Abstract:
The present application relates to method of driving an image display device comprising inserting a black data frame displaying black data between neighboring data frames alternately displaying left-eye data and right-eye data; comparing an nth frame corresponding to a current frame and an (n−2)th frame corresponding to a previous frame with each other when the data frames are input, reading a compensation value according to the comparison result from a lookup table and modulating input data of the nth frame using the read compensation value to output a modulated data; and bypassing data corresponding to the black data without modulating the data when the black data is input to output a bypass data, the application also relates to said image display device.

Description:
This application claims the benefit of Korea Patent Application No. 10-2010-0042975 filed on May 07, 2010, which is incorporated herein by reference for all purposes as if fully set forth herein. 
     BACKGROUND 
     1. Field of the Invention 
     This invention relates to an image display device and a driving method thereof for improving picture quality. 
     2. Related Art 
     With the advancement of various image processing techniques, image display systems capable of selectively displaying 2D images and 3D images are developed. 
     Methods of generating 3D images are classified into stereoscopic technique and autostereoscopic technique. 
     The stereoscopic technique uses disparity images of left and right eyes which have high 3D effect, and includes a stereoscopic method and an autostereoscopic method which are practically used. The autostereoscopic method provides an optical plate such as a parallax barrier for separating optical axes of left and right disparity images from each other in front of or behind a display screen. The stereoscopic method displays left and right disparity images having different polarization directions on a liquid crystal display panel and generates 3D images by using polarizing glasses or liquid crystal shutter glasses. 
     The stereoscopic method is further classified into a first polarizing filter method which uses a pattern retarder film and polarizing glasses, a second polarizing filter method which uses a switching liquid crystal layer and polarizing glasses, and a liquid crystal shutter glasses method. In the first and second polarizing filter methods, 3D images have low transmissivity due to the pattern retarder film or the switching liquid crystal layer which is arranged on a liquid crystal display panel to function as a polarizing filter. 
     The stereoscopic method which uses a liquid crystal shutter glasses alternately displays left-eye and right-eye images on a display frame by frame and then opens and closes left-eye and right-eye shutters of liquid crystal shutter glasses in synchronization with the display timing to generate a 3D image. The liquid crystal shutter glasses open only the left-eye shutter for an nth frame period in which a left-eye image is displayed and open only the right-eye shutter for an (n+1)th frame period in which a right-eye image is displayed to generate binocular disparity in a time division manner. 
     In the above image display systems, a liquid crystal display (LCD) is widely used as an image display device. The LCD, a hold-type display device, holds data charged in a previous frame right before new data is written because of the maintenance characteristic of liquid crystal. The response of liquid crystal is delayed when data is written. The response delay of liquid crystal causes motion blurring when a left-eye image is changed to a right-eye image or when a right-eye image is changed to a left-eye image while the LCD generates a 3D image, resulting in 3D crosstalk in the form of a ghost. 
     Various methods for improving the response characteristic of liquid crystal for 2D images are known. For example, Over Driving Control (ODC) compares previous frame data with current frame data, detects a data variation according to the comparison result, reads a compensation value corresponding to the data variation from a memory and modulates input data with the read compensation value. Referring to  FIG. 1 , the ODC modulates the current frame data into “223” larger than “191” when the previous frame data is “127” and the current frame data is “191” and modulates the current frame data into “31” smaller than “63” when the previous frame data is “191” and the current frame data is “63”, thereby adjusting data voltages applied to liquid crystal so as to improve the response characteristic of liquid crystal. Besides, Black Data Insertion (BDI) inserts a black frame between neighboring frames to improve motion blurring and thereby enhance the response characteristic of liquid crystal. 
     To improve the 3D crosstalk occurred due to the overlapping of left eye images and right eye images caused by the brightness difference, it is considered to apply the above-described methods for improving the response characteristic of liquid crystal to image display devices. 
     However, there are some problems relating to a luminance deviation using the existing ODC logic and compensation values as shown in  FIG. 2 . In  FIG. 2 , an (n−2)th frame Fn−2 represents a left-eye data frame displaying a left-eye image, an nth frame Fn represents a right-eye data frame displaying a right-eye image, and an (n−1)th frame Fn−1 represents a black data frame displaying a black image. A variation in the luminance of the nth frame Fn to which the ODC is applied is generated between a case (A) where the target gray-scale values of each frame corresponds to “180”, “0” and “150” respectively and a case (B) where the target gray-scale values of each frame corresponds to “255”, “0” and “150”, respectively. This is because liquid crystal rises due to the applied voltage such that the initial luminance Li of the nth frame Fn in the case (A) is different from the initial luminance Li of the nth frame Fn in the case (B) due to a response delay of liquid crystal even when the same compensation value is applied with reference to the target gray-scale value “0” of the (n−1)th frame Fn−1 in order to achieve the target gray-scale value “150” of the nth frame Fn. The response of liquid crystal is proportional to a gray-scale difference between the (n−2)th frame Fn−2 and the (n−1)th frame Fn−1, and thus the initial luminance Li in the case (B) is higher than the initial luminance Li in the case (A). Similar case will happen when a black data frame displaying a black image is inserted between an (n−2)th frame Fn−2 represents a left-eye data frame displaying a left-eye image and an nth frame Fn represents a right-eye data frame displaying a right-eye image. 
     SUMMARY 
     An aspect of this invention is to provide an image display device and a driving method thereof for removing 3D crosstalk without causing a luminance deviation when generating a 3D image 
     A further aspect of this invention is to provide an image display device and a driving method thereof for remarkably reducing the DC image sticking. 
     In an aspect, a method of driving an image display device comprises inserting a black data frame displaying black data between neighboring data frames alternately displaying left-eye data and right-eye data; comparing an nth frame corresponding to a current frame and an (n−2)th frame corresponding to a previous frame with each other when the data frames are input, reading a compensation value according to the comparison result from a lookup table and modulating input data of the nth frame using the read compensation value to output a modulated input data; bypassing data corresponding to the black data without modulating the data when the black data is input to output a bypass data. 
     The method may further comprise inverting the polarities of the modulated data and the bypass data according to N frame inversion (N is a multiple of 4) and applying the data having the inverted polarities to a display panel. 
     The method may further comprise storing input data of the data frames for a period corresponding to two frames. 
     The method may further comprise opening a left-eye shutter of liquid crystal shutter glasses in synchronization with display timing of the left-eye data and opening a right-eye shutter of the liquid crystal shutter glasses in synchronization with display timing of the right-eye data. 
     In another aspect, an image display device comprises a 3D processor inserting a black data frame displaying black data between neighboring data frames alternately displaying left-eye data and right-eye data; a data modulator comparing an nth frame corresponding to a current frame and an (n−2)th frame corresponding to a previous frame with each other when the data frames are input, reading a compensation value according to the comparison result from a lookup table and modulating input data of the nth frame using the read compensation value to output a modulated data; and a bypass unit bypassing data corresponding to the black data without modulating the data when the black data is input to output a bypass data. 
     The image display device may further comprise a data driver inverting the polarities of the modulated data and the bypass data according to N frame inversion (N is a multiple of 4) and applying the data having the inverted polarities to a display panel. 
     The image display device may further comprise a frame memory storing input data of the data frames for a period corresponding to two frames. 
     The image display device may further comprise liquid crystal shutter glasses having a left-eye shutter opened in synchronization with display timing of the left-eye data and a right-eye shutter opened in synchronization with display timing of the right-eye data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The implementation of this invention will be described in detail with reference to the following drawings in which like numerals refer to like elements. 
         FIG. 1  is a view for explaining a conventional Over Driving Control (ODC) method; 
         FIG. 2  illustrates a luminance deviation generated when a 3D image is produced; 
         FIG. 3  is a flowchart showing an implementation of a method of driving an image display device; 
         FIG. 4  illustrates insertion of a black data frame between data frames; 
         FIG. 5  illustrates an exemplary lookup table for comparing nth frame data and (n−2)th frame data with each other; 
         FIG. 6  is a view for explaining the effect of an implementation of the present invention; 
         FIG. 7  is a block diagram of an implementation of an image display device; and 
         FIG. 8  illustrates data polarity control according to 4 frame inversion. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an implementation of this invention will be described in detail with reference to  FIGS. 3 through 8 . 
       FIG. 3  is a flowchart showing an implementation of a method of driving an image display device,  FIG. 4  illustrates insertion of a rest frame between data frames, and  FIG. 5  illustrates an exemplary lookup table for comparing nth frame data and (n−2)th frame data with each other. 
     Referring to  FIG. 3 , it is determined whether a current driving mode is a 3D mode with reference to an input data and mode signal in operations S 10  and S 20 . 
     When it is determined that the current driving mode is the 3D mode in operation S 20 , 3D input frames composed of 3D data input from an external video source are separated into left-eye data frames L for displaying left-eye images and right-eye data frames R for displaying right-eye images in operation S 30 . A black data frame for displaying a black image is inserted between neighboring data frames L and R, as shown in  FIG. 4 , in operation S 40 . An input frame frequency is thus multiplied by four through the aforementioned data separating and inserting operations. For example, a frame frequency of 50 Hz is multiplied to 200 Hz and a frame frequency of 60 Hz is multiplied to 240 Hz. Similarly, an input frame frequency can be multiplied by 4n in the similar way. 
     It is determined whether an nth frame Fn corresponding to a current frame is the data frame L or R or the black data frame B with reference to an input vertical synchronization signal in operation S 50 . 
     When it is determined that the nth frame Fn corresponds to the data frame L or R in operation S 50 , input data of the data frame L or R is ODC-modulated using compensation values included in the first lookup table LUT 1  and output in operation S 60 . In the first lookup table LUT shown in  FIG. 5 , a previous frame with respect to the nth frame Fn is set to the (n−2)th frame Fn−2. The (n−2)th frame Fn−2 corresponds to the right-eye data frame R when the nth frame Fn is the left-eye data frame L or the (n−2)th frame Fn−2 corresponds to the left-eye data frame L when the nth frame Fn is the right-eye data frame R, and thus neighboring data frames L and R having the black data frame B arranged between them are compared with each other when ODC is applied. Accordingly, a luminance variation due to a response delay of liquid crystal is remarkably reduced as compared to a conventional method of driving an image display device. 
     When it is determined that the nth frame Fn is the black data frame B in operation S 50 , data of the black data frame B is output without being ODC-modulated in operation S 70 . 
     On the other hand, when it is determined that the current driving mode is a 2D mode in operation S 20 , a frame rate of 2D input frames composed of 2D data input from an external video source is controlled through a data interpolation method such as motion estimation motion compensation (MEMC) in operation S 80 . An input frame frequency is multiplied by four or more according to the frame rate control (FRC). 
     The frame-rate-controlled 2D data is ODC-modulated using compensation values included in a second lookup table LUT 2  in operation S 90 . The ODC modulation method is identical to the modulation methods disclosed in Korean Patent Nos. 10-2001-0032364 and 10-2001-0057119 applied by the Applicant. 
     The polarity of the data output in operations S 60 , S 70  and S 90  is inverted through N frame inversion (N is a multiple of 4) as shown in  FIG. 8  and the polarity-inverted data is applied to a liquid crystal display panel in operation S 100 . 
       FIG. 6  shows the effect of the driving method according to the present invention. 
     Referring to  FIG. 6 , when the (n−2)th and nth frames Fn−2 and Fn corresponding to left-eye and right-eye or right-eye and left-eye data frames are arranged having the (n−1)th frame Fn−1 corresponding to the black data frame located therebetween, the luminance in the nth frame Fn to which the ODC modulation is applied according to the present invention reaches “150” in both a case (A) where the target gray-scale values of each frame corresponds to “180”, “0” and “150” respectively and a case (B) where the target gray-scale values of each frame corresponds to “255”, “0” and “150” respectively. The same luminance is achieved in the two cases (A) and (B) because the data of the nth frame Fn is ODC-modulated through comparison of the nth frame Fn with the (n−2)th frame Fn−2 and modulation by different compensation value based on the comparison result. The response of liquid crystal to the (n−1)th frame Fn−1 in the case (B) where the gray-scale value of the (n−2)th frame Fn−2 is relatively large is delayed as compared to the case (A). Thus an ODC compensation value can be determined according to the gray-scale value of the (n−2)th frame Fn−2 if the data of the nth frame Fn is ODC-modulated through comparison of the nth frame Fn with the (n−2)th frame Fn−2. In  FIG. 6 , the compensation value of (B) is determined to be smaller than the compensation value of (A) to remove a luminance variation. 
       FIG. 7  illustrates an implementation of an image display device according to the present invention. 
     Referring to  FIG. 7 , the image display device includes a 3D processor  10 , a 2D processor  20 , a timing controller  30 , a data driver  40 , a gate driver  50 , a liquid crystal display (LCD) panel  60 , a back light unit  70 , and liquid crystal shutter glasses  80 . 
     The 3D processor  10  separates 3D input frames composed of 3D data input from an external video source into left-eye data frames L for displaying left-eye images and right-eye data frames R for displaying right-eye images and inserts a black data frame for displaying a black image, that is, a black data frame B, between neighboring data frames to multiply an input frame frequency. 
     The 2D processor  20  controls the frame rate of 2D input frames composed of 2D data input from an external video source through data compensation such as MEMC to multiply an input frame frequency. 
     The timing controller  30  ODC-modulates the input 2D/3D data and provides the modulated data to the data driver  40  in synchronization with the multiplied input frame frequency. The timing controller  30  generates timing control signals DDC and GDC for controlling operation timing of the data driver  40  and the gate driver  50  based on timing signals (vertical synchronization signal, horizontal synchronization signal, data enable signal, dot clock signal, etc.) input from an external system board. The timing controller  30  multiples the data timing control signal DDC and the gate timing control signal GDC to synchronize the data timing control signal DDC and the gate timing control signal GDC with the multiplied input frame frequency. The timing controller  30  generates a back light control signal CBL for controlling tuning on/off timing of the back light unit  70  based on the multiplied input frame frequency. The timing controller  30  generates a shutter control signal CST for controlling the operation of the liquid crystal shutter glasses  80  based on the multiplied input frame frequency. 
     The timing controller  30  includes a selector  31  for selectively outputting input 2D and 3D data, a frame memory  32 , a data modulator  33 , first and second lookup tables  34 A and  34 B, and a black data bypass unit  35 . The selector  31  selectively outputs the 2D data input from the 2D processor  20  and the 3D data input from the 3D processor  10  according to an external mode signal. The frame memory  32  stores the 3D data received from the selector  31  for a period corresponding to 2 frames in the 3D mode. The frame memory  32  stores only data of data frames L and R in the 3D mode. The frame memory  32  stores the 2D data received from the selector  31  for one frame period in the 2D mode. The first lookup table  34 A stores compensation values selected through comparison of the nth frame Fn and the (n−2)th frame Fn−2. The second lookup table  34 B stores compensation values selected through comparison of the nth frame Fn and the (n−1)th frame Fn−1. The compensation values of the first and second lookup tables  34 A and  34 B are loaded from an external EEPROM whenever the image display device is provided with power. Compensation values stored in the EEPROM may be updated by a user. The data modulator  33  receives the 3D data including the data frames L and R and the frame-rate-controlled 2D data from the selector  31 . The data modulator  33  compares the nth frame Fn and the (n−2)th frame Fn−2 with each other, reads compensation values according to the comparison result from the first lookup table  34 A and ODC-modulates data corresponding to the data frames L and R using the compensation values in the 3D mode. The data modulator  33  ODC-modulates the frame-rate-controlled 2D data using the compensation values included in the second lookup table  34 B in the 2D mode. The black data bypass unit  35  bypasses the black data frame B received from the selector  31  to the data driver  40 . 
     The data driver  40  converts the modulated 3D data or 2D data received from the timing controller  30  into an analog signal in response to the data control signal DDC, inverts the polarity of the modulated 3D data or 2D data through the N frame inversion (N is a multiple of 4) shown in  FIG. 8  and provides the 3D data or 2D data having the inverted polarity to data lines of the LCD panel  60 . The polarity of the data is inverted according to the N frame inversion in order to eliminate DC image sticking. A DC voltage is applied to an LCD for a long time, ions with negative charges are moved in the same motion vector direction and ions with positive charges are moved in a motion vector direction opposite to the moving direction of the ions with negative charges and polarized according to the polarity of a field applied to the liquid crystal. The quantity of accumulated ions with negative charges and the quantity of accumulated ions with positive charges increase with the lapse of time. As the quantities of accumulated ions increase, an alignment film is degraded so as to deteriorate the alignment characteristic of liquid crystal. Accordingly, when the DC voltage is applied to the LCD for a long time, image sticking is generated on a displayed image and becomes large with the lapse of time. The image sticking is rapidly generated and becomes severe when the temperature is high or the DC voltage is applied to a liquid crystal layer for a long time. The DC image sticking becomes serious when the polarity of data is inverted for every frame or for every two frames while applying BDI to the data in the 3D mode. As it can be known from  FIG. 8 , when the polarity of data is inverted according to 1 frame inversion, a data polarity corresponding to left-eye data frames L and a data polarity corresponding to right-eye data frames R are continuously maintained to be positive (+) so as to increase the DC image sticking. Furthermore, when the polarity of data is inverted according to 2 frame inversion, the data polarity corresponding to the left-eye data frames L is maintained to be positive (+) and the data polarity corresponding to the right-eye data frames R is maintained to be negative (−) to increase the DC image sticking. However, when the polarity of data is inverted according to 4 frame inversion, the data polarity corresponding to the left-eye data frames L and the data polarity corresponding to the right-eye data frames R are inverted for every four frames, and thus the DC image sticking is remarkably reduced. 
     The gate driver  50  generates a scan pulse signal in response to the gate control signal GDC and sequentially provides the scan pulse signal to gate lines of the LCD panel  60 . 
     The LCD panel  60  includes two glass substrates and a liquid crystal layer formed between the two glass substrates. The data lines and the gate lines intersecting the data lines are formed on the lower glass substrate of the LCD panel  60 . Liquid crystal cells are arranged in a matrix form in the LCD panel  60  according to the intersecting structure of the data lines and the gate lines. A black matrix, a color filter, and a common electrode of the liquid crystal cells are formed on the upper glass substrate of the LCD panel  60 . The common electrode is formed on the upper glass substrate in a vertical field driving mode such as twisted nematic (TN) mode and vertical alignment (VA) mode and formed together with pixel electrodes on the lower glass substrate in a horizontal field driving mode such as in-plane switching (IPS) mode and fringe field switching (FFS) mode. Polarizers are respectively attached to the upper and lower glass substrates of the LCD panel  60  and an alignment film for setting a pretilt angle of liquid crystal is formed between the inner sides of the upper and lower glass substrates, which come into contact with the liquid crystal. 
     The back light unit  70  includes light sources turned on according to driving power supplied from a light source driver (not shown), a light guide (or diffuser), and optical sheets. The back light unit  80  may be of direct type or edge type. The light sources may include one or more than two of a hot cathode fluorescent lamp (HCFL), a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL), and a light emitting diode (LED). The back light unit  70  may be omitted when the image display device is a reflective LCD. 
     The liquid crystal shutter glasses  80  operate in the 3D mode and include a left-eye shutter STL and a right-eye shutter STR which are electrically individually controlled. Each of the left-eye shutter STL and the right-eye shutter STR includes a first transparent substrate, a first transparent electrode formed on the first transparent substrate, a second transparent substrate, a second transparent electrode formed on the second transparent substrate, and a liquid crystal layer interposed between the first and second transparent substrates. The first transparent electrode is provided with a reference voltage and the second transparent electrode is provided with ON/OFF voltage. The left-eye shutter STL and the right-eye shutter STR transmit light from the LCD panel  70  when the ON voltage is supplied to the second transparent electrode and block the light from the LCD panel  60  when the OFF voltage is applied to the second transparent electrode. 
     As described above, the image display device and the driving method thereof according to the present invention can ODC-modulate data of the nth frame (left-eye/right-eye data frame) with reference to data of the (n−2)th frame (right-eye/left-eye data frame) such that the ODC compensation value of the nth frame can be easily controlled according to the gray-scale value of the (n−2)th frame. Accordingly, 3D crosstalk can be effectively removed without generating a luminance deviation when a 3D image is generated. 
     Furthermore, the image display device and the driving method thereof according to the present invention can remarkably reduce DC image sticking when BDI is applied to 3D images by inverting the polarity of data according to N frame inversion (N is a multiple of 4).