# Patent

## Title
Method and apparatus for driving a display device with variable reference driving signals

## Abstract

A method and an apparatus capable of increasing the video depths depending on the video content of each line in order to provide a maximum of color gradation for each given scene shall be proposed. For this purpose there is disclosed an apparatus for driving a display device including input means for receiving a digital value as video level for each pixel or cell of a line of the display device, reference signaling means for providing at least one reference driving signal and driving means for generating a driving signal on the basis of the digital value and the at least one reference driving signal. The apparatus further includes adjusting means for adjusting the at least one reference driving signal in dependence of the digital values of at least a part of the line.

## Background
This application claims the benefit, under 35 U.S.C. §119 of European Patent Application 06/300741.3, filed Jun. 30, 2006.
FIELD OF THE INVENTION
The present invention relates to a method for driving a display device including the steps of providing a digital value as video level for each pixel or cell of a line of the display device, providing at least one reference driving signal and generating a driving signal on the basis of the digital value and the at least one reference driving signal. Furthermore, the present invention relates to a respective apparatus for driving a display device.
BACKGROUND OF THE INVENTION
The structure of an active matrix OLED (organic light emitting display) or AMOLED is well known. According to FIG. 1 it comprises:

    
    
        an active matrix 1 containing, for each cell (one pixel includes a red cell, a green cell and a blue cell), an association of several TFTs T1, T2 with a capacitor C connected to an OLED material. Above the TFTs the capacitor C acts as a memory component that stores a value during a part of the video frame, this value being representative of a video information to be displayed by the cell 2 during the next video frame or the next part of the video frame. The TFTs act as switches enabling the selection of the cell 2, the storage of a data in the capacitor C and the displaying by the cell 2 of a video information corresponding to the stored data;
        a row or gate driver 3 that selects line by line the cells 2 of the matrix 1 in order to refresh their content;
        a column or source driver 4 that delivers the data to be stored in each cell 2 of the current selected line; this component receives the video information for each cell 2; and
        a digital processing unit 5 that applies required video and signal processing steps and that delivers the required control signals to the row and column drivers 3, 4.
    
    


Actually, there are two ways for driving the OLED cells 2. In a first way, each digital video information sent by the digital processing unit 5 is converted by the column drivers 4 into a current whose amplitude is directly proportional to the video level. This current is provided to the appropriate cell 2 of the matrix 1. In a second way, the digital video information sent by the digital processing unit 5 is converted by the column drivers 4 into a voltage whose amplitude is proportional to the square of the video level. This current or voltage is provided to the appropriate cell 2 of the matrix 1.
However, in principle, an OLED is current driven so that each voltage based driven system is based on a voltage to current converter to achieve appropriate cell lighting.
From the above, it can be deduced that the row driver 3 has a quite simple function since it only has to apply a selection line by line. It is more or less a shift register. The column driver 4 represents the real active part and can be considered as a high level digital to analog converter.
The displaying of a video information with such a structure of AMOLED is symbolized in FIG. 2. The input signal is forwarded to the digital processing unit that delivers, after internal processing, a timing signal for row selection to the row driver synchronized with the data sent to the column driver 4. The data transmitted to the column driver 4 are either parallel or serial. Additionally, the column driver 4 disposes of a reference signaling delivered by a separate reference signaling device 6. This component 6 delivers a set of reference voltages in case of voltage driven circuitry or a set of reference currents in case of current driven circuitry. The highest reference is used for the white and the lowest for the smallest gray level. Then, the column driver 4 applies to the matrix cells 2 the voltage or current amplitude corresponding to the data to be displayed by the cells 2.
In order to illustrate this concept, the example of a voltage driven circuitry will be taken in the rest of this document. The driver of this example uses 8 reference voltages named V0 to V7 and the video levels are built as explained in the following table 1.







TABLE 1







Gray level table from voltage driver








Video level
Grayscale voltage level





 0
V7


 1
V7 + (V6 − V7) × 9/1175


 2
V7 + (V6 − V7) × 32/1175


 3
V7 + (V6 − V7) × 76/1175


 4
V7 + (V6 − V7) × 141/1175


 5
V7 + (V6 − V7) × 224/1175


 6
V7 + (V6 − V7) × 321/1175


 7
V7 + (V6 − V7) × 425/1175


 8
V7 + (V6 − V7) × 529/1175


 9
V7 + (V6 − V7) × 630/1175


10
V7 + (V6 − V7) × 727/1175


11
V7 + (V6 − V7) × 820/1175


12
V7 + (V6 − V7) × 910/1175


13
V7 + (V6 − V7) × 998/1175


14
V7 + (V6 − V7) × 1086/1175


15
V6


16
V6 + (V5 − V6) × 89/1097


17
V6 + (V5 − V6) × 173/1097


18
V6 + (V5 − V6) × 250/1097


19
V6 + (V5 − V6) × 320/1097


20
V6 + (V5 − V6) × 386/1097


21
V6 + (V5 − V6) × 451/1097


22
V6 + (V5 − V6) × 517/1097


. . .
. . .



V1 + (V0 − V1) × 2278/3029


251 
V1 + (V0 − V1) × 2411/3029


252 
V1 + (V0 − V1) × 2549/3029


253 
V1 + (V0 − V1) × 2694/3029


254 
V1 + (V0 − V1) × 2851/3029


255 
V0









Table 1 illustrates the obtained output voltages (gray scale voltage levels) from the voltage driver for various input video levels. For instance, the reference voltages of Table 2 are used.







TABLE 2







Example of voltage references










Reference Vn
Voltage (V)














V0
3



V1
2.6



V2
2.2



V3
1.4



V4
0.6



V5
0.3



V6
0.16



V7
0










Then, the grayscale voltage levels of following Table 3 depending on video input levels according to Table 1 and Table 2 are obtained:







TABLE 3







Example of gray level voltages










Video level
Grayscale voltage level







 0
 0.00 V



 1
0.001 V



 2
0.005 V



 3
0.011 V



 4
 0.02 V



 5
0.032 V



 6
0.045 V



 7
 0.06 V



 8
0.074 V



 9
0.089 V



10
0.102 V



11
0.115 V



12
0.128 V



13
 0.14 V



14
0.153 V



15
0.165 V



16
0.176 V



17
0.187 V



18
0.196 V



19
0.205 V



20
0.213 V



21
0.221 V



22
0.229 V



. . .
. . .



250 
2.901 V



251 
2.919 V



252 
2.937 V



253 
2.956 V



254 
2.977 V



255 
 3.00 V










As can be seen in the previous paragraph current AMOLED concepts are capable of delivering 8-bit gradation per color. This can be further enhanced by using more advanced solutions like improvements on analog sub-fields.
In any case, there will be the need in the future of displays having more video-depth. This trend can be seen in the development of transmission standards based on 10-bit color channels. At the same time, various display manufacturers like PDP makers are claiming providing displays with more than 10-bit color-depth.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method and an apparatus capable of increasing the video depth depending on the video content of each line in order to provide a maximum of color gradation for a given scene. I.e., a line content picture enhancement shall be provided.
According to the present invention this object is solved by a method for driving a display device including the steps of

    
    
        providing a digital value as video level for each pixel or cell of a line of said display device,
        providing at least one reference driving signal and
        generating a driving signal on the basis of said digital value and said at least one reference driving signal, as well as
        adjusting said video level and said at least one reference driving signal in dependence of the digital values of at least a part of said line.
    
    


Furthermore, there is provided an apparatus for driving a display device including

    
    
        input means for receiving a digital value for each pixel or cell of a line of said display device,
        reference signaling means for providing at least one reference driving signal and
        driving means for generating a driving signal on the basis of said digital value and said at least one reference driving signal, as well as
        adjusting means for adjusting said video level and said at least one reference driving signal in dependence of the digital values of at least a part of said line.
    
    


Preferably, the display device is an AMOLED or a LCD. Especially, these display concepts can be improved by the above described method or apparatus.
The reference driving signal may be a reference voltage or a reference current. Each of these driving systems can profit from the present invention.
According to a further preferred embodiment, a maximum digital value of at least the part of a line is determined and when adjusting the reference driving signals, they are assigned to digital values between a minimum digital value, which is to be determined or is predetermined, and a maximum digital value. By this way, the whole range of gray scale levels is used for the video input of one line.
A further improvement can be obtained when determining a histogram of the digital values of at least the part of a line and adjusting the reference driving signals on the basis of this histogram. This results in an enhanced picture line-dependent gradation.



BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments of the invention are illustrated in the drawings showing in:
FIG. 1 a circuit diagram of an AMOLED electronic according to the prior art;
FIG. 2 a possible OLED display structure according to the prior art;
FIG. 3 a sequence of the movie “Zorro” and a corresponding line analysis diagram;
FIG. 4 a sequence of a Colombia movie and a corresponding line analysis diagram;
FIG. 5 a histogram of line 303 from the sequence “Zorro”;
FIG. 6 a histogram of line 303 with optimized reference voltages and
FIG. 7 a block diagram of a hardware embodiment of the present invention.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The main idea behind the inventive concept is based on the fact that in a video scene, the whole video dynamic range is not used on a large part of the scene. FIGS. 3 and 4 show typical examples for frames of different dynamics. FIG. 3 shows a dark picture of the movie “Zorro”. The picture has the format 4:3 with 561 lines. On the right hand side of FIG. 3 the maximum video level of each line is plotted.
FIG. 4 shows a picture of a Colombia film. The picture has the format 16:9 with 267 lines. The right hand side diagram of FIG. 4 illustrates that nearly each line is driven with a maximum video level.
Together, FIGS. 3 and 4 show that for some sequences there are strong differences in the vertical distribution of video levels. The most differences are located in dark scenes with some luminous content as illustrated by the sequence “Zorro”.
On the other hand, it is important to notice that in dark scenes the eye is much more sensitive to picture gradation. Therefore, an optimization of picture gradation for dark scenes while keeping luminous scenes quite stable would have a positive effect on the global picture quality.
As already explained, the main idea is to perform a picture line-dependent gradation by optimizing the driver reference signaling (voltage or current) to the maximum of video levels available in a line. For instance, in the sequence “Zorro” of FIG. 3, the maximum video level for line 303 is 128. Therefore, if nothing is done, from the 8-bit of available gradations (0 to 255), only 7 are used for this line (0 to 128). However, according to the present invention, the 8-bit gradation for video levels between 0 and 128 will be used. In order to do that, the reference signaling of the driver is adjusted to these 129 levels. In the present example of a voltage driven system the maximum voltage level will be adjusted to the 129/256 of the original one and all other voltages accordingly. This is illustrated in following Table 4:







TABLE 4







Example of adjusted voltage references for line 303









Reference Vn
Line 303 Voltage (Vn)
Original Voltage (Vrefn)












V0
1.5
3


V1
1.3
2.6


V2
1.1
2.2


V3
0.7
1.4


V4
0.3
0.6


V5
0.15
0.3


V6
0.08
0.16


V7
0
0









More generally, a complex function can be applied to the reference signaling under the form Sn=f(Srefn;MAX(Line)) where MAX(Line) represents the maximum video level used for a given line and Srefn the reference signaling (either voltage or current). This function can be implemented by means of LUT or embedded mathematical functions.
In the example shown in Table 4, all voltages have been modified using the same transformation




  
    V
    n
  
  =
  
    
      
        (
        
          
            Vref
            n
          
          -
          
            Vref
            7
          
        
        )
      
      ⨯
      
        
          MAX
          ⁡
          
            (
            Line
            )
          
        
        255
      
    
    +
    
      Vref
      7
    
  




where Vref0 represents the threshold voltage. This is the simplest transformation that can be used for voltage driven system since the gamma function is applied inside the OLED according to the proportionality L(x,y)∝I(x;y)=k×(V(x;y)−Vth)2 where L(x;y) represents the luminance of the pixel located at (x;y) and I(x,y) the current provided to this pixel. Indeed in a first approach, it is intended to have L(x,y)∝k×(Video(x;y))2 if one could afford to have a gamma of 2 instead of a gamma of 2.2. In this case it is easy to understand that if the Video level dynamic is modified by a factor p, then it is sufficient to modify the voltages by the same factor. In all other cases, like gamma different from 2 or current driven systems where no inherent gamma is existing a more complex transformation is mandatory for the voltage adjustment since the voltages are no more proportional to the video values.

For instance, in a current driven system there is L(x,y)=k×(I−Ith) but ideally it should be L(x,y)∝(Video(x;y))2.2. Then, a gamma transfer function of 2.2 is needed between the video level and the applied intensity. So if the video level is divided by 2, the provided intensity must be divided by 4.59 since




  
    
      L
      ⁡
      
        (
        
          x
          ,
          y
        
        )
      
    
    ∝
    
      
        (
        
          
            Video
            ⁢
            
              
            
            ⁢
            
              (
              
                x
                ;
                y
              
              )
            
          
          2
        
        )
      
      2.2
    
  
  =
  
    
      
        
          (
          
            Video
            ⁢
            
              
            
            ⁢
            
              (
              
                x
                ;
                y
              
              )
            
          
          )
        
        2.2
      
      
        2
        2.2
      
    
    .
  




The same is true for a voltage driven system and a real gamma of 2.2 is aimed. In this case, there is a transformation of 1.1 between video and voltages under the form V(x,y)∝Video(x;y)1.1 that is needed in order to have finally:

L(x,y)∝(V(x;y)−Vth)2∝(Video(x;y)1.1)2=Video(x;y)2.2 

In that case, if the maximum video is divided by 2, the voltages must be divided by 21.1=2.14.
Such a transformation is quite complex and it is often difficult to be computed on-chip. Therefore, the ideal solution is to use a LUT containing 255 inputs, each one dedicated to a maximum value. The output can be on 8-bit or more in order to define the adjusting factor. Ideally, 10-bit is mandatory.
Reverting to the example of the current driven system, if the maximum amplitude per line is 128, the output of the 256×10-bit LUT will be 225. Then the voltages will be multiplied by 225 and divided by 1024 to obtain the factor 4.59. Here, it is very difficult to perform a division in hardware excepted if a 2m divider is used that is simply a shift register. Indeed, dividing by 1024 corresponds to a shift by 10. Therefore the multiplication coefficients are always based on a 2p divider. Some further examples for such a LUT are given in Table 5 below.







TABLE 5







Example of LUT for reference signalling adjustment










LUT (Voltage
LUT (current



driven)
driven)


MAX (Line)
power of 1.1
power of 2.2












96
350
119


97
354
122


98
358
125


99
362
128


100
366
131


101
370
133


102
374
136


103
378
139


104
382
142


105
386
145


106
390
148


107
394
152


108
398
155


109
402
158


110
406
161


111
410
164


112
414
168


113
418
171


114
422
174


115
426
178


116
431
181


117
435
184


118
439
188


119
443
191


120
447
195


121
451
199


122
455
202


123
459
206


124
463
210


125
467
213


126
472
217


127
476
221


128
480
225


129
484
229


130
488
233


131
492
237


132
496
241


133
500
245


134
505
249


135
509
253


136
513
257


137
517
261


138
521
265









In parallel to that the video levels must be modified accordingly to benefit of the enhanced gradation. In that case




  
    L
    out
  
  =
  
    
      L
      in
    
    ⨯
    
      255
      
        MAX
        ⁡
        
          (
          Line
          )
        
      
    
  




applies. Here also the transformation should be better implemented via a LUT with 256 inputs corresponding to the 256 possible values for MAX(Line) and an output corresponding to a coefficient on 10-bit or more.

In the previous paragraph, a simple solution is shown based on adjusting the reference signaling range to the maximal available video level in a line. A more advanced concept would lead in an optimization of the gradation between the more used video levels. Such enhanced concept of picture line-dependent gradation will be based on a histogram analysis performed on each line. The example of the sequence “Zorro” and the line 303 shall be taken from such histogram analysis with the previous approach for voltage adjustment.
FIG. 5 shows in a histogram analysis the repartition of video levels for the line 303 of the sequence “Zorro” (FIG. 3). The vertical lines represent the new adjusted voltages from the first embodiment presented in connection with Table 4. The reference voltages are represented according to the example from Table 1 and the video level is adjusted according to the equation




  
    V
    n
  
  =
  
    
      
        (
        
          
            Vref
            n
          
          -
          
            Vref
            0
          
        
        )
      
      ⨯
      
        
          MAX
          ⁡
          
            (
            Line
            )
          
        
        255
      
    
    +
    
      
        Vref
        0
      
      .
    
  




Now, for all examples simply a gamma of 2 shall be used. For this case, the new correspondence between video levels and voltages is shown in Table 6.







TABLE 6







Adjusted gray level table from voltage driver










Video level
Grayscale voltage level







 0
V7



 0.5
V7 + (V6 − V7) × 9/1175



 1
V7 + (V6 − V7) × 32/1175



 1.5
V7 + (V6 − V7) × 76/1175



 2
V7 + (V6 − V7) × 141/1175



 2.5
V7 + (V6 − V7) × 224/1175



 3
V7 + (V6 − V7) × 321/1175



 3.5
V7 + (V6 − V7) × 425/1175



 4
V7 + (V6 − V7) × 529/1175



 4.5
V7 + (V6 − V7) × 630/1175



 5
V7 + (V6 − V7) × 727/1175



 5.5
V7 + (V6 − V7) × 820/1175



 6
V7 + (V6 − V7) × 910/1175



 6.5
V7 + (V6 − V7) × 998/1175



 7
V7 + (V6 − V7) × 1086/1175



 7.5
V6



 8
V6 + (V5 − V6) × 89/1097



 8.5
V6 + (V5 − V6) × 173/1097



 9
V6 + (V5 − V6) × 250/1097



 9.5
V6 + (V5 − V6) × 320/1097



 10
V6 + (V5 − V6) × 386/1097



 10.5
V6 + (V5 − V6) × 451/1097



 11
V6 + (V5 − V6) × 517/1097



. . .
. . .



125.5
V1 + (V0 − V1) × 2278/3029



126
V1 + (V0 − V1) × 2411/3029



126.5
V1 + (V0 − V1) × 2549/3029



127
V1 + (V0 − V1) × 2694/3029



127.5
V1 + (V0 − V1) × 2851/3029



128
V0










As it can be seen on FIG. 5, the maximum of video levels are located between level 15 (V5) and level 95 (V2) but this is not the location where the finest gradation is obtained. However, the finest gradation is obtained when reference voltages are near together. This example shows that the gradation obtained with this driver with voltages computed according to the first embodiment is not optimized to this particular line structure.
Therefore, according to a further embodiment there is provided an adaptation of the video transformation and voltage levels to adjust finest gradation where the maximum of video levels are distributed. In order to implement this concept, a first table is needed representing the driver behavior, which means the number of levels represented by each voltage. This is illustrated in Table 7 for the example of Table 1. A full voltage reference table for the driver chosen as example is given in Annex 1.







TABLE 7







Example of voltage references video rendition










Reference Vn
Amount of levels














V7
0



V6
15



V5
16



V4
32



V3
64



V2
64



V1
32



V0
32










It is generally known that a histogram of a picture represents, for each video level, the number of times this level is used. Such a histogram table is computed for a given line and described as HISTO[n], where n represents the possible video levels used for the input picture (at least 8 bit or more). In order to simplify the exposition, an input signal limited to 8-bit (256 discrete levels) will be taken.
Now, the main idea is based on a computation of video level limits for each voltage. Such a limit represents the ideal number of pixels that should be coded inside each voltage. Ideally, this will be based on a percentage of the number of pixels per line. For example, for a display with 720 pixels per lines (720×3 cells) the voltage V5 should be used to encode at least 720×3×16/255=135 cells. Based on this assumption the following Table 8 is obtained.







TABLE 8







Example of voltage references limitation










Amount of
Limit with


Reference Vn
levels
320 cells












V7
0
0


V6
15
127


V5
16
135


V4
32
271


V3
64
542


V2
64
542


V1
32
271


V0
32
271









The limits of this table are stored in an array LIMIT[k] with LIMIT[0]=0, LIMIT[1]=127, . . . , LIMIT[7]=271.
Now, for each line following exemplary computation is performed:















LevelCount = 0



Range = 1



For (l=0; l<255; l++)



{



  LevelCount = LevelCount + HISTO[l]



  If (LevelCount > LIMIT[Range])



  {



    LevelCount = 0



    LEVEL_SELECT[Range]=l



    Range++



  }



}










From this computation a table of video levels LEVEL_SELECT[k] results that represents the video level at the transition between the voltage k-1 and k. The results for line 303 are given in Table 9 below, which is based on Annex 2.







TABLE 9







Results of analysis for line 303












Level
Occurrence
Accumulation
Decision
















 0
27
27
Range 1



 1
13
40
Range 1



 2
1
41
Range 1



 3
2
43
Range 1



 4
3
46
Range 1



 5
4
50
Range 1



 6
3
53
Range 1



 7
0
53
Range 1



 8
1
54
Range 1



 9
1
55
Range 1



10
2
57
Range 1



11
0
57
Range 1



12
5
62
Range 1



13
7
69
Range 1



14
4
73
Range 1



15
8
81
Range 1



16
9
90
Range 1



17
19
109
Range 1



18
29
138
Range 2



19
50
188
Range 2



20
35
223
Range 2



21
37
260
Range 2



22
24
284
Range 3



23
26
310
Range 3



. . .
. . .



116 
0
2149
Range 7



117 
2
2151
Range 7



118 
1
2152
Range 7



119 
0
2152
Range 7



120 
1
2153
Range 7



121 
0
2153
Range 7



122 
0
2153
Range 7



123 
2
2155
Range 7



124 
0
2155
Range 7



125 
1
2156
Range 7



126 
1
2157
Range 7



127 
2
2159
Range 7



128 
1
2160
Range 7










Table 9 shows that:

    
    
        Levels [0-17] are used in Range 1→voltage V6→LEVEL_SELECT[1]=18
        Levels [18-21] are used in Range 2→voltage V5→LEVEL_SELECT[2]=22
        Levels [22-31] are used in Range 3→voltage V4→LEVEL_SELECT[3]=32
        Levels [32-40] are used in Range 4→voltage V3→LEVEL_SELECT[4]=41
        Levels [41-51] are used in Range 5→voltage V2→LEVEL_SELECT[5]=52
        Levels [52-60] are used in Range 6→voltage V1→LEVEL_SELECT[6]=61
        Levels [61-128] are used in Range 7→voltage V0→LEVEL_SELECT[7]=128

LEVEL_SELECT[0]=0.

    
    


The result is illustrated in FIG. 6 showing a possible optimization of the voltages repartition according to the video levels repartition. The example of algorithm used here for this optimization should be seen as an example since other computations with similar achievements are possible. Indeed, it could be better to reduce a bit more the gap V1 to V0 in the above example. This can be achieved by a more complicated system.
As soon as the optimal voltages repartition for a given line is defined, two types of adjustment should be performed to display a correct but improved picture:

    
    
        First the adaptation of the voltages themselves—this computation is similar to the computation done in the previous embodiment. In that case the following equation applies:
    
    






  
    V
    n
  
  =
  
    
      
        (
        
          
            Vref
            n
          
          -
          
            Vrefr
            
              n
              -
              1
            
          
        
        )
      
      ⨯
      
        (
        
          
            
              LEVEL_SELECT
              ⁡
              
                [
                n
                ]
              
            
            -
            
              LEVEL_SELECT
              ⁡
              
                [
                
                  n
                  -
                  1
                
                ]
              
            
          
          
            LIMIT
            ⁡
            
              [
              n
              ]
            
          
        
        )
      
    
    +
    
      V
      
        n
        -
        1
      
    
  




    
    
        with n≧1
        Then, the modification of the video levels to suit the new voltages distribution. In that case for a level located in Range n the luminance value is:
    
    






  
    L
    out
  
  =
  
    
      
        (
        
          
            L
            
              i
              ⁢
              
                
              
              ⁢
              n
            
          
          -
          
            LEVEL_SELECT
            ⁡
            
              [
              
                n
                -
                1
              
              ]
            
          
        
        )
      
      ⨯
      
        (
        
          
            LIMIT
            ⁡
            
              [
              n
              ]
            
          
          
            
              LEVEL_SELECT
              ⁡
              
                [
                n
                ]
              
            
            -
            
              LEVEL_SELECT
              ⁡
              
                [
                
                  n
                  -
                  1
                
                ]
              
            
          
        
        )
      
    
    +
    
      

    
    ⁢
    
      
    
    ⁢
    
      TRANS
      ⁡
      
        [
        
          n
          -
          1
        
        ]
      
    
  




With the table transition being an accumulation of the LIMIT[k] values so that




  
    TRANS
    ⁡
    
      [
      k
      ]
    
  
  =
  
    
      ∑
      
        p
        =
        0
      
      
        p
        =
        k
      
    
    ⁢
    
      
        LIMIT
        ⁡
        
          [
          k
          ]
        
      
      .
    
  




Consequently, one gets TRANS[0]=0, TRANS[1]=16, TRANS[1]=32, TRANS[2]=64, TRANS[3]=128, TRANS[4]=192, TRANS[5]=224 and TRANS[6]=256.

The results of the previous computations are given in Tables 10 and 11 below:







TABLE 10







Computed new voltages for line 303










Vref
Vline 303













V7
0.00 V
0.00 V


V6
0.16 V
0.19 V


V5
0.30 V
0.23 V


V4
0.60 V
0.32 V


V3
1.40 V
0.43 V


V2
2.20 V
0.57 V


V1
2.60 V
0.68 V


V0
3.00 V
1.52 V
















TABLE 11







Computed new video levels for line 303










Lin
Lout














 0
0



 1
0.833333



 2
1.666667



 3
2.5



 4
3.333333



 5
4.166667



 6
5



 7
5.833333



 8
6.666667



 9
7.5



10
8.333333



11
9.166667



12
10



13
10.83333



14
11.66667



15
12.5



16
13.33333



17
14.16667



18
15



. . .
. . .



116 
249.2687



117 
249.7463



118 
250.2239



119 
250.7015



120 
251.1791



121 
251.6567



122 
252.1343



123 
252.6119



124 
253.0896



125 
253.5672



126 
254.0448



127 
254.5224



128 
255










As already explained the complex computations are most of the cases replaced by LUTs. In the situation of the video level adjustment described as:




  
    L
    out
  
  =
  
    
      
        (
        
          
            L
            
              i
              ⁢
              
                
              
              ⁢
              n
            
          
          -
          
            LEVEL_SELECT
            ⁡
            
              [
              
                n
                -
                1
              
              ]
            
          
        
        )
      
      ⨯
      
        (
        
          
            LIMIT
            ⁡
            
              [
              n
              ]
            
          
          
            
              LEVEL_SELECT
              ⁡
              
                [
                n
                ]
              
            
            -
            
              LEVEL_SELECT
              ⁡
              
                [
                
                  n
                  -
                  1
                
                ]
              
            
          
        
        )
      
    
    +
    
      

    
    ⁢
    
      
    
    ⁢
    
      TRANS
      ⁡
      
        [
        
          n
          -
          1
        
        ]
      
    
  




A 8-bit LUT takes as input the value LEVEL_SELECT[n]−LEVEL_SELECT[n−1] and delivers a certain factor (more than 10-bit resolution is mandatory) to perform the division. The rest are only multiplications and additions that can be done in real time without any problem.
As already said, the example is related to a simple gamma of 2 in a voltage driven system to simplify the exposition. For a different gamma or for a current driven system, the computations must be adjusted accordingly by using adapted LUTs.
FIG. 7 illustrates an implementation of the inventive solution. The input signal 11 is forwarded to a line analysis block 12 that performs for each input line the required parameters extraction like the highest video level per line or even histogram analysis. This block 12 requires a line memory to delay the whole process of a line. Indeed, the results of the line analysis are obtained only at the end of the line but the modifications to be done on this line must be performed on the whole line.
After the analysis and the delay of the line, the video levels are adjusted in a video adjustment block 13. Here the new video levels Lout are generated on the basis of the original video levels Lin. The video signal with the new video levels is input to a standard OLED processing unit. 14. Column driving data are output from this unit 14 and transmitted to a column driver 15 of an AMOLED display 16. Furthermore, the standard OLED processing unit 14 produces row driving data for controlling the row driver 17 of the AMOLED display 16.
Analysis data of line analysis block 12 are further provided to a voltage adjustment block 18 for adjusting a reference voltages being provided by a reference signaling unit 19. This reference signaling unit 19 delivers reference voltages Vrefn to the column driver 15. For adjusting the reference voltages, the voltage adjustment block 18 is synchronized onto the row driving unit 17.
The control data for programming the specific reference voltages are forwarded from voltage adjustment block 18 to the reference signaling unit 19. The adaptation of the voltages as well as that of the video levels is done on the basis of LUTs and computation.
In case of a current driven system, the reference signaling is performed with currents and block 18 takes care of a current adjustment.
The invention is not limited to the AMOLED screens but can also be applied to LCD displays or other displays using reference signaling means.










Annex 1 - Full driver voltage table








Level
Voltage











0
V7


1
V7 + (V6 − V7) × 9/1175


2
V7 + (V6 − V7) × 32/1175


3
V7 + (V6 − V7) × 76/1175


4
V7 + (V6 − V7) × 141/



1175


5
V7 + (V6 − V7) × 224/



1175


6
V7 + (V6 − V7) × 321/



1175


7
V7 + (V6 − V7) × 425/



1175


8
V7 + (V6 − V7) × 529/



1175


9
V7 + (V6 − V7) × 630/



1175


10
V7 + (V6 − V7) × 727/



1175


11
V7 + (V6 − V7) × 820/



1175


12
V7 + (V6 − V7) × 910/



1175


13
V7 + (V6 − V7) × 998/



1175


14
V7 + (V6 − V7) × 1086/



1175


15
V6


16
V6 + (V5 − V6) × 89/1097


17
V6 + (V5 − V6) × 173/



1097


18
V6 + (V5 − V6) × 250/



1097


19
V6 + (V5 − V6) × 320/



1097


20
V6 + (V5 − V6) × 386/



1097


21
V6 + (V5 − V6) × 451/



1097


22
V6 + (V5 − V6) × 517/



1097


23
V6 + (V5 − V6) × 585/



1097


24
V6 + (V5 − V6) × 654/



1097


25
V6 + (V5 − V6) × 723/



1097


26
V6 + (V5 − V6) × 790/



1097


27
V6 + (V5 − V6) × 855/



1097


28
V6 + (V5 − V6) × 917/



1097


29
V6 + (V5 − V6) × 977/



1097


30
V6 + (V5 − V6) × 1037/



1097


31
V5


32
V5 + (V4 − V5) × 60/



1501


33
V5 + (V4 − V5) × 119/



1501


34
V5 + (V4 − V5) × 176/



1501


35
V5 + (V4 − V5) × 231/



1501


36
V5 + (V4 − V5) × 284/



1501


37
V5 + (V4 − V5) × 335/



1501


38
V5 + (V4 − V5) × 385/



1501


39
V5 + (V4 − V5) × 434/



1501


40
V5 + (V4 − V5) × 483/



1501


41
V5 + (V4 − V5) × 532/



1501


42
V5 + (V4 − V5) × 580/



1501


43
V5 + (V4 − V5) × 628/



1501


44
V5 + (V4 − V5) × 676/



1501


45
V5 + (V4 − V5) × 724/



1501


46
V5 + (V4 − V5) × 772/



1501


47
V5 + (V4 − V5) × 819/



1501


48
V5 + (V4 − V5) × 866/



1501


49
V5 + (V4 − V5) × 912/



1501


50
V5 + (V4 − V5) × 957/



1501


51
V5 + (V4 − V5) × 1001/



1501


52
V5 + (V4 − V5) × 1045/



1501


53
V5 + (V4 − V5) × 1088/



1501


54
V5 + (V4 − V5) × 1131/



1501


55
V5 + (V4 − V5) × 1173/



1501


56
V5 + (V4 − V5) × 1215/



1501


57
V5 + (V4 − V5) × 1257/



1501


58
V5 + (V4 − V5) × 1298/



1501


59
V5 + (V4 − V5) × 1339/



1501


60
V5 + (V4 − V5) × 1380/



1501


61
V5 + (V4 − V5) × 1421/



1501


62
V5 + (V4 − V5) × 1461/



1501


63
V4


64
V4 + (V3 − V4) × 40/2215


65
V4 + (V3 − V4) × 80/2215


66
V4 + (V3 − V4) × 120/



2215


67
V4 + (V3 − V4) × 160/



2215


68
V4 + (V3 − V4) × 200/



2215


69
V4 + (V3 − V4) × 240/



2215


70
V4 + (V3 − V4) × 280/



2215


71
V4 + (V3 − V4) × 320/



2215


72
V4 + (V3 − V4) × 360/



2215


73
V4 + (V3 − V4) × 400/



2215


74
V4 + (V3 − V4) × 440/



2215


75
V4 + (V3 − V4) × 480/



2215


76
V4 + (V3 − V4) × 520/



2215


77
V4 + (V3 − V4) × 560/



2215


78
V4 + (V3 − V4) × 600/



2215


79
V4 + (V3 − V4) × 640/



2215


80
V4 + (V3 − V4) × 680/



2215


81
V4 + (V3 − V4) × 719/



2215


82
V4 + (V3 − V4) × 758/



2215


83
V4 + (V3 − V4) × 796/



2215


84
V4 + (V3 − V4) × 834/



2215


85
V4 + (V3 − V4) × 871/



2215


86
V4 + (V3 − V4) × 908/



2215


87
V4 + (V3 − V4) × 944/



2215


88
V4 + (V3 − V4) × 980/



2215


89
V4 + (V3 − V4) × 1016/



2215


90
V4 + (V3 − V4) × 1052/



2215


91
V4 + (V3 − V4) × 1087/



2215


92
V4 + (V3 − V4) × 1122/



2215


93
V4 + (V3 − V4) × 1157/



2215


94
V4 + (V3 − V4) × 1192/



2215


95
V4 + (V3 − V4) × 1226/



2215


96
V4 + (V3 − V4) × 1260/



2215


97
V4 + (V3 − V4) × 1294/



2215


98
V4 + (V3 − V4) × 1328/



2215


99
V4 + (V3 − V4) × 1362/



2215


100
V4 + (V3 − V4) × 1396/



2215


101
V4 + (V3 − V4) × 1429/



2215


102
V4 + (V3 − V4) × 1462/



2215


103
V4 + (V3 − V4) × 1495/



2215


104
V4 + (V3 − V4) × 1528/



2215


105
V4 + (V3 − V4) × 1561/



2215


106
V4 + (V3 − V4) × 1593/



2215


107
V4 + (V3 − V4) × 1625/



2215


108
V4 + (V3 − V4) × 1657/



2215


109
V4 + (V3 − V4) × 1688/



2215


110
V4 + (V3 − V4) × 1719/



2215


111
V4 + (V3 − V4) × 1750/



2215


112
V4 + (V3 − V4) × 1781/



2215


113
V4 + (V3 − V4) × 1811/



2215


114
V4 + (V3 − V4) × 1841/



2215


115
V4 + (V3 − V4) × 1871/



2215


116
V4 + (V3 − V4) × 1901/



2215


117
V4 + (V3 − V4) × 1930/



2215


118
V4 + (V3 − V4) × 1959/



2215


119
V4 + (V3 − V4) × 1988/



2215


120
V4 + (V3 − V4) × 2016/



2215


121
V4 + (V3 − V4) × 2044/



2215


122
V4 + (V3 − V4) × 2072/



2215


123
V4 + (V3 − V4) × 2100/



2215


124
V4 + (V3 − V4) × 2128/



2215


125
V4 + (V3 − V4) × 2156/



2215


126
V4 + (V3 − V4) × 2185/



2215


127
V3


128
V3 + (V2 − V3) × 31/2343


129
V3 + (V2 − V3) × 64/2343


130
V3 + (V2 − V3) × 97/2343


131
V3 + (V2 − V3) × 130/



2343


132
V3 + (V2 − V3) × 163/



2343


133
V3 + (V2 − V3) × 196/



2343


134
V3 + (V2 − V3) × 229/



2343


135
V3 + (V2 − V3) × 262/



2343


136
V3 + (V2 − V3) × 295/



2343


137
V3 + (V2 − V3) × 328/



2343


138
V3 + (V2 − V3) × 361/



2343


139
V3 + (V2 − V3) × 395/



2343


140
V3 + (V2 − V3) × 429/



2343


141
V3 + (V2 − V3) × 463/



2343


142
V3 + (V2 − V3) × 497/



2343


143
V3 + (V2 − V3) × 531/



2343


144
V3 + (V2 − V3) × 566/



2343


145
V3 + (V2 − V3) × 601/



2343


146
V3 + (V2 − V3) × 636/



2343


147
V3 + (V2 − V3) × 671/



2343


148
V3 + (V2 − V3) × 706/



2343


149
V3 + (V2 − V3) × 741/



2343


150
V3 + (V2 − V3) × 777/



2343


151
V3 + (V2 − V3) × 813/



2343


152
V3 + (V2 − V3) × 849/



2343


153
V3 + (V2 − V3) × 885/



2343


154
V3 + (V2 − V3) × 921/



2343


155
V3 + (V2 − V3) × 958/



2343


156
V3 + (V2 − V3) × 995/



2343


157
V3 + (V2 − V3) × 1032/



2343


158
V3 + (V2 − V3) × 1069/



2343


159
V3 + (V2 − V3) × 1106/



2343


160
V3 + (V2 − V3) × 1143/



2343


161
V3 + (V2 − V3) × 1180/



2343


162
V3 + (V2 − V3) × 1217/



2343


163
V3 + (V2 − V3) × 1255/



2343


164
V3 + (V2 − V3) × 1293/



2343


165
V3 + (V2 − V3) × 1331/



2343


166
V3 + (V2 − V3) × 1369/



2343


167
V3 + (V2 − V3) × 1407/



2343


168
V3 + (V2 − V3) × 1445/



2343


169
V3 + (V2 − V3) × 1483/



2343


170
V3 + (V2 − V3) × 1521/



2343


171
V3 + (V2 − V3) × 1559/



2343


172
V3 + (V2 − V3) × 1597/



2343


173
V3 + (V2 − V3) × 1635/



2343


174
V3 + (V2 − V3) × 1673/



2343


175
V3 + (V2 − V3) × 1712/



2343


176
V3 + (V2 − V3) × 1751/



2343


177
V3 + (V2 − V3) × 1790/



2343


178
V3 + (V2 − V3) × 1829/



2343


179
V3 + (V2 − V3) × 1868/



2343


180
V3 + (V2 − V3) × 1907/



2343


181
V3 + (V2 − V3) × 1946/



2343


182
V3 + (V2 − V3) × 1985/



2343


183
V3 + (V2 − V3) × 2024/



2343


184
V3 + (V2 − V3) × 2064/



2343


185
V3 + (V2 − V3) × 2103/



2343


186
V3 + (V2 − V3) × 2143/



2343


187
V3 + (V2 − V3) × 2183/



2343


188
V3 + (V2 − V3) × 2223/



2343


189
V3 + (V2 − V3) × 2263/



2343


190
V3 + (V2 − V3) × 2303/



2343


191
V2


192
V2 + (V1 − V2) × 40/1638


193
V2 + (V1 − V2) × 81/1638


194
V2 + (V1 − V2) × 124/



1638


195
V2 + (V1 − V2) × 168/



1638


196
V2 + (V1 − V2) × 213/



1638


197
V2 + (V1 − V2) × 259/



1638


198
V2 + (V1 − V2) × 306/



1638


199
V2 + (V1 − V2) × 353/



1638


200
V2 + (V1 − V2) × 401/



1638


201
V2 + (V1 − V2) × 450/



1638


202
V2 + (V1 − V2) × 499/



1638


203
V2 + (V1 − V2) × 548/



1638


204
V2 + (V1 − V2) × 597/



1638


205
V2 + (V1 − V2) × 646/



1638


206
V2 + (V1 − V2) × 695/



1638


207
V2 + (V1 − V2) × 745/



1638


208
V2 + (V1 − V2) × 795/



1638


209
V2 + (V1 − V2) × 846/



1638


210
V2 + (V1 − V2) × 897/



1638


211
V2 + (V1 − V2) × 949/



1638


212
V2 + (V1 − V2) × 1002/



1638


213
V2 + (V1 − V2) × 1056/



1638


214
V2 + (V1 − V2) × 1111/



1638


215
V2 + (V1 − V2) × 1167/



1638


216
V2 + (V1 − V2) × 1224/



1638


217
V2 + (V1 − V2) × 1281/



1638


218
V2 + (V1 − V2) × 1339/



1638


219
V2 + (V1 − V2) × 1398/



1638


220
V2 + (V1 − V2) × 1458/



1638


221
V2 + (V1 − V2) × 1518/



1638


222
V2 + (V1 − V2) × 1578/



1638


223
V1


224
V1 + (V0 − V1) × 60/3029


225
V1 + (V0 − V1) × 120/



3029


226
V1 + (V0 − V1) × 180/



3029


227
V1 + (V0 − V1) × 241/



3029


228
V1 + (V0 − V1) × 304/



3029


229
V1 + (V0 − V1) × 369/



3029


230
V1 + (V0 − V1) × 437/



3029


231
V1 + (V0 − V1) × 507/



3029


232
V1 + (V0 − V1) × 580/



3029


233
V1 + (V0 − V1) × 655/



3029


234
V1 + (V0 − V1) × 732/



3029


235
V1 + (V0 − V1) × 810/



3029


236
V1 + (V0 − V1) × 889/



3029


237
V1 + (V0 − V1) × 969/



3029


238
V1 + (V0 − V1) × 1050/



3029


239
V1 + (V0 − V1) × 1133/



3029


240
V1 + (V0 − V1) × 1218/



3029


241
V1 + (V0 − V1) × 1304/



3029


242
V1 + (V0 − V1) × 1393/



3029


243
V1 + (V0 − V1) × 1486/



3029


244
V1 + (V0 − V1) × 1583/



3029


245
V1 + (V0 − V1) × 1686/



3029


246
V1 + (V0 − V1) × 1794/



3029


247
V1 + (V0 − V1) × 1907/



3029


248
V1 + (V0 − V1) × 2026/



3029


249
V1 + (V0 − V1) × 2150/



3029


250
V1 + (V0 − V1) × 2278/



3029


251
V1 + (V0 − V1) × 2411/



3029


252
V1 + (V0 − V1) × 2549/



3029


253
V1 + (V0 − V1) × 2694/



3029


254
V1 + (V0 − V1) × 2851/



3029


255
V0



















Annex 2 - Histogram of line 303 from sequence “Zorro”










Level
Occurrence














0
27



1
13



2
1



3
2



4
3



5
4



6
3



7
0



8
1



9
1



10
2



11
0



12
5



13
7



14
4



15
8



16
9



17
19



18
29



19
50



20
35



21
37



22
24



23
26



24
19



25
23



26
12



27
24



28
26



29
23



30
25



31
31



32
56



33
54



34
64



35
61



36
78



37
42



38
59



39
61



40
75



41
78



42
61



43
41



44
55



45
52



46
43



47
48



48
42



49
42



50
46



51
45



52
28



53
29



54
27



55
26



56
28



57
25



58
25



59
33



60
39



61
38



62
38



63
25



64
23



65
12



66
11



67
22



68
13



69
5



70
4



71
5



72
6



73
13



74
8



75
3



76
7



77
6



78
4



79
2



80
2



81
2



82
4



83
5



84
3



85
3



86
6



87
2



88
1



89
3



90
2



91
0



92
3



93
0



94
1



95
1



96
0



97
1



98
0



99
1



100
0



101
0



102
0



103
1



104
1



105
1



106
0



107
2



108
0



109
0



110
1



111
1



112
0



113
1



114
0



115
0



116
0



117
2



118
1



119
0



120
1



121
0



122
0



123
2



124
0



125
1



126
1



127
2



128
1



129
0



130
0



131
0



132
0



133
0



134
0



135
0



136
0



137
0



138
0



139
0



140
0



141
0



142
0



143
0



144
0



145
0



146
0



147
0



148
0



149
0



150
0



151
0



152
0



153
0



154
0



155
0



156
0



157
0



158
0



159
0



160
0



161
0



162
0



163
0



164
0



165
0



166
0



167
0



168
0



169
0



170
0



171
0



172
0



173
0



174
0



175
0



176
0



177
0



178
0



179
0



180
0



181
0



182
0



183
0



184
0



185
0



186
0



187
0



188
0



189
0



190
0



191
0



192
0



193
0



194
0



195
0



196
0



197
0



198
0



199
0



200
0



201
0



202
0



203
0



204
0



205
0



206
0



207
0



208
0



209
0



210
0



211
0



212
0



213
0



214
0



215
0



216
0



217
0



218
0



219
0



220
0



221
0



222
0



223
0



224
0



225
0



226
0



227
0



228
0



229
0



230
0



231
0



232
0



233
0



234
0



235
0



236
0



237
0



238
0



239
0



240
0



241
0



242
0



243
0



244
0



245
0



246
0



247
0



248
0



249
0



250
0



251
0



252
0



253
0



254
0



255
0

## Claims

1. Method for driving a display device with at least one variable reference driving signal for displaying video level with variable video depth, comprising:
providing a digital value as video level for each pixel or cell of a line of said display device,
providing at least one reference driving signal and
generating a driving signal on the basis of said digital value and said at least one reference driving signal,
adjusting said at least one reference driving signal dependent on a change of digital values of video levels of at least a part of said line representing a range of video levels by
a transformation of said range of video levels of said line to a maximum number of available video levels for displaying video levels with variable video depth to perform a picture line-dependent alteration of a number of gradations by adjusting said at least one reference driving signal to the video levels in said at least part of said line and a number of video levels corresponding to said range of video levels for displaying video level with variable video depth and original luminance on the display device.

2. Method according to claim 1, wherein said display device is one of an AMOLED display and a LCD display.

3. Method according to claim 1, wherein said reference driving signal is one of a reference voltage and a reference current.

4. Method according to claim 1, wherein a maximum digital value of said at least part of a line is determined and when adjusting said at least one reference driving signal, said at least one reference driving signal is assigned to digital values between a minimum digital value which is to be determined or is predetermined, and said maximum digital value.

5. Method according to claim 1, wherein a histogram of the digital values of said at least part of a line is determined and said at least one reference driving signal is adjusted on the basis of said histogram.

6. Apparatus for driving a display device with at least one variable reference driving signal for displaying video level with variable video depth including
an input for receiving a digital value for each pixel or cell of a line of said display device,
a reference signaling unit for providing at least one reference driving signal and
a driver for generating a driving signal on the basis of said digital value and said at least one reference driving signal,
an adjustment block for adjusting said at least one reference driving signal in dependence of a range of video levels representative of the digital values of video levels of at least a part of said line,
a line analysis block providing for each input line the highest video level for said at least part of said line and
a video adjustment block to generate new video levels on the basis of said range of video levels according to a maximum number of available video levels for displaying video level with variable video depth, wherein said adjustment block is connected to said line analysis block to perform a picture line-dependent alteration of a number of gradations by adjusting said at least one reference driving signal being provided by a reference signaling unit to the video levels in said at least part of said line for driver reference signaling a number of video levels corresponding to said range of video levels for displaying video level with variable video depth and original luminance on the display device.

7. Apparatus according to claim 6, wherein said display device is one of an AMOLED display and a LCD display.

8. Apparatus according to claim 6, wherein said reference signaling unit provides ones of reference voltages and reference currents as reference driving signal.

9. Apparatus according to claim 6, wherein said line analysis block further determines a maximum digital value of said at least part of a line and for providing said maximum digital value to said adjustment block, so that said adjustment block is capable of assigning said at least one reference driving signal to digital values between a minimum digital value, which is to be determined or is predetermined, and said maximum digital value.

10. Apparatus according to claim 6, wherein said line analysis block further determines a histogram of the digital values of said at least part of a line and for controlling said adjustment block so that said at least one reference driving signal is adjusted on the basis of said histogram.