1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to a response time accelerator system and method for driving a liquid crystal display.
2. Description of the Related Art
Current LCD technologies have problems associated with liquid crystal response time. Thus, because the response time of the liquid crystal forming pixels in an LCD panel is relatively slow, a user sees an after image, when a TV displays a large number of moving images.
Each liquid crystal cell within an LCD panel allows light to pass through or blocks the light when an applied bias voltage rotates a lattice. Usually, liquid crystals cannot respond in real-time to data changes since the time required to respond to the bias voltage is on the order of tens of milliseconds and bias voltage applied across the crystal varies from frame to frame (1/75 seconds at SXGA resolution). For example, if the bias voltage applied across the liquid crystal within an LCD panel is set to a gray level of 255 for 8-bit image data, the brightness level after the actual response of the liquid crystal is less than 255, which causes vertical stripe patterns to generate an after image. The liquid crystal response characteristics is aggravated as the frame period decreases due to increased resolution.
The most commonly used method for preventing liquid crystal panel after image retention due to a slow response is to appropriately preprocess image data prior to processing it in a source driver for driving a liquid crystal panel. To implement this technique, a response time accelerator (RTA) has been adopted. However, the RTA has many problems.
In order for the liquid crystal to keep up with current frame data, the RTA basically compares a previous frame data with the current frame data, interpolates the two frame data and finds a new current frame data with respect to which a response time can be accelerated according to the result of the comparison. The current data input to the RTA in real time is compared with the previous frame data stored in a memory such as SDRAM outside the RTA.
A method for accelerating a response time applied to a conventional RTA involves storing four to six most significant bits (MSB's) of RGB (Red, Green, Blue) data as frame data in an external memory such as SDRAM. However, when storing only the MSBs (e.g., n MSBs) in an external memory such as SDRAM as frame data, a pixel data truncation error (PDTE) occurs. If only the MSBs are used as previous frame data Pn−1, the number of gray level levels that can be compared between the previous frame data Pn−1 and current frame data Pn is (2n×2n) where n bits Pn−1[7:8−n] of previous frame data Pn−1 and n bits Pn[7:8−n] of current frame data Pn are used. This causes an error to occur. That is, a quantization error occurs because the number of gray levels available for comparison is reduced to (2n×2n) from “256×256”, which is the number of gray levels available when 8-bit data is entirely used for each frame data. Here, the term “quantization error” is given because the error occurs sporadically due to unused 8-n LSBs.
For example, assuming a value derived by shifting n bits Pn[7:8-n] of the current frame data Pn to the right by 8-n bit positions is K (e.g., Pn[3:0] if n=4), the quantization error occurs when outputting an overshoot value exceeding or an undershoot value below the current frame data to the panel at gray level K*2(8−N). Eventually, since the quantization error occurs periodically at gray level produced by multiplying 2(8−N) by the value derived by shifting current frame data for each gray level by 8-n bit positions, noise is generated at regular intervals in vertical stripes to be displayed on a liquid crystal panel. The above description, as previously mentioned, is under the assumption that the LCD uses 8-bit RGB data for each pixel making it possible to display 256 gray levels.
Here, the new current frame data found by interpolation of the previous frame data Pn−1 and the current frame data Pn is usually stored in a table memory within an RTA. The tabular values stored in the built-in table memory for interpolation, defining overshoot exceeding or undershoot below the current frame data, are experimentally determined based on the liquid crystal characteristics of a panel. For example, given the fact that the response time of liquid crystal in the panel is relatively slow, if the current frame data is greater than the previous frame data, the new current frame data is assigned a tabular value greater than actual data, which is then output to the panel. Similarly, if the current frame data is less than the previous frame data, the current frame data is assigned a tabular value less than the actual data, which is then output to the panel.
Another drawback of the method for accelerating a response time implemented using a conventional RTA is that it limits the amount of overshoot and undershoot for each output data to a gray level range from 0 to 255 since the number of bits of RGB image data is limited to 8. Therefore, if the current frame data Pn has an extremely large value (around a gray level of 255) or an extremely small value (around a gray level of 0), the output data does not have sufficient amount of overshoot or undershoot. For example, if the current frame data Pn has a maximum gray level of 255, it is impossible to give an overshoot greater than the current frame data Pn to the output data since the overshoot is limited to the maximum gray level of 255. Thus, if data output from the panel is at a gray level of 255, the liquid crystal panel will return a value less than 255 in response. Such an overshoot or undershoot limitation makes it difficult to improve a response time.