Abstract:
A display device includes a self light-emitting display in which data electrodes and scanning electrodes are arranged in matrix form, and a modulating voltage is applied to the data electrode side while a threshold voltage is applied to the scanning electrode side, the device including: a signal level range determination means for digitally processing an input signal to determine a signal level range of the input signal for every prescribed frame number unit; a threshold voltage control means for controlling a threshold voltage based upon a determination result by the signal level range determination means; and an input signal correction means for correcting an input signal level based upon a determination result by the signal level range determination means.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a display device comprising a self light-emitting display. 
   2. Description of the Related Art 
   As shown in  FIG. 1 , a self light-emitting display  10  is known in which data electrodes and scanning electrodes are arranged in matrix form, and a modulating voltage is applied by a data driver  11  to the data electrode side while a threshold voltage is applied by an operation driver  12  to the scanning electrode side. The display  10  as thus configured is called a passive display. 
     FIG. 2  shows a light-emitting characteristic of a self light-emitting element for use in the self light-emitting display. 
   As shown in  FIG. 2 , the self light-emitting element starts emitting light upon application of a voltage not lower than a light-emission starting voltage “Vstart”. The light-emitting luminance increases as the voltage applied to the self light-emitting element becomes higher. 
   In a display device comprising the above-mentioned self light-emitting display, for example, a threshold voltage “Vth” corresponding to the light-emission starting voltage “Vstart” is sequentially applied to each scanning electrode of the self light-emitting display, as shown in  FIG. 3 . A modulating voltage “Vmod” of 0 to Vmod max , according to the signal level, is applied to the data electrodes of the self light-emitting display. As a result, a voltage of “Vth+Vmod(0≦Vmod≦Vmod max )” is applied to a light-emitting element which is an intersection of the scanning electrode and the data electrode, and light is emitted at a luminance based upon the applied voltage. 
   In such a conventional display device (first conventional example), there is a problem in that “Vmod max ” cannot be set sufficiently large due to performance of the data driver  11  and thereby high luminance cannot be obtained. 
   For obtaining high luminance, a display device (second conventional example) has already been developed in which “Vth” is set higher than the light-emission starting voltage “Vstart”, as shown in  FIG. 4 . In the second conventional example, however, there is a problem in that the light-emitting element emits light in some quantity even with the signal level set to 0 (the modulating voltage “Vmod” set to 0), thereby leading to degradation in contrast. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a display device capable of promoting high luminance with respect to an image whose signal level is totally high, without degrading contrast. 
   It is also an object of the present invention to provide a display device which is capable of high luminance display with respect to an image whose signal level is totally high, and is capable of reduction in black level with respect to an image whose signal level is totally low, thereby allowing promotion of improvement in contrast and an increase in luminance. 
   A first display device according to the present invention comprises a self light-emitting display, in which data electrodes and scanning electrodes are arranged in matrix form, and a modulating voltage is applied to the data electrode side while a threshold voltage is applied to the scanning electrode side, the device comprising: a signal level range determination means for digitally processing an input signal to determine a signal level range of the input signal for every prescribed frame number unit; a threshold voltage control means for controlling a threshold voltage based upon a determination result by the signal level range determination means; and an input signal correction means for correcting an input signal level based upon a determination result by the level range detection means. 
   As the threshold voltage control means, for example, a means is used by which a threshold voltage is set low when the signal level range determined by the signal level range determination means is the entire level range except for a high luminance part, whereas the threshold voltage is set high when the signal level range determined by the signal level range determination means is the entire level range except for a low luminance part. 
   As the input signal correction means, for example, a means is used by which an input signal is corrected such that the signal level range is extended to the high luminance side when the signal level range determined by the signal level range determination means is the entire level range except for a high luminance part, whereas an input signal is corrected such that the signal level range is extended to the low luminance side when the signal level range determined by the signal level range determination means is the entire level range except for a low luminance part. 
   The display device may comprise a scene change detection means, and the signal level range determination means may update the determination result of the signal level range of the input signal only when a scene change is detected by the scene change detection means. 
   A second display device according to the present invention comprises an active display, the device comprising: a signal level range determination means for digitally processing an input signal to determine a signal level range of the input signal for every prescribed frame number unit; a driving power-supply voltage control means for controlling a driving power-supply voltage of the active display based upon a determination result by the signal level range determination means; and an input signal correction means for correcting an input signal level based upon a determination result by the level range detection means. 
   As the driving power-supply voltage control means, for example, a means is used by which the driving power-supply voltage is set low when the signal level range determined by the signal level range determination means is the entire level range except for a high luminance part, whereas the driving power-supply voltage is set high when the signal level range determined by the signal level range determination means is the entire level range except for a low luminance part. 
   As the input signal correction means, for example, a means is used by which an input signal is corrected such that the signal level range is extended to the high luminance side when the signal level range determined by the signal level range determination means is the entire level range except for a high luminance part, whereas the input signal is corrected such that the signal level range is extended to the low luminance side when the signal level range determined by the signal level range determination means is the entire level range except for a low luminance part. 
   The display device may comprise a scene change detection means, and the signal level range determination means may update the determination result of the signal level range of the input signal only when a scene change is detected by the scene change detection means. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a configuration of a self light-emitting display. 
       FIG. 2  is a graph showing a light-emitting characteristic of a self light-emitting element for use in the self light-emitting display. 
       FIG. 3  is a graph showing a relationship among a threshold voltage, a modulating voltage and a signal level when the threshold voltage “Vth” is set to a light-emission starting voltage “Vstart”. 
       FIG. 4  is a graph showing the relationship among the threshold voltage, the modulating voltage and the signal level when the threshold voltage “Vth” is set higher than the light-emission starting voltage “Vstart”. 
       FIG. 5  is a graph showing an example of control in a first conventional example shown in  FIG. 3 . 
       FIG. 6  is a graph showing a concept of the present invention. 
       FIG. 7  is a block diagram showing an electric configuration of a display device comprising a self light-emitting display such as an inorganic EL display. 
       FIG. 8  is a schematic diagram for describing an action of a determination portion  32  in a signal level detection portion  3 . 
       FIGS. 9   a  and  9   b  show results of controls when the classification result is “A”.  FIG. 9   a  is a graph showing the result of the control in the first conventional example;  FIG. 9   b  is a graph showing the result of the control in a method of the present invention (present method). 
       FIGS. 10   a  and  10   b  show results of controls when the classification result is “C”.  FIG. 10   a  is a graph showing the result of the control in the first conventional example;  FIG. 10   b  is a graph showing the result of the control in the present method. 
       FIGS. 11   a  and  11   b  show results of controls when the classification result is “B”.  FIG. 11   a  is a graph showing the result of the control in the first conventional example;  FIG. 11   b  is a graph showing the result of the control in the present method. 
       FIG. 12  is a graph showing an example of control in a second conventional example shown in  FIG. 4 . 
       FIG. 13  is a graph showing a concept of the present invention. 
       FIGS. 14   a  and  14   b  show results of controls when the classification result is “A”.  FIG. 14   a  is a graph showing the result of the control in the second conventional example;  FIG. 14   b  is a graph showing the result of the control in the present method. 
       FIGS. 15   a  and  15   b  show results of controls when the classification result is “C”.  FIG. 15   a  is a graph showing the result of the control in the second conventional example;  FIG. 15   b  is a graph showing the result of the control in the present method. 
       FIG. 16  is a block diagram showing an electric configuration of a display device according to a third example. 
       FIG. 17  is a view of an electric circuit showing a basic pixel configuration of an active display. 
       FIG. 18  is a graph showing a light-irradiating characteristic of a self light-emitting element for use in the active display (self light-emitting display). 
       FIG. 19  is a graph showing an example of the conventional control. 
       FIG. 20  is a graph showing a concept of the present invention. 
       FIG. 21  is a block diagram showing an electric configuration of a display device comprising a self light-emitting display such as an inorganic EL display. 
       FIGS. 22   a  and  22   b  show results of controls when the classification result is “A”.  FIG. 22   a  is a graph showing the result of the control in the conventional example;  FIG. 22   b  is a graph showing the result of the control in the present method. 
       FIGS. 23   a  and  23   b  show results of controls when the classification result is “C”.  FIG. 23   a  is a graph showing the result of the control in the conventional example;  FIG. 23   b  is a graph showing the result of the control in the present method. 
       FIGS. 24   a  and  24   b  show results of controls when the classification result is “B”.  FIG. 24   a  is a graph showing the result of the control in the conventional example;  FIG. 24   b  is a graph showing the result of the control in the present method. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following, examples of the present invention are described by reference to  FIGS. 5 to 24 . 
   [A] EXAMPLE 1 
   First, an example in the case of applying this example to a passive display is described. 
   [1] Description of Concept of the Present Invention 
   It is assumed that, when a signal level is expressed by eight bits, the minimum value of the signal level in one screen is 128 and the maximum value thereof is 255. 
     FIG. 5  shows an example of the control in the first conventional example shown in  FIG. 3 . In the first conventional example, the threshold voltage “Vth” is set to the light-emission starting voltage “Vstart”. As shown in  FIG. 5 , in the frame, the applied voltage to the light-emitting element is “Vb” when the signal level is the minimum value (128) of the frame, whereas the applied voltage to the light-emitting element is “Vth+Vmod MAX ” when the signal level is the maximum value (255) of the frame. Accordingly, the range of the light-emission luminance is the luminance range of “Lb” to “Lc”, which corresponds to the range of the applied voltage of “Vb” to “Vth+Vmod MAX ”. 
   In the present invention, as shown in  FIG. 6 , the threshold voltage “Vth” is set to “Va” which is a higher value than the light-emission starting voltage “Vstart”. The signal level “S” is then corrected to “S−(255−S)×GAIN”. As a result, when the signal level in this frame is the maximum value (255) of the frame, the applied voltage to the light-emitting element is “Va+Vmod MAX ”. Further, when the signal level in this frame is the minimum value (128) of the frame, the applied voltage to the light-emitting element is “Vb”, for example. Accordingly, the range of the light-emission luminance is the luminance range of “Lb” to “Ld”, which corresponds to the range of the applied voltage of “Vb” to “Va+Vmod MAX ”, thereby improving the luminance level. 
   [2] Description of Configuration of Display Device 
     FIG. 7  shows an electric configuration of a display device comprising a self light-emitting display such as an inorganic EL display. 
   An input signal (8-bit digital signal) is sent to a signal level detection portion (signal level range determination means)  3 , as well as to a frame memory  1 . The input signal stored in the frame memory  1  is sent to a data driver  11  of a self light-emitting display  10  after the signal level has been corrected by a signal level control portion (input signal correction means)  2 . A scanning driver  12  of the self light-emitting display  10  is controlled by a threshold voltage control portion (threshold voltage control means)  4 . The signal level detection portion  3  gives a control signal to the threshold voltage control portion  4  as well as to give a control signal to the signal level control portion  2 . 
   [2-1] Description of Signal Level Detection Portion  3   
   The signal level detection portion  3  comprises a maximum/minimum value detection portion  31  and a determination portion  32 . The maximum/minimum value detection portion  31  extracts the maximum value “MAX” and the minimum value “MIN” of an input signal for every one frame (or every several frames), and then gives the extracted values to the determination portion  32 . 
   Based upon the maximum value “MAX” and the minimum value “MIN” given by the maximum/minimum value detection portion  31 , the determination portion  32  produces a gain “GAIN” and a classification determination signal “Class”, to be given to the signal level control portion  2 , and a set value “VTH” for controlling a threshold voltage to be given to the threshold voltage control portion  4 . The gain “GAIN” is a coefficient for correcting an input signal. The classification determination signal “Class” is a determination signal for indicating a classification determined based upon the maximum value “MAX” and the minimum value “WIN”. The set value “VTH” is a set value for determining a threshold voltage “Vth”. 
   The action of the determination portion  32  is described. As shown in  FIG. 8 , based upon the signal maximum value “MAX”, the signal minimum value “MIN”, and previously set standard values: minA, maxA, minC and maxC in one frame, the determination portion  32  first determines to which of four classifications: A, B, C and D, a range where the signal maximum value “MAX” and the signal minimum value “MIN” are present belongs. 
   As shown in  FIG. 8 , each of the standard values: minA, maxA, minC and maxC, has been set such that the relationship: 0=minA&lt;minC&lt;maxA&lt;maxC=255, is maintained in the range (0 to 255) where an input signal can be present. 
   When the signal maximum value “MAX” and the signal minimum value “MIN” in one frame are present in a range not smaller than “minC” and not larger than “maxA”, the classification of this range is determined to be “B”. The classification B represents the case where the range of the signal level in one frame is in the intermediate part of the entire level range. 
   When the signal maximum value “MAX” and the signal minimum value “MIN” in one frame are present in a range not smaller than “minA” and not larger than “maxA”, and the classification B does not apply, the classification of the range is determined to be “A”. The classification A represents the case where the range of the signal level in one frame is the entire level range except for a high luminance part. 
   When the signal maximum value “MAX” and the signal minimum value “MIN” in one frame are present in a range not smaller than “minC” and not larger than “maxC”, and the classification B does not apply, the classification of the range is determined to be “C”. The classification C represents the case where the range of the signal level in one frame is the entire level range except for a low luminance part. 
   When none of the classifications A, B and C applies, the classification of the range is determined to be “D”. The classification D represents the case where the range of the signal level in one frame is a broad range from the low luminance part through the high luminance part. 
   Next, based upon the classification results, the determination portion  32  determines a classification determination signal “Class”, a gain “GAIN”, and a set value “VTH” as follows: 
   When the classification result is “B”: 
   Class=2, GAIN=Gb, VTH=Vthb 
   When the classification result is “A”: 
   Class=0, GAIN=Ga, VTH=Vtha 
   When the classification result is “C”: 
   Class=1, GAIN=Gc, VTH=Vthc 
   When the classification result is “D”: 
   Class=0, GAIN=0, VTH=Vthd 
   Herein, the scales of the set values are expressed by: Vtha(=Vthd)&lt;Vthb&lt;Vthc. It is to be noted that, in this example, “Vtha” is assumed to have been set to the light-emission starting voltage “Vstart”. “Ga”, “Gb” and “Gc” are set to values in a range larger than 0 and smaller than 1. 
   [2-2] Description Of Signal Level Control Portion  2   
   The action of the signal level control portion  2  is described. The signal level control portion  2  corrects a level of an input signal “S” based upon the classification determination signal “Class” and the gain “GAIN”, given by the signal level detection portion  3 , using the following formula (1). Herein, “SS” represents a signal after the correction (an output signal of the signal level control portion  2 ). 
   When Class=0 (the classification is “A” or “D”)
 
 SS=S+S* GAIN
 
When Class=1 (the classification is “C”)
 
 SS=S− (255− S )*GAIN
 
When Class=2 (the classification is “B”)
 
 SS=S− (Max− S )*GAIN  (1)
 
   When Class=1, the correction formula in the case of Class=2 may be used. Although “GAIN” is set by classification in the above example, “GAIN” may be set more adaptively according to the maximum value and the minimum value in one screen. It should be noted that, although the signal level control portion  2  produces the output signal “SS” based upon the above formula (1), tables representing a relation between the input signal “S” and the output signal “SS” in the respective cases of Class=0, Class=1 and Class=2 may be previously prepared, and based upon these tables, the input signal “S” may be corrected. 
   [2-3] Description of Threshold Voltage Control Portion  4   
   The action of the threshold voltage control portion  4  is described. The threshold voltage control portion  4  controls a threshold voltage, based upon the set value “VTH” given by the signal level detection portion  3 . That is, when the classification result is “B”, “VTH” is equivalent to “Vthb”, and thus the scanning driver  12  is controlled such that the threshold voltage “Vth” becomes equivalent to “Vthb”. When the classification result is “A”, “VTH” is equivalent to “Vtha”, and thus the scanning driver  12  is controlled such that the threshold voltage “Vth” becomes equivalent to “Vtha”. When the classification result is “C”, VTH is equivalent to “Vthc”, and thus the scanning driver  12  is controlled such that the threshold voltage “Vth” becomes equivalent to “Vthc”. When the classification result is “D”, “VTH” is equivalent to “Vthd”, and thus the scanning driver  12  is controlled such that the threshold voltage “Vth” becomes equivalent to “Vthd”. 
   [3] Description of Control Results 
   The result of the control of (MIN=0, MAX=128) when the classification result is “A” is described.  FIG. 9   a  shows the result of the control in the first conventional example described using  FIG. 3 .  FIG. 9   b  shows the result of the control in the example of the present invention (hereinafter referred to as the present method). 
   In the first conventional example, the input signal level is not corrected, whereas in the present method, the input signal “S” is corrected based upon the formula: SS=S+S*GAIN, and the range of the input signal level is thus extended to the high luminance side. It is thereby possible in the present method to make the luminance higher than that of the first conventional example. 
   The result of the control of (MIN=128, MAX=255) when the classification result is “C” is described.  FIG. 10   a  shows the result of the control in the first conventional example described using  FIG. 3 .  FIG. 10   b  shows the result of the control in the example of the present invention (hereinafter referred to as the present method). 
   In the first conventional example, the threshold voltage “Vth” is set to “Vstart”, whereas in the present method, the threshold voltage “Vth” is set to “Vthc” (&gt;Vstart). Further, in the present method, since the input signal “S” is corrected based upon the formula: SS=S−(255−S)*GAIN, the range of the input signal level is extended to the low luminance side. It is thereby possible in the present method to increase the light-emission luminance on the high luminance side so as to improve contrast. 
   The result of the control of (MIN=64, MAX=192) when the classification result is “B” is described.  FIG. 11   a  shows the result of the control in the first conventional example described using  FIG. 3 .  FIG. 11   b  shows the result of the control in the example of the present invention (hereinafter referred to as the present method). 
   In the first conventional example, the threshold voltage “Vth” is set to “Vstart”, whereas in the present method, the threshold voltage “Vth” is set to “Vthb” (&gt;Vstart). Further, in the present method, since the input signal “S” is corrected based upon the formula: SS=S−(MAX−S)*GAIN, the range of the input signal level is extended to the low luminance side. Further, in the present method, the light-emission luminance is higher on the high luminance side than in the conventional example, due to the shift of “Vth”. It is thereby possible in the present method to increase the light-emission luminance on the high luminance side so as to improve contrast. 
   [B] EXAMPLE 2 
   [1] Description of Concept of the Present Invention 
   It is assumed that, when a signal level is expressed by eight bits, the minimum value of the signal level in one screen is 0 and the maximum value thereof is 128. 
     FIG. 12  shows an example of control in the second conventional example shown in  FIG. 4 . In the second conventional example, the threshold voltage “Vth” is set to “Va” which is a higher value than the light-emission starting voltage “Vstart”. As shown in  FIG. 12 , in the frame, the applied voltage to the light-emitting element is “Va” when the signal level is the minimum value (0) of the frame, whereas the applied voltage is “Vb” when the signal level is the maximum value (128) of the frame. Accordingly, the range of the light-emission luminance is the luminance range of “La” to “Lb”, which corresponds to the range of the applied voltage of “La” to “Vb”. 
   In the present invention, as shown in  FIG. 13 , the threshold voltage “Vth” is set to the light-emission starting voltage “Vstart”. The signal level “S” is then corrected to “S+S*GAIN”. As a result, when the signal level in this frame is the minimum value (0) of the frame, the applied voltage to the light-emitting element is “Vstart (V 0 )”. Further, when the signal level in this frame is the maximum value (128) of the frame, the applied voltage to the light-emitting element is “Vb”. Accordingly, the range of the light-emission luminance is the luminance range of “L 0 ” to “Lb”, which corresponds to the range of the applied voltage of “V 0 ” to “Vb”, thereby improving contrast. 
   [2] Description of Configuration of Display Device 
   The configuration of the display device is the same as that of Example 1. Namely, the display device is configured as shown in  FIG. 7 . However, the process of the determination portion  32  in the signal level detection portion  3  and the process of the signal level control portion  2  are different from those in Example 1. 
   [2-1] Description of Signal Level Detection Portion  3   
   The signal level detection portion  3  comprises a maximum/minimum value detection portion  31  and a determination portion  32 . The maximum/minimum value detection portion  31  extracts the maximum value “MAX” and the minimum value “MIN” of an input signal for every one frame, and then gives the extracted values to the determination portion  32 . 
   Based upon the maximum value “MAX” and the minimum value “MIN” given by the maximum/minimum value detection portion  31 , the determination portion  32  produces a gain “GAIN” and a classification determination signal “Class”, to be given to the signal level control portion  2 , and a set value “VTH” for controlling a threshold voltage to be given to the threshold voltage control portion  4 . The gain “GAIN” is a coefficient for correcting an input signal. The classification determination signal “Class” is a determination signal for indicating a classification determined based upon the maximum value “MAX” and the minimum value “MIN”. The set value “VTH” is a set value for determining a threshold voltage “Vth”. 
   The action of the determination portion  32  is described. As shown in  FIG. 8 , based upon the signal maximum value “MAX”, the signal minimum value “MIN”, and previously set standard values: minA, maxA, minC and maxC in one frame, the determination portion  32  first determines to which of four classifications: A, B, C and D, a range where the signal maximum value “MAX” and the signal minimum value “MIN” are present belongs. 
   As shown in  FIG. 8 , each of the standard values: minA, maxA, minC and maxC, has been set such that the relationship: 0=minA&lt;minC&lt;maxA&lt;maxC=255, is maintained in the range (0 to 255) where an input signal can be present. 
   When the signal maximum value “MAX” and the signal minimum value “MIN” in one frame are present in a range not smaller than “minC” and not larger than “maxA”, the classification of this range is determined to be “B”. The classification B represents the case where the range of the signal level in one frame is in the intermediate part of the entire level range. 
   When the signal maximum value “MAX” and the signal minimum value “MIN” in one frame are present in a range not smaller than “minA” and not larger than “maxA”, and the classification B does not apply, the classification of the range is determined to be “A”. The classification A represents the case where the range of the signal level in one frame is the entire level range except for a high luminance part. 
   When the signal maximum value “MAX” and the signal minimum value “MIN” in one frame are present in a range not smaller than “minC” and not larger than “maxC”, and the classification B does not apply, the classification of the range is determined to be “C”. The classification C represents the case where the range of the signal level in one frame is the entire level range except for a low luminance part. 
   When none of the classifications A, B and C applies, the classification of the range is determined to be “D”. The classification D represents the case where the range of the signal level in one frame is a broad range from the low luminance part through the high luminance part. 
   Next, based upon the classification results, the determination portion  32  determines a classification determination signal “Class”, a gain “GAIN”, and a set value “VTH” as follows: 
   When the classification result is “B” or “D”: 
   Class=0, GAIN=0, VTH=Vthb 
   When the classification result is “A”: 
   Class=0, GAIN=Ga, VTH=Vtha 
   When the classification result is “C”: 
   Class=1, GAIN=Gb, VTH=Vthc 
   Herein, the scales of the set values are expressed by: Vtha&lt;Vthb&lt;Vthc. It is to be noted that, in this example, “Vtha” is assumed to have been set to the light-emission starting voltage “Vstart”. “Ga” and “Gb” are set to values in a range not smaller than 0 and not larger than 1. 
   [2-2] Description of Signal Level Control Portion  2   
   The action of the signal level control portion  2  is described. The signal level control portion  2  corrects a level of an input signal “S” based upon the classification determination signal “Class” and the gain “GAIN”, given by the signal level detection portion  3 , using the following formula (2). Herein, “SS” represents a signal after the correction (an output signal of the signal level control portion  2 ). 
   When Class=0 (the classification is “A”, “B” or “D”)
 
 SS=S+S* GAIN
 
When Class=1 (the classification is “C”)
 
 SS=S− (255− S )*GAIN  (2)
 
   It is to be noted that, although the signal level control portion  2  produces the output signal “SS” based upon the above formula (2), tables representing a relation between the input signal “S” and the output signal “SS” in the respective cases of Class=0 and Class=1 may be previously prepared, and based upon these tables, the input signal “S” may be corrected. 
   [2-3] Description of Threshold Voltage Control Portion  4   
   The action of the threshold voltage control portion  4  is described. The threshold voltage control portion  4  controls a threshold voltage, based upon the set value “VTH” given by the signal level detection portion  3 . That is, when the classification result is “B” or “D”, “VTH” is equivalent to “Vthb”, and thus the scanning driver  12  is controlled such that the threshold voltage “Vth” becomes equivalent to “Vthb”. When the classification result is “A”, “VTH” is equivalent to “Vtha”, and thus the scanning driver  12  is controlled such that the threshold voltage “Vth” becomes equivalent to “Vtha”. When the classification result is “C”, VTH is equivalent to “Vthc”, and thus the scanning driver  12  is controlled such that the threshold voltage “Vth” becomes equivalent to “Vthc”. 
   [3] Description of Control Results 
   The result of the control of (MIN=0, MAX=128) when the classification result is “A” is described.  FIG. 14   a  shows the result of the control in the second conventional example described using  FIG. 4 .  FIG. 14   b  shows the result of the control in the example of the present invention (hereinafter referred to as the present method). 
   In the second conventional example, the threshold voltage “Vth” is set to “Vthb”, which is a higher value than “Vstart”, whereas in the present method, the threshold voltage “Vth” is set to “Vtha” (=Vstart&lt;Vthb). Further, in the present method, since the input signal “S” is corrected based upon the formula: SS=S+S*GAIN, the range of the input signal level is extended to the high luminance side. It is thereby possible in the present method to decrease the light-emission luminance (reduce the black level) on the low luminance side so as to improve contrast. 
   The result of the control of (MIN=128, MAX=255) when the classification result is “C” is described.  FIG. 15   a  shows the result of the control in the second conventional example described using  FIG. 4 .  FIG. 15   b  shows the result of the control in the example of the present invention (hereinafter referred to as the present method). 
   In the second conventional example, the threshold voltage “Vth” is set to “Vthb”, whereas in the present method, the threshold voltage “Vth” is set to “Vthc” (&gt;Vthb). Further, in the present method, since the input signal “S” is corrected based upon the formula: SS=S−(255−S)*GAIN, the range of the input signal level is extended to the low luminance side. It is thereby possible in the present method to increase the light-emission luminance on the high luminance side so as to improve contrast. 
   It is to be noted that, when the classification result is “B” or “D”, “SS” is equivalent to “S” and “VTH” is equivalent to “Vthb”, and thus the result of the control in the present method is the same as that in the second conventional example. 
   Although, in above Examples 1 and 2, the input signal level is corrected and the threshold voltage “Vth” is controlled by calculating “Class”, “GAIN” and “VTH” in units of one frame, such correction and control may also be performed by calculating “Class”, “GAIN” and “VTH” in units of one horizontal line. 
   [C] EXAMPLE 3 
   In Examples 1 and 2, although the signal level detection portion  3  updates the signal level detection result (“Class”, “GAIN” and “VTH”) for every one frame (every several frames), the signal level detection portion  3  may be arranged to update the signal level detection result (“Class”, “GAIN” and “VTH”) only when a scene change is detected. 
     FIG. 16  shows an electric configuration of a display device. In  FIG. 16 , the same constituents as those in  FIG. 7  are provided with the same reference numerals as in  FIG. 7 . Hence descriptions of those constituents are omitted in FIG.  16 . 
   In this display device, a scene change detection portion  5  is provided for detecting whether a scene has changed or not between the present frame and a frame immediately preceding to the present frame, based upon an input signal of the present frame and an input signal of the immediately preceding frame, obtained from the frame memory  1 . As the scene change detection portion  5 , for example, a detection device is used which detects whether or not the scene has changed between the preceding frame and the present frame, based upon a detection result of an action between the frames. 
   Upon detection of the scene change, the scene change detection portion  5  transmits this information to the signal level detection portion  3 . The signal level detection portion  3  updates the signal level detection result (“Class”, “GAIN” and “VTH”) only when the scene change has been detected and outputs it. When the scene change has not been detected, the signal level detection portion  3  continues to output the previous signal level detection result (“Class”, “GAIN” and “VTH”. 
   In Example 3, it is possible to prevent flicker from occurring due to variations in luminance level in each frame. 
   [D] EXAMPLE 4 
   Next, an example in the case of applying the present invention to an active display is described. 
   [1] Description of Active Display. 
     FIG. 17  shows a basic pixel configuration of an active display. 
   A circuit for one pixel of an active display (self light-emitting display) is constituted of a switch TFT  101 , a capacitor  102 , a drive TFT  103 , and an inorganic EL element (light-emitting device)  104 . 
   A display signal “Data” is applied to a drain of the switch TFT  101  through a data line  111 . A selection signal “SCAN” is applied to the gate of the switch TFT  101  through a scanning line  112 . The source of the switch TFT  101  is connected with the gate of the drive TFT  103 , and also grounded through the capacitor  102 . A driving power-supply voltage “VDD” is applied to the drain of the drive TFT  103  through a power-supply line  113 . The source of the drive TFT  103  is connected with the anode of the inorganic EL element  104 . The cathode of the inorganic EL element  104  is grounded. 
   The switch TFT  101  is on/off-controlled by the selection signal “SCAN”. The capacitor  102  is charged by the display signal “Data” supplied through the switch TFT  101  when the switch TFT  101  is ON. The charging voltage is maintained when the switch TFT  101  is OFF. The drive TFT  103  provides the inorganic EL element  104  with a current according to a holding voltage of the capacitor  102  to be added to the gate. 
     FIG. 18  shows a light-irradiating characteristic of a self light-emitting element for use in the active display (self light-emitting display). 
   As shown in  FIG. 18 , the self light-emitting element starts emitting light upon application of an applied voltage “Data” which is not lower than the light-emission starting voltage “Vstart”. The light-emission luminance increases as the applied voltage “Data” to the self light-emitting element becomes higher. However, when the signal level is the maximum value (255), the applied voltage to the light-emitting element becomes equivalent to the drive power-supply voltage “VDD”. 
   [2] Description of Concept of the Present Invention 
   It is assumed that, when a signal level is expressed by eight bits, the minimum value of the signal level in one screen is 128 and the maximum value thereof is 255. 
     FIG. 19  shows an example of the control in a conventional example. In the conventional example, the driving power-supply voltage “VDD” is set to “VDDstd”. As shown in  FIG. 19 , in the frame, the applied voltage to the light-emitting element is “Vb” when the signal level is the minimum value (128) of the frame, whereas the applied voltage the light-emitting element is “VDD” when the signal level is the maximum value (255) of the frame. Accordingly, the range of the light-emission luminance is the luminance range of “Lb” to “Le”, which corresponds to the range of the applied voltage of “Vb” to “VDD”. 
   In the present invention, as shown in  FIG. 20 , the driving power-supply voltage “VDD” is set to “VDDp” which is a higher value than “VDDstd”. The signal level “S” is then corrected to “S−(255−S)×GAIN”. As a result, when the signal level in this frame is the maximum value (255) of the frame, the applied voltage to the light-emitting element is “VDDp”. Further, when the signal level in this frame is the minimum value (128) of the frame, the applied voltage to the light-emitting element is “Vb”, for example. Accordingly, the range of the light-emission luminance is the luminance range of “Lb” to “Ld”, which corresponds to the range of the applied voltage of “Vb” to “VDDp”, thereby improving the luminance level. 
   [3] Description of Configuration of Display Device 
     FIG. 21  shows an electric configuration of a display device comprising a self light-emitting display such as an inorganic EL display. 
   An input signal (8-bit digital signal) is sent to a frame memory  201 , a signal level detection portion (signal level range determination means)  203 , and a timing control portion  205 . The input signal stored in the frame memory  201  is sent to a data line  111  of a self light-emitting display  110  after the signal level has been corrected by a signal level control portion (input signal correction means)  202 . A scanning line  112  of the self light-emitting display  110  is controlled by the timing control portion  205 . The power-supply line  113  of the self light-emitting display  110  is controlled by a voltage control portion (driving power-supply voltage control means)  204 . The signal level detection portion  203  gives a control signal to the signal level control portion  202  and also gives a control signal to the voltage control portion  204 . 
   [3-1] Description of Signal Level Detection Portion  3   
   The signal level detection portion  203  comprises a maximum/minimum value detection portion  231  and a determination portion  232 . The maximum/minimum value detection portion  231  extracts the maximum value “MAX” and the minimum value “MIN” of an input signal for every one frame (or every several frames), and then gives the extracted values to the determination portion  232 . 
   Based upon the maximum value “MAX” and the minimum value “MIN” given by the maximum/minimum value detection portion  231 , the determination portion  232  produces a gain “GAIN” and a classification determination signal “Class”, to be given to the signal level control portion  202 , and a set value “VDD” for controlling a voltage to be given to the voltage control portion  204 . The gain “GAIN” is a coefficient for correcting an input signal. The classification determination signal “Class” is a determination signal for indicating a classification determined based upon the maximum value “MAX” and the minimum value “MIN”. The set value “VDD” is a set value for determining a driving power-supply voltage. 
   The action of the determination portion  232  is described. As shown in  FIG. 8 , based upon the signal maximum value “MAX”, the signal minimum value “MIN”, and previously set standard values: minA, maxA, minC and maxC in one frame, the determination portion  232  first determines to which of four classifications: A, B, C and D, a range where the signal maximum value “MAX” and the signal minimum value “MIN” are present belongs. 
   As shown in  FIG. 8 , each of the standard values: minA, maxA, minC and maxC, has been set such that the relationship: 0=minA&lt;minC&lt;maxA&lt;maxC=255, is maintained in the range (0 to 255) where an input signal can be present. 
   When the signal maximum value “MAX” and the signal minimum value “MIN” in one frame are present in a range not smaller than “minC” and not larger than “maxA”, the classification of this range is determined to be “B”. The classification B represents the case where the range of the signal level in one frame is in the intermediate part of the entire level range. 
   When the signal maximum value “MAX” and the signal minimum value “MIN” in one frame are present in a range not smaller than “minA” and not larger than “maxA”, and the classification B does not apply, the classification of the range is determined to be “A”. The classification A represents the case where the range of the signal level in one frame is the entire level range except for a high luminance part. 
   When the signal maximum value “MAX” and the signal minimum value “MIN” one frame are present in a range not smaller than “minC” and not larger than “maxC”, and the classification B does not apply, the classification of the range is determined to be “C”. The classification C represents the case where the range of the signal level in one frame is the entire level range except for a low luminance part. 
   When none of the classifications A, B and C applies, the classification of the range is determined to be “D”. The classification D represents the case where the range of the signal level in one frame is a broad range from the low luminance part through the high luminance part. 
   Next, based upon the classification results, the determination portion  232  determines a classification determination signal “Class”, a gain “GAIN”, and a set value “VDD” as follows: 
   When the classification result is “B”: 
   Class=2, GAIN=Gb, VDD=VDDb 
   When the classification result is “A”: 
   Class=0, GAIN=Ga, VDD=VDDa 
   When the classification result is “C”: 
   Class=1, GAIN=Gc, VDD=VDDc 
   When the classification result is “D”: 
   Class=0, GAIN=0, VTH=VDDd 
   Herein, the scales of the set values are expressed by: VDDa(=VDDd)&lt;VDDb&lt;VDDc. It is to be noted that “Ga”, “Gb” and “Gc” are set to values in a range not smaller than 0 and not larger than 1. 
   [3-2] Description of Signal Level Control Portion  202   
   The action of the signal level control portion  202  is described. The signal level control portion  202  corrects a level of an input signal “S” based upon the classification determination signal “Class” and the gain “GAIN”, given by the signal level detection portion  203 , using the following formula (3). Herein, “SS” represents a signal after the correction (an output signal of the signal level control portion  202 ). 
   When Class=0 (the classification is “A” or “D”)
 
 SS=S+S* GAIN
 
When Class=1 (the classification is “C”)
 
 SS=S− (255− S )*GAIN
 
When Class=2 (the classification is “B”)
 
 SS=S− (MAX− S )*GAIN  (3)
 
   When Class=1, the correction formula in the case of Class=2 may be used. Although “GAIN” is set by classification in the above example, “GAIN” may be set more adaptively according to the maximum value and the minimum value in one screen. It should be noted that, although the signal level control portion  202  produces the output signal “SS” based upon the above formula (3), tables representing a relation between the input signal “S” and the output signal “SS” in the respective cases of Class=0, Class=1 and Class=2 may be previously prepared, and based upon these tables, the input signal “S” may be corrected. 
   [3-3] Description of Voltage Control Portion  204   
   The action of the voltage control portion  204  is described. The voltage control portion  204  controls a driving power-supply voltage, based upon the set value “VDD” given by the signal level detection portion  203 . That is, when the classification result is “B”, “VDD” is equivalent to “VDDb”, and thus the driving power-supply voltage “VDD” to be supplied to the power-supply line  113  is controlled so as to become equivalent to “VDDb”. When the classification result is “A”, “VDD” is equivalent to “VDDa”, and thus the driving power-supply voltage “VDD” to be supplied to the power-supply line  113  is controlled so as to become equivalent to “VDDa”. When the classification result is “C”, “VDD” is equivalent to “VDDc”, and thus the driving power-supply voltage “VDD” to be supplied to the power-supply line  113  is controlled so as to become equivalent to “VDDc”. When the classification result is “D”, “VDD” is equivalent to “VDDd”, and thus the driving power-supply voltage “VDD” to be supplied to the power-supply line  113  is controlled so as to become equivalent to “VDDd”. 
   [4] Description of Control Results 
   The result of the control of (MIN=0, MAX=128) when the classification result is “A” is described.  FIG. 22   a  shows the result of the control in the conventional example described using  FIG. 19 .  FIG. 22   b  shows the result of the control in the example of the present invention (hereinafter referred to as the present method). 
   Both in the conventional Example and the present method, the driving power-supply voltage “VDD” is set to “VDDa (=VDDstd). In the conventional example, the input signal level is not corrected, whereas in the present method, the input signal “S” is corrected based upon the formula: SS=S+S*GAIN, and the range of the input signal level is thus extended to the high luminance side. It is thereby possible in the present method to make the luminance higher than that of the conventional example. 
   The result of the control of MIN=128, MAX=255) when the classification result is “C” is described.  FIG. 23   a  shows the result of the control in the conventional example described using  FIG. 19 .  FIG. 23   b  shows the result of the control in the example of the present invention (hereinafter referred to as the present method). 
   In the conventional example, the driving power-supply voltage “VDD” is set to “VDDa(=VDDstd), whereas in the present method, the driving power-supply voltage “VDD” is set to “VDDc” (&gt;VDDa). Further, in the present method, since the input signal “S” is corrected based upon the formula: SS=S−(255−S)*GAIN, the range of the input signal level is extended to the low luminance side. It is thereby possible in the present method to increase the light-emission luminance on the high luminance side. 
   The result of the control of (MIN=64, MAX=192) when the classification result is “B” is described.  FIG. 24   a  shows the result of the control in the conventional example described using  FIG. 19 .  FIG. 24   b  shows the result of the control in the example of the present invention (hereinafter referred to as the present method). 
   In the conventional example, the driving power-supply voltage “VDD” is set to “VDDa” (=VDDstd), whereas in the present method, the driving power-supply voltage “VDD” is set to “VDDb” (&gt;VDDa). Further, in the present method, since the input signal “S” is corrected based upon the formula: SS=S−(MAX−S)*GAIN, the range of the input signal level is extended to the low luminance side. Further, in the present method, the light-emission luminance is higher on the high luminance side than in the conventional example, due to the shift of “VDD”. It is thereby possible in the present method to increase the light-emission luminance on the high luminance side so as to improve contrast. 
   In Example 4, although the signal level detection portion  203  updates the signal level detection result (“Class”, “GAIN” and “VDD”) for every one frame (every several frames), the signal level detection portion  203  may be arranged to update the signal level detection result (“Class”, “GAIN” and “VDD”) only when a scene change is detected.