1. Field of the Invention
The present invention relates to a method for driving a bistable display.
The present invention further relates to an apparatus for driving a bistable display.
The present invention further relates to a system comprising a bistable display and an apparatus for driving the same.
2. Related Art
Multistable displays, such as electrophoretic displays, have a plurality of pixels, which may be settable with a first operating luminance level, a second operating luminance level and an intermediate operating luminance level. Electrowetting based displays are another example of a multistable display technology. Also LCD based displays have been developed having a multistable behavior. Typically, multistable displays are reflection type displays. Accordingly the luminance level is determined by a reflection level. Alternatively, a transmission type multistable display may be displayed, wherein the luminance level is determined by a transmission level. Conventionally, multistable displays are denoted as “bistable displays”. This denotation will be used throughout the description. In the following the wording “luminance level” will also be briefly denoted as “luminance”.
Usually, the first operating luminance level relates to “white”, the second operating luminance level relates to “black” and the intermediate operating luminance level relates to “grey”. In order to change image content on an electrophoretic display, new image information is written for a certain amount of time, for example during a period of 300 ms-600 ms. The refresh rate of the active-matrix is usually higher (for example 20 ms frame time for a 50 Hz display and 10 ms frame time for a 100 Hz display). Changing pixels of such display from black to white, for example, requires the pixel capacitors to be charged to a suitable control voltage for 200 ms to 300 ms, in the case where a pulse-width modulation principle is used. During this time the white particles drift towards the top (common) electrode, while the black particles drift towards the bottom electrode, for example an active-matrix back plane. Nevertheless, in order to rule out effects of earlier states of the display updating to an accurately defined new state requires an update time that is about three times longer, e.g. in the range of 600 to 900 ms. Switching to black requires a control voltage of a different polarity, and applying substantially 0 V on the pixel substantially preserves its condition. Addressing such electrophoretic display for a short time with a certain voltage will result in a situation wherein a mixture of white and black particles is visible. Alternatively, electrophoretic displays exist that use only one type of particle. Therein the perceived grey value is determined by the position of the particles with respect to the electrodes. Because the particles are very small human eyes integrate various ratios of black and white particles to shades/levels of grey. Such condition is regarded as an intermediate reflection level.
Bistable displays may have an infinite number of microstates depending on the momentaneous position and velocity of the particles that determine the luminance of the pixel. However, for practical purposes it will be presumed that the state of the pixel is one of a predetermined number of states that corresponds to a respective one of that predetermined number of grey values that is controlled by the apparatus for driving the display.
WO 2009/078711 describes a method and apparatus for controlling an electronic display having a plurality of pixels settable in a plurality of reflection levels comprising a first level, a second level and a plurality of intermediate levels. The intermediate levels form a substantially equidistant partition of a dynamic range between the first level and the second level. The method comprises the step of setting the pixels to a preparatory intermediate level immediately prior to setting the pixels in a desired level selectable from said plurality of levels. The preparatory intermediate level can be selected from two or more levels. Subsequently, pulse width modulation is used to set the pixels in said desired level starting from the selected preparatory level.
Pixels of the known electrophoretic display have a limited bit depth. For example, a 4 bit pixel has 24=16 grey levels. In order to enable 32 levels (distinct shades) the pixels have to be controlled with a 5-bit driving scheme. For the known electrophoretic display for an equidistant partition of a full dynamic range of a pixel (e.g., between lightest to darkest shades), increasing the bit depth could require increasing the frame rate. Increasing the frame rate generally increases power consumption and potentially leads to a shorter product lifetime. Also, increasing the bit depth requires a higher accuracy and robustness of the method to control the display used to obtain the equidistant partitioning of the dynamic range.