EP1895496A2

EP1895496A2 - Method and apparatus for driving a display device with variable reference driving signals

Info

Publication number: EP1895496A2
Application number: EP07111198A
Authority: EP
Inventors: Sébastien Weitbruch; Rainer Schweer; Sylvain Thiebaud
Original assignee: Thomson Licensing SAS
Current assignee: THOMSON LICENSING
Priority date: 2006-06-30
Filing date: 2007-06-27
Publication date: 2008-03-05
Also published as: EP1895496A3

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 (16) including input means (11) for receiving a digital value as video level for each pixel or cell of a line of the display device (16), reference signalling means (19) for providing at least one reference driving signal and driving means (15) 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 (18) for adjusting the at least one reference driving signal in dependence of the digital values of at least a part of the line.

Description

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 signalling delivered by a separate reference signalling 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)x9/1175
2	V7+(V6-V7)x32/1175
3	V7+(V6-V7)x76/1175
4	V7+(V6-V7)×141/1175
5	V7+(V6-V7)x224/1175
6	V7+(V6-V7)x321/1175
7	V7+(V6-V7)x425/1175
8	V7+(V6-V7)x529/1175
9	V7+(V6-V7)x630/1175
10	V7+(V6-V7)x727/1175
11	V7+(V6-V7)x820/1175
12	V7+ (V6-V7)x910/1175
13	V7+(V6-V7)x998/1175
14	V7+(V6-V7)×1086/1175
15	V6
16	V6+ (V5-V6) x89/1097
17	V6+ (V5-V6) x173/1097
18	V6+ (V5-V6) x250/1097
19 20	V6+ (V5-V6) x320/1097 V6+ (V5-V6) x386/1097
21	V6+ (V5-V6) x451/1097
22	V6+ (V5-V6) x517/1097
...	...
	V1+(V0-V1)x2278/3029
251	V1+(V0-V1)x2411/3029
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	0.00V
1	0.001V
2	0.005V
3	0.011V
4	0.02V
5	0.032V
6	0.045V
7	0.06V
8	0.074V
9	0.089V
10	0.102V
11	0.115V
12	0.128V
13	0.14V
14	0.153V
15	0.165V
16	0.176V
17	0.187V
18	0.196V
19	0.205V
20	0.213V
21	0.221V
22	0.229V
...	...
250	2.901V
251	2.919V
252	2.937V
253	2.956V
254	2.977V
255	3.00V

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 signalling 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 optimised 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. Figures 3 and 4 show typical examples for frames of different dynamics. Figure 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 Figure 3 the maximum video level of each line is plotted.
Figure 4 shows a picture of a Colombia film. The picture has the format 16:9 with 267 lines. The right hand side diagram of Figure 4 illustrates that nearly each line is driven with a maximum video level.
Together, Figures 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 signalling (voltage or current) to the maximum of video levels available in a line. For instance, in the sequence "Zorro" of Figure 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 signalling 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 signalling under the form S_n=f(Sref_n ; 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 $V_{n} = ({Vref}_{n} - {Vref}_{7}) \times \frac{MAX (Line)}{255} + {Vref}_{7}$
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)-V_th )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- I_th) 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 $L (x y) \propto {(\frac{Video (x . y)}{2})}^{2.2} = \frac{{(Video (x . y))}^{2.2}}{2^{2.2}} .$
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: $L (x y) \propto {(V (x, y) - V_{t h})}^{2} \propto {({Video (x . y)}^{1.1})}^{2} = Video {(x y)}^{2.2}$
In that case, if the maximum video is divided by 2, the voltages must be divided by 2^1.1=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 256x10-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 2^m 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 2^p divider. Some further examples for such a LUT are given in Table 5 below.

Table 5: Example of LUT for reference signalling adjustment

MAX (Line)	LUT (Voltage driven) power of 1.1	LUT (current driven) 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 $L_{out} = L_{in} \times \frac{255}{MAX (Line)}$
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 signalling 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.
Figure 5 shows in a histogram analysis the repartition of video levels for the line 303 of the sequence "Zorro" (Figure 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 $V_{n} = ({Vref}_{n} - {Vref}_{0}) \times \frac{MAX (Line)}{255} + {Vref}_{0} .$

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)x9/1175
1	V7+(V6-V7)x32/1175
1.5	V7+(V6-V7)x76/1175
2	V7+(V6-V7)x141/1175
2.5	V7+(V6-V7)x224/1175
3	V7+(V6-V7)x321/1175
3.5	V7+(V6-V7)x425/1175
4	V7+(V6-V7)x529/1175
4.5	V7+(V6-V7)x630/1175
5	V7+(V6-V7)x727/1175
5.5	V7+(V6-V7)x820/1175
6	V7+(V6-V7)x910/1175
6.5	V7+(V6-V7)x998/1175
7	V7+(V6-V7)x1086/1175
7.5	V6
8	V6+ (V5-V6) x89/1097
8.5	V6+ (V5-V6) x173/1097
9	V6+ (V5-V6)x250/1097
9.5	V6+ (V5-V6) x320/1097
10	V6+ (V5-V6) x386/1097
10.5	V6+(V5-V6)x451/1097
11	V6+(V5-V6)x517/1097
...	...
125.5	V1+(V0-V1)x2278/3029
126	V1+(V0-V1)x2411/3029
126.5	V1+(V0-V1)x2549/3029
127	V1+(V0-V1)x2694/3029
127.5	V1+(V0-V1)x2851/3029
128	V0

As it can be seen on Figure 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 (720x3 cells) the voltage V5 should be used to encode at least 720x3x16/255 = 135 cells. Based on this assumption the following Table 8 is obtained. Table 8: Example of voltage references limitation

Reference Vn Amount of levels Limit with 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 (1=0; 1<255; 1++)
 {
     LevelCount = LevelCount + HISTO[1]
     If (LevelCount > LIMIT[Range])
     {
          LevelCount = 0
          LEVEL_SELECT[Range]=1
          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	Range1
1	13	40	Range1
2	1	41	Range1
3	2	43	Range1
4	3	46	Range1
5	4	50	Range1
6	3	53	Range1
7	0	53	Range1
8	1	54	Range1
9	1	55	Range1
10	2	57	Range1
11	0	57	Range1
12	5	62	Range1
13	7	69	Range1
14	4	73	Range1
15	8	81	Range1
16	9	90	Range1
17	19	109	Range1
18	29	138	Range2
19	50	188	Range2
20	35	223	Range2
21	37	260	Range2
22	24	284	Range3
23	26	310	Range3
...		...
116	0	2149	Range7
117	2	2151	Range7
118	1	2152	Range7
119	0	2152	Range7
120	1	2153	Range7
121	0	2153	Range7
122	0	2153	Range7
123	2	2155	Range7
124	0	2155	Range7
125	1	2156	Range7
126	1	2157	Range7
127	2	2159	Range7
128	1	2160	Range7

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

with LEVEL_SELECT[0]=0.

The result is illustrated in Figure 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: $V_{n} = ({Vref}_{n} - {Vref}_{n - 1}) \times (\frac{LEVEL_SELECT [n] - LEVEL_SELECT [n - 1]}{LIMIT [k]}) + V_{n - 1}$

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: $L_{out} = (L_{in} - LEVEL_SELECT [n - 1]) \times (\frac{LIMIT [n]}{LEVEL_SELECT [n] - LEVEL_SELECT [n - 1]}) + TRANS [n - 1]$

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

TRANS [k] = \sum_{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_{in} - LEVEL_SELECT [n - 1]) \times (\frac{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.

Figure 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 signalling unit 19. This reference signalling unit 19 delivers reference voltages Vref_n 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 signalling 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 signalling 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 signalling 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+ (V6V7)×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)×145/2343
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+(VO-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+(VO-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

Method for driving a display device (16) including the steps of
- providing a digital value as video level for each pixel or cell of a line of said display device (16),

- 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,
characterized by
- 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.
Method according to claim 1, wherein said display device (16) is an AMOLED or a LCD.
Method according to claim 1 or 2, wherein said reference driving signal is a reference voltage or a reference current.
Method according to one of the preceding claims, wherein a maximum digital value of said at least part of a line is determined and when adjusting said at least 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.
Method according to one of the claims 1 to 3, wherein a histogram of the digital values of said at least part of a line is determined and said at least one reference driving signals is adjusted on the basis of said histogram.
Apparatus for driving a display device (16) including
- input means for receiving a digital value for each pixel or cell of a line of said display device (16),

- reference signalling means (19) for providing at least one reference driving signal and

- driving means (15) for generating a driving signal on the basis of said digital value and said at least one reference driving signal,
characterized by

- adjusting means (18) 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.
Apparatus according to claim 6, wherein said display device (16) is an AMOLED or a LCD.
Apparatus according to claim 6 or 7, wherein said reference signalling means (19) provides reference voltages or reference currents as reference driving signals.
Apparatus according to one of the claims 6 to 8, further including analysing means (12) for determining a maximum digital value of said at least part of a line and for providing said maximum digital value to said adjusting means, so that said adjusting means (18) 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.
Apparatus according to one of the claims 6 to 8, further including analysing means (12) for determining a histogram of the digital values of said at least part of a line and for controlling said adjusting means (18) so that said at least one reference driving signal is adjusted on the basis of said histogram.