The present invention relates to a method for displaying an image in an active matrix display device and more particularly in an active matrix OLED (Organic Light Emitting Display) display. This method has been more particularly but not exclusively developed for video application.
The structure of an active matrix OLED or AM-OLED is well known. It comprises:
Actually, there are two ways for driving the OLED cells. In a first way, each digital video information sent by the digital processing unit is converted by the column drivers into a current whose amplitude is proportional to the video information. This current is provided to the appropriate cell of the matrix. In a second way, the digital video information sent by the digital processing unit is converted by the column drivers into a voltage whose amplitude is proportional to the video information. This current or voltage is provided to the appropriate cell of the matrix.
From the above, it can be deduced that the row driver 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 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 AM-OLED is the following. 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 drivers. The data transmitted to the column driver are either parallel or serial. Additionally, the column driver disposes of a reference signaling delivered by a separate reference signaling device. This component 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 black level. Then, the column driver applies to the matrix cells the voltage or current amplitude corresponding to the data to be displayed by the cells.
In order to illustrate this concept, an example of a voltage driven circuitry is described below. Such a circuitry will also used in the rest of the present specification for illustrating the invention. The driver taken as example uses 8 reference voltages named V0 to V7 and the video levels are built as shown below:
A more complete table is given in Annex 1. This table illustrates the output voltage for various input video levels. The reference voltages used are for example the following ones:
Actually, there are three ways for making colored displays
The invention is more particularly adapted to the displays of
The use of three different OLED materials (one par color) implies that they all have different behaviors. This means that they all have different threshold voltages and different efficiencies as illustrated by
Thus a white color temperature of 6400° K (x=0.313; y=0.328) is achieved by using 100% of the red, 84% of the green and 95% of the blue.
If one driver with only one set of reference signals (voltages or currents) for the 3 colors is used and if the maximum voltage to be applied to the cells is 7 Volts (=Vmax), the voltage range must be from 3V to 7V but only a part of the available dynamic can be used and all corrections must be done digitally. Such a correction will reduce the video dynamic of the whole display.
Since the video levels between 3V and 7V are defined with 256 bits, it means that the green component is displayed with only a few digital levels. The red component uses a bit more gray level but this is still not enough to provide a satisfying picture quality. A solution would be to use specific drivers having for all three color outputs a different reference signaling but such drivers are either not available or quite expensive.
It is an object of the present invention to propose a method to remedy to these drawbacks.
According to the invention, this object is solved by a method for displaying a picture in an active matrix organic light emitting display having a plurality of luminous elements each dedicated to a colour component among at least three colour components of pixels of a picture, wherein the luminance generated by each of said luminous elements is based on the intensity of a signal supplied to said luminous element, the intensity of said signal being defined as a function of reference signals. It comprises the following steps:
The three colour components are for example a red component, a green component and a blue component.
In a first embodiment, the red component is displayed during the first sub-frame with the set of reference signals dedicated to said colour component, the green component is displayed during the second sub-frame with the set of reference signals dedicated to said colour component and the blue component is displayed during the third sub-frame with the set of reference signals dedicated to said colour component.
In a preferred embodiment, the red, green and blue components are displayed during the first sub-frame with the set of reference signals dedicated to the green component, the red and blue components are displayed during the second sub-frame with the set of reference signals dedicated to the red component and the blue component is displayed during the third sub-frame with the set of reference signals dedicated to said colour component.
Advantageously, the durations of the sub-frame are different and are chosen for reducing the voltages applied to the luminous elements in order to increase the lifetime of the luminous elements. For example, the duration of the first sub-frame is lower than the duration of the second sub-frame and the duration of the second sub-frame is lower than the duration of the third sub-frame.
Advantageously, the three sub-frames are interleaved such that two consecutive rows of pixels are addressed sequentially for displaying different colour components.
The invention concerns also a display device comprising
The digital processing unit is designed to control the row driver and to deliver video information and reference signals to the column driver such that the picture is addressing at least three times during the video frame and that the video frame is split into at least three sub-frames, at least one colour component being associated to each subframe, and during each sub-frame, the associated colour component is displayed with a set of reference signals dedicated to said colour component.
Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. In the drawings:
The invention presented here is a specific addressing that can be used in a standard active matrix OLED. The idea is to have a set of reference voltages (or currents) for each colour and to address three times per frame the luminous elements of the display such that the video frame is divided into three sub-frames, each sub-frame being adapted to display mainly a dedicated color by using the corresponding set of reference voltages. The main color to be displayed changes at each sub-frame as the set of reference voltages.
For example, the red colour is displayed during the first sub-frame with the set of reference voltages dedicated to the red colour, the green colour is displayed during the second sub-frame with the set of reference voltages dedicated to the green colour and the blue colour is displayed during the third sub-frame with the set of reference voltages dedicated to the blue colour.
The invention will be explained in more detail in reference to
According to the invention, it is now possible to adjust the 8 reference voltages (or currents) at each sub-frame. The only particularity is that the lowest reference voltages must be kept equal to the lowest threshold voltage of the three colors. Indeed, displaying a blue component means having red and green components equal to zero, which means equal to V7 in our example that is the lowest reference voltage. So, this voltage must be low enough to have them really black. In the example of
V7(R)=V7(B)=V7(G)=VRth.
The only additional requirement is the necessity of addressing the matrix three times faster.
An example of reference voltages for the green component is given below
In reference to
An example of reference voltages for the red component is given below
In reference to
An example of reference voltages for the blue component is given below
In a more general manner, the colour component having the highest luminosity capabilities (in our example, the green component) is displayed only in the first sub-frame. The colour component having the lowest luminosity capabilities (in our example, the blue component) is displayed in the three sub-frames. And the colour component having in-between luminosity capabilities (in our example, the red component) is displayed during two sub-frames.
Advantageously, the duration of the three sub-frames are different and are adapted in order to avoid increasing too much the voltages of a dedicated color component. The color temperature of the display can be adjusted by varying the active time duration of each color component (duration of the sub-frame). This improvement is illustrated by
This invention can also be improved because the display device implementing it can suffer from an artifact called “color break-up”. It is working like a display device based on color-multiplexing by a color-wheel like a DLP (Digital Light Processing) display device for instance. This artifact can be observed when the eye is moving rapidly or while following a rapid movement. It is illustrated by
According to the invention, it is proposed to do a color interleaving line by line. Indeed, in
And finally
Thus, at the end of the 3 sub-periods (which corresponds to the end of the video frame), all the rows have been addressed with voltages based on the 3 sets of reference voltages (currents).
This interleaved mode reduces the visibility of the color break-up. Furthermore, it represents a simple solution that does not require any modification of the active matrix layout. As previously, the data driver is working three times faster than in a classical display device, i.e. a 180 Hz in a 60 hz mode and at 150 Hz in a 50 Hz mode. In this operation mode, it is no more possible to have different active time per colour component.
These two solutions have the advantage of not requiring any modification of the active matrix layout of the display device.
The invention is not restricted to the disclosed embodiments. Various modifications are possible and are considered to fall within the scope of the claims, e.g. other OLED materials with other threshold voltages and efficiencies can be used; a higher number of sub-frames can be used; other color component or group of colour components can be displayed during the sub-frames; the color components can also be displayed in a different order.
Number | Date | Country | Kind |
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05292435.4 | Nov 2005 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2006/068409 | 11/13/2006 | WO | 00 | 5/14/2008 |