Abstract:
Disclosed herein is a power consumption reduction device including: a region-adaptive gray level conversion unit; wherein the gray level conversion unit is operable to convert n1 bits of gray level information for a low gray level region into m1 (&lt;n1) bits of gray level information, further operable to convert n2 bits of gray level information for an intermediate gray level region into m2 (≦n2) bits of gray level information, and still further operable to convert n3 bits of gray level information for a high gray level region into m3 (&lt;n3) bits of gray level information, and the gray level conversion unit converts a gray level of an input video signal so that m1≦m2, m3≦m2 and n1+n2+n3&gt;m1+m2+m3 are all satisfied.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present invention contains subject matter related to Japanese Patent Application JP 2006-247463 filed in the Japan Patent Office on Sep. 13, 2006, the entire contents of which being incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention described in the present specification relates to a technology for reducing power consumption while at the same time keeping decline in visibility in high ambient illuminance conditions to a minimum and also to a technology for providing improved visibility while at the same time keeping increase in power consumption to a minimum. 
         [0004]    The invention proposed by the inventors includes a power consumption reduction device, a visibility improvement device, a self-luminous display apparatus, an image processing device, electronic equipment, a power consumption reduction method, a visibility improvement method, and a computer program. 
         [0005]    2. Description of the Related Art 
         [0006]    Today, flat panel displays are finding widespread application in various types of electronic equipment. As a result, these displays are used in ever more diverse operating conditions. For example, such displays are increasingly used in extremely high ambient illuminance conditions. 
         [0007]    Incidentally, screens suffer a sharp decline in visibility if used under high illuminance conditions. In this case, the screen brightness has to be increased to provide improved visibility. 
         [0008]    Japanese Patent Laid-Open No. 2004-109170 discloses a peak brightness control technique of changing the peak brightness in accordance with the luminance of ambient light. That is, the technique disclosed includes increasing the peak brightness in light conditions, and reducing the peak brightness in dark conditions. 
       SUMMARY OF THE INVENTION 
       [0009]    However, increasing the screen brightness typically leads to higher power consumption. With a self-luminous flat panel display in particular, higher screen brightness results directly in higher power consumption. Further, increased power consumption translates directly into shorter usage time in the case of mobile electronic equipment. 
         [0010]    Therefore, the inventors propose a power consumption reduction device having a region-adaptive gray level conversion unit. The gray level conversion unit converts n1 bits of gray level information for a low gray level region into m1 (&lt;n1) bits of gray level information. Further, the gray level conversion unit converts n2 bits of gray level information for an intermediate gray level region into m2 (≦n2) bits of gray level information. Still further, the gray level conversion unit converts n3 bits of gray level information for a high gray level region into m3 (&lt;n3) bits of gray level information. The gray level conversion unit converts a gray level of an input video signal so that the conditions that m1≦m2, m3≦m2 and n1+n2+n3&gt;m1+m2+m3 are all satisfied. 
         [0011]    If ambient illuminance is high, low and high gray level regions typically decline in visibility as compared to an intermediate gray level region. The technique proposed by the inventors provides active reduction of gray level information in these gray level regions. This permits reduction of power consumption while not affecting the actual visibility. 
         [0012]    It should be noted that improved visibility can be achieved as compared to the existing art if the peak brightness is increased to the extent that power consumption is reduced as a result of this gray level conversion. That is, the screen visibility can be enhanced while maintaining power consumption constant. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a view illustrating an example of functional configuration of a power consumption reduction device; 
           [0014]      FIG. 2  is a view illustrating an example of internal configuration of a by-region gray level conversion unit; 
           [0015]      FIG. 3  is a view explaining an example of setting gray level regions for a mean gray level and the relationship thereof with assignment of gray level information; 
           [0016]      FIGS. 4A ,  4 B and  4 C are views illustrating how the setting of each gray level region changes with different mean gray levels; 
           [0017]      FIG. 5  is a view illustrating examples of gray level conversion formulas for different gray level regions; 
           [0018]      FIG. 6  is a view illustrating an example of functional configuration of a display device; 
           [0019]      FIGS. 7A and 7B  are views explaining a duty pulse signal; 
           [0020]      FIG. 8  is a view illustrating the connection relationship between a pixel circuit and peripheral circuits; 
           [0021]      FIG. 9  is a view explaining a gray level region setting procedure; 
           [0022]      FIG. 10  is a view explaining a gray level conversion procedure; 
           [0023]      FIG. 11  is a view explaining reduction in power consumption; 
           [0024]      FIG. 12  is a view illustrating another example of internal configuration of the by-region gray level conversion unit; 
           [0025]      FIG. 13  is a view illustrating an example of structure of a conversion table; 
           [0026]      FIG. 14  is a view illustrating an example of conversion table setting procedure; 
           [0027]      FIG. 15  is a view illustrating another example of functional configuration of the power consumption reduction device; 
           [0028]      FIG. 16  is a view illustrating another example of internal configuration of the by-region gray level conversion unit; 
           [0029]      FIG. 17  is a view illustrating another example of conversion table setting procedure; 
           [0030]      FIG. 18  is a view illustrating an example of functional configuration of a visibility improvement device; 
           [0031]      FIG. 19  is a view illustrating an example of power consumption calculation procedure; 
           [0032]      FIG. 20  is a characteristic curve illustrating the correspondence between a gray level and a current level; 
           [0033]      FIGS. 21A and 21B  are views explaining duty pulse signal variation control; 
           [0034]      FIG. 22  is a view explaining change in peak brightness as a result of duty pulse signal control; 
           [0035]      FIG. 23  is a view explaining an example of incorporation into electronic equipment; 
           [0036]      FIG. 24  is a view explaining another example of incorporation into electronic equipment; 
           [0037]      FIG. 25  is a view explaining an example of the power consumption reduction device incorporated in electronic equipment; 
           [0038]      FIG. 26  is a view explaining another example of the power consumption reduction device incorporated in electronic equipment; 
           [0039]      FIG. 27  is a view explaining still another example of the power consumption reduction device incorporated in electronic equipment; 
           [0040]      FIG. 28  is a view explaining still another example of the power consumption reduction device incorporated in electronic equipment; 
           [0041]      FIG. 29  is a view explaining still another example of the power consumption reduction device incorporated in electronic equipment; 
           [0042]      FIGS. 30A and 30B  are views illustrating other examples of setting gray level regions; 
           [0043]      FIGS. 31A and 31B  are views illustrating other examples of assigning gray level information to gray level regions; 
           [0044]      FIG. 32  is a view illustrating another example of assigning gray level information to gray level regions; 
           [0045]      FIG. 33  is a view explaining change in peak brightness level as a result of supply voltage control; 
           [0046]      FIG. 34  is a view illustrating the connection relationship between the pixel circuit and peripheral circuits; and 
           [0047]      FIGS. 35A and 35B  are views explaining other examples of setting the duty pulse signal. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0048]    A description will be made below about power consumption reduction and visibility improvement techniques according to an embodiment of the present invention. 
         [0049]    It should be noted that known or publicly known technologies of the technical field concerned will be applied to those portions not specifically illustrated or described. 
         [0050]    It is also to be noted that the embodiments described below are preferred embodiments of the invention. The present invention is not limited thereto. 
       (A) Embodiment 1 
     (A-1) Functional Configuration of Power Consumption Reduction Device 
       [0051]      FIG. 1  is a view illustrating an example of functional configuration of a power consumption reduction device. 
         [0052]    A power consumption reduction device  1  includes a mean gray level calculation unit  3  and a by-region gray level conversion unit  5 . 
         [0053]    The mean gray level calculation unit  3  is a processing device operable to calculate a mean gray level (APL: average picture level) per frame based on a video signal. It should be noted that the mean gray level may be calculated on a frame-by-frame basis or as a mean level per frame of video signal input during a plurality of frames. 
         [0054]    The by-region gray level conversion unit  5  is a processing device operable to preserve much gray level information in a given range set around a mean gray level and also actively reduce gray level information in the low and high gray level regions if ambient brightness is high. It should be noted that when ambient brightness is not high (when ambient brightness is lower than a determination threshold level), the same unit  5  outputs the video signal as is without converting it. 
         [0055]      FIG. 2  is a view illustrating an example of internal configuration of the by-region gray level conversion unit  5 . The same unit  5  includes a gray level region setting unit  11  and a calculation unit  13 . 
         [0056]    If ambient brightness is high, the gray level region setting unit  11  sets low, intermediate and high gray level regions based on a mean gray level. When ambient brightness is not high, the same unit  11  stops setting gray level regions. 
         [0057]    In the present embodiment, the gray level region setting unit  11  performs the calculations of (mean gray level−total gray level region/2) and (mean gray level+total gray level region/2). Then the same unit  11  sets three gray level regions based on these two gray levels. 
         [0058]    That is, the same unit  11  sets, as an intermediate region, a region between (mean gray level−total gray level region/2) and (mean gray level+total gray level region/2). Further, the same unit  11  sets, as a low gray level region, a region smaller than (mean gray level−total gray level region/2). Still further, the same unit  11  sets, as a high gray level region, a region greater than (mean gray level+total gray level region/2). 
         [0059]      FIG. 3  illustrates an example of setting gray level regions for a mean gray level. The example illustrated in  FIG. 3  assumes that the video signal is eight bits wide (video signal with 256 gray levels). Therefore, the intermediate gray level region is set as 128 gray levels wide. On one hand, the boundary between the low and intermediate gray level regions is given by subtracting 64 from a mean gray level. On the other hand, the boundary between the intermediate and high gray level regions is given by adding 64 to the mean gray level. 
         [0060]    For example, if the mean gray level is 128, the low gray level region is from 1 to 64. Similarly, the intermediate and high gray level regions are respectively from 65 to 191 and from 192 to 256. 
         [0061]      FIGS. 4A ,  4 B and  4 C illustrate how the setting of each gray level region changes in accordance with different mean gray levels. It should be noted that we assume that the width of the intermediate gray level region remains unchanged irrespective of different mean gray levels.  FIG. 4A  illustrates a case where the mean gray level is small. In this case, the low gray level region is narrow, whereas the high gray level region is wide. 
         [0062]      FIG. 4B  illustrates a case where the mean gray level is some intermediate value which is neither small nor large. In this case, the low and high gray level regions are almost equal in width.  FIG. 4C  illustrates a case where the mean gray level is large. In this case, the low gray level region is wide, whereas the high gray level region is narrow. 
         [0063]    The calculation unit  13  performs gray level conversion by arithmetic operation. The same unit  13  carries out conversion according to the gray level region to which the video signal (gray level) for each pixel belongs. 
         [0064]    In the present embodiment, we assume that gray level information (bit count) assigned to each gray level region is set in advance. 
         [0065]    In the case illustrated in  FIG. 3 , four bits (16 gray levels) of gray level information are assigned to the low gray level region. Six bits (64 gray levels) are assigned to the intermediate gray level region. Four bits (16 gray levels) are assigned to the high gray level region. 
         [0066]    As a result of gray level conversion by the calculation unit  13 , therefore, a video signal containing gray level information of 256 gray levels is converted into a video signal containing gray level information of 96 gray levels (=16 gray levels+64 gray levels+16 gray levels). 
         [0067]      FIG. 5  illustrates each of the formulas applied to each of the gray level regions to which a video signal belongs. Naturally,  FIG. 5  illustrates a case where the intermediate gray level region is half the size of total gray level region when an eight-bit video signal is given. 
         [0068]    In gray level conversion, two processes are performed. The first process is a division process which divides the value, obtained by normalizing an input gray level in each gray level region, by a unit step value (step count calculation process in the same gray level region). The second process is a multiplication process which multiplies the calculated step count by a unit step value (output gray level calculation process). It should be noted that, for the intermediate and high gray level regions, an additional process is also performed which adds a gray level (offset) for the origin of each gray level region to the calculation result. 
         [0069]    It should be noted that, in the calculation formulas illustrated in  FIG. 5 , an operator NINT refers to an integer generation process by rounding. 
         [0070]    For example, when the mean gray level is 128, a video signal (gray level) belonging to the low gray level region is converted into a video signal whose gray level changes in a step form in units of four gray levels. 
         [0071]    Similarly, when the mean gray level is 128, a video signal (gray level) belonging to the intermediate gray level region is converted into a video signal whose gray level changes in a step form in units of two gray levels. 
         [0072]    Similarly, when the mean gray level is 128, a video signal (gray level) belonging to the high gray level region is converted into a video signal whose gray level changes in a step form in units of two gray levels. 
         [0073]      FIG. 3  illustrates this input and output relationship as a bold line in a step form. It should be noted that the input and output relationship is linear as shown by a fine line in  FIG. 3  when the gray level conversion is not performed. The result of the gray level conversion performed selectively according to the ambient brightness as described above is output to a display device  7 . 
       (A-2) Configuration of the Display Device 
       [0074]    In the present embodiment, we assume that an organic EL display, a type of self-luminous display device, is used as a display device. 
         [0075]      FIG. 6  illustrates an example of functional configuration of the display device  7 . The display device  7  includes a timing generator  21 , a data line driver  23 , scan drivers  25  and  27 , a supply voltage source  29  and an organic EL display panel  31 . 
         [0076]    The timing generator  21  is a processing device operable to generate various timing signals necessary for screen display based on a timing signal contained in a video signal given by the power consumption reduction device  1 . The timing generator  21  generates, for example, a write pulse. 
         [0077]    The data line driver  23  is a circuit device operable to drive the data line of the organic EL display panel  31 . 
         [0078]    The data line driver  23  converts a gray level specifying the luminescent brightness of each pixel into an analog voltage level and supplies the analog voltage to the data line. 
         [0079]    The scan driver  25  is a circuit device operable to select a gate line, which is provided for selection of a horizontal line to which a gray level is written, in a line sequential manner. This selection signal is supplied to the organic EL display panel  31  as a write pulse. In the present embodiment, the scan driver  25  outputs a write pulse to each of the horizontal lines. 
         [0080]    The scan driver  27  is a circuit device operable to drive a gate line provided to supply a duty pulse signal. Here, the duty pulse signal refers to a signal which gives the duration of light-on time in one frame period. An example of a duty pulse signal is illustrated in  FIGS. 7A and 7B .  FIG. 7A  illustrates a vertical synchronizing pulse which gives the maximum duration of light-on time.  FIG. 7B  illustrates an example of a duty pulse signal. In the case of  FIG. 7B , the low-level period of the duty pulse signal is the duration of light-on time in one frame period. In the present embodiment, we assume that the light-on time is fixed. 
         [0081]    The supply voltage source  29  is a circuit device operable to supply a supply voltage (analog voltage) to be applied to the anode of the organic EL device. In the present embodiment, the supply voltage source  29  generates a constant voltage. 
         [0082]    The organic EL display panel  31  is a display device with organic EL devices arranged in a matrix form. It should be noted that the organic EL display panel  31  is designed for color display. Therefore, one pixel on the display includes sub-pixels for three colors of RGB. 
         [0083]      FIG. 8  illustrates the connection relationship between a pixel circuit  41  and peripheral circuits. 
         [0084]    The pixel circuit  41  includes a data switch device T 1 , a capacitor C 1 , a current supply device T 2 , and a light-on period control device T 3 . 
         [0085]    Here, the data switch device T 1  is a transistor operable to control loading (writing) of a voltage level given via the data line. The timing of loading the voltage level is given for each horizontal line. 
         [0086]    The capacitor C 1  is a storage device operable to store the loaded voltage level for a period of one frame. Even if data is written by a line sequential scan, use of the capacitor C 1  provides light emission similar to that by a frame sequential scan. 
         [0087]    The current supply device T 2  is a transistor operable to supply a drive current suited for the voltage level of the capacitor C 1  to an organic EL device D 1 . 
         [0088]    The light-on period control device T 3  is a transistor operable to control the light-on time of the organic EL device D 1  within one frame. 
         [0089]    The light-on period control device T 3  is disposed in series with the supply path of drive current. The organic EL device D 1  is lit while the light-on period control device T 3  is on. On the other hand, the organic EL device D 1  is unlit while the light-on period control device T 3  is off. 
         [0090]    The signal applied to the light-on period control device T 3  is the duty pulse signal described earlier ( FIG. 7B ). 
       (A-3) Gray Level Conversion Process 
       [0091]    The gray level conversion performed when ambient brightness is high will be described below. It should be noted that the gray level conversion is carried out if ambient brightness information from an ambient light sensor is greater than the determination threshold level. 
         [0092]      FIG. 9  illustrates the procedure for setting gray level regions. It should be noted that the operation steps illustrated in  FIG. 9  are performed every frame. 
         [0093]    First, the power consumption reduction device  1  calculates a mean gray level per frame (S 1 ). 
         [0094]    Next, the power consumption reduction device  1  sets low, intermediate and high gray level regions according to the mean gray level (S 2 ). 
         [0095]    More specifically, when a low gray level region is set, the power consumption reduction device  1  sets conversion calculation parameters for each gray level region (S 3 ). More specifically, the same device  1  sets parameters other than the input gray level in the calculation formulas described in  FIG. 5 . 
         [0096]    Following the setting of parameters, the power consumption reduction device  1  performs the steps illustrated in  FIG. 10  for each pixel. 
         [0097]    First, the same device  1  determines whether the input gray level falls within the low gray level region (S 11 ). 
         [0098]    When the determination is affirmative, the same device  1  performs gray level conversion for the low gray level region (S 12 ). 
         [0099]    In contrast, if the determination is negative, the power consumption reduction device  1  determines whether the input gray level falls within the intermediate gray level region (S 13 ). 
         [0100]    When the determination is affirmative, the power consumption reduction device  1  performs gray level conversion for the intermediate gray level region (S 14 ). 
         [0101]    On the other hand, if the determination is negative, the power consumption reduction device  1  performs gray level conversion for the high gray level region (S 15 ). 
         [0102]    A series of the operation steps illustrated in  FIG. 10  will be repeated for all pixels making up a frame. As a result, the video signal containing 256 gray levels is converted into a video signal containing 96 gray levels, which will be displayed on the screen. 
       (A-4) Effect of Gray Level Conversion 
       [0103]    As described above, much gray level information is assigned to the intermediate gray level region while at the same time reducing gray level information. This permits reduction of power consumption without degrading visibility even in high ambient brightness conditions. 
         [0104]      FIG. 11  visually illustrates how power consumption is reduced. In  FIG. 11 , the areas in which power consumption is reduced and the amount of reduction are shown by a black filled-in pattern. The amount of reduction in power consumption is significant in the low and high gray level regions where gray level information has been significantly reduced. 
         [0105]    It should be noted that, as described earlier, the observable difference in contrast is inherently small in high ambient brightness conditions. In addition, preserving much gray level information for the intermediate gray level region, which has been set relative to the mean gray level, keeps the decline in visibility to a minimum. That is, power consumption can be positively reduced without adversely affecting visibility. 
         [0106]    In particular, if an organic EL display is used outdoors, this reduced power consumption can be used to extend the operation time. 
       (B) Embodiment 2 
       [0107]    Here, a description will be made about a case where a by-region gray level conversion function is implemented using a gray level conversion table. It should be noted that the basic system configuration is identical to that of  FIG. 1  described in relation to the embodiment 1, except for the internal configuration of the by-region gray level conversion unit. 
         [0108]      FIG. 12  illustrates the internal configuration of a by-region gray level conversion unit  51 . 
         [0109]    The by-region gray level conversion unit  51  includes a table selection unit  53  and a conversion table  55 . 
         [0110]    The table selection unit  53  selects an optimal conversion table based on the mean gray level if ambient brightness is high. The same unit  53  stops conversion (or selects a conversion table in which the input and output gray levels are the same) when ambient brightness is not high. 
         [0111]    The conversion table  55  includes a plurality of sets of conversion tables prepared in advance in anticipation for a calculated mean gray level. To be exact, as many conversion tables as 256 gray levels should be prepared. Practically, however, a plurality of representative sets of tables is incorporated in consideration of the frequency of use and the rate of change in gray level after conversion. As a result, the table selection unit  53  selects a conversion table which contains a calculated mean gray level within the estimated range. 
         [0112]      FIG. 13  illustrates the structure of the conversion table  55 . As illustrated in  FIG. 13 , the conversion table  55  stores a correspondence between input and output gray levels. Naturally, the correspondence satisfies the gray level conversion formulas for different gray level regions described in relation to the embodiment 1. 
         [0113]    It should be noted that although the conversion table illustrated in  FIG. 13  stores a correspondence between all 256 input gray levels and their associated output gray levels. Alternatively, however, the conversion table may store the part of the correspondence in which the output gray level changes. Then, for those input gray levels with no associated output gray levels, the output gray level may be read which is associated with the input gray level which is smaller than and closest to the input gray level in question. Such an arrangement will permit reduction of storage capacity for storing the conversion table  55 . 
         [0114]      FIG. 14  illustrates the procedure for setting the conversion table. It should be noted that the operation steps illustrated in  FIG. 14  are performed every frame. 
         [0115]    Also in this case, the mean gray level calculation unit  3  calculates a mean gray level per frame (S 21 ). 
         [0116]    Next, the by-region gray level conversion unit  51  sets a conversion table with low, intermediate and high gray levels specified according to the mean gray level (S 22 ). 
         [0117]    From here onward, gray level conversion is performed continuously on a pixel-by-pixel basis using the selected conversion table. 
         [0118]    Use of a conversion table as in the present embodiment eliminates the need to incorporate a high performance signal processing unit. Use of a conversion table is also effective if the screen size is large and if the number of bits of input video signal is large. 
       (C) Embodiment 3 
       [0119]    Here, a description will be made about a case where a by-region gray level conversion function is implemented based on genre information attached to video signal. It should be noted that genre information is given as information attached to video signal. 
         [0120]      FIG. 15  illustrates an example of functional configuration of a power consumption reduction device  61 . 
         [0121]    The power consumption reduction device  61  includes a genre information acquisition unit  63  and a by-region gray level conversion unit  65 . 
         [0122]    The genre information acquisition unit  63  is a processing device operable to acquire genre information attached to video signal. Genre information relates to details of program such as news, entertainment and sports. It should be noted that genre information is described, for example, in coded data format or in text data format with tags as defined by the data format. 
         [0123]    The by-region gray level conversion unit  65  is a processing device operable to preserve much gray level information in the intermediate gray level region and actively reduce gray level information in the low and high gray level regions if ambient brightness is high. It should be noted that when ambient brightness is not high, the by-region gray level conversion unit  65  outputs the video signal as is without converting it. 
         [0124]      FIG. 16  illustrates an example of internal configuration of the by-region gray level conversion unit  65 . The by-region gray level conversion unit  65  includes a table selection unit  71  and a conversion table  73 . 
         [0125]    The table selection unit  71  selects an optimal conversion table based on genre information if ambient brightness is high. The same unit  71  stops conversion (or selects a conversion table in which the input and output gray levels are the same) when ambient brightness is not high. 
         [0126]    The conversion table  73  includes a plurality of sets of conversion tables prepared in advance on a genre information by genre information basis. Also in the case of the conversion table  73 , to be exact, as many conversion tables as 256 gray levels should be prepared. Practically, however, a plurality of representative sets of tables is incorporated in consideration of the frequency of use and the rate of change in gray level after conversion. As a result, the table selection unit  73  selects a conversion table which contains a mean gray level specific to each genre within the estimated range. 
         [0127]    The individual tables of the conversion table  73  are the same in structure as those of the conversion table  55  described in relation to the embodiment 2. 
         [0128]      FIG. 17  illustrates the procedure for setting the conversion table. It should be noted that the operation steps illustrated in  FIG. 17  are performed every frame. 
         [0129]    In this case, the genre information acquisition unit  63  acquires genre information attached to the video signal (S 31 ). 
         [0130]    Next, the by-region gray level conversion unit  65  sets a conversion table with low, intermediate and high gray levels specified according to the mean gray level (S 32 ). 
         [0131]    From here onward, gray level conversion is performed continuously on a pixel-by-pixel basis using the selected conversion table. 
         [0132]    Reference to genre information as in the present embodiment eliminates the need to calculate a mean gray level per frame, thus permitting gray level conversion suitable for input video signal. 
         [0133]    As described above, one conversion table is used for each program in the method based on reference to genre information. 
         [0134]    Therefore, this prevents frequent switching of gray level conversion during a program, thus keeping the load on the signal processing system low. 
         [0135]    It should be noted that the present embodiment may be combined with the arrangement based on reference to mean gray level described in the embodiment 2. In this case, if there is a large difference between a mean gray level of the entire program and that per frame, priority may be given to the gray level conversion based on the mean gray level calculated per frame. 
       (D) Embodiment 4 
       [0136]    In the three embodiments described above, primary emphasis was placed on reduction of power consumption by gray level conversion performed for each gray level region. 
         [0137]    However, reduced power consumption can be effectively used to actively provide improved visibility. 
         [0138]      FIG. 18  illustrates an example of functional configuration of a visibility improvement device  81  of the type described above. It should be noted that the visibility improvement device  81  includes the power consumption reduction device  1  illustrated in  FIG. 1  as a basic component thereof. In  FIG. 18 , therefore, like components as those in  FIG. 1  are designated by the same numerals. 
         [0139]    The visibility improvement device  81  includes the mean gray level calculation unit  3 , the by-region gray level conversion unit  5 , power consumption calculation units  83  and  85 , and a peak brightness control unit  87 . A description will be made below about the power consumption calculation units  83  and  85 , and the peak brightness control unit  87 . 
         [0140]    The power consumption calculation unit  83  is a processing device operable to calculate power consumption prior to gray level conversion. On the other hand, the power consumption calculation unit  85  is a processing device operable to calculate power consumption following gray level conversion. 
         [0141]      FIG. 19  illustrates an example of process steps common to both the power consumption calculation units  83  and  85 . In the power consumption calculation, the gray level for each pixel is converted into a current level first (S 31 ). 
         [0142]    In this conversion, a gray level-to-current level conversion table illustrated in  FIG. 20  is referred to. As illustrated in  FIG. 20 , the current level has the property to increase non-linearly with respect to the gray level due to the gamma characteristic of the organic EL device. Therefore, the gray level is converted into an appropriate current level according to the correspondence registered in advance. 
         [0143]    Next, the power consumption calculation units  83  and  85  calculate panel current consumption (sum of current consumptions of all pixels) in an entire one frame period (S 32 ). This calculation is carried out over a period from one vertical synchronizing signal input to the next. 
         [0144]    When the panel current level is obtained, the power consumption calculation units  83  and  85  each multiply the panel current level by the supply voltage level to calculate the power consumption (S 33 ). The power consumption calculated by a series of above steps is supplied to the peak brightness control unit  87  by each of the calculation units  83  and  85 . 
         [0145]    The peak brightness control unit  87  refers to the value, obtained by dividing the power consumption prior to gray level conversion by that following gray level conversion, as a peak brightness incremental factor. By doing so, the peak brightness control unit  87  controls the peak brightness of the display device  7  so that the incremental factor is satisfied. That is, the peak brightness control unit  87  controls the peak brightness so that the power consumption of the display device  7  is almost the same as prior to gray level conversion. 
         [0146]    In the present embodiment, the peak brightness control is accomplished by varying the low-level period of the duty pulse signal as illustrated in  FIG. 21 . The larger the proportion of the low-level period of the signal in a one-frame period, the longer the organic EL device is lit. Conversely, the smaller the proportion of the low-level period of the signal in a one-frame period, the shorter the organic EL device is lit. 
         [0147]    That is, power consumption changes with change in the low-level period of the duty pulse signal. It should be noted that the peak brightness control unit  87  controls the output timing of the duty pulse signal in response to reception of a timing signal for video signal. 
         [0148]    In the present embodiment, reduction in power consumption achieved by gray level conversion can be used to provide higher peak brightness. This permits highly visible display even in high ambient brightness conditions. The present embodiment provides a highly visible display screen despite the fact that power consumption remains the same as in the case where gray level conversion is not performed as described in the embodiment 1. 
       (D) Examples of Incorporation 
       [0149]    Here, a description will be made about examples of incorporation of the aforementioned power consumption reduction device or visibility improvement device into electronic equipment. First, examples of incorporation of the power consumption reduction device into electronic equipment will be described. 
         [0000]    (a) Incorporation into Self-Luminous Display Apparatus 
         [0150]    The power consumption reduction device  1  can be incorporated into a self-luminous display apparatus  91  as illustrated in  FIG. 23 . A display device  93  and a power consumption reduction device  95  are incorporated in the self-luminous display apparatus  91  illustrated in  FIG. 23 . 
         [0151]    It should be noted that the power consumption reduction device  95  can be implemented by a small-scale circuit. Therefore, the same device  95  can be accommodated in an IC (integrated circuit) or other circuitry incorporated into the display device  93 . 
         [0152]    For example, if the display device  93  has a device configuration as described with reference to  FIG. 6 , the power consumption reduction device  95  can be incorporated in part of the timing generator  21  ( FIG. 6 ). 
         [0153]    As described above, if the power consumption reduction device  95  is incorporated in part of the existing processing circuit, there is no need to change the layout or incorporation space, thus making the embodiment advantageous in terms of manufacturing cost. 
         [0000]    (b) Image Processing Device 
         [0154]    The aforementioned power consumption reduction device can also be incorporated in an image processing device  111 . The image processing device  111  is provided as an external device to supply a video signal to a self-luminous display device  101  as illustrated in  FIG. 24 . 
         [0155]      FIG. 24  illustrates a case where the image processing device  111  is directly connected to the self-luminous display device  101 . However, the image processing device  111  is also applicable when it is connected to the self-luminous display device  101  via the Internet or other network. 
         [0156]    The image processing device  111  illustrated in  FIG. 24  includes an image processing unit  113  and a power consumption reduction device  115 . It should be noted that the details of the process performed by the image processing unit  113  depend upon the application installed. 
         [0000]    (c) Other Examples of Incorporation 
         [0157]    The power consumption reduction and visibility improvement devices may be incorporated in a variety of electronic equipment in addition to the apparatus described earlier. It should be noted that although incorporation is possible irrespective of whether electronic equipment is portable or stationary, there should be, as a precondition, at least a likelihood that the display device may be used in high ambient brightness conditions. 
         [0000]    (c1) Broadcast Wave Reception Apparatus 
         [0158]    The power consumption reduction device may be incorporated in a broadcast wave reception apparatus. 
         [0159]      FIG. 25  illustrates an example of functional configuration of a broadcast wave reception apparatus. A broadcast wave reception apparatus  121  includes a display device  123 , a system control unit  125 , an operation unit  127 , a storage medium  129 , a power supply  131  and a tuner  133  as its major components. 
         [0160]    It should be noted that the system control unit  125  includes, for example, a microprocessor. The system control unit  125  controls the overall operation of the system. The operation unit  127  includes not only mechanical controls but also a graphic user interface. 
         [0161]    The storage medium  129  is used as a storage area of not only data for images and video to be displayed on the display device  123  but also firmware and application programs. A battery power supply is used as the power supply  131  when the broadcast wave reception apparatus  121  is portable. Needless to say, a commercial power supply is used when the broadcast wave reception apparatus  121  is stationary. 
         [0162]    The tuner  133  selectively receives the wave of the channel selected by the user from among incoming broadcast waves. 
         [0163]    The configuration of this broadcast wave reception apparatus is applicable, for example, to television program receivers, radio program receivers and electronic equipment incorporating the broadcast wave reception function. 
         [0000]    (c2) Audio Apparatus 
         [0164]      FIG. 26  illustrates an example of functional configuration of an audio apparatus serving as a player when the power consumption reduction device is applied. 
         [0165]    An audio apparatus  141  serving as a player includes a display device  143 , a system control unit  145 , an operation unit  147 , a storage medium  149 , a power supply  151 , an audio processing unit  153  and a speaker  155  as its major components. 
         [0166]    Also in this case, the system control unit  145  includes, for example, a microprocessor. The same unit  145  controls the overall operation of the system. The operation unit  147  includes a graphic user interface as well as mechanical controls. 
         [0167]    The storage medium  149  serves as a storage area of firmware and application programs as well as audio data. The storage medium  149  is also used to store music data. A semiconductor storage medium, hard disk drive or other medium is used as the storage medium  149 . 
         [0168]    A battery power supply is used as the power supply  151  when the audio apparatus  141  is portable. Naturally, a commercial power supply is used when the audio apparatus  141  is stationary. 
         [0169]    The audio processing unit  153  is a processing device operable to process an audio data signal. The same unit  153  decompresses compressed and coded audio data. The speaker  155  outputs reproduced sounds. 
         [0170]    It should be noted that if the audio apparatus  141  is used as a recorder, a microphone is connected in place of the speaker  155 . In this case, the audio processing unit  153  is capable of compressing and coding audio data. 
         [0171]    The configuration of this audio apparatus is applicable, for example, to portable musical equipment and mobile phones. 
         [0000]    (c3) Communication Apparatus 
         [0172]      FIG. 27  illustrates an example of functional configuration of a communication apparatus when the power consumption reduction device is applied. A communication apparatus  161  includes a display device  163 , a system control unit  165 , an operation unit  167 , a storage medium  169 , a power supply  171  and a communication unit  173  as its major components. 
         [0173]    It should be noted that the system control unit  165  includes, for example, a microprocessor. The same unit  165  controls the overall operation of the system. The operation unit  167  includes a graphic user interface as well as mechanical controls. 
         [0174]    The storage medium  169  is used as a storage area of firmware and application programs as well as data files for images and video to be displayed on the display device  163 . A battery power supply is used as the power supply  171  when the communication apparatus  161  is portable. Naturally, a commercial power supply is used when the communication apparatus  161  is stationary. 
         [0175]    The communication unit  173  is a radio device operable to exchange data with external equipment. The configuration of this communication apparatus is applicable, for example, to stationary telephone sets, mobile phones and portable electronic equipment incorporating the communication function. 
         [0000]    (c4) Image Pickup Apparatus 
         [0176]      FIG. 28  illustrates an example of functional configuration of an image pickup apparatus when the power consumption reduction device is applied. An image pickup apparatus  181  includes a display device  183 , a system control unit  185 , an operation unit  187 , a storage medium  189 , a power supply  191  and an image pickup unit  193  as its major components. 
         [0177]    It should be noted that the system control unit  185  includes, for example, a microprocessor. The same unit  185  controls the overall operation of the system. The operation unit  187  includes a graphic user interface as well as mechanical controls. 
         [0178]    The storage medium  189  is used as a storage area of firmware and application programs as well as data files for images and video to be displayed on the display device  183 . A battery power supply is used as the power supply  191  when the image pickup apparatus  181  is portable. Naturally, a commercial power supply is used when the image pickup apparatus  181  is stationary. 
         [0179]    The image pickup unit  193  includes, for example, a CMOS sensor and a signal processing unit operable to process an output signal from the CMOS sensor. The configuration of this image pickup apparatus is applicable, for example, to digital cameras, video camcorders and portable electronic equipment incorporating the image pickup function. 
         [0000]    (c5) Information Processing Apparatus 
         [0180]      FIG. 29  illustrates an example of functional configuration of a portable information processing apparatus when the power consumption reduction device is applied. An information processing apparatus  201  includes a display device  203 , a system control unit  205 , an operation unit  207 , a storage medium  209  and a power supply  211  as its major components. 
         [0181]    It should be noted that the system control unit  205  includes, for example, a microprocessor. The same unit  205  controls the overall operation of the system. The operation unit  207  includes a graphic user interface as well as mechanical controls. 
         [0182]    The storage medium  209  is used as a storage area of firmware and application programs as well as data files for images and video to be displayed on the display device  203 . A battery power supply is used as the power supply  211  when the information processing apparatus  201  is portable. Naturally, a commercial power supply is used when the information processing apparatus  201  is stationary. 
         [0183]    The configuration of this information processing apparatus is applicable, for example, to game machines, electronic books, electronic dictionaries, computers and measuring instruments. It should be noted that if the configuration thereof is applied to a measuring instrument, a detection signal from the sensor (detection device) is fed to the system control unit  205 . 
       (E) Other Embodiments 
       [0184]    (a) In the aforementioned embodiments, a description was made about a case where ambient brightness information is fed via an ambient light sensor. 
         [0185]    However, ambient brightness information may be given through manipulation of the user interface as a signal adapted to switch between processes. In this case, power consumption reduction or visibility improvement operation is performed at the discretion of the user. 
         [0000]    (b) In the aforementioned embodiments, a description was made about a case where an eight-bit video signal is given. However, a video signal having other bit count may be given. For example, a 10- or 12-bit video signal may be given.
 
(c) In the aforementioned embodiments, a description was made about a case where 128 gray levels are assigned to the intermediate gray level region. However, the number of gray levels assigned to the intermediate gray level region is arbitrary. For example, a fewer number of gray levels such as 100, or a larger number of gray levels such as 150, may be assigned.
 
(d) In the aforementioned embodiments, a description was made about a case where the low gray level region is converted into 16 gray levels (four bits), the intermediate gray level region into 64 gray levels (six bits), and the high gray level region into 16 gray levels (four bits).
 
         [0186]    However, the amount of gray level information assigned to each gray level region is arbitrary. For example, the low gray level region may be converted into four gray levels (two bits), the intermediate gray level region into 32 gray levels (five bits), and the high gray level region into four gray levels (two bits) as illustrated in  FIGS. 31A and 31B . This provides further reduced power consumption. 
         [0000]    (e) In the aforementioned embodiments, a description was made about a case where output gray level information is reduced as compared to input gray level information for all gray level regions. 
         [0187]    As illustrated in  FIG. 32 , however, output gray level information may be reduced as compared to input gray level information for the low and high gray level regions, with input gray level information preserved as is for the intermediate gray level region. 
         [0188]    The embodiment illustrated in  FIG. 32  preserves as much of gray level information as possible for the intermediate gray level while at the same time providing reduced power consumption as compared to the embodiment 1. Nevertheless, in high ambient brightness conditions, part of gray level information for the intermediate gray level region is also corrupted. Therefore, the preserved gray level information will not necessarily translate into improved visibility. 
         [0000]    (f) In the aforementioned embodiments, a description was made about a case where the peak brightness level is controlled by controlling the low-level period of the duty pulse signal. 
         [0189]    However, the peak brightness level control can also be accomplished by controlling the supply voltage level applied to the display device, as illustrated in  FIG. 33 . The peak brightness level has the property to increase non-linearly with increase in supply voltage as illustrated in the same figure. 
         [0190]      FIG. 34  illustrates an example of circuit configuration of a pixel circuit  221  capable of controlling the peak brightness by varying the supply voltage. 
         [0191]    This pixel circuit is basically identical in circuit configuration to that of the embodiment 1 ( FIG. 8 ). It should be noted that the pixel circuit in  FIG. 34  differs from that in the embodiment 1 in that two separate power lines are provided, one adapted to supply a potential to the anode of an organic EL device D 1 , and the other for a capacitor C 1 . This makes it possible to vary the current level supplied to the organic EL device D 1  even if the charge (gray level) stored in the capacitor C 1  remains unchanged. 
         [0000]    (g) In the aforementioned embodiments, a description was made about a case where the duty pulse signal is output once per frame ( FIGS. 7 and 21 ). 
         [0192]    As illustrated in  FIG. 35 , however, the duty pulse signal may be output once per horizontal period. 
         [0000]    (h) In the aforementioned embodiments, a description was made about a case where an organic EL display panel is used as the display device. 
         [0193]    However, other self-luminous display device may be used instead as the display device. 
         [0194]    For example, an inorganic EL, FED or PDP display apparatus may be used. 
         [0000]    (i) The entire processing functionality of both the power consumption reduction and visibility improvement devices described in the aforementioned embodiments can be implemented in hardware or software form. Further, the entire processing functionality thereof can be implemented by using hardware and software in combination so that a share of the functionality is assigned to hardware and software.
 
(j) The aforementioned embodiments can be modified in various manners within the scope of the spirit of the invention. Further, various modifications and applications created or combined based on the description herein are also possible.
 
         [0195]    It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factor in so far as they are within the scope of the appended claims or the equivalents thereof.