The structure of an active matrix OLED or AMOLED is well known. According to FIG. 1 it comprises:                an active matrix 1 containing, for each 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 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 proportional to the video information. 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 video information. This current or voltage is provided to the appropriate cell 2 of the matrix 1.
However, in principal, 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 AM-OLED 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.
A grayscale rendition without frequency doubling (e.g. case of 60 Hz or beyond) has been presented in the previous international patent application WO 05/104074 of the present applicant and will be used here as background reference. The idea was to split an analog frame as it is used today in a multiple of analog sub-frames similar to that being used in a PDP. However, in PDP each sub-frame can be only controlled in a digital way (fully ON or OFF) whereas in the concept presented there each sub-frame will be an analog one having variable amplitude, (compare FIG. 3). The number of sub-frames SF0 to SFN must be equal or higher than two and its real number will depend on the refreshing rate of the AMOLED (time required to update the value located in each pixel).
FIG. 3 illustrates an example based on a split of the original video frame in 6 sub-frames (SF0 to SF5). This number is only given as an example.
The six sub-frames SF0 to SF5 have respective durations D0 to D5. During each of the sub-frames SF0 to SF5 a respective elementary data signal corresponding to the signal amplitude is used for displaying a grayscale level. In FIG. 3 the independent analog amplitude is indicated by double arrows.
A threshold Cmax represents the maximum data value of the sub-frames. The amplitude of each elementary data signal, i.e. the amplitude depicted in FIG. 3 for each sub-frame, is either Cblack or higher than Cmin, wherein Cblack designates the amplitude of the elementary data signal to be applied to a cell for disabling light emission. Cmin, which is higher than Cblack, is a threshold that represents a value of a data signal above which the working of the cell is considered as good (fast ride, good stability). Furthermore, a refresh cycle is applied between two sub-frames in order to update the information stored in the capacitor C (compare FIG. 1).
FIGS. 4 and 5 illustrate the rendition of the white level (video level 255) for two possibilities of Cmax as disclosed before (Cmax=C255 or Cmax>C255).
The sub-frame structure of FIG. 4 would lead to a light emission similar to that of a CRT whereas the emission of white based on the sub-frame structure of FIG. 5 is similar to conventional methods.
Both solutions are equivalent for the low level rendition. In the same way the solutions are similar for the rendition of low levels up to mid gray concerning the motion rendition. However, the concept described in FIG. 4 has the advantage of offering a better motion rendition for all levels specifically in the range of high levels. Generally, the solution of FIG. 4 presents much more advantages. However, the maximal driving signals Cmax used for some sub-frames is much higher and could have an impact on the display lifetime. This item will define which concept should be used (a compromise between both is also realistic).
Another main advantage of the solution of FIG. 4 is that the analog amplitude of a sub-frame is defined via a driver as presented on FIG. 2. If the driver is a 6-bit driver for instance, for each sub-frame there is the possibility to have a 6-bit resolution on its analog amplitude. Finally, due to the split of the frames in many sub-frames, each one being on 6-bit basis, one can dispose of much more bits due to the combination of sub-frames.
Beside this grayscale rendition without frequency doubling the concept of grayscale rendition with frequency doubling (e.g. case of 50 Hz or large screen) is also known.
Derived from evolution, humans were hunters who needed a very strong acuity in the middle of their visual field to lock their prey. At the same time, they needed the possibility to detect a danger (slight movement of wild animals, enemy . . . ) on the periphery of their visual field as illustrated in FIG. 6. Therefore, the retina is a non-homogeneous neurosensory layer. Its central part (fovea) provides a maximal acuity in terms of spatial resolution whereas the peripheral region is more sensitive to movement (temporal resolution). This peripheral sensitivity to temporal frequencies is graphically described in FIG. 7 for different levels of luminance. This eye behavior is the source of the large-area flickering effect that appears on the visual field periphery only. In addition, this effect strongly evolves with the luminance of the scene.
In the case of new flat display technology, the brightness of the screen is limited by the panel efficacy, which is constantly improved. This brightness improvement combined with increasing screen sizes will increase the perception of the large area flickering for the customer's eye up to a real disturbing effect.
In the case of standard AMOLED driving, there is no real notion of temporal frequency since the signal is constant among the whole frame and is not a pulse as it is the case in a CRT. Therefore, there is also no real problem of large-area flickering. However, when performing a pulsing grayscale rendition as shown in FIG. 4, a notion of flicker is introduced again.