Abstract:
A display device has a measuring circuit ( 14 ) to detect flicker due to the presence of a DC voltage by monitoring the pixel voltage and, if necessary, modifying driving signals.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a display device comprising an electro-optical material between two substrates, at least one of which is transparent, and a first substrate is provided with at least one picture electrode at the location of a pixel, each pixel being coupled to a row electrode and a column electrode, control means comprising first drive means for applying a selection signal to the row electrodes and second drive means for applying a data signal to the column electrodes. 
     Display devices of this type are used in, for example, televisions, monitors, laptop computers, etc. 
     Usually, the second substrate comprises one or more counter electrodes but this is not strictly necessary as in, for example, the case of “in-plane switching” (IPS). 
     Display devices of the type described above are generally known and are usually driven by means of alternating voltages across the pixels (AC driving) so as to prevent degeneration of the liquid crystal materials. Nevertheless, it has been found that, due to different causes, a parasitic DC component may be produced across the layer of liquid crystal material. This is particularly the case when the pixels have an asymmetrical structure, as is the case, for example, in reflective display devices (in which the display device comprises a reflector, or the picture electrodes on one of the substrates are reflecting). 
     Said DC component affects the drive of a pixel differently for opposite polarities in successive frame times. When the absolute voltages across a pixel in successive frame times (at the same data) differ, this will give rise to flicker at half the frequency of the frame frequency used (generally 50 or 60 Hz) which is very clearly visible in the image. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is, inter alia, an object of the invention to provide a display device of the type described above, in which the above-mentioned drawbacks are at least partly obviated. 
     To this end, a display device according to the invention is characterized in that the display device comprises a measuring element, and the control means comprise means for applying a voltage to the measuring element during a selection period and for measuring the variation of the voltage across the measuring element after the selection period, and for adapting, dependent on the measured voltage variation, at least one of the control voltages of the display device generated by the control means. 
     The control voltage to be adapted is, for example, a voltage of a line selection signal, a data signal, a reference voltage of the display device (for example, a reset voltage, or the voltage across a control electrode) or, when the second substrate comprises at least one counter electrode, the voltage of a signal across the counter electrode. 
     The invention is applicable to display devices of the passive and active type. 
     For example, a line selection period of the display device is chosen for the selection period. 
     It is found that the parasitic DC component may give rise to differences in the (measured) voltage variation in the different (positive and negative) frame periods. When driving pixels, this difference leads to said flicker. By comparing the variation of the voltage after two consecutive selection periods (or in two consecutive frame periods) with each other and by adapting one of the control voltages, dependent on the measured result, the flicker is reduced considerably. 
     In a first implementation (active display device), each pixel is coupled to the row electrode or the column electrode via a switching element. In this case, the measuring element may be constituted by, for example, a row of pixels, but preferably the display device is provided with a row of extra (dummy) pixels. 
     The measured voltage difference can be compared with a variation stored in advance in the control means, for example when the display device is adjusted in advance. 
     A preferred embodiment of a display device according to the invention is, however, characterized in that the control means comprise means for reversing the sign of the polarity of the voltage across the measuring element and for measuring the difference between the voltage directly after the selection period and the voltage just before a subsequent selection period, and means for adapting the control voltage of the display device in such a way that the absolute value of the voltage difference for both polarities is substantially the same. A small number of measurements may then be sufficient. In this case, measurement and correction take place continuously. In another implementation, measurement and correction are performed once, for example when switching on the display device, or periodically. 
     When used in passive displays, for example, a measuring element (outside the actual display section) is used which is directly driven from the control means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     In the drawings: 
     FIG. 1 is a diagrammatic cross-section of a part of a display device while 
     FIG. 2 shows diagrammatically an equivalent circuit diagram of a part of a display device according to the invention, and 
     FIGS. 3 to  5  show drive signals and an internal signal of the display device, while 
     FIG. 6 shows diagrammatically an equivalent circuit diagram of a part of a display device according to the invention, and 
     FIG. 7 shows a detail of FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a diagrammatic cross-section of a part of a liquid crystalline display device  1 , for example of the size of a few pixels, comprising a liquid crystal cell with a twisted nematic liquid crystalline material  2  which is present between two substrates  3 ,  4  of, for example, glass provided with, for example, transparent (ITO) picture electrodes  6  and a reflecting counter electrode  5 . The device further comprises two polarizers  7 ,  8  whose directions of polarization are mutually crossed perpendicularly. The cell further comprises orientation layers  9  which orient the liquid crystalline material on the inner walls of the substrates. In this case, the liquid crystalline material has a positive optical anisotropy and a positive dielectric anisotropy. If the electrodes  5 ,  6  are energized by an electric voltage, the molecules and hence the directors direct themselves to the field. 
     The voltage across the picture electrodes is determined by the drive mode. FIG. 2 shows diagrammatically a picture display device  1  which is driven with active switching elements, in this example thin-film transistors. It comprises a matrix of pixels  18  at the area of crossings of row or selection electrodes  17  and column or data electrodes  11 . The row electrodes are consecutively selected by means of a row driver  16 , while the column electrodes are provided with data via a data register  10 . If necessary, incoming data  13  is first processed in a processor  15 . Mutual synchronization between the row driver  16  and the data register  10  takes place via drive lines  12 . 
     Drive signals from the row driver  16  select the picture electrodes via thin-film transistors (TFTs)  19  whose gate electrodes  40  are electrically connected to the row electrodes  17 , and the source electrodes  21  are electrically connected to the column electrodes  11 . The signal present at the column electrode  11  is transferred via the TFT to a picture electrode of a pixel  18  coupled to the drain electrode  22 . The other picture electrodes are connected to, for example one (or more) common counter electrode(s)  24 . 
     In this embodiment, the display device of FIG. 1 also comprises an auxiliary capacitor  23  at the location of each pixel. In this embodiment, the auxiliary capacitor is connected between the common point of the drain electrode  22  and the pixel in a given row of pixels, on the one hand, and the row electrode of the previous row of pixels, on the other hand. Other configurations are alternatively possible, for example, an auxiliary capacitor between said common point or one of the subsequent rows of pixels (or a previous row). It is to be noted that these auxiliary capacitors do not occur in all display devices based on TFTs. 
     To prevent picture deviations, the display device of FIG. 2 comprises an extra row electrode  17 ′. 
     FIGS. 3 and 4 show drive signals of the display device. At the instant t 0 , a row electrode  17  is energized by means of a signal V sel  (FIG.  3 ), while simultaneously data signals V d  (FIG. 4) are presented to the column electrodes  11 . After a line selection time t L  has elapsed, a subsequent row of electrodes is selected at instant t 1 , etc. After some time, for example, a field time or a frame time, usually 20 msec or 16.7 msec, said row electrode  17  is energized again at instant t 2  by means of a signal V sel , while simultaneously inverted data signals V d  are presented to the column electrodes  11  (in the case of an unchanged picture). After a line selection time t L  has elapsed, the next row of electrodes is selected at instant t 3 , etc. This is repeated again at the instant t 4 . 
     Since the data voltages are inverted at every subsequent selection of the pixel, the voltage across the pixel changes polarity, as is shown in FIG.  5 . During the period when the pixel is not selected, the absolute value of this pixel voltage decreases. The voltage decrease is also dependent on the polarity, notably, but not exclusively, in pixels having an asymmetrical structure, as is the case in reflective display devices or when different materials or thicknesses of, for example, orientation layers on both substrates are used. The voltage decrease is determined by a DC component which is inherent in the structure of the device. After the (positive) selection during the period t 0 -t 1 , the (absolute value of the) pixel voltage V P  decreases from V 1  to V 2 ′ (broken line  25  in FIG. 5) during the other part t 1 -t 2  of the frame time (t f ) without special measures. Likewise, the (absolute value of the) pixel voltage V P  decreases from V 3  to V 4 ′ (broken line  25 ′ in FIG. 5) during the other part t 3 -t 4  of the frame time after the (negative) selection during the period t 2 -t 3 . Since the voltage decrease for the two polarities ΔV a =V 1 -V 2 ′ and ΔV b =V 4 ′-V 3  is asymmetrical, this is visible as flicker at half the time frequency (25 or 30 Hz). 
     According to the invention, the variation of the voltage across one or more pixels is measured; preferably, a row of dummy pixels (not used for the actual display) is controlled for this purpose by means of, for example, the data voltage for medium grey, and one or more control voltages are adapted in such a way that (in this example) ΔV a =ΔV b . The control voltages to be adapted may be data or selection voltages but also, for example the voltage across the counter electrode. The voltage across the pixel then has the variation as shown by the solid lines  26 ,  26 ′ (ΔV a  =V 1 -V 2 =ΔV b =V 4 -V 3 ). Instead of measuring the voltage difference between the voltages at the beginning and the end of the non-selection period, it is also possible to measure at several positions in the time-voltage curve (or a voltage integral may be determined). 
     In the case of passive display devices, the switches  19  are absent. Pixels are now defined by overlapping parts of row and column electrodes. For the purpose of measurement, one or more measuring elements are provided which are driven and measured by means of extra electrodes (for example) via the processor  15 . Dependent on the measured result, for example, the line selection voltages are adapted. 
     FIG. 6 shows diagrammatically the display device with a picture area  27  and a row of dummy pixels  28 , which functions as a measuring element and is selected by means of a selection electrode  17 ″. The signal across the pixels  28  is simultaneously applied via a measuring electrode  37  to a measuring section  14  of the processor  15  in which the values V 1 , V 2 , V 2 ′, V 3 , V 4 , V 4 ′ are stored and compared. If necessary, one of the drive signals is adapted until ΔV a =V 1 -V 2  =ΔV b =V 4 -V 3 . For the purpose of measurement (and possible adaptation of the bias voltage), the common counter electrode  24  is also connected via a measuring electrode  37  in this embodiment. 
     Corresponding signals for the picture area  27  are adapted in a similar manner. Instead of a row of dummy pixels, a pixel from the picture area  27  may also be used as a measuring element, for example, prior to its actual use (when the display device is switched on). 
     The signal across the dummy pixels  28  is applied via the measuring electrode  37  in the measuring section  14  to an input section  30  with an operational amplifier  29  (FIG.  7 ). Its output is applied to a sample-and-hold section  31  via switches  35  successively at the instants t 1 , t 2 , t 3 , t 4  so that the voltages V 1  , V 2 , (V 2 ′), V 3 , V 4 , (V 4 ′) are stored via operational amplifiers  29 ′ and subsequently processed in a differential amplifier  32  consisting of operational amplifiers  29 ″ and resistors  33 ,  33 ′. The difference voltages ΔV a , ΔV b  are compared in the output amplifier  36  consisting of operational amplifiers  29 ′″ and resistors  34 ,  34 ′. Dependent on a possible voltage at the output of the output amplifier  29 ′″, one or more control voltages are adapted until ΔV a =ΔV b  or, if necessary, ΔV a =ΔV b +c, in which c is a constant voltage. 
     It will be evident that many variations within the scope of the invention can be conceived by those skilled in the art. For example, the measuring electrodes may also be spread on the surface of the display device. 
     In summary, the invention relates to a display device comprising a measuring circuit for detecting flicker due to, for example, DC offset (in both active and passive display devices) and, if necessary, adapting control voltages. The invention is based on each novel characteristic feature and each combination of characteristic features.