Abstract:
A liquid crystal display (LCD) device comprises an image signal processing unit that selectively compensates a current frame upon determining that it is part of a sequence of changing images as opposed to a sequence of still images. The image signal processing device comprises an encoding/decoding unit that generates comparison frame decoding data by encoding and decoding comparison frame data, generates reference frame decoding data by encoding and decoding reference frame data, and a determining unit that sets a comparison range based on effective bits in the comparison frame decoding data and effective bits in the reference frame decoding data, and compares the comparison frame decoding data and the reference frame decoding data within the comparison range.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2011-0022887 filed on Mar. 15, 2011, the disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The inventive concept relates generally to liquid crystal display (LCD) technology. More particularly, the inventive concept relates to methods and apparatuses for driving an LCD device to improve image quality. 
     An LCD device comprises a liquid crystal panel having a liquid crystal layer disposed between two substrates, a backlight unit that provides light to the liquid crystal panel, and a driving circuit that drives the liquid crystal panel to display a sequence of image signals. The performance of the LCD device is limited by the response time of the liquid crystal layer, as the response time affects the rate at which images appear on the liquid crystal panel. 
     Researchers have developed methods to improve response time of the liquid crystal layer by comparing an image signal of a previous frame with an image signal of a current frame and then generating a compensated image signal for the current frame. In these methods, a frame memory stores the image signal of the previous frame, and the stored image signal is compressed in order to reduce the required capacity of the frame memory. 
     Unfortunately, these methods suffer from a variety of shortcomings that can deteriorate the quality of displayed images. For example, where noise is present in an image signal of a still image, it may be recognized as an image signal of a moving picture, and the image signal may be unnecessarily compensated such that the noise is amplified. The noise may also be amplified while the image signal is compressed and then restored, which can further deteriorate the image quality of a LCD device. In addition, in a moving picture signal, an error may occur when compressing and restoring an image signal, such that a pixel-shaking problem arises due to the error. 
     SUMMARY OF THE INVENTION 
     Embodiments of the inventive concept provide methods of driving an LCD device that can reduce image quality deterioration due to noise. Embodiments of the inventive concept also provide methods of driving an LCD device that can reduce image quality deterioration due to errors that occur during image compression and restoration. 
     In one embodiment, a method of driving an LCD device comprises generating comparison frame decoding data by encoding and decoding comparison frame data in a first mode, generating reference frame decoding data by encoding and decoding reference frame data in a second mode, setting a comparison range as a first effective range or a second effective range, wherein the first effective range corresponds to effective bits in the comparison frame decoding data, and the second effective range corresponds to effective bits in the reference frame decoding data, and comparing the comparison frame decoding data and the reference frame decoding data within the comparison range. 
     In another embodiment, a method of driving an LCD device comprises generating comparison frame decoding data and reference frame decoding data by encoding and decoding comparison frame data and reference frame data, respectively, generating comparison frame filtering data by filtering the comparison frame decoding data, determining whether the reference frame data and the comparison frame data are the same by comparing the comparison frame decoding data and the reference frame decoding data, and upon determining that the reference frame data and the comparison frame data are not the same, compensating the reference frame data based on the reference frame data and the comparison frame filtering data and outputting reference frame compensation data. 
     In another embodiment, an image signal processing unit for an LCD device comprises an encoding/decoding unit that generates comparison frame decoding data by encoding and decoding comparison frame data, generates reference frame decoding data by encoding and decoding reference frame data, and a determining unit that sets a comparison range based on effective bits in the comparison frame decoding data and effective bits in the reference frame decoding data, and compares the comparison frame decoding data and the reference frame decoding data within the comparison range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate selected embodiments of the inventive concept. In the drawings, like reference numbers indicate like features. 
         FIG. 1  is a block diagram of an LCD device according to an embodiment of the inventive concept. 
         FIG. 2  is a block diagram of an image signal processing unit of an LCD device according to an embodiment of the inventive concept. 
         FIG. 3  illustrates mode information, effective bits, and error information that correspond to different encoding modes of an encoding unit in an LCD device. 
         FIG. 4  is a block diagram of a determining unit of  FIG. 2  according to an embodiment of the inventive concept. 
         FIG. 5  is a block diagram of the determining unit of  FIG. 2  according to another embodiment of the inventive concept. 
         FIG. 6  is a block diagram of an image signal processing unit of an LCD device according to another embodiment of the inventive concept. 
         FIG. 7  illustrates an example of previous frame filtering data filtered by a filtering unit of  FIG. 6 . 
         FIG. 8  is a block diagram of a filtering unit in the image signal processing unit of  FIG. 6 . 
         FIGS. 9A through 9C  illustrate examples of filters of  FIG. 8 . 
         FIG. 10  is a block diagram of an image signal processing unit of an LCD device according to another embodiment of the inventive concept. 
         FIG. 11  is a flowchart illustrating a method of driving an LCD device according to an embodiment of the inventive concept. 
         FIG. 12  is a flowchart illustrating a method of driving an LCD device according to another embodiment of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the inventive concept are described below with reference to the corresponding drawings. These embodiments are presented as teaching examples and should not be construed to limit the scope of the inventive concept. 
     In the description that follows, where a feature is referred to as being “on”, “connected to,” or “coupled with” another feature, it can be directly on the other feature, or intervening features may also be present. However, where a feature is referred to as being “directly on”, “directly connected to” or “directly coupled with” another feature, it will be understood that there are no intervening features. The term “and/or” indicates any combination of one or more of a list of items. 
     Although the terms “first” and “second”, etc., are used to describe various features, these terms are not limiting of the features. Rather, they are merely used to distinguish between different features. Terms in the singular form may encompass plural forms as well, unless the context or description indicates otherwise. Terms such as “comprise,” “comprising,” “include,” or “including” are used to indicate the presence of a recited feature, but they do not exclude the presence of additional features. Unless indicated to the contrary, all terms including descriptive or technical terms should be construed as having meanings understood by ordinary skill in the art. 
       FIG. 1  is a block diagram of an LCD device  1  according to an embodiment of the inventive concept. 
     Referring to  FIG. 1 , LCD device  1  comprises a liquid crystal panel  10 , a timing controller  20 , a data driver  30 , and a gate driver  40 . 
     Liquid crystal panel  10  comprises an upper substrate and a lower substrate that are combined while facing each other, and a liquid crystal interposed between the upper and lower substrates. Liquid crystal panel  10  further comprises a plurality of pixels  12  arrayed in a matrix. Each of pixels  12  comprises a thin film transistor (TFT)  14 , a liquid crystal capacitor  16 , and a storage capacitor  18 . 
     Liquid crystal panel  10  still further comprises a plurality of gate lines GL 1  through GLn that extend in a row direction and are separated from each other in a column direction, and a plurality of data lines DL 1  through DLm that are disposed to cross gate lines GL 1  through GLn while data lines DL 1  through DLm extend in a column direction and are separated from each other in a row direction. TFT  14  is connected to a corresponding gate line GL 1  among gate lines GL 1  through GLn and is connected to a corresponding data line DL 1  among data lines DL 1  through DLm. Liquid crystal capacitor  16  and storage capacitor  18  are connected to TFT  14 . 
     Timing controller  20  receives image data DATA and an external control signal ECS from an external source. Timing controller  20  comprises a control signal processing unit  22  that generates a data control signal DCS and a gate control signal GCS based on external control signal ECS and provides data control signal DCS and gate control signal GCS to data driver  30  and gate driver  40 , respectively. Timing controller  20  further comprises an image signal processing unit  100  that generates image compensation data DATA′ by adjusting, or compensating, image data DATA and provides image compensation data DATA′ to data driver  30 . 
     Image signal processing unit  100  receives image data DATA comprising previous frame data D 1  and current frame data D 2 . Previous frame data D 1  is encoded and decoded in a first mode to produce previous frame decoding data. Current frame data D 2  is encoded and decoded in a second mode to produce current frame decoding data. Image signal processing unit  100  sets one of a first effective range and a second effective range as a comparison range, wherein the first effective range corresponds to effective bits ensuring that an error is not in the data encoded and decoded in the first mode, and the second effective range corresponds to effective bits ensuring that an error is not in the data encoded and decoded in the second mode. 
     Image signal processing unit  100  compares the previous frame decoding data and the current frame decoding data within the comparison range. Based on the comparison, image signal processing unit  100  determines whether current frame data D 2  is a moving picture or a still image. Where current frame data D 2  is determined to be a moving picture, image signal processing unit  100  compensates current frame data D 2  and outputs the compensated current frame data so as to improve a response time. However, where current frame data D 2  is determined to be a still image, image signal processing unit  100  outputs current frame data D 2  without compensation. 
     Image signal processing unit  100  may also generate comparison frame filtering data by filtering the previous frame decoding data. Where it is determined that current frame data D 2  is a still image, it is not necessary to compensate current frame data D 2 . However, if it is determined that current frame data D 2  is a moving picture, image signal processing unit  100  compensates current frame data D 2  based on current frame data D 2  and the comparison frame filtering data, and it outputs the compensated current frame data. 
     Data driver  30  converts image compensation data DATA′ received from timing controller  20  into an analogue data voltage using data control signal DCS, and provides the analogue data voltage to data lines DL 1  through DLm of liquid crystal panel  10 . 
     Gate driver  40  generates gate signals using gate control signal GCS, and respectively provides the gate signals to gate lines GL 1  through GLn. 
       FIG. 2  is a block diagram of an image signal processing unit  100   a  according to an embodiment of the inventive concept. This embodiment represents one example of image signal processing unit  100  of LCD device  1 . 
     Referring to  FIG. 2 , image signal processing unit  100   a  comprises an encoding/decoding unit  110 , a frame storage unit  120 , a determining unit  200 , and a compensating unit  130 . 
     Image signal processing unit  100   a  receives image data DATA from an external source. If image data DATA is data of a still image, image signal processing unit  100   a  does not compensate image data DATA, and it outputs image data DATA as image compensation data DATA′. However, if image data DATA is data of a moving picture, image signal processing unit  100   a  compensates image data DATA and outputs image compensation data DATA′. 
     Image data DATA comprises previous frame data PF_org and current frame data CF_org that have a difference of one frame. Previous frame data PF_org and current frame data CF_org may be whole data of consecutive two frames. For example, it may be data corresponding to all pixels of a liquid crystal panel. In another example, previous frame data PF_org and current frame data CF_org may be partial data of consecutive two frames, i.e., data corresponding to some pixels, e.g., 2×2, 2×3, or 3×3 pixels, or they may be data of specific pixels of consecutive two frames. In other examples, previous frame data PF_org and current frame data CF_org comprise multiple units of data corresponding to three colors, e.g., red (R), green (G), and blue (B). Pixels corresponding to previous frame data PF_org and pixels corresponding to current frame data CF_org are the same pixels in the liquid crystal panel. 
     Hereinafter, current frame data CF_org may be referred to as reference frame data, and previous frame data PF_org may be referred to as comparison frame data. In  FIG. 2 , previous frame data PF_org and previous frame encoding data PF_enc, which are enclosed by parentheses, are previously received and typically comprise as much as one frame. 
     For convenience of explanation, it may be assumed that each of previous frame data PF_org and current frame data CF_org is data corresponding to one pixel of a single color. However, the inventive concept is not limited thereto and thus each of previous frame data PF_org and current frame data CF_org may be data corresponding to three colors, or may be data corresponding to all pixels or some pixels of a frame. In certain contexts below, each of previous frame data PF_org and current frame data CF_org may be a group of multiple units of data which correspond to three (R, G, and B) colors in a single pixel. 
     Encoding/decoding unit  110  receives image data DATA comprising previous frame data PF_org and current frame data CF_org, and generates previous frame decoding data PF_dec and current frame decoding data CF_dec. Encoding/decoding unit  110  comprises an encoding unit  112 , a first decoding unit  116 , and a second decoding unit  114 . 
     At an n−1 th  frame time, encoding unit  112  receives and encodes previous frame data PF_org, and then generates previous frame encoding data PF_enc. Previous frame encoding data PF_enc is stored in frame storage unit  120  for a time period of one frame. 
     At an n th  frame time, encoding unit  112  receives current frame data CF_org. Encoding unit  112  encodes current frame data CF_org and then generates current frame encoding data CF_enc. Current frame encoding data CF_enc is decoded by second decoding unit  114  and then converted to the current frame decoding data CF_dec. 
     Previous frame encoding data PF_enc that is stored in frame storage unit  120  is decoded by first decoding unit  116  and then is converted to the previous frame decoding data PF_dec. Because previous frame encoding data PF_enc is stored in frame storage unit  120  for a time period of one frame, previous frame decoding data PF_dec and current frame decoding data CF_dec may be generated substantially at a same time. 
     Current frame encoding data CF_enc is also stored in frame storage unit  120  for a time period of one frame and is compared with next frame data (not shown) to be received at an n+1 th  frame time. A relationship between current frame data CF_org and the next frame data (not shown) is the same as a relationship between previous frame data PF_org and current frame data CF_org, and thus a description of the next frame data (not shown) will be omitted to avoid redundancy. 
     Encoding unit  112  performs encoding to decrease a size of current frame data CF_org. To allow comparison between whole pixel data of a current frame and whole pixel data of a previous frame, the whole pixel data of the previous frame is stored in frame storage unit  120 . However, as the resolution of the liquid crystal panel increases, a size of a whole pixel data of one frame increases accordingly. Thus, frame storage unit  120  may require expansion to store the whole pixel data of one frame. However, where the capacity of frame storage unit  120  is increased, the manufacturing costs are also increased. To address this problem, encoding unit  112  may perform encoding, e.g., compression, to decrease an amount of data to be stored in frame storage unit  120 . 
     Encoding unit  112  can perform encoding in various encoding modes. In one encoding mode, for example, predetermined lower bits of data may be removed. In another encoding mode, only a difference value from adjacent data may be stored. In yet another encoding mode, the number of lower bits to be removed may be adjusted according to a data value. Where current frame data CF_org comprises first color (e.g., red) data, second color (e.g., green) data, and third color (e.g., blue) data, according to the encoding modes, three lower bits may be removed with respect to the second color data, and four lower bits may be removed with respect to the first color data and the third color data. Where data is decoded after an encoding process, some information of the data may be lost, or the decoded data may include an error. Also, according to the encoding modes, an amount of lost information may vary. 
       FIG. 3  illustrates examples of the encoding modes of encoding unit  112 . For instance,  FIG. 3  illustrates encoding modes that can be used to encode image data comprising red color data, green color data, and blue color data. In the examples of  FIG. 3 , the red color data, the green color data, and the blue color data are 8-bit data. 
     More specifically,  FIG. 3  illustrates mode information, effective bits, and error information that correspond to different encoding modes of encoding unit  112 . The mode information is included in encoded data to allow a decoding unit to recognize an encoding mode of the encoded data. Where encoding and decoding are performed in each encoding mode, the effective bits are the bits that are the same after the encoding and the decoding are performed. For example, where 8 bits are encoded and decoded, and two lower bits are removed through this process, the remaining 6 bits are deemed to be effective bits. 
     In an effective bit section of  FIG. 3 , effective bits are labelled with corresponding bit numbers. Error information, as distinct from effective bits, indicates the number of bits among decoded data that may have an error. In the example where there are six effective bits, the error information is 2. An effective range may be calculated based on the effective bits and the error information. Also, the effective range may be a portion corresponding to the effective bits from among all bits of data. The effective range may also be expressed as a value obtained by subtracting a value of the error information from the number of all bits of the data. That is, in the above case where the data is 8 bits and the error information is 2, the effective range can be expressed as 6. 
     Although a mode and a sub-mode are separately illustrated in  FIG. 3 , the mode and the sub-mode can be referred to as an encoding mode. The mode and the sub-mode of  FIG. 3  are examples and do not limit the inventive concept. 
     The encoding mode performed by encoding unit  112  varies according to data to be encoded. For example, where a data value is close to 0 or close to a maximum value (e.g., if the data is 8-bit, the maximum value is 255), the data value may be unrecognizable to human eyes, so lower bits may be removed. Also, where a current frame data value and an adjacent frame data value are similar to each other, a difference between two units of adjacent data may be stored by using a small number of bits. 
     Where current frame data CF_org is a group of multiple units of 2×2 pixel data, the encoding mode may vary according to a disposition of the 2×2 pixel data. For example, where values of the units of 2×2 pixel data are the same, the values are the same in a vertical direction, the values are the same in a horizontal direction, or the values are the same except for one, patterns of these cases may be defined as encoding modes, respectively. 
     After data is encoded and decoded according to all of the encoding modes, the data before encoding may be compared with multiple units of encoded and decoded data, and then an encoding mode may be automatically selected according to a predetermined rule, in consideration of a size of the encoded data and a size of an error. 
     Thus, an encoding mode in which current frame data CF_org is encoded may be different from an encoding mode in which previous frame data PF_org is encoded. Hereinafter, the encoding mode in which previous frame data PF_org is encoded is referred to as a first mode, and the encoding mode in which current frame data CF_org is encoded is referred to as a second mode. 
     Previous frame encoding data PF_enc and current frame encoding data CF_enc comprises first mode information indicating the first mode, and second mode information indicating the second mode, respectively. 
     Second decoding unit  114  receives current frame encoding data CF_enc and then extracts the second mode information indicating a mode in which current frame encoding data CF_enc is encoded. Afterward, current frame encoding data CF_enc is decoded according to the second mode information. As a result, second decoding unit  114  generates current frame decoding data CF_dec. As described above, current frame decoding data CF_dec comprises an error of current frame data CF_org. 
     First decoding unit  116  receives previous frame encoding data PF_enc from frame storage unit  120  and extracts the first mode information indicating a mode in which previous frame encoding data PF_enc is encoded. Afterward, previous frame encoding data PF_enc is decoded according to the first mode information. As a result, second decoding unit  114  generates previous frame decoding data PF_dec. 
     Determining unit  200  receives previous frame encoding data PF_enc, current frame encoding data CF_enc, previous frame decoding data PF_dec, and current frame decoding data CF_dec, and determines whether previous frame data PF_org and current frame data CF_org are equal to each other. By doing so, determining unit  200  determines whether current frame data CF_org is a moving picture or a still image. Determining unit  200  provides a determination result S to compensating unit  130 . Determining unit  200  comprises a comparison range setting unit  210 , an error information storage unit  220 , a comparison data generating unit  230 , and a comparing unit  240 . 
     Comparison range setting unit  210  receives previous frame encoding data PF_enc and current frame encoding data CF_enc, and it extracts the first mode information and the second mode information from the received data. Comparison range setting unit  210  refers to the effective bits or the error information based on the encoding mode stored in error information storage unit  220 , and then it sets a comparison range in which the previous frame decoding data PF_dec and the current frame decoding data CF_dec are to be compared. Comparison range setting unit  210  generates effective data SD corresponding to the comparison range. The comparison range is an effective range with respect to the first mode or an effective range with respect to the second mode. For example, the comparison range may be a smaller effective range from among the effective range with respect to the first mode and the effective range with respect to the second mode. 
     Error information storage unit  220  stores mode information and effective bits or error information for each encoding mode. For example, error information storage unit  220  can stores the mode information and the effective bits or error information of  FIG. 3 . 
     Comparison data generating unit  230  receives effective data SD, previous frame decoding data PF_dec, and current frame decoding data CF_dec, and it generates previous frame comparison data PF_SD and current frame comparison data CF_SD. Comparing unit  240  receives and compares previous frame comparison data PF_SD and current frame comparison data CF_SD, and then it generates the signal S indicating whether previous frame comparison data PF_SD and current frame comparison data CF_SD are the same. For example, where previous frame comparison data PF_SD and current frame comparison data CF_SD are the same, S may be logic ‘0’, and where previous frame comparison data PF_SD and current frame comparison data CF_SD are different, S may be logic ‘1’. 
     Compensating unit  130  receives current frame data CF_org and previous frame decoding data PF_dec, and it outputs compensation data DATA′. Compensating unit  130  comprises a look-up table  132 , a data compensating unit  134 , and a selecting unit  136 . 
     Where the signal S is logic ‘0’, it indicates that current frame data CF_org is a still image, so compensating unit  130  outputs current frame data CF_org without compensation. However, where the signal S is logic ‘1’, it indicates that current frame data CF_org is a moving picture, so compensating unit  130  compensates current frame data CF_org and outputs the compensated current frame data as image compensation data DATA′. To compensate current frame data CF_org, compensating unit  130  refers to look-up table  132 . 
     Look-up table  132  stores compensation data for previous data and current data. In general, if a value of the current data is greater than a value of the previous data, the compensation data has a value greater than the current data. Conversely, if the value of the current data is less than the value of the previous data, the compensation data has a value less than the current data. If the previous data and the current data are the same, the compensation data is the same as the current data. 
     For example, where the number of frames per second is 50 fps, a time period for displaying one frame is 20 ms. In this regard, a response time of the liquid crystal panel may be decreased in a manner that a voltage corresponding to the compensation data is applied to a pixel of the liquid crystal panel during a time period from 0 ms to 10 ms, and a voltage corresponding to the current data is applied to the liquid crystal panel during a time period from 10 ms to 20 ms. 
     For example, where the value of the previous data is 0, and the value of the current data is 48, the value of the compensation data may be 155. By applying a voltage corresponding to the value of the compensation data, i.e., 155, to a pixel during a time period from 0 ms to 10 ms, liquid crystal capacitor  16  (refer to  FIG. 1 ) and storage capacitor  18  (refer to  FIG. 1 ) of the pixel may be rapidly charged. However, at 10 ms, the voltage charged in liquid crystal capacitor  16  and storage capacitor  18  may be less than a voltage corresponding to the value of the current data, i.e., 48. Afterward, during a time period from 10 ms to 20 ms, the voltage corresponding to the value of the current data, i.e., 48, is applied to the pixel, so that the pixel may emit light corresponding to the current data. 
     In the embodiment of  FIG. 2 , compensating unit  130  comprises selecting unit  136  and data compensating unit  134 . According to the signal S, selecting unit  136  outputs current frame data CF_org or previous frame decoding data PF_dec as selection data SF. For example, where the signal S is logic ‘0’, selecting unit  136  outputs current frame data CF_org, and where the signal S is logic ‘1’, selecting unit  136  outputs the previous frame decoding data PF_dec. 
     Data compensating unit  134  receives current frame data CF_org and selection data SF. Here, selection data SF is regarded as previous frame decoding data PF_dec. Data compensating unit  134  refers to look-up table  132  and then outputs current frame compensation data corresponding to current frame data CF_org and selection data SF. Compensation data DATA′ includes the current frame compensation data. 
     Image signal processing unit  100   a  decreases the noise in current frame data CF_org, or previous frame data PF_org is displayed on a screen. In general, noise is frequently incurred in a process of quantizing an analogue signal into a digital signal. Due to the noise, although previous frame data PF_org and current frame data CF_org are the same, current frame data CF_org may be determined as a moving picture. 
     Also, although the quantization noise has a relatively very small value, the quantization noise may be amplified during an encoding process. For example, where previous frame data PF_org and current frame data CF_org are exactly the same, they are encoded and decoded in the same encoding mode. However, previous frame data PF_org and current frame data CF_org that become different from each other due to the noise may be encoded and decoded in different encoding modes. Also, because they are encoded and decoded in the different encoding modes, a difference between previous frame decoding data PF_decand current frame decoding data CF_dec may increase. As a result, current frame data CF_org may be determined as a moving picture. 
     However, image signal processing unit  100   a  sets different comparison ranges according to the encoding modes so that, although noise is in current frame data CF_org or previous frame data PF_org, image signal processing unit  100   a  may correctly determine whether current frame data CF_org is a moving picture, i.e., whether to perform a compensation operation. Thus, it is possible to prevent unnecessary data compensation from being performed due to the noise. 
       FIG. 4  is a block diagram illustrating an example of determining unit  200  of  FIG. 2  according to an embodiment of the inventive concept. 
     Referring to  FIG. 4 , determining unit  200  comprises comparison range setting unit  210 , comparison data generating unit  230 , and comparing unit  240 . Error information storage unit  220  illustrated in  FIG. 2  is not illustrated in  FIG. 4 . However, error information storage unit  220  may store mode information and effective bits for each of encoding modes. 
     Comparison range setting unit  210  comprises a first effective data generating unit  212  and a second effective data generating unit  214 . 
     First effective data generating unit  212  receives previous frame encoding data PF_enc and extracts first mode information in the previous frame encoding data PF_enc. First effective data generating unit  212  refers to the effective bits stored in error information storage unit  220  and then generates first effective data SD 1  corresponding to the first mode information. 
     Second effective data generating unit  214  receives current frame encoding data CF_enc, extracts second mode information in the current frame encoding data CF_enc, and generates second effective data SD 2  corresponding to the second mode information. 
     For example, referring to  FIG. 3 , where the first mode information is 0100 xxx, first effective data SD 1  may be 1111 1000(R) 1111 1000(G) 1111 1000(B). Also, where the second mode information is 1101 01x, second effective data SD 2  may be 1111 0000(R) 1111 1000(G) 1111 0000(B). Here, it is assumed that previous frame data PF_org and current frame data CF_org include multiple units of data corresponding to three colors, respectively, and each of the units of data corresponding to three colors is 8 bits. Thus, a total number of bits of each of previous frame data PF_org and current frame data CF_org is 24 bits. 
     Comparison range setting unit  210  comprises a first logic unit  216  that performs an AND operation on bits of first effective data SD 1 , and bits of second effective data SD 2 . First logic unit  216  receives first effective data SD 1  and second effective data SD 2 , and then generates comparison data CD. In the above example, comparison data CD is 1111 0000(R) 1111 1000(G) 1111 0000(B). Comparison data CD indicates a comparison range in which bits of previous frame decoding data PF_dec and bits of current frame decoding data CF_dec are compared with each other. Also, comparison data CD corresponds to effective data SD of  FIG. 2 . 
     Comparison data generating unit  230  comprises a second logic unit  232  and a third logic unit  234 , wherein second logic unit  232  performs an AND operation on bits of comparison data CD and the bits of previous frame decoding data PF_dec, and third logic unit  234  performs an AND operation on the bits of comparison data CD and the bits of current frame decoding data CF_dec. 
     Second logic unit  232  generates previous frame comparison data PF_SD. In the aforementioned example, previous frame comparison data PF_SD is obtained by masking lower 4 bits of first data R, lower 3 bits of second data G, and lower 4 bits of third data B, which are of the previous frame decoding data PF_dec. 
     Also, third logic unit  234  generates current frame comparison data CF_SD. In the above example, current frame comparison data CF_SD may be obtained by masking lower 4 bits of first data R, lower 3 bits of second data G, and lower 4 bits of third data B, which are of current frame decoding data CF_dec. 
     Comparing unit  240  determines whether previous frame comparison data PF_SD and the current frame comparison data CF_SD are the same. 
     Thus, for example, due to quantization noise or an encoding error, lower 4 bits of first data R, lower 3 bits of second data G, and lower 4 bits of third data B, which are among previous frame decoding data PF_dec, may be different from lower 4 bits of first data R, lower 3 bits of second data G, and lower 4 bits of third data B, which are among current frame decoding data CF_dec. 
     In this case, determining unit  200  sets a comparison range according to an encoding mode and compares previous frame decoding data PF_dec and current frame decoding data CF_dec only within the comparison range so that determining unit  200  determines that previous frame decoding data PF_dec and current frame data CF_org are the same. That is, determining unit  200  determines that current frame data CF_org is a still image. Accordingly, it is possible to prevent unnecessary data compensation being performed due to the quantization noise or the encoding error. 
       FIG. 5  is a block diagram illustrating a determining unit  200   a  according to an embodiment of the inventive concept. This embodiment represents another example of determining unit  200  of  FIG. 2 . 
     Referring to  FIG. 5 , determining unit  200   a  comprises a comparison range setting unit  210   a , a comparison data generating unit  230   a , and a comparing unit  240   a . Error information storage unit  220  of  FIG. 2  is not illustrated in  FIG. 5 . However, error information storage unit  220  may store mode information and effective bits for each of encoding modes. 
     Comparison range setting unit  210   a  comprises a first error information extracting unit  212   a  and a second error information extracting unit  214   a.    
     First error information extracting unit  212   a  receives previous frame encoding data PF_enc and extracts first mode information in previous frame encoding data PF_enc. First error information extracting unit  212   a  refers to error information stored in error information storage unit  220  and then extracts first error information EI 1  corresponding to the first mode information. 
     Second error information extracting unit  214   a  receives current frame encoding data CF_enc, extracts second mode information in current frame encoding data CF_enc, and extracts second error information EI 2  corresponding to the second mode information. 
     For example, referring to  FIG. 3 , where first mode information is 0100 xxx, first error information EI 1  may be  3 (R),  3 (G), and  3 (B). Also, where the second mode information is 1101 01x, second error information EI 2  may be  4 (R),  3 (G), and  4 (B). Here, it is assumed that previous frame data PF_org and current frame data CF_org comprise multiple units of data corresponding to three colors, respectively, and each of the units of data corresponding to three colors is 8 bits. Thus, a total of previous frame data PF_org and current frame data CF_org is 24 bits. Also, a total of a previous frame decoding data PF_dec and a current frame decoding data CF_dec is 24 bits. 
     Comparison range setting unit  210   a  comprises a shift value generating unit  216   a  that generates a shift value Vsft, which is a greater value among a value of first error information EI 1  and second error information EI 2 . In the above example, shift value Vsft may be, for instance,  4 (R),  3 (G), and  4 (B). Shift value Vsft corresponds to a comparison range in which previous frame decoding data PF_dec and current frame decoding data CF_dec are to be compared, or to effective data SD of  FIG. 2 . 
     Comparison data generating unit  230   a  comprises a first shifter  232   a  that shifts previous frame decoding data PF_dec by as much as shift value Vsft, and a second shifter  234   a  that shifts current frame decoding data CF_dec by as much as shift value Vsft. 
     In the above example, first shifter  232   a  generates a previous frame shift data PF_sft by shifting a first data (R) by as much as 4 bits, shifting a second data (G) by as much as 3 bits, and shifting a third data (B) by as much as 4 bits, wherein the first, second, and third data (R), (G), and (B) are among previous frame decoding data PF_dec. Previous frame shift data PF_sft comprises first data (R) of 4 bits, second data (G) of 5 bits, and third data (B) of 4 bits. 
     Second shifter  234   a  generates a current frame shift data CF_sft by shifting a first data (R) by as much as 4 bits, by shifting a second data (G) by as much as 3 bits, and by shifting a third data (B) by as much as 4 bits, wherein the first, second, and third data (R), (G), and (B) are among current frame decoding data CF_dec. Current frame shift data CF_sft comprises the first data (R) of 4 bits, the second data (G) of 5 bits, and the third data (B) of 4 bits. 
     Comparing unit  240   a  determines whether previous frame shift data PF_sft and current frame shift data CF_sft are equal to each other. For example, although previous frame data PF_org and current frame data CF_org are equal to each other, due to quantization noise or an encoding error, lower 4 bits of the first data (R), lower 3 bits of the second data (G), and lower 3 bits of the third data (B) of the previous frame decoding data PF_dec may become different from lower 4 bits of the first data (R), lower 3 bits of the second data (G), and lower 3 bits of the third data (B) of the current frame decoding data CF_dec. However, by performing the shifting operation, lower 4 bits of the first data (R), lower 3 bits of the second data (G), and lower 3 bits of the third data (B) of the previous frame decoding data PF_dec, and lower 4 bits of the first data (R), lower 3 bits of the second data (G), and lower 3 bits of the third data (B) of the current frame decoding data CF_dec do not remain in the previous frame shift data PF_sft and the current frame shift data CF_sft, so that comparing unit  240   a  may determine that the previous frame shift data PF_sft and the current frame shift data CF_sft are equal to each other. Accordingly, it is possible to prevent unnecessary data compensation being performed due to the quantization noise or the encoding error. 
       FIG. 6  is a block diagram of an image signal processing unit  100   b  of an LCD device, according to an embodiment of the inventive concept. This embodiment represents another example of image signal processing unit  100  of  FIG. 1 . 
     Referring to  FIG. 6 , image signal processing unit  100   b  comprises encoding/decoding unit  110 , frame storage unit  120 , determining unit  140 , a filtering unit  300 , and compensating unit  130 . Because encoding/decoding unit  110 , frame storage unit  120 , and compensating unit  130  are described above with reference to  FIG. 2 , further descriptions of these features will be omitted to avoid redundancy. Hereinafter, parts of image signal processing unit  100   b  that are different from parts of image signal processing unit  100   a  of  FIG. 2  will be described. 
     In the description that follows, it is assumed that previous frame data PF_org and current frame data CF_org correspond to two pixels. However, the inventive concept is not limited to this number of pixels, and previous frame data PF_org and current frame data CF_org may correspond to other numbers of pixels, e.g., 2×2, 2×3, or 3×3 pixels. 
     Filtering unit  300  provides previous frame filtering data PF_flt to compensating unit  130  by filtering previous frame decoding data PF_dec. Deviation values of data values in previous frame filtering data PF_flt may be decreased compared to those of data values in previous frame decoding data PF_dec. 
     Determining unit  140  determines whether previous frame decoding data PF_dec and current frame decoding data CF_dec are the same, and provides a determination result S to compensating unit  130 . 
     Where determination result S indicates that previous frame decoding data PF_dec and current frame decoding data CF_dec are not the same, compensating unit  130  compensates current frame data CF_org based on current frame data CF_org and previous frame filtering data PF_flt, and outputs current frame compensation data. The current frame compensation data corresponding to current frame data CF_org and previous frame filtering data PF_flt is defined in look-up table  132  of  FIG. 2 . Where determination result S indicates that previous frame decoding data PF_dec and current frame decoding data CF_dec are the same, compensating unit  130  outputs current frame data CF_org without compensating it. The current frame compensation data may be included in image compensation data DATA′. 
       FIG. 7  illustrates an example of previous frame filtering data PF_flt filtered by filtering unit  300  of  FIG. 6 . 
     Referring to  FIGS. 6 and 7 , an original data value of each of first through third pixels in a first frame is 15, and an original data value of each of fourth through sixth pixels in the first frame is 127. Also, an original data value of each of first through fourth pixels in a second frame is 15, and an original data value of each of fifth through sixth pixels in the second frame is 127. Similarly, in a third frame and a fourth frame, a pixel having an original data value of 127 is shifted one by one in a right direction. 
     In this case, a decoding data value of each of the first and second pixels in the first frame is 15, which is the same as the original data value. The reason why an error does not occur is that encoding and decoding are performed by a unit comprised of two units of pixel data, and the original data values of the first and second pixels in an encoding unit are equal to each other. An encoding mode in this case may indicate that data values of pixels in the encoding unit are equal to each other. 
     However, a decoding data value of the third pixel is 0, and a decoding data value of the fourth pixel is 112. Because the original data values of the third and fourth pixels are different from each other, an error may have occurred in the encoding and decoding. Thus, encoding may be performed to remove lower 4 bits of the third and fourth pixels. Errors of the third and fourth pixels are 15. Again, decoding data values of the fifth and sixth pixels may be 127, which is the same as the original data value. 
     In the second frame, the original data values of the first and second pixels, the original data values of the third and fourth pixels, and the original data values of the fifth and sixth pixels are equal to each other so that encoding and decoding may be performed without an error. The third frame may be encoded and decoded in a similar manner to the first frame, and the fourth frame may be encoded and decoded in a similar manner to the second frame. 
     If filtering unit  300  is omitted, an operation of compensating unit  130  is performed based on previous frame decoding data PF_dec and current frame data CF_org. In general, the response time of compensating unit  130  is proportional to a difference between current frame data CF_org and the previous frame decoding data PF_dec. Thus, the fourth pixel of the second frame has a response time proportional to 97, which is a difference between the original data value (i.e., 15) of the second frame and the decoding data value (i.e., 112) of the first frame. On the other hand, the fifth pixel of the third frame has a response time proportional to 112, which is a difference between the original data value (i.e., 15) of the third frame and the decoding data value (i.e., 127) of the second frame. Similarly, the sixth pixel of the fourth frame has a response time that is proportional to 97. Thus, the response times significantly vary as values that are proportional to 97, 127, and 97, and a pixel shaking problem may arise. 
     However, where filtering unit  300  provides previous frame filtering data PF_flt to compensating unit  130 , the pixel shaking problem tends to decrease. For example, in the first frame, a filtering data value of the second pixel becomes 13, which is decreased by as much as 2 compared to the decoding data value of the second pixel. Also, a filtering data value of the fifth pixel becomes 125, which is decreased by as much as 2, compared to the decoding data value of the fifth pixel. However, a filtering data value of the third pixel becomes 16, and a filtering data value of the fourth pixel becomes 120. 
     In the second frame, a filtering data value of the fourth pixel becomes 29, and a filtering data value of the fifth pixel becomes 123. In the third frame, similar to the first frame, a filtering data value of the fourth pixel becomes 13, a filtering data value of the fifth pixel becomes 16, and a filtering data value of a sixth pixel becomes 120. 
     Where filtering unit  300  is included in image signal processing unit  100 , the fourth pixel of the second frame has a response time proportional to 105, which is a difference between the original data value (i.e., 15) of the second frame and the filtering data value (i.e., 120) of the first frame. On the other hand, the fifth pixel of the third frame has a response time proportional to 108, which is a difference between the original data value (i.e., 15) of the third frame and the filtering data value (i.e., 123) of the second frame. Similarly, the sixth pixel of the fourth frame has a response time that is proportional to 105. Thus, the response time is almost constant at values that are proportional to 105, 108, and 105, and the pixel shaking problem may be significantly reduced. 
       FIG. 8  is a block diagram of filtering unit  300  in image signal processing unit  100   b  of  FIG. 6 . 
     Referring to  FIG. 8 , filtering unit  300  comprises one or more filters  312 ,  314 , and  316 , and a selecting unit  318  for selecting one of these filters. Filtering unit  300  further comprises a mode and error information extracting unit  320  (M/E extracting unit  320 ) and a coefficient adjusting unit  330 . Coefficient adjusting unit  330  comprises an error information-based coefficient adjuster  332 , a data-based coefficient adjuster  334 , and a look-up table-based coefficient adjuster  336 . Look-up table-based coefficient adjuster  336  comprises a basic look-up table  338  and a current look-up table  337 . 
     Filters  312 ,  314 , and  316  can be spatial filters for filtering previous frame decoding data PF_dec, and they may have different sizes or shapes. For example, first filter  312  may have a 2×3 size, second filter  314  may have a 3×3 size, and n th  filter  316  may have a cross-shape. For explanation purposes, it will be assumed that all of the filters  312 ,  314 , and  316  have the same 2×3 size. Examples of filters  312 ,  314 , and  316  are illustrated in  FIGS. 9A through 9C . 
     Referring to  FIGS. 9A through 9C , each of filters  312 ,  314 , and  316  has a central coefficient c 0  positioned at a center of a lower row, and neighboring coefficients c 1  through c 5  surrounding central coefficient c 0 . Central coefficient c 0  is a coefficient by which filtering pixel data whose value is changed by filtering is to be multiplied, and the neighboring coefficients c 1  through c 5  are to be multiplied by multiple units of neighboring pixel data, respectively, that are adjacent to the filtering pixel data. The value of the filtering pixel data is obtained by adding a multiplication value of the filtering pixel data before filtering and the central coefficient c 0  to a multiplication value of the units of neighboring pixel data corresponding to the neighboring coefficients c 1  through c 5 , and then dividing the sum of the addition by the sum of the neighboring coefficients c 1  through c 5 . To perform filtering using filters  312 ,  314 , and  316 , previous frame decoding data PF_dec and previous frame data PF_org may include not only the filtering pixel data but also the units of neighboring pixel data. 
     First filter  312  can be, for instance, a low pass filter. Central coefficient c 0  of first filter  312  may be 3, and neighboring coefficients c 1  through c 5  may be 1. Second filter  314  may be a Gaussian filter. Central coefficient c 0  of second filter  314  may be 8, some neighboring coefficients c 1 , c 3 , and c 5  may be 2, and residual neighboring coefficients c 2  and c 4  may be 1. The n th  filter  316  may be a minimum filter, and its central coefficient c 0  may be 11 and its neighboring coefficients c 1  through c 5  may be 1. 
     The coefficients of filters  312 ,  314 , and  316  may be optimized by repeating a test. Also, the coefficients of filters  312 ,  314 , and  316  may be optimized with respect to the basic look-up table  338 , which is random. If compensating unit  130  of  FIG. 6  uses another look-up table, it is necessary to change the coefficients of filters  312 ,  314 , and  316 , as described in further detail below. 
     M/E extracting unit  320  receives previous frame encoding data PF_enc and extracts information about an encoding mode, i.e., first mode information. M/E extracting unit  320  refers to error information storage unit  220  of  FIG. 2  and then extracts error information corresponding to a first mode. 
     Filters  312 ,  314 , and  316  may be optimized while corresponding to encoding modes. For example, first filter  312  may be optimized to a first encoding mode, second filter  314  may be optimized to a second encoding mode, and n th  filter  316  may be optimized to an n th  encoding mode. In another example, filters  312 ,  314 , and  316  may be optimized while corresponding to multiple units of error information. For example, the first encoding mode may be optimized for a case where error information is 4, the second encoding mode may be optimized for a case where error information is 5, and the n th  encoding mode may be optimized for a case where error information is 6. 
     M/E extracting unit  320  generates a filter selection signal S_flt for selecting filters  312 ,  314 , and  316  from mode or error information extracted from previous frame encoding data PF_enc. Filter selection signal S_flt is provided to selecting unit  318 , which selects one of filters  312 ,  314 , and  316  to filter previous frame decoding data PF_dec. Although M/E extracting unit  320  is described with respect to the error information, functions of M/E extracting unit  320  can also be performed with respect to effective bits. 
     Coefficient adjusting unit  330  adjusts central coefficients c 0  and neighboring coefficients c 1  through c 5  of filters  312 ,  314 , and  316 . 
     As illustrated in  FIG. 8 , coefficient adjusting unit  330  comprises error information-based coefficient adjuster  332 , data-based coefficient adjuster  334 , and look-up table-based coefficient adjuster  336 . However, it is not necessary for coefficient adjusting unit  330  to incorporate all of error information-based coefficient adjuster  332 , data-based coefficient adjuster  334 , and look-up table-based coefficient adjuster  336 . In other words, some of these features may be omitted from coefficient adjusting unit  330 . 
     Error information-based coefficient adjuster  332  determines whether or not to filter previous frame decoding data PF_dec based on error information about previous frame encoding data PF_enc. For example, where the error information is less than a predetermined reference value, error information-based coefficient adjuster  332  adjusts central coefficients c 0  of filters  312 ,  314 , and  316  to 1 and adjusts the neighboring coefficients c 1  through c 5  of filters  312 ,  314 , and  316  to 0 so as not to filter previous frame decoding data PF_dec. Accordingly, previous frame decoding data PF_dec may be output as previous frame filtering data PF_flt. For example, the predetermined reference value may be 4. As illustrated in  FIG. 7 , the pixel shaking problem may arise due to encoding and decoding errors. However, when the errors are relatively small, the pixel shaking problem is reduced so that filtering may be omitted with respect to an encoding mode with a small error. 
     In another example, effective bits corresponding to an encoding mode may be extracted from M/E extracting unit  320 . In this case, where an effective range corresponding to the effective bits is less than a predetermined reference effective range, error information-based coefficient adjuster  332  adjusts central coefficients c 0  of filters  312 ,  314 , and  316  to 1 and adjusts neighboring coefficients c 1  through c 5  of filters  312 ,  314 , and  316  to 0. 
     Data-based coefficient adjuster  334  adjusts central coefficient c 0 , and neighboring coefficients c 1  through c 5  corresponding to the units of neighboring pixel data, based on a difference between the filtering pixel data and each of the units of neighboring pixel data. Here, it is assumed that a neighboring coefficient corresponding to one of the units of neighboring pixel data which is calculated with respect to its difference from the filtering pixel data is referred to as a corresponding neighboring coefficient cc, and a value of the corresponding neighboring coefficient cc is c. Data-based coefficient adjuster  334  divides the difference between the filtering pixel data and the neighboring pixel data into several blocks, and then adjusts coefficients c 0  through c 5 . 
     In some examples, data-based coefficient adjuster  334  divides the difference between the filtering pixel data and the neighboring pixel data into three blocks and then adjusts coefficients c 0  through c 5 . For example, if the difference between the filtering pixel data and the neighboring pixel data is less than 32, data-based coefficient adjuster  334  may not adjust central coefficient c 0  and the corresponding neighboring coefficient cc. Where the difference between the filtering pixel data and the neighboring pixel data is greater than or equal to 32 and less than 64, data-based coefficient adjuster  334  may increase the central coefficient c 0  by as much as c/2, and may reduce the corresponding neighboring coefficient cc by as much as c/2. Where the difference between the filtering pixel data and the neighboring pixel data is greater than or equal to 64, data-based coefficient adjuster  334  increases central coefficient c 0  by as much as c, and adjusts the corresponding neighboring coefficient cc to 0. 
     In another example, data-based coefficient adjuster  334  divides the difference between the filtering pixel data and the neighboring pixel data into five blocks and then adjusts coefficients c 0  through c 5 . For example, if the difference between the filtering pixel data and the neighboring pixel data is less than 32, data-based coefficient adjuster  334  does not adjust central coefficient c 0  and the corresponding neighboring coefficient cc. If the difference between the filtering pixel data and the neighboring pixel data is greater than or equal to 32 and less than 96, data-based coefficient adjuster  334  increases central coefficient c 0  by as much as c/4, and it reduces the corresponding neighboring coefficient cc by as much as c/4. If the difference between the filtering pixel data and the neighboring pixel data is greater than or equal to 96 and less than 160, data-based coefficient adjuster  334  increases central coefficient c 0  by as much as c/2, and it reduces the corresponding neighboring coefficient cc by as much as c/2. If the difference between the filtering pixel data and the neighboring pixel data is greater than or equal to 160 and less than 224, data-based coefficient adjuster  334  increases central coefficient c 0  by as much as 3c/4, and reduces the corresponding neighboring coefficient cc by as much as 3c/4. 
     If the difference between the filtering pixel data and the neighboring pixel data is greater than or equal to 224, data-based coefficient adjuster  334  increases central coefficient c 0  by as much as c, and it adjusts the corresponding neighboring coefficient cc to 0. 
     Look-up table-based coefficient adjuster  336  comprises the basic look-up table  338  that is used to calculate the coefficients of filters  312 ,  314 , and  316 . Also, the look-up table-based coefficient adjuster  336  comprises or may access the current look-up table  337  that is actually used by image signal processing unit  100  of  FIG. 2 . The current look-up table  337  may be the same as the look-up table  132  in data compensating unit  134  of  FIG. 2 , and the look-up table-based coefficient adjuster  336  may access the look-up table  132  and then may obtain current frame compensation data. The look-up table-based coefficient adjuster  336  may receive current frame data CF_org and the previous frame decoding data PF_dec. 
     Look-up table-based coefficient adjuster  336  adjusts the number of filters  312 ,  314 , and  316  according to the current look-up table  337 . For example, look-up table-based coefficient adjuster  336  refers to basic look-up table  338  and then extracts basic compensation data corresponding to current frame data CF_org and previous frame decoding data PF_dec. Also, look-up table-based coefficient adjuster  336  refers to current look-up table  337  and then extracts actual compensation data corresponding to current frame data CF_org and previous frame decoding data PF_dec. Here, it is assumed that a value of previous frame decoding data PF_dec is D 1 , a value of current frame data CF_org is D 2 , a value of basic compensation data is D 3 , and a value of the actual compensation data is D 4 . A basic compensation ratio R 1  can be defined as a ratio in which previous frame decoding data PF_dec is increased to the basic compensation data and current frame data CF_org, and it can be calculated by (D 3 −D 1 )/(D 2 −D 1 ). An actual compensation ratio R 2  can be defined as a ratio in which the previous frame decoding data PF_dec is increased to the actual compensation data and current frame data CF_org, and it can be calculated by (D 4 −D 1 )/(D 2 −D 1 ). 
     Look-up table-based coefficient adjuster  336  calculates a weight w based on basic compensation ratio R 1  and actual compensation ratio R 2 . Weight w can be defined as a ratio of basic compensation ratio R 1  and actual compensation ratio R 2 , that is, R 2 /R 1 . Thus, weight w may be calculated by (D 4 −D 1 )/(D 3 −D 1 ). Look-up table-based coefficient adjuster  336  adjusts coefficients c 0  through c 5  by multiplying or dividing central coefficients c 0  or neighboring coefficients c 1  through c 5  of filters  312 ,  314 , and  316  by weight w. For example, look-up table-based coefficient adjuster  336  can multiply neighboring coefficients c 1  through c 5  by weight w while maintaining central coefficients c 0  of filters  312 ,  314 , and  316 . Also, look-up table-based coefficient adjuster  336  can multiply central coefficients c 0  by a reciprocal number of the weight w while maintaining the neighboring coefficients c 1  through c 5  of filters  312 ,  314 , and  316 . 
       FIG. 10  is a block diagram of an image signal processing unit  100   c  of an LCD device according to another embodiment of the inventive concept. This embodiment represents one example of image signal processing unit  100  of LCD device  1 , and it can be formed by combining features of image signal processing unit  100   a  of  FIG. 2  with features of image signal processing unit  100   b  of  FIG. 6 . 
     Referring to  FIG. 10 , image signal processing unit  100   c  comprises encoding/decoding unit  110 , frame storage unit  120 , determining unit  200 , filtering unit  300 , and a compensating unit  130 . Encoding/decoding unit  110 , frame storage unit  120 , and compensating unit  130  are implemented in the same manner as described above with reference to  FIG. 2 . Determining unit  200  of  FIG. 10  can be implemented similar to determining unit  200  of  FIG. 4  or determining unit  200   a  of  FIG. 5 , and filtering unit  300  can be implemented similar to filtering unit  300  of  FIG. 6  or filtering unit  300  of  FIG. 8 . 
       FIG. 11  is a flowchart illustrating a method of driving an LCD device according to an embodiment of the inventive concept. 
     Referring to  FIG. 11 , the method begins by generating previous frame decoding data PF_dec and current frame decoding data CF_dec (S 110 ). Previous frame decoding data PF_dec is generated by encoding and decoding previous frame data PF_org in a first mode. Current frame decoding data CF_dec is generated by encoding and decoding current frame data CF_org in a second mode. 
     Next, a comparison range is set (S 120 ). For example, the comparison range can be set to a first effective range for the first mode, or a second effective range for the second mode. Thereafter, previous frame decoding data PF_dec and current frame decoding data CF_dec are compared (S 130 ). Previous frame decoding data PF_dec and current frame decoding data CF_dec are compared within the comparison range set in operation S 120 . 
       FIG. 12  is a flowchart illustrating a method of driving an LCD device according to another embodiment of the inventive concept. 
     Referring to  FIG. 12 , the method begins by generating previous frame decoding data PF_dec and current frame decoding data CF_dec (S 210 ). Previous frame decoding data PF_dec can be generated, for example, by encoding and decoding previous frame data PF_org. Current frame decoding data CF_dec can be generated, for example, by encoding and decoding current frame data CF_org. 
     Next, previous frame filtering data PF_flt is generated (S 220 ). Previous frame filtering data PF_flt can be obtained, for example, by filtering the previous frame decoding data PF_dec. Then, it is determined whether previous frame data PF_org and current frame data CF_org are equal to each other (S 230 ). For this determination, the previous frame decoding data PF_dec and the current frame decoding data CF_dec are compared to each other. If it is determined that previous frame data PF_org and current frame data CF_org are not equal to each other (S 230 =NO), current frame data CF_org is compensated for based on previous frame filtering data PF_flt and current frame data CF_org (S 240 ). However, if it is determined that previous frame data PF_org and current frame data CF_org are equal to each other (S 230 =YES), current frame data CF_org is output (S 250 ). 
     While the inventive concept has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood that various changes in form and details may be made therein without departing from the scope of the following claims.