has a direction opposite to the velocity at the other edge, the vertical difference between the positions of the light spot on the left and right parts of the screen is 12.8 mm. The deviation is proportionally smaller between the vertical edges. Thus, a rotation of the displayed picture is visible on the screen. This detrimental effect is very troublesome, particularly in monitor uses and/or when displaying test pictures, for example a chequer board pattern. This error is also present in monochrome display tubes, but in colour display tubes, in which three electron beams are generated, convergence errors are also visible. It is true that colour display devices are generally equipped with a demagnetizing circuit for eliminating colour purity errors. This circuit reduces the described deflection error to some extent but cannot eliminate it completely. For this purpose, use can be made of a winding surrounding the neck or the cone of the display tube, i.e. perpendicular to the longitudinal axis of the display tube, through which winding a direct current flows which can be adjusted by the user. The magnetic field thereby generated can be adjusted in such a way that it compensates the effects of the external magnetic field. However, to ensure that the envisaged compensation is satisfactory under all circumstances, an automatic compensation circuit is used in the device according to the invention. If the field strength of the magnetic field is measured, the direct current required for the compensation is also known.

Fig. 1 shows such a compensation circuit which forms part of the device according to the invention. In Fig. 1 , the reference numeral 1 denotes a magnetic field sensor of, for example the Philips type KMZ10A1 which is an element utilizing the magnetoresistive effect of a current-conducting magnetic material. A signal processing element (linear variable differential transformer signal conditioner) 2 of, for example the Philips type NE5521N comprises a clock generator 3, an operational amplifier 4 and a synchronous demodulator 5. Via two transistors 7 and 8 of opposite conductivity types, generator 3 supplies short current pulses of approximately +/-0.28 A having a duration of 60 μs and a frequency of approximately 100 Hz to a coil 6 which is arranged in the vicinity of element 1. The current through the coil either flows from the collector of transistor 7 or to the collector of transistor 8 and may thus reverse its direction. In this way the field strength- voltage characteristic of element 1 is utilized, in which a strong magnetic field causes the polarity of the voltage across the element to reverse. Consequently, the influence of magnetic stray fields and of an offset, whose polarity now remains constant, is eliminated. A switched voltage which is present across element 1 is applied to an input of amplifier 4 via a capacitor 9. At the other input of amplifier 4, the voltage is applied to the central junction point A of element 1. The gain of amplifier 4 is determined by the value of two resistors 10 and 11. The voltage amplified by amplifier 4 is applied to demodulator 5 which demodulates this voltage at the same frequency as that of the square-wave voltage of element 1. The output voltage of demodulator 5 is smoothed by means of a low-pass filter comprising a resistor 12 and a capacitor 13. The measured magnetic field strength is thus converted into a differential voltage of approximately 25 mV per A/m between the output voltage of demodulator 5 and the voltage at point A smoothed by a filter comprising a resistor 14 and a capacitor 15. For a better stability of the circuit, both filters 12, 13 and 14, 15 have approximately the same time constant of 0.7 s.

The voltage across capacitor 15 is the input voltage of an operational amplifier 16 connected as a buffer stage, whose output is connected to a capacitor 17. The voltage REF across capacitor 17 serves as a reference voltage. A non-inverting input of an operational amplifier 18 for temperature compensation receives the voltage across capacitor 13, while an inverting input receives the voltage REF via a silicon temperature sensor 19 of, for example the Philips type KTY81. A resistor 20 connected to the inverting input of amplifier 18 and a potentiometer 21 connected thereto, whose other end is connected to a direct voltage, provide for the bias voltage of amplifier 18. The non-inverting input of an operational transconductance amplifier 22 of, for example the Philips type NE55-17A receives the output voltage of amplifier 18, while the inverting input of amplifier 22 is connected to voltage REF. The multiplication factor and hence the sensitivity of amplifier 22 are adjusted by means of a resistor 23 and a potentiometer 24, whose other end is connected to a direct voltage. The output current of amplifier 22 flows through a resistor 25 which is connected to voltage REF and the voltage across resistor 25 is applied via a resistor 26 to the output stage to be described hereinafter.

It is achieved by means of the compensation circuit of Fig. 1 that the geometry of the displayed picture is independent of the position of the picture display device with respect to external magnetic fields, and particularly the earth's magnetic field, and of the field strengths of these fields. For example, when the present circuit was tested, strong external magnetic fields of opposite polarities were presented without errors remaining visible, while the smoothing filters only caused a short delay when the polarity was reversed. Moreover, the convergence errors caused by the magnetic fields are partly eliminated. Small geometry errors in the displayed picture may be caused by the display tube, which is known per se and is not shown in the Figures, or by the deflection coil, which is not shown either, for deflecting the electron beam(s) generated in the tube, or by the combination formed by both elements. Such errors may be corrected by means of additional circuits. The invention is based on the recognition that the winding for compensating the rotational error caused by external magnetic fields may also serve for correcting the errors mentioned above, which corrections and said compensation are independent of each other. One of the errors to be corrected is a rotation of the displayed picture, in which the lines written on the display screen are not displayed horizontally but are tilted to some extent. This error, which is independent of the rotational error caused by external magnetic fields and which may be considered to be a deflection error, is caused, for example by a small rotation of the deflection coil on the neck of the display tube with respect to the desired position. Another error is a trapezium error in which, for example the upper horizontal lines are displayed actually horizontally, but the horizontal lines there below and maximally the lowest horizontal lines are displayed tilted. This error is a tolerance error of the combination formed by the display tube and the deflection coil.

The trapezium error may be considered to be a field-frequency deflection error and can therefore be corrected by means of a field-frequency signal. Fig. 2 shows a circuit for this purpose, in which a field-frequency sawtooth-shaped signal is generated. To ensure that the adjustment of the trapezium correction does not have any influence on the horizontal rotation, it is necessary that the value of the sawtooth-shaped signal is constant for a given line of the field. This is shown in Fig. 3 in which the level of the upper horizontal line, i.e. the initial value of the sawtooth shape, is constant. Fig. 3 shows the sawtooth shape for different adjustments, each sawtooth shape having a duration of one field period T. Such a signal may be obtained, for example from the voltage which is present across a feedback resistor located in the path of the sawtooth-shaped field deflection current and by shifting and amplifying this voltage.

However, in the described embodiment, the sawtooth-shaped signal in Fig. 2 is not generated in the manner described, but from an incoming field signal. A capacitor 31 connected to the output of an operational amplifier 32 is charged by means of a resistor 33, connected to the inverting input of amplifier 32, and a potentiometer 34, the other terminal of which is connected to a direct voltage. The non-inverting input of the amplifier is connected to the reference voltage REF. The field blanking pulses originating from a synchronizing pulse separating stage (not shown in Fig. 2) are applied to the base of a transistor 35 which switches two transistors 36 and 37 of opposite conductivity types, which transistors are connected in parallel with capacitor 31 so that the voltage present across the capacitor is short-circuited by one of the transistors during the occurrence of the pulses. Consequently, a field-frequency sawtooth-shaped voltage which is applied via a resistor 38 to the output stage to be described hereinafter is present across the integration capacitor 31. The direction of variation of the sawtooth shape can be adjusted by means of potentiometer 34 because the voltage at the inverting input of amplifier 32, whose initial value is equal to voltage REF, will be higher or lower than this voltage, and also the amplitude of the sawtooth shape can be adjusted. Fig. 3 shows that the sawtooth shape obtained has an amplitude of, for example +6/-5 V and is superimposed on a constant voltage of, for example 5 V. The correction current is zero at the top of the displayed picture and this current is maximal at the bottom. Similarly as described above for the vertical edges of the picture, the horizontal lines written on the display screen are tilted under the upper horizontal edge of the picture and maximally at the lower horizontal edge, as compared with the case without correction. The trapezium error is thus corrected by means of the correct amplitude of the sawtooth-shaped voltage.

Fig. 4 shows the output stage of the circuit according to the invention and also the facility for correcting the rotational error not caused by external magnetic fields. This correction is ensured by a resistor 41 and a potentiometer 42 connected to a supply voltage, resistor 41 being connected to the inverting input of an operational amplifier 43 which, together with the other amplifiers 16, 18 and 32, forms part of an integrated circuit of, for example the Philips type LM324. The direct voltage at the inverting input is adjusted with respect to voltage REF by means of potentiometer 42. Resistor 26 which transfers the magnetic field strength signal and resistor 38 which transfers the vertical sawtooth-shaped signal are also connected to said input. The non-inverting input of amplifier 43 is connected to voltage REF. The three signals from resistors 26, 38 and 41 are amplified by means of the amplifier and added together. The output signal of amplifier 43 controls the winding L via two buffer transistors 44 and 45 of opposite conductivity types, which winding surrounds the neck of the display tube for compensating the rotational error caused by external magnetic fields and for correcting the above-mentioned deflection errors. A resistor 46 arranged in series with winding L and a network 47 connected to the junction point of winding L and resistor 46 are fed back to the inverting input of amplifier 43 for compensating deviations caused by temperature and tolerances. The terminal of resistor 46 not connected to winding L is grounded, while one of the supply voltages of transistors 44 and 45 is positive and the other is negative, so that the current through winding L can flow in both directions. In the embodiment described, the winding has approximately 400 turns having a resistivity of 210 Ω ± 10% and a diameter of approximately 200 mm. The maximum voltage across winding L is approximately +/-10 V.

By adjusting potentiometer 42, the intensity and the direction of a DC component flowing through winding L are adjusted for correcting the rotational error which is not caused by an external magnetic field. This adjustment is performed at an earth's magnetic field without influence, i.e. with the display screen being directed towards the east, and this after the display tube has been demagnetized and after the difference voltage at the inputs of amplifier 22 has subsequently been set at zero by means of potentiometer 21. Subsequently the upper lines are adjusted horizontally by means of potentiometer 42, whereafter the trapezium correction is performed by means of potentiometer 34. Finally, the picture display device is directed towards the north, where the earth's magnetic field is maximal, and the display tube is again demagnetized, whereafter potentiometer 24 is adjusted in such a way that the upper lines are displayed horizontally in the same manner as in the situation where the device is directed towards the east. Turning the picture display device now has no influence on the rotation of the picture. It will be noted that the adjustments mentioned above can be performed in a simple manner by means of potentiometers connected to DC voltage sources. In a different manner, adjustable DC voltages can be applied to the relevant points of the circuit. Two partial circuits have been described hereinbefore, each circuit being used for correcting a given deflection error, in which the winding L is already available for compensating the deflection errors caused by external magnetic fields is utilized. It will be evident that other deflection errors can be corrected in a similar way, with correction currents being generated each with a variation as a function of time which is determined by the error to be corrected.

Claims

CLAIMS:

1. A picture display device comprising a display tube, a deflection circuit for generating deflection currents through a deflection coil for deflecting one or more electron beams generated in the display tube in the horizontal (line) direction and the vertical (field) direction, and a compensation circuit (Fig. 1) for generating a direct current through a compensation winding (L) surrounding the display tube for correcting a deflection error caused by an external magnetic field, characterized in that the picture display device also comprises a correction circuit (Fig. 2; Fig. 4) for generating a correction current through the compensation winding (L) so as to correct a deflection error caused by the display tube and/or the deflection coil.

2. A picture display device as claimed in Claim 1, characterized in that the correction circuit has a facility (41-47) for generating a correction direct current through the compensation winding (L).

3. A picture display device as claimed in Claim 1 , characterized in that the correction circuit has a facility (33-38, 43-45) for generating a field-frequency sawtooth- shaped correction current through the compensation winding (L).

4. A picture display device as claimed in Claim 2 or 3, characterized in that the direction and intensity of said correction current are adjustable (41, 42; 33, 34).

5. A picture display device as claimed in Claim 3, characterized in that the correction circuit comprises an integrator (31-34) for integrating incoming field pulses for generating said correction current.

6. A picture display device as claimed in Claim 3, characterized in that the intensity of the field-frequency sawtooth-shaped current has a constant value for a given line of the field (Fig. 3) which is independent of the adjustment of the current.

7. A picture display device as claimed in Claim 1, characterized in that the correction circuit comprises an adder stage (43) for adding the correction current to the compensation current.

8. A picture display device as claimed in Claim 7, characterized by a feedback network (46, 47) arranged between the compensation winding (L) and the adder stage (43).

9. A picture display device as claimed in Claim 1 , characterized in that the correction circuit has a reference voltage which is. derived from a magnetic field sensor.

10. A method of correcting deflection errors in a picture display device which comprises a display tube, a deflection circuit for generating deflection currents through a deflection coil for deflecting one or more electron beams generated in the display tube in the horizontal (line) direction and the vertical (field) direction, and a compensation circuit for generating a direct current through a compensation winding (L) surrounding the display tube for correcting a deflection error caused by an external magnetic field, characterized in that a correction current flowing through the compensation winding is generated for correcting a deflection error caused by the display tube and/or the deflection coil.

11. A method as claimed in Claim 10, characterized in that said correction current is adjusted in direction and, from zero, in intensity by means of a direct voltage (41, 42; 33, 34).

12. A method as claimed in Claim 10, characterized in that the voltage across a feedback resistor arranged in the path of the field deflection current is shifted and amplified for generating said correction current.

13. A method as claimed in any one of the preceding Claims 10 to 12, in which the picture display device is oriented in such a way that the influence of the external magnetic field is zero and in which the compensation direct current is subsequently set at zero (21), characterized in that the intensity and the direction of the correction direct current are adjusted (42) in such a way that the upper lines are displayed horizontally, whereafter the field-frequency sawtooth-shaped correction current is adjusted (34) in such a way that the other lines are displayed horizontally, and in which finally the picture display device is directed in such a way that the influence of the external magnetic field is maximal, while the sensitivity of an amplifier (22) amplifying a signal dependent on the magnetic field strength is adjusted (24) in such a way that the upper lines are displayed horizontally in the same way as in the situation in which the device is directed in the first-mentioned way.