WO2005104079A1

WO2005104079A1 - Enhanced overdrive for displays

Info

Publication number: WO2005104079A1
Application number: PCT/IB2005/051195
Authority: WO
Inventors: Roel Van Woudenberg; Hendrikus W. Groot Hulze
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-04-26
Filing date: 2005-04-12
Publication date: 2005-11-03

Abstract

The invention relates to a driver (4) for driving a pixel (3) of a display panel (2). The driver (4) comprises a signal processor (5) which supplies a drive voltage (V) to the pixel (3) of the display panel (2) in response to an input signal (L). The input signal (L) represents the desired luminance (Ld) of the pixel during an addressing period (TP). When a luminance change of the pixel is requested during an addressing period (TP), the supplied drive voltage (V) is adapted to obtain an average luminance in the addressing period (TP) which is substantially equal to the desired luminance (Ld) during that addressing period (TP). The luminance, perceived by a viewer, during the addressing period (TP) during which a luminance change is requested is now equal to the desired luminance (Ld), reducing blurred edges and ghost images.

Description

Enhanced overdrive for displays

FIELD OF THE INVENTION: The invention relates to a driver for driving a pixel of a display panel. The invention also relates to a display module comprising such a driver, an apparatus comprising such a display module and a method of driving a pixel of a display panel.

BACKGROUND OF THE INVENTION: LCD display modules are increasingly used for displaying motion pictures and TV signals. Fast moving objects within a picture are a challenge to an LCD display module. The reason is the response time of the pixels of an LCD display module to a required change in luminance. The document "Fast Response IPS-LCD Using Feed-Backward Overdrive

Technology" [D.Nakano and T.Minami, IBM research Tokyo Res. Lab., presented during the ninth International Display module Workshop IDW '02, Japan, December 4-6 2002] discloses a method to improve the response time of the pixels using feedback overdrive. Without overdrive, when a luminance change of a pixel is required, a drive voltage is applied to the pixel such that a desired luminance will be reached in the end. The luminance of the pixel gradually changes from a starting luminance to the desired luminance. If motion pictures or TV signals have to be displayed, the required change in luminance needs to be achieved within a short time period, the so called frame period. The frame period is the duration during which a single image of a motion picture or TV signal is supplied to the display module. Or said differently, during the frame period or addressing period all the pixels of the display panel are addressed once to receive a drive voltage. When applying the drive voltage, necessary for achieving the desired luminance to the pixel, the actual luminance of the pixel lags behind the desired luminance, due to the inertia of the pixel. It may take several addressing periods until the desired luminance is achieved, causing blurred edges or ghost images. To shorten the response time of a pixel, the prior art referred to applies an overdrive voltage. The level of the overdrive voltage exceeds the level of the drive voltage required to obtain the desired luminance in the end and thus targets at an overdrive luminance exceeding the desired luminance. When applying the overdrive voltage it usually takes several addressing periods until the overdrive luminance would be achieved. But, when the overdrive voltage is selected carefully, the luminance achieved at the end of a single addressing period is equal to the desired luminance. Now, the desired luminance is reached within one addressing period and thus the response time of the pixel is artificially increased. The overdrive voltage required to achieve the desired luminance depends on the required luminance change and the starting luminance, and further depends on other variables, for example, on the type of display module and the frame rate at which the display is operated. Therefore, the overdrive voltages usually are listed in Look Up Tables (LUT). A further improvement of the overdrive approach is achieved by using the actually achieved luminance at the end of the previous addressing period as starting luminance for the luminance change of the current addressing period. This is called feedback overdrive and is disclosed in the earlier mentioned prior art document. It has been observed that blurred edges and ghost images are still present when using feedback overdrive for displaying fast moving images.

SUMMARY OF THE INVENTION: It is an object of the invention to further reduce blurred edges and ghost images. A first aspect of the invention provides a driver comprising a signal processor for adapting the drive voltage of the pixels as claimed in claim 1. A second aspect of the invention provides a display module as claimed in claim 7. A third aspect of the invention provides an apparatus as claimed in claim 9. A fourth aspect of the invention provides a method for driving a pixel of a display module as claimed in claim 10. Advantageous embodiments are defined in the dependent claims. The driver in accordance with the first aspect of the invention drives a pixel of a display panel. The driver comprises a signal processor for receiving an input signal which represents the desired luminance of the pixel in successive addressing periods. The signal processor supplies a drive voltage to the pixel in the addressing period. The signal processor is arranged to determine the drive voltage necessary to obtain a luminance change of the pixel. The luminance has an average value which is substantially equal to the desired luminance. The effect of adapting the drive voltage of the pixel according to the invention is that the average value of the luminance of the pixel is substantially equal to the desired . luminance. When a luminance change is required during a specific addressing period, the drive voltage adapted according to the invention is applied to the pixel. This causes the average value of the luminance of the pixel to be substantially equal to the desired luminance of the pixel during the specific addressing period. By generating the average value of the luminance of the pixel to be substantially equal to the desired luminance in an addressing period in which a luminance change is required, blurred edges and ghost images are further reduced. The inventors have seen that blurred edges and ghost images mainly occur during addressing periods where a luminance change takes place. It has been observed that the luminance, as experienced by the human eye, is an average luminance over the addressing period rather than a final value of the luminance reached at the end of an addressing period. Overdrive, as it has been used up to now, achieves a luminance change wherein the desired luminance is reached at the end of the addressing period. This implies that each one of the pixels which require a luminance change, have an average luminance which cannot be substantially equal to the desired luminance and thus blurred edges and ghost images will be observed by the human eye. By adapting the drive voltage according to the invention, the average value of the luminance of each one of the pixels which require a luminance change is substantially equal to the desired luminance. The human eye experiences the desired luminance at the appropriate pixels already during the addressing period within which the luminance change takes place, thus substantially reducing blurred edges and ghost images. A further effect of the invention is that a luminance overshoot or undershoot occurs at the end of the addressing period in which a luminance change is induced. To enable the average luminance to be equal to the desired luminance, the resulting luminance at the end of the addressing period needs to exceed the desired luminance, thus creating the overshoot or undershoot. The drive voltages of subsequent time periods all need to be adapted according to the invention, ensuring that the average value of the luminance of the subsequent addressing periods are equal to the desired luminance value levels of the corresponding subsequent addressing periods. US 2002/0044115 discloses an LCD drive method in which the average luminance over a so called frame period is equal to the desired luminance. To achieve this average luminance, the LC cells are addressed a plurality of times, preferably twice, within this so-called frame period. During the first half of the mentioned frame period the supplied drive voltage results in a luminance overshoot or undershoot and during the second half of the so-called frame period the supplied drive voltage results in the desired luminance. This prior art discloses a method wherein the average luminance over the whole frame period is equal to the desired luminance, but wherein it is essential that the LC cells are addressed a plurality of times within each frame period. Consequently, several addressing periods are present during a single one of the so-called frame periods to obtain the desired average value. In contrast, in the present invention only a single addressing period is present in the frame period to obtain the desired average value in a single addressing period. However, luminance changes within motion pictures and TV signals need to be achieved within a short time period and current LC cells are unable to respond fast enough to achieve the response disclosed in this prior art. Therefore, the method disclosed in this prior art may result in a reduction of blurred edges or ghost images, but will lead to a too slow refresh rate of the display due to the current refresh rate specifications of the current LC cells and therefore does not disclose a practical implementation. In an embodiment in accordance with the invention as claimed in claim 2, the display panel is arranged to be illuminated by a scanning illuminator for producing light for a particular subgroup of pixels during a predetermined time window of the addressing period. The signal processor is arranged for supplying a drive voltage to the pixels of the particular subgroup to obtain a luminance change. The changing luminance has an average value over the predetermined time window of the addressing period which is substantially equal to the desired luminance multiplied by a ratio of a duration of the addressing period (TP) and a duration of the time window (T). The scanning illuminator scans across the display panel, successively illuminating particular sub-groups of pixels. During one addressing period, the whole display module is scanned and all pixels have been illuminated. While scanning, each pixel of the display module will only be illuminated by the scanning backlight during a sub- period of the addressing period. This means that the transmission of each pixel of the display module is only relevant during the sub-period during which the pixel is illuminated by the scanning illuminator. The drive voltage of the pixels is determined such that the average value of the luminance of the pixel during the sub-period is equal to the desired luminance multiplied by a ratio of a duration of the addressing period and a duration of the time window. This multiplication is required to obtain the desired luminance which is the luminance during the time window averaged over the addressing period. The benefit of the current embodiment is that only during the sub-period of the addressing period the average value of the luminance needs to be equal to the desired luminance multiplied by a ratio of a duration of the addressing period and a duration of the time window to reduce blurred edges and ghost images. In an embodiment in accordance with the invention as claimed in claim 4, the signal processor of the driver comprises a first Look Up Table for determining a drive value of the drive voltage. A cell of the first Look Up Table provides for combinations of a desired luminance value and a previous luminance value the corresponding drive value. The desired luminance value represents the desired luminance and the previous luminance value is equal to the desired luminance value or the drive value of the preceding addressing period. One of the advantages of using a Look Up Table to determine the necessary drive value is that characteristics of the display module can be easily incorporated into the table. In an embodiment in accordance with the invention as claimed in claim 5, the signal processor of the driver comprises, next to the first Look Up Table, a second Look Up Table for determining a resulting luminance value. A cell of the second Look Up Table provides for combinations of the desired luminance value and the previous luminance value or for combinations of the drive value and the previous luminance value the corresponding resulting luminance value. The resulting luminance value represents the luminance which will be achieved by the pixel at the end of the current addressing period when the drive voltage resulting from the drive value is applied to the pixel. The signal processor of the driver as claimed in claim 5 further comprises a frame memory for storing the resulting luminance and for providing the previous luminance to the first Look Up Table and to the second Look Up Table. The resulting luminance value is stored in the frame memory and used as previous luminance value in the next addressing period. The benefit of this embodiment is that the previous luminance value, which will be used in the next addressing period, represents an actually achieved luminance during the current addressing period due to the applied drive voltage. One of the effects of the current invention, as mentioned earlier, is that a luminance overshoot or undershoot occurs at the end of the addressing period with respect to the desired luminance level. This enables the average value of the luminance to be equal to the desired luminance. The resulting luminance value which is determined from the second Look Up Table takes the luminance overshoot or undershoot into account and enables the previous luminance value to represent the actual luminance reached by the pixel at the end of the current addressing period. The drive value for the next addressing period will use the resulting luminance value of the current addressing period as previous luminance value, thus taking the luminance overshoot or undershoot into account. In an embodiment of the driver, the first Look Up Table is converted into a third Look Up Table, comprising a delta-drive value rather than a drive value and/or the second Look Up Table is converted into a fourth Look Up Table, comprising a delta-resulting luminance value rather than a resulting luminance value. For determining the drive value from the third Look Up Table, the delta-drive value needs to be added either to the desired luminance value or to the previous luminance value. For determining the resulting luminance value from the fourth Look Up Table, the delta-resulting luminance value needs to be added either to the previous luminance value, or to the desired luminance value or to the drive value. One benefit of these embodiments may be that the delta- values stored in the Look Up Tables require less storage capacity, which leads to cost reduction. Another benefit of these embodiments is that it enables a small adaptation of the drive value or the resulting luminance value by simply applying a correction factor. The delta-drive value or to the delta- resulting luminance value is multiplied with the correction factor to correct for, for example, a small change in the frame rate. In an embodiment of the Look Up Table the drive value or resulting luminance value are determined using interpolation methods between the cells of the Look Up Table. The benefits of using interpolation methods are that either the size of the Look Up Table can be smaller, reducing the required storage capacity, or the accuracy of the Look Up Table can be increased without increasing the storage capacity. In an embodiment of the Look Up Table, the drive value or resulting luminance value are determined using a functional representation of the characteristics of the pixels as is listed in the Look Up Table. The values, which are combined in the Look Up Table to determine the cell from which the content is provided, are the input variables of the functional representation. So, instead of a Look Up Table a calculator may be present for calculating drive values. The benefit of this embodiment is that no storage capacity is necessary. 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 module according to the invention, Fig. 2A and 2B show the drive voltage supplied to a pixel using a prior art overdrive system for a luminance change, and the luminance response of the pixel, Fig. 2C and 2D show the drive voltage supplied to a pixel using the drive voltage determined according to the invention, and the perceived luminance of the pixel, Figs. 3 show a configuration of the signal processor within the driver having a frame memory and a first Look Up Table, wherein Fig. 3 A shows a basic configuration, Fig. 3B shows a feed- forward configuration, and Fig. 3C shows a feed-back configuration, Fig. 4 shows an example of a first Look Up Table, Figs. 5 A and 5B show two configurations of the signal processor within the driver wherein a second Look Up Table is added for determining a resulting luminance value, Fig. 6 shows an example of a second Look Up Table, Figs. 7A and 7B show two configurations of the signal processor within the driver wherein both look up tables comprise delta- alues, and Fig. 8 shows a configuration of the signal processor within the driver wherein the fourth Look Up Table is replace by a functional representation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS: Fig. 1 shows a block-diagram of a display module 1 according to the invention. The display module 1 comprises a display panel 2, comprising a two-dimensional array of pixels 3, driver electronics 4 and an optional illuminator 6 for illuminating the pixels 3. The illuminator 6 for illuminating the pixels 3 may be a continuous light source or a flashing or scanning light source, for example, a scanning or scrolling backlight when direct- lit displays are used and a scrolling lighting when reflective displays are used, and is well know to one skilled in the art. The driver electronics 4, further also referred to as driver 4, of the display module 1 comprise a signal processor 5 to receive an input signal representing a luminance L for each one of the pixels 3 of the display panel 2. The signal processor 5 determines a drive voltage V which sets the luminance of each one of the pixels 3 to a desired luminance. The input signal is provided to the driver 4 in an addressing period. When the display module 1 is used for showing motion pictures or TV signals, the repetition frequency at which the images of the motion picture or TV signal are supplied to the display module 1 is the frame rate. The addressing period TP has the duration of a frame. Although the description of the luminance transition and the determination of the required drive voltage V is focused on a single pixel 3 of the display module 1, it is valid for each one of the pixels 3 of the display module 1. In Fig. 1 three successive frames 7 are shown of a single pixel 3: FI, F2 and

F3. The current luminance Lp of the pixel 3 is shown in the first frame FI in Fig. 1 and represents a relatively dark pixel 3. In the second frame F2 the luminance of the pixel 3 needs to change from the current luminance Lp to the desired luminance Ldl. In the third frame F3 the luminance Ld2 should remain equal to the luminance Ldl of the second frame, so: Ldl = Ld2. The drive voltages V through which to achieve the desired luminance in frame F2 and F3 in accordance with the prior art are shown in Figs. 2 A to 2B. Fig. 2A and 2B show the drive voltage V supplied to a pixel 3 using a prior art overdrive system for a luminance change, and the perceived luminance PL of the pixel 3, respectively versus time t. During a frame FI, which lasts from tl to t2 a drive voltage Vp is supplied. When no overdrive is applied, the luminance transition from frame FI to F2 is achieved by supplying a drive voltage Vd to the pixel 3. Because of the inertia of the pixel 3, the perceived luminance PL of the pixel 3 does not change instantaneously, but changes gradually from Lp (previous luminance) to Ld (desired luminance). To ensure that the required luminance change takes place within one addressing period TP, overdrive is performed, wherein the overdrive voltage Vo is used which targets the luminance Lo exceeding the desired luminance Ld (see Fig. 2B). The overdrive voltage Vo is selected such that the luminance at the end of the addressing period lasting from instant t2 to the instant t3, further referred to as frame period F2, is equal to the desired luminance Ld. The overdrive voltage Vo exceeds the drive voltage Vd with which in the end (after more than one addressing period TP) the desired luminance Ld would be reached. Between the frames F2 and F3 no luminance change is required so that the drive voltage V supplied to the pixel 3 at the beginning of the third frame F3 is reduced to the desired drive voltage Vd for holding the luminance of the pixel 3 at the desired luminance Ld. It can be clearly seen from Fig. 2B that the average luminance over the addressing period F2, or over a predefined time window T within the addressing period F2 is not equal to the desired luminance Ld. Furthermore, the deviation from the desired luminance Ld increases when the duration of the time window T is increased or when the time window T is moved towards t2 within the addressing period F2. The human eye experiences an average luminance for each one of the addressing periods TP and will experience the luminance of the addressing period F2 as too low, resulting in blurred edges and ghost images. This can be improved by implementing the drive scheme according to the invention, as shown in Figs. 2C and 2D. Fig. 2C and 2D show the drive voltage V supplied to a pixel 3 using the drive voltage determined according to the invention, and the perceived luminance PL of the pixel 3, respectively versus time t. When at frame F2 a luminance change is introduced for pixel 3, the driver 4 supplies, for the same transition, a drive voltage Veol exceeding the overdrive voltage Vo as defined in Fig. 2A. This drive voltage Veol targets the luminance Leo, which would be reached if this drive voltage is maintained for several frame periods. The resulting luminance Leol of the pixel 3 at the end of the addressing period TP of the frame F2 exceeds the desired luminance Ld as can be clearly seen from Fig. 2D. However the supplied drive voltage Veol is selected such that the average luminance La during a predefined time window T within the addressing period F2, when averaged over the whole addressing period F2, is equal to the desired luminance Ld. The average luminance La is thus equal to the desired luminance Ld multiplied by a ratio of a duration of the addressing period TP and a duration of the predefined time window T. Therefore, the human eye will not experience the luminance of the addressing period F2 as too low and thus the visibility of blurred edges and ghost images decreases. It has to be noted that although the curve showing the perceived luminance PL is also shown outside the time window T, usually, the pixel 3 will not be illuminated during this period in time. When the luminance in frame F3 needs to be equal to the luminance in frame F2, a new drive voltage Veo2 needs to be determined which will result in a luminance Leo2 of the pixel 3 at the end of the frame period F3 which again exceeds the desired luminance Ld, but in this case shows a lower luminance than the desired luminance Ld. This new drive voltage Veo2 again is selected such that the average luminance La during the predefined time window T within addressing period F3, when averaged over the whole addressing period F3, is equal to the desired luminance Ld. Again the human eye will experience an average luminance over the addressing period F3, which is equal to the desired luminance Ld and thus the visibility of blurred edges or ghost images decreases. If the duration or location within the addressing period TP of the predetermined time window T changes, the drive voltages Veol and Veo2 need to be adapted such that the average value La of the luminance within the changed time window is equal to the desired luminance Ld. The same reasoning is valid when the luminance between frame FI and frame F2 changes from a high luminance to a low luminance. In an embodiment of the display module 1, more than one predetermined time window may be present within the addressing period TP. For example, the display module 1 may have a scanning illuminator 6 which illuminates each pixel 3 during several separate predetermined time windows within the addressing period TP. The drive voltage V supplied to the pixel 3 to introduce a luminance change must be adapted to obtain the correct average value of the luminance over the addressing period TP. In fact, the average over the addressing period TP of the sum of the integrals over the luminance within each one of the time windows should be equal to the desired luminance Ld. Or, said differently, the sum of average values per time window, respectively multiplied with a corresponding ratio should be equal to the desired luminance Ld. The corresponding ratio is the duration of the respective time window divided by the duration of the addressing period TP. Fig. 3 A shows a basic configuration of the signal processor 5 within the driver 4 having a frame memory FM and a first Look Up Table LUTl. In this basic configuration, the signal processor 5 uses a first Look Up Table LUTl for determining the drive value DVdp of the drive voltage. The first Look Up Table LUTl uses a desired luminance value LVd, which is related to the desired luminance Ld, and a previous luminance value LVp, supplied by the frame memory FM, as input. The previous luminance value LVp represents the luminance of the pixel 3 at the end of the previous addressing period TP. Fig. 3B shows a feed- forward configuration, in which the previous luminance value LVp is determined from the desired luminance value LVd. The frame memory FM stores the desired luminance value LVd during one addressing period TP and supplies this delayed luminance value LVd as the previous luminance value LVp to the signal processor 5. Fig. 3C shows a feed-back configuration, in which the previous luminance value LVp is determined from the drive value DVdp. In this configuration, the frame memory FM stores the drive value DVdp during one addressing period TP and supplies this delayed drive value DVdp as the previous luminance value LVp to the signal processor 5. Fig. 4 shows an example of a first Look Up Table LUTl. The use of look up tables is common within display modules for determining drive values. The drive values listed in the look up tables are depending on several variables, for example, on the type of display module used and on the temperature at which the display module operates and on the frame-rate. In this first Look Up Table LUTl, the drive values DVdp for a specific addressing period TP are selected such that the resulting average value of the luminance over a predefined time window T is equal to the desired luminance Ld for that addressing period TP. In the first Look Up Table LUTl, the previous luminance value LVp is listed in the most left column and the desired luminance value LVd is listed in the top row. For each combination of the previous luminance value LVp and the desired luminance value LVd, a drive value DVdp is found at the crossing point within the first Look Up Table LUTl. Thus, the difference with the prior art is that this first Look Up Table LUTl comprises different values in that now the supplied drive voltage Veol, Veo2 results in an average luminance La during the time window T. Consequently, the average luminance over the whole addressing period is equal to the desired luminance Ld. Figs. 5 A and 5B show two configurations of the signal processor 5 within the driver 4 wherein a second Look Up Table LUT2 is added for determining a resulting luminance value LVr. In these configurations the signal processor 5 comprises two look up tables LUTl, LUT2 and a frame memory FM for determining the drive value DVdp. The resulting luminance value LVr represents a luminance actually achieved at the end of the addressing period TP in response to the applied drive voltage V, depending on the characteristics of the display panel 2. In a first configuration (Fig. 5 A) the resulting luminance value LVr is determined using the same input parameters as are used for determining the drive value DVdp from the first Look Up Table LUTl. In a second configuration (Fig. 5B) the resulting luminance value LVr is determined using the drive value DVdp, which is determined from the first Look Up Table LUTl, and the previous luminance value LVp as input parameters. In both configurations the resulting luminance value LVr is determined according to the second Look Up Table LUT2. The benefit of the embodiment shown in Fig. 5B is that when the first Look Up Table LUTl is changed due to, for example, a change in scanning frequency, the second Look Up Table LUT2 can remain identical. This . is especially beneficial when a dynamic backlight is used. The resulting luminance value LVr is then stored in the frame memory FM for the duration of one addressing period TP. In the next addressing period TP the frame memory FM provides the stored resulting luminance value LVr as the previous luminance value LVp to both the first Look Up Table LUTl and the second Look Up Table LUT2. The resulting luminance value LVr provided by the second Look Up Table LUT2 may depend on the same variables as the first Look Up Table LUTl, for example, on the type of display module 1 used and on the temperature at which the display module 1 operates. The benefit of these configurations is that the actually achieved luminance at the end of the previous addressing period TP is used for determining the drive value DVdp for the current addressing period TP. This can be explained in more detail when referring back to the Fig. 2D. The resulting luminance value LVr determined by the second Look Up Table LUT2 during the addressing period F2 is equal to an overshoot luminance Leol which is the luminance achieved at the end of the addressing period F2, resulting from the drive voltage Veol. For the next addressing period F3 a new drive voltage Veo2 needs to be determined such that the average value La of the luminance over a predefined time window T, when averaged over the whole addressing period F3, is equal to the desired luminance Ld. The parameters used by the first Look Up Table LUTl are the desired luminance value LVd and the previous luminance value LVp, which in this case is equal to the overshoot luminance Leol as determined by the second Look Up Table LUT2 in the previous addressing period F2. During the addressing period F3, the second Look Up Table LUT2 determines a new resulting luminance value LVr, being an undershoot luminance Leo2, which again will be used as previous luminance value LVp in the following addressing period TP. If the actually achieved luminance at the end of the previous addressing period TP is not used as the previous luminance value LVp for determining the drive value DVdp, the overshoot Leol as shown in Fig. 2D is not taken into account in determining the drive value DVdp of the subsequent addressing period F3. This may result in a drive voltage V in the subsequent addressing period F3 in which the average value La of the luminance over a predetermined time window T of the addressing period TP, when averaged over the whole addressing period TP, is not substantially equal to the desired luminance Ld of the addressing period F3. Fig. 6 shows an example of a second Look Up Table LUT2 as it may be used in Fig. 5B. The principle of the look up table is identical to what is described in Fig. 4. The second Look Up Table LUT2 shown in Fig. 6 uses the drive value DVdp and the previous luminance value LVp as input parameters for determining the resulting luminance value LVr. As indicated in Fig. 5 A also the desired luminance value LVd can be used as input parameter, next to the previous luminance value LVp. Then, of course, also the content of the second Look Up Table LUT2 has to be adapted. Figs. 7A and 7B show two configurations of the signal processor 5 within the driver 4 wherein both look up tables comprise delta- values. Also in these configurations, the signal processor 5 comprises two look up tables LUT3, LUT4 for determining respectively the drive value DVdp and the resulting luminance value LVr. The first Look Up Table LUTl is converted into a third Look Up Table LUT3 by subtracting one of the input values, (either the desired luminance value LVd or the previous luminance value LVp) from the values provided in the cells of the first Look Up Table LUTl (being the drive value DVdp). This results in a third Look Up Table LUT3 comprising delta-drive values ΔDVdp. The second Look Up Table LUT2 is converted into a fourth Look Up Table LUT4 by subtracting one of the input values (either one value from the combination of the desired luminance value LVd and the previous luminance value LVp or one value from the combination of the drive value DVdp and the previous luminance value LVp) from the values provided in the cells of the second Look Up Table, being the resulting luminance value LVr. This results in a fourth Look Up Table LUT4 comprising delta-resulting luminance values ΔLVr. The signal processor 5 further comprises a frame memory FM for storing the resulting luminance value LVr during one addressing period TP and supplies this delayed resulting luminance value LVr as the previous luminance value LVp to the signal processor 5 for use in both look up tables LUT3, LUT4. The difference with the configurations as shown in Figs. 5A and 5B is that the cells of the look up tables LUT3, LUT4 comprise delta-values (ΔDVdp, respectively ΔLVr) which need to be added to an input value of the look up tables LUT3, LUT4 to achieve the required drive value DVdp and the required resulting luminance value LVr, as can be seen in Figs. 7A and 7B. One benefit of these embodiments is that the delta- values stored in the look up tables LUT3, LUT4 may require less storage capacity, which leads to cost reduction. Another benefit of these embodiments is that it enables a small adaptation of the drive value or the resulting luminance value by simply introducing a correction factor. The delta-drive value or to the delta-resulting luminance value is multiplied with the added correction factor, correcting for the required small adaptation. Fig. 8 shows a configuration of the signal processor 5 within the driver 4 wherein the fourth Look Up Table LUT4 is replace by a functional representation F. The functional representation F uses as inputs the delta-drive value ΔDVdp of the third Look Up Table LUT3 and the desired luminance value LVd. The delta-drive value ΔDVdp is corrected by using a gain value G (G<1). Adding the corrected delta-drive value ΔDVdp to the desired luminance value LVd results in an estimated resulting luminance value LVer. The frame memory FM stores the estimated resulting luminance value LVer during one addressing ~ period TP and supplies this delayed estimated resulting luminance value LVer as the previous luminance value LVp to the signal processor 5. Because the estimated resulting luminance LVer is a calculated value, it may not be as strongly related to the actually achieved luminance (see Leol, Leo2 of Fig. 2D) at the end of the addressing period TP as when the fourth Look Up Table LUT4 would have been used. The benefit of this embodiment is that one look up table can be omitted, thus reducing the cost. 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. The driver for driving the display panel may, for example, be a discrete electronic circuit or an integrated circuit which comprises a Programmable Logic Controller, or a Microprocessor being suitably programmed to perform the driving of the display panel, or an other Digital Signal Processing circuit. The display module comprising the display panel may, for example, be a Matrix Display module, an Active Matrix Display module, a Liquid Crystal Display module, or an Active Matrix Liquid Crystal Display module. The display module may be, for example, a back-lit transmissive, or a front-lit reflective module. The apparatus comprising the display module may be, for example, a computer monitor, a TV receiver, a gaming computer, or a pocket computer such as a Personal Digital Assistant. 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 driver (4) for driving a pixel (3) of a display panel (2), wherein the driver (4) comprises a signal processor (5) for receiving an input signal (L) representing a desired luminance of the pixel (3) during an addressing period (TP) and for supplying a drive voltage (V) to the pixel (3) to obtain, in the addressing period (TP), a changing luminance having an average value being substantially equal to the desired luminance (Ld; Ldl; Ld2).

2. A driver (4) as claimed in claim 1, wherein the display panel (2) comprises pixels (3), wherein the display panel (2) is arranged for being illuminated by a scanning illuminator (6) for producing light for a particular sub-group of the pixels during a predetermined time window (T) of the addressing period (TP) only, and wherein the signal processor (5) is arranged for supplying the drive voltage (V) to the pixels of the particular subgroup to obtain the changing luminance having an average value (La) over the predetermined time window (T) of the addressing period (TP) being substantially equal to the desired luminance (Ld; Ldl; Ld2) multiplied by a ratio of a duration of the addressing period (TP) and a duration of the time window (T).

3. A driver (4) as claimed in claim 2, wherein the predetermined time window (T) has a duration equal to the addressing period (TP).

4. A driver (4) as claimed in claim 1, wherein the signal processor (5) comprises a first Look Up Table (LUTl) for determining a drive value (DVdp) of the drive voltage (V), wherein a cell of the first Look Up Table (LUTl) comprises the drive value (DVdp) for combinations of a desired luminance value (LVd) and a previous luminance value (LVp), the desired luminance value (LVd) representing the desired luminance (Ld; Ldl; Ld2) and the previous luminance value (LVp) being a desired luminance value (LVd) or a drive value (DVdp) of a previous addressing period, being a further addressing period directly preceding the first mentioned addressing period (TP), and a frame memory (FM) for storing the desired luminance value (LVd) or the drive value (DVdp) to supply the previous luminance value (LVp) to the first Look Up Table (LUTl).

5. A driver (4) as claimed in claim 1, wherein the signal processor (5) comprises a first Look Up Table (LUTl) for determining a drive value (DVdp) of the drive voltage (V), wherein a cell of the first Look Up Table (LUTl) comprises the drive value (DVdp) for combinations of a desired luminance value (LVd) and a previous luminance value (LVp), the desired luminance value (LVd) representing the desired luminance (Ld; Ldl; Ld2) and the previous luminance value (LVp) being a resulting luminance value (LVr) of a previous addressing period, being a further addressing period directly preceding the first mentioned addressing period (TP), the resulting luminance value (LVr) representing a luminance of the pixel (3) achieved at the end of the first mentioned addressing period (TP) due to the applied drive voltage (V) a second Look Up Table (LUT2) for determining the resulting luminance value (LVr), wherein a cell of the second Look Up Table (LUT2) comprises the resulting luminance value (LVr) for combinations of the desired luminance value (LVd) and the previous luminance value (LVp) or for combinations of the desired luminance value (LVd) and the drive value (DVdp), and the a frame memory (FM) for storing the resulting luminance value (LVr) to supply the previous luminance value (LVp) to the first Look Up Table (LUTl) and to the second Look Up Table (LUT2).

6. A driver (4) as claimed in claim 4 or 5, wherein the first Look Up Table (LUTl) of the signal processor (5) comprises a calculator for calculating the drive value (DVdp) of the drive voltage (V) from a desired luminance value (LVd) and a previous luminance value (LVp) with a predetermined function.

7. A display module (1) comprising a display panel (2) and the driver (4) of claim 1 or 2 for driving the display panel (2).

8. A display module (1) as claimed in claim 7 when dependent on claim 2, wherein the display module (1) further comprises an illuminator (6) for illuminating the display panel (2) to illuminate the pixel (3) during the predetermined time window (T) of the addressing period (TP) only.

9. An apparatus comprising a display module (1) as claimed in claim 7.

10. A method of driving a pixel (3) of a display panel (2), wherein the method comprises: receiving an input signal (L) representing a desired luminance (Ldl; Ld2) of the pixel (3) during an addressing period (TP), determining a drive voltage (V) to obtain, in the addressing period (TP), a changing luminance having an average value being substantially equal to the desired luminance (Ldl ; Ld2), and supplying the drive voltage (V) to the pixel (3).