WO1996005550A1

WO1996005550A1 - Data processing system comprising a user interface

Info

Publication number: WO1996005550A1
Application number: PCT/IB1995/000622
Authority: WO
Inventors: Kofi Afolabi Anthony Makinwa
Original assignee: Philips Electronics N.V.; Philips Norden Ab
Priority date: 1994-08-12
Filing date: 1995-08-08
Publication date: 1996-02-22

Abstract

A data processing system comprises a user interface with an image display panel with row and column electrodes. Drive means generate electric signals on the row and column electrodes in order to generate an image on the image display panel. The interface comprises a signal pick-up member which can be displaced relative to the image display panel in order to pick-up measurement signals in response to the electric signals for generating the image. The signal pick-up member is coupled to measuring means in order to derive a position of the signal pick-up member relative to the image display panel from the measurement signals. The measuring means are arranged to detect the measurement signals in synchronism with the generation of at least a sub-set of the electric signals. The electric signals for generating the image thus also serve to determine the position of the signal pick-up member.

Description

Data processing system comprising a user interface.

The invention relates to a data processing system, comprising a user interface provided with an image display panel with row and column electrodes and drive means coupled to generate electric signals on the row and column electrodes in order to generate an image on the image display panel, which interface is arranged to generate localization signals on the row and/or column electrodes and comprises a signal pick-up member which can be displaced relative to the image display panel in order to pick up measurement signals in response to the localization signals, which signal pick-up member is coupled to measuring means in order to derive from said measurement signals a position of the signal pick-up member relative to the image display panel . A system of this kind is known from EP 504728. In the system disclosed in the cited publication the control of the electrodes is subdivided into two periods: a display period and a measuring period. During the display period the electric signals producing the image on the panel are generated. During the measuring period, localization signals are applied to selected electrodes, said localization signals generating the measurement signal in the signal pick-up member, however, without causing an image change. The amplitude of the measurement signal generated is dependent on the distance between the signal pick-up member and the electrode whereto the signal is applied. The position of the signal pick-up member relative to the image display panel can thus be derived from a comparison of the amplitude of various measurement signals generated by means of signals on electrodes in different locations on the image display panel.

The use of a separate measuring period, however, requires time which cannot be used for controlling the image display. If too much time is used, the quality of the image display suffers. Therefore, it is desirable to reduce the time required for measurement, notably because a position can then also be measured more frequently without affecting the quality of image display.

In an alternative embodiment, in conformity with said publication high- frequency localization signals (of a frequency which is much higher than that of the signals for controlling the image display) are applied to the electrodes during the display period. These high-frequency test signals also generate a measurement signal which has an amplitude which is dependent on the distance between the signal pick-up member and the electrode carrying the high-frequency signal. The position can thus also be derived therefrom.

The use of high-frequency test signals necessitates a specific adaptation of the panel drive means in order to enable position detection. This has a cost-increasing effect. It is inter alia an object of the invention to provide a data processing system in which the additional time required for the measurement is reduced without special steps being required as regards the drive means.

The data processing system in accordance with the invention is characterized in that the measuring means are arranged to detect the measurement signals in synchronism with the generation of at least a sub-set of the electric signals for generating the image, the sub-set of the electric signals serving at least as a sub-set of the localization signals. The additional time required for a measurement is reduced by using localization signals formed by signals serving to generate the image.

An embodiment of the data processing system in accordance with the invention is characterized in that said sub-set of electric signals concerns electric signal level transitions which are independent of the contents of an image. The measurement can thus be performed independently of the image contents. It is also possible to utilize signals which are dependent on the image contents, for example by predicting associated measurement signals for feasible positions (or positions expected on the basis of previous positions) and by choosing the position whose predicted signal deviates least from the measured signal.

However, this requires a substantially larger amount of computing capacity than when use is made of signals which are independent of the contents.

A further embodiment of the data processing system in accordance with the invention is characterized in that the drive means are arranged to switch on and off a row selection signal on a selected row electrode, an image signal remaining present on one or more of the column electrodes during said switching on or off, and that the measuring means are arranged to derive the position transversely of the row electrodes from a change in the measurement signal from before to after the switching on or off of the row selection signal. The switching on or off results in a signal level transition which is excellently suitable for measuring the position.

An embodiment of the data processing system in accordance with the invention is characterized in that it comprises a signal generator which is coupled to the column electrodes in order to apply a column-dependent set of localization signals thereto during a measuring period, said set having substantially no effect on the image displayed, and that the measuring means are arranged to derive the position transversely of the columns from a response of the measurement signal to the set of test signals. The position transversely of the row electrodes is thus measured during the generation of the image and the position transversely of the columns is measured during a measuring period in which the image remains unchanged. If no row electrodes are activated during this measuring period, the image will remain unchanged except for parasitic effects. The image will also remain substantially unchanged when only one or a few row electrodes in the periphery of the image are selected during the measurement period. Notably the blanking period between the writing of successive images is a suitable measuring period. An embodiment of the data processing system in accordance with the invention is characterized in that the measuring means are arranged to measure the position transversely of the columns and/or the rows several times during the control of one and the same image. The position is thus measured at a frequency of more than once per image. This allows for better following of movements of the signal pick-up member. Because the measurement signals are detected at least partly simultaneously with the generating of the image, the system can perform several measurements per image without excessively affecting the image display quality.

An embodiment of the data processing system in accordance with the invention is characterized in that the drive means are arranged to select first a first part of the row electrodes for image display and subsequently a second part of the row electrodes which spatially alternate with the row electrodes of the first part on the image display panel. This reduces the intervals during which no row electrodes in the vicinity of the signal pick¬ up member are selected, so that the position transversely of the row electrodes can be measured more often. An embodiment of the data processing system in accordance with the invention is characterized in that the measuring means comprise a hold circuit for holding the measurement signal occurring prior to switching on or off, and a comparator circuit for comparing the signal held by the hold circuit with the measurement signal after switching on or off. The measurement signal can thus be simply detected. An embodiment of the data processing system in accordance with the invention is characterized in that the hold circuit is arranged to make the measurement signal held vary for simulating the variation of the measurement signal in the presence of an average image signal in the absence of switching on or off. Disturbances of the position measurement which are dependent on the image contents can thus be reduced. These and other advantageous aspects of the data processing system in accordance with the invention will be described in detail hereinafter with reference to some Figures; therein

Fig. 1 shows a data processing system comprising a user interface; Fig. 2 is a diagrammatic side elevation of an image display panel with a signal pick-up member;

Fig. 3 shows signals generated by the drive unit;

Fig. 4 shows a measuring circuit;

Fig. 5 shows a voltage variation on the row electrodes, and Fig. 6 shows peak-to-peak transitions of a signal on the output of the measuring circuit.

Fig. 1 shows a data processing system 10 comprising a user interface. The system 10 includes a processing unit 12 and an image display panel 14. The processing unit 12 comprises an output which is coupled to the panel 14 via a drive unit 16. The system 10 also comprises a signal pick-up member 15 which is shown, by way of example, as a measuring pin and which is coupled to the processing unit 12 via a measuring unit 18. An additional output of the drive unit 16 is also coupled to the measuring unit 18.

Fig. 2 is a diagrammatic side elevation of an image display panel 20 with a signal pick-up member. The image display panel 20 comprises a stack formed by a first glass layer 21, a set of row electrodes 22, 22a (one of which (22) is visible in a transverse view) with intermediate pixel electrodes 22b, a liquid crystal matrix 23, a black mask 24, a set of column electrodes 25, a second glass layer 26, a transparent, conductive layer 27, and a diffuser 28. Adjacent the diffuser there is arranged a light source 29. The signal pick-up member is shown, by way of example, as a measuring pen which comprises a measuring head 200, a housing 202 and a signal lead 206. A capacitance 204 is symbolically shown between the measuring head 200 and the row electrode 22.

When the measuring head 200 is held over a row electrode 22 of the display panel, a capacitance 204 is formed between the measuring head 200 and the row electrode 22. A capacitance will in principle be formed also between the measuring head 200 and all row electrodes 22a. However, the capacitance will be highest between the measuring head 200 and the row electrode 22 situated directly underneath the measuring head. During operation the housing 202 of the measuring pen is connected to a voltage which is fixed relative to the reference for the voltages on the row electrodes 22, 22a and the column electrodes 25; the measuring head is insulated from the housing. As a result, the capacitance from the signal lead 206 to the row electrodes 22, 22a and the column electrodes 25 is negligibly small.

In response to changes in the voltage on the row electrodes 22, 22a and the column electrodes 25 a signal is formed on the signal lead 206 via the capacitance 204. The magnitude of this signal is a measure of the distance between the measuring head 200 and the row electrode 22 or column electrode 25 on which the change occurs. The measuring unit 18 derives the position of the measuring head 200 from these signals. In accordance with the invention, the signals whereby the image on the panel 14 is generated are utilized for this purpose. The control aspects involved in an embodiment of the invention will be briefly described.

During operation the processing unit 12 generates image contents composed of individual pixels for display on the image display panel 14. A pixel is displayed in that the drive unit 16 applies an electric field across the liquid crystal matrix 23, via the row electrodes 22, 22a and the column electrodes 25, at the area of the pixel on the display panel 14, the field strength being dependent on the desired intensity value of the pixel. This electric field locally polarizes the liquid crystal matrix 23, so that it transmits more or less light (originating from the light source 29 via the diffuser 28) in dependence on the location.

The drive unit 16 applies the electric fields locally across the panel 14 by selecting a row of pixels by means of a row electrode 22, and by applying voltages for respective pixels in the relevant row by means of respective column electrodes 25. The drive unit repeats this operation for different rows until the entire image has been written.

Fig. 3 shows signals generated by the drive unit in order to control image display as a function of time. A first trace 30 represents the voltage on a column electrode 25; a second trace 32 represents the voltage on the first row electrode 22, and a third trace 34 represents the voltage on another row electrode 22a. The other traces 36, 38 relate to signals in the measuring unit 18 which will be described in detail hereinafter.

A row of pixels is written every 32 microseconds. To this end, the drive unit 16 controls the voltage 32 on the selected row electrode 22 to a "select" level, whereas the voltage 34 on the other row electrodes remains at a "hold" level. The voltage on the row electrode remains at the "select" level, for example for approximately 19 microseconds. For the period during which the selected row electrode 22 remains at the "select" level the drive unit 16 keeps the voltage 30 on the column electrode 25 at a level which corresponds to the intensity of the pixel situated at the intersection of this column electrode 25 and the selected row electrode 22. Via the capacitance 204 a measurable signal as represented by the fourth trace 36 of Fig. 3 appears on the measuring head 200. This signal exhibits a transition in response to the transition of the voltage on the row electrode 22 to the "select" level. The amplitude of this transition is dependent on the value of the capacitance 204 and is a measure of the distance between the measuring head 200 and the row electrode 22. (A capacitance also exists between the pixel electrodes 22b and the measuring head 200. The effect of this capacitance is comparable to that of the capacitance 204 between the row electrode 22 and the measuring head 200, because the voltage on these pixel electrodes 22b varies in response to voltage variations on the adjacent row electrode 22). The amplitude of the transition can be measured, for example by converting the signal on the measuring head into a digital signal (by means of an A/D converter in the measuring unit 18) and by subsequently digitally calculating, in the processing unit 12, the difference between the signal levels (extrapolated, if necessary) before and after the transition in the voltage on the row electrode 22. However, this can also be very simply carried out in an analog measuring circuit.

Fig. 4 shows a measuring circuit for use in the measuring unit 18 of an embodiment of a data processing system in accordance with the invention. The measuring head 200 in this circuit is coupled to the input of an input amplifier 40 via the signal lead 206. An output of the input amplifier 40 is coupled to the inputs of a differential amplifier 46, i.e. directly as well as via a sample-and-hold circuit 42. The input of the sample-and- hold circuit 42 is coupled to a first terminal of a capacitor 422, via successively a buffer amplifier 420 and a switch 44. The second terminal of the capacitor 422 is connected to ground. The drive unit 16 is coupled to the control input of the switch 44. A resistor 424 is connected parallel to the capacitor 422. The first terminal of the capacitor 422 is coupled to the output of the sample-and-hold circuit 42. The input amplifier 40 comprises air inverting amplifier 400 whose output is coupled to the input via a parallel connection of a capacitor 402 and a resistor 404. The values of the resistor 404 and the capacitor 402 are chosen so that the associated RC time is longer than the time required to write a row of pixels.

Fig. 3 shows, as a function of time, a number of signals occurring during use of the measuring circuit shown in Fig. 4. The fourth trace 36 represents the signal picked up by the measuring head 200 as it occurs on the output of the input amplifier 40. A fifth trace 38 shows the signal as it occurs on the output of the differential amplifier 46.

The fourth trace 36 also represents (dashed) the variation of the signal on the measuring head 200 in the absence of signal variations on the row electrode 22. The deviation of this variation is a response to the transition of the voltage on the row electrode 22 from the "hold" level to the "select" level and back again. This deviation is larger as the measuring head 200 is situated nearer to the row electrode 22 on which the voltage transition is induced. Furthermore, the deviation is independent of the image contents because the amplitude of the transition from the "hold" level to the "select" level is not dependent on the image contents and because the transition occurs in a time interval in which the contents- dependent voltage level on the column electrodes 25 remains the same.

The sample-and-hold circuit 42 is utilized to measure the deviation; the switch 44 in the circuit 42 is controlled by means of a signal from the drive unit 16 which indicates when the signal on one or more of the row electrodes 22, 22a changes. Just before this happens (but so long after the changing of the voltage on the column electrode 25 that the response thereto has been smoothed) this signal renders the switch 44 non-conductive. Some time later, before the drive unit changes the voltage on the column electrode 25 again, the switch 44 is rendered conductive again. Thus, a voltage difference temporarily occurs, i.e. when the switch 44 is not conductive, across the input of the differential amplifier 46. This results in a pulse-shaped signal on the output of the differential amplifier 46 as represented by the fifth trace 38 of Fig. 4. The amplitude of this pulse-shaped signal is dependent on the distance between the selected row electrode 22 and the measuring head 200. This amplitude is substantially independent of the image contents. Because the drive unit 16 successively activates a series of adjacent row electrodes 22, 22a, the signal on the output of the differential amplifier 46 will contain a series of pulse-shaped signals whose amplitude initially increases as the drive unit 16 activates row electrodes 22, 22a which are situated increasingly nearer to the measuring head 200. Subsequently, the amplitudes decrease again when row electrodes 22, 22a are activated which are situated increasingly further from the measuring head 200.

The maximum amplitude occurs at the instant at which the drive unit 16 selects the row electrode 22 situated nearest to the measuring head 200. The measuring unit 18 derives the position of the measuring head 200 relative to the panel 14 therefrom. This can be realised by taking the position as the location of the row electrode 22 whose selection leads to the maximum amplitude or, if more accurate positioning is required, by interpolation between the response to the selection signals on different row electrodes 22, 22a. This interpolation can be performed, for example by means of a family of modelled amplitude series, each for a respective different location of the measuring head. The measuring head 18 then selects the location corresponding to the series best matching the measured amplitudes. In Fig. 3, fourth trace 36, the variation of the signal on the output of the input amplifier which would occur in the absence of a change of the voltage on the row electrodes 22, 22a is denoted by a dashed line. This signal is not completely constant in time, because only capacitive coupling exists between the measuring head 200 and the electrodes 22, 25 of the panel 14, so that the transmission inevitably has a "high-pass" characteristic. The speed of decay of this signal is co-dependent on the image contents. This decay reduces the accuracy of the position measurement. If a higher accuracy is desired, this decay can be approximately compensated for.

To this end, a resistor 424 is inserted parallel to the capacitor 422 in the sample-and-hold circuit 42 in Fig. 4. This resistor serves to ensure that the voltage on the output of the sample-and-hold circuit 42 decays in exactly the same way as the voltage with which it is compared in the differential amplifier 46 would decay in the absence of signal transitions on the row electrodes 22, 22a and of a mean signal on the column electrodes 25. This largely compensates for the effect of the voltage on the column electrodes 25. If even higher accuracy is desired, the decay can also be calculated on the basis of a measured position of the measuring head 200 transversely of the column electrodes and the known contents of the signal on the column electrodes near that position. This quantity can be subtracted from the measured signal variation on the output of the differential amplifier 46. The resistor 424 can then be dispensed with. The position of the measuring head 200 can also be derived when more complex signals are used to control the image contents on the panel 14. To illustrate this, first a more complex control signal for an image display panel 14 will be discussed.

When liquid crystal displays are used, it is desirable that the mean pixel voltage in time is substantially zero. In order to achieve this, the drive unit 16 periodically reverses (for example, every other row) the polarity used to select the row electrodes 22, 22a. As a result, the polarity of the signal pulses on the output of the differential amplifier 46 periodically changes.

For control of image display the panel 14 comprises switching elements which are arranged between the row electrodes 22, 22a and the pixel electrodes 22b. The switching elements ensure that only the pixel electrodes 22c associated with the selected row electrode 22 are charged upon selection, and that the charge built up does not change when the drive unit 16 does not select the associated row electrode 22. For the switching elements use can be made of, for example two-terminal non-linear elements or switching transistors. When two-terminal non-linear elements are used, it has been found that the image contents are preferably always written on the panel with the same polarity.

Fig. 5 shows a voltage variation on the row electrodes 22, 22a etc. in order to ensure that the mean voltage is also maintained substantially zero. The Figure shows the voltage variation on a column electrode 25 and on three neighbouring row electrodes 22, 22a during the writing of two successive images.

In a first image the drive unit 16 ensures that the selection pulse 50a on a first row electrode 22a is preceded by a reset pulse 51a of opposite polarity. The selection pulses 50b,c on the neighbouring second and third row electrodes 22 are not preceded by a reset pulse in the first image. In the second image the situation is the other way around: the drive unit 16 ensures that the selection pulses 52b, c on the second and the third row electrode 22 are preceded by reset pulses 53b, c of opposite polarity, and that the selection pulse 54 for the first row electrode 22a is not preceded by a reset pulse. This is repeated, shifted in time, for successive row electrodes 22, 22a and for successive images.

Fig. 6 shows (in inverted form) the peak-to-peak transitions of the signal on the output of the differential amplifier 46 as a function of time over a time interval in which a series of adjacent row electrodes 22, 22a are successively selected. This signal comprises an introductory segment in which it hardly changes, a central segment in which it first exhibits an increasing amplitude and subsequently a decreasing amplitude because of the successive selection of row electrodes 22, 22a which are initially situated increasingly nearer to and subsequently increasingly further from the measuring head 200, and a terminal segment in which it hardly changes again. The maximum amplitude corresponds to the instant of selection of the row electrode 22 situated directly underneath the measuring head 200.

Signal amplitude still exists far from this instant. This is due to the i age- contents-dependent contribution of the voltages on the column electrodes. In order to improve the measuring accuracy, this contribution can be corrected, if necessary, on the basis of the time- dependency of the signal on the column lines which can be calculated on the basis of the image contents. However, usually this is not necessary.

Fig. 6 shows positive as well as negative peaks. These peaks are caused by positive and negative transitions 55, 56 in the selection signals shown in Fig. 5. The response to signal transitions 56 around an instant at which the drive unit 16 varies the voltage on only one row electrode 22 at a time and the voltage on the column electrode 25 does not change are more sensitive to the position of the measuring head 200. For improved positional accuracy the position of the measuring head 200 is preferably derived exclusively from these signal transitions. They correspond to the positive peaks in Fig. 6.

The position in a direction transversely of the row electrodes 22, 22a can be suitably measured by means of the described technique. The measurement of the position in the direction of the row electrodes 22, 22a is derived from the response of the signal on the measuring head to signal variations on the column lines. In its simplest form, this is realised in that the drive unit 16 applies test signals to the column electrodes 25 in the blanking period between successive images; no row electrodes 22, 22a are then selected. Different groups of neighbouring column electrodes 25 then successively receive a voltage pulse, different groups being situated in different locations on the panel 14; in the same way as described above for the row electrodes, the position of the measuring head 200 transversely of the column electrodes 25 can be determined therefrom.

It is to be noted that this technique for determining the position in a direction transversely of the column electrodes 25 does not form part of the invention. This technique and further techniques for deriving the position transversely of the column electrodes 25 is described in cited European Patent Application No. 504728. In accordance with the invention, however, it has been found that this technique can be used notably when it is applied to an active matrix panel (i.e. a pixel on which a switching element is provided for each pixel).

For many applications it is desirable to obtain position measurements of the measuring head 200 at a frequency which is higher than the image replacement frequency. To this end, it is advantageous not to select the row electrodes 22, 22a in the sequence in which they are situated on the panel 14, but in a sequence such that row electrodes are selected with shorter intermediate time intervals in the vicinity of any arbitrary position in which the measuring head 200 may be present. (If the image information is supplied in a different sequence, it must be stored in a memory so as to obtain the desired sequence).

Shorter intermediate time intervals are obtained, for example by writing the panel 14 in the interlace mode by means of the drive unit. To this end, a series of row electrodes 22, 22a which extends substantially across the entire panel is successively selected so as to control the information displayed by the panel 14. Between successive row electrodes of this series each time a number of other row electrodes are situated, which other row electrodes themselves are successively selected after all row electrodes of said series have been selected. From the response of the signal on the measuring head to the selection of the row electrodes of the series and the selection of the other row electrodes, the position of the measuring head 200 transversely of the row electrodes can be derived at a frequency which is higher than the image frequency.

The position transversely of the column electrodes can be determined at a frequency of more than once per image by interposing measuring periods in which test signals are appbed to the column electrodes 25, without selection of row electrodes 22, 22a, and by deriving the position therefrom.

It will be evident that the invention is not restricted to the described embodiments. Even though the invention has been described on the basis of a liquid crystal display, it can be used equally well for other panels comprising row and column electrodes, such as plasma displays and electroluminescent displays. Instead of a capacitive signal pick¬ up member, use can also be made of a magnetic signal pick-up member. In liquid crystal displays, however, a capacitive signal pick-up member is to be preferred because of the small currents occurring during the writing on this panel. Furthermore, the spherical shape of the measuring head is advantageous because the voltages picked up do not depend on the orientation of the measuring pen, but other measuring head shapes are also feasible.

Furthermore, instead of the measuring pen a signal pick-up member of a different shape, for example a loupe, can be used without objection. On the transparent conductive layer 27, reducing interference between the signal picked up by the measuring head 200 and fields from the light source 29, can also be omitted, provided that sufficient signal is picked up (or noise suppression is applied).

Furthermore, in Fig. 2 the row electrodes 22, 22a and the pixel electrodes 22b are provided on the side of the panel 14 which faces the measuring head 200. The pixel electrodes 22b are isolated from one another and are connected to the row electrodes 22, 22a via switching elements. Because of the insulation between the pixel electrodes and the fact that the pixel electrodes 22b are usually isolated from the row electrodes 22, 22a by the switching elements, the electric field of the column electrodes 25 can readily extend as far as the measuring head 200, so that a strong signal can be picked up from the column electrodes 25. Therefore, the side of the panel 14 on which loose pixel electrodes 22b are situated is preferably accessible as the viewing side of the panel. If the column electrodes 25 and the black mask 24 (preventing light transmission between the column electrodes 25) were provided on the side of the panel 14 which faces the measuring head 200, substantially less signal would be picked up from the row electrodes 22, 22a; depending on the desired accuracy, however, it may still suffice in given circumstances.

Claims

CLAIMS:

1. A data processing system, comprising a user interface provided with an image display panel with row and column electrodes and drive means coupled to generate electric signals on the row and column electrodes in order to generate an image on the image display panel, which interface is arranged to generate localization signals on the row and/or column electrodes and comprises a signal pick-up member which can be displaced relative to the image display panel in order to pick up measurement signals in response to the localization signals, which signal pick-up member is coupled to measuring means in order to derive from said measurement signals a position of the signal pick-up member relative to the image display panel, characterized in that the measuring means are arranged to detect the measurement signals in synchronism with the generation of at least a sub-set of the electric signals for generating the image, the sub-set of the electric signals serving at least as a sub¬ set of the localization signals.

2. A data processing system as claimed in Claim 1, in which said sub-set of electric signals concerns electric signal level transitions which are independent of the contents of an image.

3. A data processing system as claimed in Claim 2, characterized in that the drive means are arranged to switch on and off a row selection signal on a selected row electrode, an image signal remaining present on one or more of the column electrodes during said switching on or off, and that the measuring means are arranged to derive the position transversely of the row electrodes from a change in the measurement signal from before to after the switching on or off of the row selection signal.

4. A data processing system as claimed in any one of the Claims 1 to 3, characterized in that it comprises a signal generator which is coupled to the column electrodes in order to apply a column-dependent set of localization signals thereto during a measuring period, said set having substantially no effect on the image displayed, and that the measuring means are arranged to derive the position transversely of the columns from a response of the measurement signal to the set of test signals.

5. A data processing system as claimed in Claim 4, characterized in that the signal generator is arranged to generate the test signals during a blanking period of a signal controlling the image.

6. A data processing system as claimed in Claim 5, characterized in that the measuring means are arranged to measure the position transversely of the columns and/or the rows several times during the control of one and the same image.

7. A data processing system as claimed in Claim 6, characterized in that the drive means are arranged to select ifirst a first part of the row electrodes for image display and subsequently a second part of the row electrodes which spatially alternate with the row electrodes of the first part on the image display panel.

8. A data processing system as claimed in Claim 3, characterized in that the measuring means comprise a hold circuit for holding the measurement signal occurring prior to switching on or off, and a comparator circuit for comparing the signal held by the hold circuit with the measurement signal after switching on or off.

9. A data processing system as claimed in Claim 8, characterized in that the hold circuit is arranged to make the measurement signal held vary for simulating the variation of the measurement signal in the presence of an average image signal in the absence of switching on or off.

10. Measuring means for use in a data processing system as claimed in any one of the Claims 1 to 8.