1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an apparatus and method for driving an LCD device.
2. Discussion of the Related Art
Liquid Crystal Display (LCD) devices have been used in many different types of electronic equipment. The LCD devices display images by adjusting the light transmittance of liquid crystal cells according to a video signal. Active matrix type LCD devices have a switching element formed for every liquid crystal cell and are well suited for displaying moving images. Thin film transistors (TFTs) are the devices primarily used as the switching element in the active matrix type LCD device.
However, LCD devices generally have a relatively slow response speed attributable to the inherent viscosity and elasticity of a liquid crystal, as can be seen from the following Equations 1 and 2:
                              τ          r                ∝                              γ            ⁢                                                  ⁢                          d              2                                            Δɛ            ⁢                                                                          V                  a                  2                                -                                  V                  F                  2                                                                                                      Equation        ⁢                                  ⁢        1                            where τr is a rise time for response to a voltage is applied to the liquid crystal, Va is the applied voltage, VF is a Frederick transition voltage at which liquid crystal molecules start to be inclined, d is a liquid crystal cell gap, and γ is the rotational viscosity of the liquid crystal molecules; and        
                              τ          F                ∝                              γ            ⁢                                                  ⁢                          d              2                                K                                    Equation        ⁢                                  ⁢        2                            where τF is a falling time for returning the liquid crystal molecules to their original positions by an elastic restoration force after the voltage applied to the liquid crystal is turned off, and K is the inherent elastic modulus of the liquid crystal.        
For a twisted nematic (TN) mode LCD device, although the response speed of the liquid crystal may vary with the physical properties and cell gap of the liquid crystal, rise times of 20 to 80 ms and the falling times of 20 to 30 ms are typical. Because these typical liquid crystal response times are longer than a moving image frame period (for example 16.67 ms for moving images according to the National Television Standards Committee (NTSC) standard), the voltage charged on the liquid crystal may not reach the desired level before the next frame data is presented, as shown in FIG. 1. The slow response results in motion blurring in which an afterimage is left on the LCD display panel.
As illustrated in FIG. 1, a related art LCD device may fail to display a desired color and brightness for a moving image because, when data VD is changed from one level to another level, the corresponding display brightness level BL is unable to reach the desired value due to the slow response of the LCD device. As a result, motion blurring occurs in moving images, causing degradation in contrast ratio and display quality.
As one method for overcoming the low response speed of the LCD device, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO 99/09967 propose a method for modulating data using a look-up table (referred to hereinafter as an ‘over-driving method’). The principle of this over-driving method of the related art is illustrated in FIG. 2.
As may be appreciated with reference to FIG. 2, the related art over-driving method includes modulating input data VD to produce modulated data MVD, and applying the modulated data to a liquid crystal cell to obtain a desired brightness level MBL. By this method, the response speed of a liquid crystal is rapidly accelerated by increasing |Va2−VF2| in the Equation 1 for a variation in the input data from that of a previous frame period in order to obtain the desired brightness level corresponding to the luminance of the input data in one frame period.
Accordingly, a related art LCD device using the over-driving method is able to compensate for a slow response of a liquid crystal by modulation of a data value to reduce or eliminate motion blurring in a moving image to display a picture having the desired color and brightness.
In order to reduce the memory storage used in implementing over-driving, the related art over-driving method performs modulation by comparing only respective most significant bits (MSB) of a previous frame (Fn−1) and current frame (Fn) with each other, as shown in FIG. 3. In other words, the related art over-driving method compares respective most significant bit data (MSB) of the previous frame (Fn−1) and current frame (Fn) with each other to determine whether there is a variation between the two most significant bit data (MSB). If there is a variation between the two most significant bit data (MSB), the corresponding modulated data (MRGB) is selected from a look-up table as most significant bit data (MSB) of the current frame (Fn).
FIG. 4 illustrates a related art over-driving apparatus implementing the above-described over-driving method.
As illustrated in FIG. 4, the related art over-driving apparatus includes a frame memory 43 connected to a most significant bit bus line 42, and a look-up table 44 connected in common with the output terminals of the most significant bit bus line 42 and frame memory 43.
The frame memory 43 stores most significant bit data (MSB) for one frame period and supplies the stored data to the look-up table 44. In the related art over-driving apparatus of FIG. 4, the most significant bit data (MSB) includes the four most significant bits of 8-bit source data (RGB).
The look-up table 44 compares most significant bit data (MSB) of a current frame (Fn) supplied from the most significant bit bus line 42 with most significant bit data (MSB) of a previous frame (Fn−1) inputted from the frame memory 43, as in Table 1 below, and selects modulated data (MRGB) corresponding to the comparison result. The modulated data (MRGB) is combined with least significant bit data (LSB) from a least significant bit bus line 41 and then supplied to an LCD device.
Where the most significant bit data (MSB) is limited to four bits, the modulated data (MRGB) registered in the look-up table 44 of the over-driving apparatus and method is as follows:
TABLE 1Current Frame0123456789101112131415Previous00134679101112141515151515Frame10124579101112131415151515201235789101213141515151530123568910111214141515154001246791011121314151515500023578911121314151515600013468910111314151515700012457810111214141515800012356891112131415159000123467910121314151510000012457810111314151511000002356791112141515120000013457810121315151300000123468101113141514000000123579111314151500000001246911131415
In the above Table 1, the leftmost column represents the data voltage (VDn−1) of the previous frame (Fn−1) and the uppermost row represents the data voltage (VDn) of the current frame (Fn). The contents of Table 1 are look-up table information obtained by expressing four most significant bits in decimal form.
The related art apparatus and method for driving the LCD device is used to provide a high response speed of liquid crystal during a gray-to-gray change between previous frame and current frames. In comparison to the variation associated with a black-to-white change, the gray-to-gray change involves a relatively small voltage difference between frames, so that the liquid crystal response for each gray-to-gray change without overdriving would be slow or non-linear, resulting in poor color change in a moving image or degradation in picture quality.
In the above-described over-driving apparatus a digital memory, such as the look-up table 44, is used in the generation of modulated data (MRGB) in the comparison of the data of the previous frame (Fn−1) with that of the current frame (Fn). The use of the digital memory increases chip size as well as manufacturing costs for the LCD device.