Abstract:
An image display apparatus includes a light-source driving circuit that drives a light source to cyclically change brightness. A control unit controls a pixel to turn on and off over a plurality of subfields in a field so that transmittance or reflectance in the pixel over the plurality of subfields in the field corresponds to a gray level specified for the pixel. The control unit also arranges the plurality of subfields such that two or more gray-level defining subfields maintain a predetermined relationship with respect to the cyclical change in brightness of the light source caused by drive of a light-source driving circuit.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a technique for improving characteristics in representing gradation by turning on or off a pixel over subfields into which one field is divided. 
         [0003]    2. Related Art 
         [0004]    As is well known, a display panel has a display element, such as a liquid crystal element, for each pixel and changes a transmittance or reflectance of light for each pixel. In recent years, a projection display apparatus (projector) that emits light to such a display panel to generate a reduced image and projects an enlarged image of the reduced image using an optical system is becoming popular. One example of a technique for representing shades of gray in a display panel is a technique for dividing one field into a plurality of subfields, turning on or off a display element (pixel) for each of the subfields, changing a proportion of a period of time when the pixel is in an on (or off) state in one field, and thereby representing shades of gray (see, for example, JP-A-2007-148417). 
         [0005]    With the above technique, gradation is represented in accordance with the proportion of a period of time when a pixel is in an on (or off) state in one field, so only two values for on and off levels are required for a driving voltage for a display element. Accordingly, the above technique can achieve a simpler structure by, for example, eliminating a digital-to-analog (D/A) converting circuit, in comparison with the traditional voltage modulation scheme. 
         [0006]    A light source for emitting light to a display panel is AC-driven to achieve long life. When the light source is AC-driven, because the luminance of light emitted is changed, even with subfields having the same period, the subfields have different transmittances (reflectances) when a pixel is turned on or off. This causes an adverse effect on characteristics for representing gradation. 
       SUMMARY 
       [0007]    An advantage of some aspects of the invention is that it provides a technique for representing gradation by turning on or off a pixel over each of subfields into which one field is divided with a reduced adverse effect on characteristics therefor even when a light source is AC-driven. 
         [0008]    According to an aspect of the invention, An image display apparatus includes a light source that emits light, a light-source driving circuit that drives the light source, a display panel that includes a plurality of pixels and that changes a transmittance or reflectance of the light emitted by the light source for each of the plurality of pixels, and a control unit. The control unit controls the display panel such that the transmittance or reflectance in each of the plurality of pixels has a value corresponding to an on or off state in accordance with a gray level specified for the pixel over each of a plurality of subfields into which one field is divided on a time line and arranges the plurality of subfields such that two or more gray-level defining subfields other than an adjusting subfield among the plurality of subfields maintains a predetermined relationship with respect to light-source driving circuit that drives the light source to cyclically change brightness. With this configuration, because the arrangement of the gray-level defining subfields maintains a predetermined relationship with respect to a change in luminance of the light source, gradation can be represented without being affected by the change in the lamp. 
         [0009]    In this case, the control unit may arrange the subfields such that a shortest subfield that has a shortest period among the gray-level defining subfields is contained in time when a luminance of the light emitted from the light source has a minimum or maximum value. With this arrangement, characteristics for representing gradation in a darker side or lighter side can be improved. 
         [0010]    In this case, that the adjusting subfield may preferably be arranged immediately before the shortest subfield. Furthermore, the control unit may preferably set the transmittance or reflectance of each of the pixels to a value corresponding to either one of the on and off states over the adjusting subfield irrespective of the gray level. If responsiveness in turning on or off a pixel in the shortest subfield is low, the actual transmittance or reflectance would be displaced depending on the immediately preceding state. In contrast, with the above configuration, irrespective of the gray level, the initial state in turning on or off in the shortest subfield can be made uniform. The invention can be directed to not only an image display apparatus but also a method for controlling the image display apparatus and, in turn, to an electronic device including the image display apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
           [0012]      FIG. 1  is a block diagram that illustrates a configuration of an image display apparatus according to one embodiment of the invention. 
           [0013]      FIG. 2  illustrates a configuration of a display panel in the image display apparatus. 
           [0014]      FIG. 3  illustrates an operation occurring in each subfield. 
           [0015]      FIG. 4  illustrates a field structure in the display panel. 
           [0016]      FIG. 5  illustrates another field structure in the display panel. 
           [0017]      FIG. 6  illustrates still another field structure in the display panel. 
           [0018]      FIG. 7  illustrates a configuration of a projector that uses the image display apparatus. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0019]    An embodiment of the invention is described below with reference to the drawings.  FIG. 1  is a block diagram that illustrates a configuration of an image display apparatus according to the present embodiment. As illustrated in this drawing, an image display apparatus  1  includes a display panel  10 , a timing control circuit  30 , a field memory  32 , a look-up table (LUT)  34 , a light-source driving circuit  40 , and a light source  42 . 
         [0020]      FIG. 2  illustrates a configuration of the display panel  10 . The display panel  10  can be an active-matrix liquid-crystal panel, for example. In the display panel  10 , a Y driver  130  and an X driver  140  are arranged in the vicinity of a display area  100 . In the display area  100  in the present embodiment, 1080 scan lines  112  extend in a row direction (horizontally in the drawing) and 1920 data lines  114  extend in a column direction (vertically in the drawing). Each of the data lines  114  are electrically insulated from each of the scan lines  112 . A pixel  110  is arranged so as to correspond to the intersection of each of the 1080 scan lines  112  and each of the 1920 data lines  114 . In the present embodiment, the pixels  110  are arranged in a matrix with 1080 rows and 1920 columns. However, the invention is not intended to be limited to this arrangement. 
         [0021]    The Y driver  130  sequentially selects the 1080 scan lines  112 , applies a voltage VH corresponding to an H level to a selected scan lines  112 , and applies a voltage VL corresponding to an L level to the other unselected scan lines  112 . More specifically, as illustrated in  FIG. 3 , the Y driver  130  supplies the 1st to 1080th scan lines  112  with scan signals G 1  to G 1080 , respectively, being logic signals in which a start pulse Dy having a pulse width corresponding to one period of a clock signal Cly is delayed in sequence by half of the period of the clock signal Cly and the pulse width is reduced by one-half so as not to have an overlapping portion. The X driver  140  supplies the 1st to 1920th data lines  114  with data signals d 1  to d 1920  to be supplied to the pixels  110  at the selected scan line  112 , respectively. 
         [0022]    As is well known, for an active-matrix liquid-crystal panel, each of the pixels  110  has a liquid crystal element including liquid crystal disposed between a pixel electrode and a common electrode, and the liquid crystal element has a transmittance (in a transmissive display; for a reflective display, a reflectance) in accordance with a stored voltage. Here, when a scan line  112  is in the H level, a data signal supplied to the data lines  114  is applied to the pixel electrode of each of the pixels  110  positioned in the scan line  112 , and a difference voltage between the common electrode and the data signal is written therein. Because the voltage written in the pixel  110  is retained by the capacitive nature of the liquid crystal element even when the voltage of the scan line  112  is in the L level, that pixel  110  has a transmittance in accordance with the difference voltage written when the scan line  112  is in the H level. In the present embodiment, the liquid crystal elements operate in a normally black mode in which the transmittance has a minimum value when the stored voltage is zero and the transmittance gradually increases with an increase in the stored voltage. 
         [0023]    In representing gradation using a traditional analog scheme, a data signal for a voltage modulated according to a gray level is written in a liquid crystal element. With this analog scheme, unevenness in displaying resulting from a wiring resistance may occur or an additional D/A converting circuit may be necessary. In the present embodiment, a data signal has two levels of an ON voltage for turning on a liquid crystal element and an OFF voltage for turning off the liquid crystal element. To represent gradation using only two levels of ON and OFF voltages for a liquid crystal element as described above, a driving process (subfield driving process) may be required in which one field being a fundamental period is divided into a plurality of subfields, a liquid crystal element is made to retain a difference voltage corresponding to an ON or OFF voltage on a subfield basis, a proportion of a period of time for the on (or off) state is changed in accordance with the gray level for each pixel. One field used herein is a time period required to create a single image and is constant at 16.7 milliseconds (the inverse of 60 Hz) being a period for a vertical scan signal contained in a synchronous signal Sync supplied from an external higher level circuit. In the present embodiment, one field is divided into subfields sf 0  to sf 18 . The details thereof will be described later. 
         [0024]    The timing control circuit  30  corresponds to a control unit. The timing control circuit  30  defines one field being a reference for use in driving the display panel  10  from a synchronous signal Sync supplied from an external higher level circuit (not shown) and a light-source control signal Lf, defines the start point of each of the subfields sf 0  to sf 18 , and generates various signals for driving the display panel  10 . Because the synchronous signal Sync contains a vertical scan signal Vsync defining a vertical scan of image data Din, the timing control circuit  30  in the present embodiment defines one field being the reference for use in driving the display panel  10  on the basis of the vertical scan signal Vsync. 
         [0025]    The field memory  32  stores image data Din of at least one field under the control of the timing control circuit  30  and also reads the stored image data Din in units of the subfields sf 0  to sf 18 . Here, the image data Din is digital data specifying levels of gray of the pixels  110  on a pixel basis and supplied from an external higher level circuit in synchronization with the synchronous signal Sync. More specifically, one frame of the image data Din for the 1080 rows×1920 columns is supplied in synchronization with vertical and horizontal scans defined by the synchronous signal Sync. 
         [0026]    The LUT  34  is a table that stores data Dsf indicating on and off for combinations of levels of gray and subfields. More specifically, the table is used for converting a gray level defined by image data Din read from the field memory  32  into data Dsf defining ON or OFF for each of the subfields sf 0  to sf 18 . Here, to converting into the data Dsf, in addition to the image data Din, information indicating what subfield corresponds thereto is necessary. Because of this, the timing control circuit  30  supplies the LUT  34  with data Nsf indicating a subfield number. Thus, the LUT  34  outputs the data Dsf corresponding to a gray level specified by the image data Din read form the field memory  32  and corresponding to a subfield indicated by the data Nsf. 
         [0027]    The light source  42  can be a high-pressure discharge lamp, for example, and emits white light to the display area  100 . For achieving long life of the light source, it is necessary to AC-drive the light source  42  at a constant frequency. Because of this, the light-source driving circuit  40  generates a light-source driving signal Ld using a recommended frequency for the light source  42  and supplies it to the light source  42 . In such a way, the light source  42  is AC-driven at its recommended frequency and thus has an increased life. Here, in the present embodiment, as illustrated in  FIG. 4 , the light-source driving signal Ld has a ramp waveform that linearly increases from a minimum value to a maximum value in a ramp period being the inverse of the recommended frequency. The light source  42  driven in response to the light-source driving signal Ld is in a relatively dark state at the point corresponding to the minimum value of the light-source driving signal Ld and in a relatively light state at the point corresponding to the maximum value. The light-source driving circuit  40  supplies the timing control circuit  30  with a light-source control signal Lf synchronized with the light-source driving signal Ld. The light-source control signal Lf is a pulse signal that becomes an H level when the light-source driving signal Ld changes from the maximum value to the minimum value. 
         [0028]    The light-source driving signal Ld is constant for the recommended frequency of the light source  42 , and that frequency is not synchronized with a synchronous signal Sync. Here, in the present embodiment, when the frequency of the light-source driving signal Ld is four or more times larger than the frequency of the vertical scan signal Vsync contained in the synchronous signal Sync, the light-source driving signal Ld becomes the minimum value at least three times in one field period. That is, when the period from the minimum value to the maximum value of the light-source driving signal Ld is one period, three continuous periods of the light-source driving signal Ld are always contained in one field period. 
         [0029]    In the present embodiment, by the use of this, the timing control circuit  30  defines subfields in the display panel  10 , as illustrated in  FIG. 4 . More specifically, in one field period defined by the vertical synchronization signal Vsync, the timing control circuit  30  uses a term Ta corresponding to the three continuous periods of the light-source control signal Lf (light-source driving signal Ld) starting from the time when a first pulse of the light-source control signal Lf is supplied, as the subfields sf 1  to sf 18  for defining levels of gray in the first row, and uses the other period as the subfield sf 0  for adjustment in the first row. In other words, in one field period, the term Ta corresponding to the three continuous periods of the light-source control signal Lf is assigned to the gray-level defining subfields sf 1  to sf 18 , and the arrangement of the subfields sf 1  to sf 18  is synchronized with respect to the light-source driving signal Ld. Because the light-source driving signal Ld is not synchronized with the synchronous signal Sync, the remaining of one field period other than the subfields sf 1  to sf 18  is assigned to the adjusting subfield sf 0  and is used for adjusting the time (timing). Among the gray-level defining subfields sf 1  to sf 18 , the subfields sf 1 , sf 7 , and sf 13 , which have the shortest time period, are arranged immediately after a pulse of the light-source control signal Lf is output, i.e., immediately after the light-source driving signal Ld is at the minimum value. The subfields at the second and subsequent rows have a relationship in which delays sequentially occur from the first row by the amount of sequential selection of the scan lines by the Y driver  130 . 
         [0030]    The details of conversion of such subfields performed by use of the LUT  34  are outlined below. That is, an off state is set in all of the subfields sf 1  to sf 18  for the darkest gray level; the sum of lengths of periods of subfields set in an on state increases as a lighter gray level is specified. Actual details of the conversion are experimentally determined in consideration of electrical optical properties and response properties of a liquid crystal element. In the present embodiment, the adjusting subfield sf 0  is uniformly set in an off state, irrespective of the gray level specified by image data. 
         [0031]    In such a configuration, the timing control circuit  30  defines a reference period in driving the display panel  10  in accordance with the vertical scan signal vsync as one field, outputs a start pulse Dy at the point in time when a first pulse of the light-source control signal Lf is supplied after a pulse of the vertical scan signal Vsync is supplied, and outputs the start pulse Dy every time a time of period of each of the subfields sf 1  to sf 18  elapses after that point in time of supplying the first pulse. When the start pulse Dy is supplied, the Y driver  130  sequentially supplies the 1st, 2nd, 3rd, . . . , to 1080th scan lines  112  with the scan signals G 1 , G 2 , G 3 , . . . , G 1080 , respectively, being in an H level (selection voltage VH), as illustrated in  FIG. 2 . 
         [0032]    In accordance with selection of a scan line  112  by the Y driver  130 , the timing control circuit  30  reads image data elements Din for one row of pixels in the 1st to 1920th columns positioned in a scan line where the scan signal is in the H level. In the LUT  34 , the image data elements Din for one row in that scan line are converted into the respective data elements Dsf corresponding to the subfield numbers indicated by the data elements Nsf. The timing control circuit  30  transfers the data elements Dsf obtained by the conversion corresponding to the pixels in the 1st to 1920th columns to the X driver  140 . The X driver  140  reconverts the transferred data elements Dsf corresponding to the pixels in the 1st to 1920th columns into ON and OFF voltages indicated by the respective data elements Dsf, and supplies the voltages as the data signals d 1  to d 1920  to the 1st to 1920th data lines  114  in synchronism with the time when the scan signal for the corresponding row is turned to the H level under the control of the timing control circuit  30 . When the scan line  112  has the selection voltage VH corresponding to the H level, the ON and OFF voltages supplied to the data lines  114  are applied to their respective pixel electrodes. 
         [0033]    To prevent an alternating-current component from being applied to the liquid crystal, ON voltages have two polarities: positive and negative. These positive and negative polarities are alternately used for each field. Because of this, for example, when an ON voltage is specified in an odd-numbered field, the X driver  140  supplies the positive ON voltage, whereas when an ON voltage is specified in an even-numbered field, the X driver  140  supplies the negative ON voltage. Positive voltages and negative voltages used herein indicate voltages higher than and voltages lower than a voltage applied to the common electrode, respectively. An OFF voltage can be a voltage of the common electrode without the use of different polarities as long as the OFF voltage makes the difference voltage of the liquid crystal element zero. 
         [0034]    An operation of supplying a data signal and writing it into a pixel in such a way is repeated every time the 1st to 1080th scan lines  112  are sequentially selected in each of the gray-level defining subfields sf 1  to sf 18  and the adjusting subfield sf 0 . Therefore, ON or OFF writing corresponding to their respective levels of gray is carried out in each of the pixels in the 1st to 1080th rows for each of the subfields sf 1  to sf 18 , and OFF writing is uniformly carried out in the subfield sf 0  irrespective of levels of gray. 
         [0035]    In driving subfields in the normally black mode, when all the subfield are turned off in one field, the darkest level of gray is represented. A gray level one step lighter than the darkest level is represented by turning on one of the shortest subfields. Accordingly, in the normally black mode, the width of a level of gray represented in a darker side is determined by the shortest subfield period as long as the luminance of the light source is constant. In the present embodiment, the shortest subfields sf 1 , sf 7 , and sf 13  are arranged in synchronization with the points in time when the luminance of the light source  42  is in the minimum level. Thus, without changing the length of a subfield period, a dark gray level can be represented by the amount of change in luminance of the light source. 
         [0036]    If the length of the period of a subfield is short in writing ON or OFF in accordance with the gray level, the transmittance in that subfield may different depending on the ON or OFF state of an immediately preceding subfield because of its low response speed. For example, the actual transmittance in a subfield having a short period when a pixel is turned off in the case where the pixel was in an on state in the immediately preceding subfield is leaning to the on state than that in the case where the pixel was in an off state in the immediately preceding subfield. In the present embodiment, for the subfield sf 0 , OFF writing is uniformly carried out on all of the pixels, and the shortest subfield sf 1  is positioned immediately after the subfield sf 0 . Accordingly, in the present embodiment, when a pixel is turned on or off in the subfield sf 1 , because the actual transmittance in the immediately preceding subfield sf 0  is always off, gradation can be stably represented. Additionally, because uniformly turning off the pixels in the subfield sf 0  makes the transmittance be in the minimum level, a change in luminance of the light source  42  is not actually identified. This can make a difference in levels of gray be less identifiable. When an image to be displayed is light averagely, the transmittance may be increased on the whole by turning on in the subfield sf 0 . In either case, pixels are uniformly turned on or off in the adjusting subfield sf 0 . 
         [0037]    In a normally white mode, in contrast to the normally black mode, when pixels are turned off over all of the subfields in one field, the lightest gray level is represented. A level one step darker than the lightest level is represented by turning on one of the shortest subfields. Accordingly, in the normally white mode, the width of a level of gray represented in a lighter side is determined by the shortest subfield period. Therefore, the shortest subfields may be arranged in synchronization with the points in time when the light source  42  is lightest. 
         [0038]    In the above embodiment, the recommended frequency for driving the light source  42  does not match a multiplied frequency of the vertical scan signal. However, the recommended frequency for driving the light source  42  may match a multiplied frequency of the vertical scan signal. The vertical scan for the display panel  10  is defined by the vertical synchronization signal Vsync. However, it may be determined so as to be a submultiple of the recommended frequency for driving the light source  42 . In either case, when the recommended frequency for driving the light source  42  matches a multiple of the vertical scan frequency for the display panel  10 , there is no need to have the adjusting subfield sf 0 , as illustrated in  FIG. 5 . Thus, it may be merely required that subfields be arranged such that the light-source control signal Lf is synchronized with the vertical scan signal Vsync, i.e., with a change in luminance of the light source  42 . 
         [0039]    In the above embodiment, the lengths of periods of subfields constituting one field are different. However, they may be the same. To have the same length in the subfields constituting one fields, as illustrated in  FIG. 6 , in one field period defined by the vertical scan signal Vsync, the term Ta corresponding to n continuous periods of the light-source control signal Lf from when a first pulse of the light-source control signal Lf is supplied may be used as a gray-level defining subfield at the 1st row, and the other may be used as an adjusting subfield at the 1st row. 
         [0040]    Further, in the above embodiment, the light-source driving signal Ld has a ramp waveform that rises from left to right. However, it may have other waveforms, such as a sinusoidal waveform or a triangular waveform. The light-source control signal Lf being a pulse signal output at the beginning of a ramp period may alternatively be digital data indicating the luminance of the light source. In addition to a liquid crystal panel, various kinds of panel may also be used as the display panel as long as the display panel needs a light source. For example, a digital mirror element may be used as the display panel. 
       Electronic Device 
       [0041]    One example of an electronic device that uses an image display apparatus according to the above embodiment is a projector that uses the above-described display panel  10  as a light valve. The projector is described below with reference to  FIG. 7 , which is a plan view that illustrates a configuration of this projector. As illustrated in this drawing, a projector  2100  includes a lamp unit  2102  including a white light source, such as a halogen lamp, therein. Light emitted from the lamp unit  2102  is divided into light beams for three primary colors, red (R), green (G), and blue (B), by three mirrors  2106  and two dichroic mirrors  2108 , and the beams are guided to light valves  10 R,  10 G, and  10 B corresponding to the primary colors R, G, and B, respectively. Because the optical path of the beam for B is longer than those of the other colors R and G, the beam for B is guided through a relay lens system  2121  including an incident lens  2122 , a relay lens  2123 , and an exit lens  2124 . 
         [0042]    In the projector  2100 , three sets of image display apparatuses each including the display panel  10  are disposed for the colors R, G, and B. The projector  2100  has a configuration in which an image data element for each of the colors R, G, and B is supplied from an external higher level circuit. The configuration of each of the light valves  10 R,  10 G, and  10 B is substantially the same as the display panel  10  in the above-described embodiment. The light valves  10 R,  10 G, and  10 B are driven in a subfield basis in response to data signals for R, G, and B supplied from timing control circuits (not shown in  FIG. 7 ) provided for the respective colors. The beams modulated by the light valves  10 R,  10 G, and  10 B enter a dichroic prism  2112  from three respective directions. In the dichroic prism  2112 , the beams for R and B are refracted by 90 degrees, whereas the beam for G travels in a straight line. Accordingly, after images for the colors are combined, a color image is projected onto a screen  2120  by a projection lens  2114 . 
         [0043]    There is no need to have a color filter in each of the light valves  10 R,  10 G, and  10 B because the dichroic mirrors  2108  enable them to receive their respective beams corresponding to the primary colors R, G, and B. Images obtained by beams passing through each of the light valves  10 R and  10 B are projected after being reflected by the dichroic prism  2112 , whereas an image obtained by a beam passing through the light valve  10 G is projected as it is. Thus, the horizontal scan directions performed by the light valves  10 R and  10 B are opposite to the horizontal scan direction performed by the light valve  10 G such that left-right reversed images are displayed. 
         [0044]    Examples of the electronic device include, in addition to the projector illustrated in  FIG. 7 , a television, a viewfinder type or direct-view type video tape recorder, a car navigation system, a pager, an electronic organizer, a calculator, a word processor, a work station, a videophone, a point-of-sale (POS) terminal, a digital still camera, a cellular phone, and a device having a touch panel. An image display apparatus according to the invention can be used in these various electronic devices. 
         [0045]    The entire disclosure of Japanese Patent Application No. 2008-016607, filed Jan. 28, 2008 is expressly incorporated by reference herein.