Patent Publication Number: US-7903129-B2

Title: Image display device

Description:
TECHNICAL FIELD 
     The present invention relates to image display devices, such as liquid crystal display devices and electroluminescence display devices. 
     BACKGROUND ART 
     Image display devices, such as liquid crystal display devices, are disadvantageous in that, when displaying a moving image, boundaries with different display brightness are visually perceived as blurry. Such a deterioration in moving image display performance is due to the following two factors. The first factor is that display elements have a response speed slower than a speed corresponding to one frame cycle of video. A known technology for making up for such a lack of response speed of the display elements is overshoot drive (also referred to as “overdrive”). Overshoot drive is a method for forcibly driving display elements at high speed by applying voltage higher or lower than voltage for achieving a desired gradation level, in accordance with the direction in which the gradation level of a video signal changes (increases or decreases). Overshoot drive is disclosed in, for example, Patent Document 1. 
     The second factor is that hold-type display elements are used for keeping brightness at an approximately constant level over one frame cycle of video. Concerning this point, Patent Document 2 describes that, in the case of an image display device provided with hold-type display elements, when displaying a moving image, the moving image might appear blurry due to the viewer&#39;s following line of sight. To prevent such a moving image blur, the hold time of the display elements is required to be shortened, and specific methods known for this are frame interpolation drive for shortening the length of one frame cycle of video, and time-division gradation drive (pseudo-impulse drive) for approximation to impulse-type light emission as in CRTs. Frame interpolation drive using motion vectors is disclosed in, for example, Patent Document 3, and time-division gradation drive is disclosed in, for example, Patent Document 4. 
     In addition, Patent Document 5 discloses a drive mode combining frame interpolation drive, in which a frame for compensating for motion of an object image is created and interposed between frames to achieve an increased refresh rate, and overdrive. In this drive mode, an overdrive correction circuit is disposed at the subsequent stage of a frame converter circuit. The frame converter circuit suppresses occurrence of any moving image blur due to the viewer&#39;s following line of sight, while the overdrive correction circuit makes up for the lack of response-speed of the liquid crystal display elements, which improves moving image display performance. 
     [Patent Document 1] Japanese Patent No. 2650479 Publication 
     [Patent Document 2] Japanese Laid-Open Patent Publication No. 9-325715 
     [Patent Document 3] Japanese Laid-Open Patent Publication No. 2001-42831 
     [Patent Document 4] Japanese Laid-Open Patent Publication No. 2005-173573 
     [Patent Document 5] Japanese Laid-Open Patent Publication No. 2005-91454 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     As mentioned above, although Patent Document 5 discloses a method for improving moving image display performance using frame interpolation drive and overshoot drive, this method is not applicable to image display devices performing time-division gradation drive. Therefore, when attempting to improve moving image display performance of image display devices with display elements having a slow response speed by suppressing occurrence of any moving image blur due to the viewer&#39;s following line of sight via time-division gradation drive, a new problem arises where false contours appear on the screen. 
     Referring to  FIGS. 27 to 29 , the cause of the appearance of false contours will be described. An image display device in which brightness is distributed to two sub-frames obtained by halving one frame cycle of video into first and second portions, such that the sum of time integration values of brightness for the two sub-frames is equal to brightness for one frame cycle, is taken as an example of the image display device performing time-division gradation drive. However, it should be noted that brightness is preferentially distributed to the second sub-frame, and the first sub-frame has brightness distributed thereto only when the maximum brightness is distributed to the second sub-frame. 
     A case is considered where the gradation level of a video signal provided to fifteen horizontally-neighboring pixels (display elements) in this image display device changes as shown in  FIG. 27 .  FIG. 27  shows a gradation level change over six frame cycles of the video signal provided to the fifteen pixels horizontally neighboring across the boundary between two regions displayed on the screen, one being a region where the gradation level is 0 during the first sub-frame and 255 during the second sub-frame, the other being a region where the gradation level is 0 during both the first and second sub-frames, the boundary moving horizontally rightward at a rate of two pixels per frame. 
     In this case, if the display elements have a response speed slower than a speed corresponding to one sub-frame-cycle of video, brightness of a pixel of interest shown in  FIG. 27  changes, for example, as shown in  FIG. 28 . Note that brightness shown in  FIG. 28  is the value converted to the gradation level in hold-type display. Brightness of the pixel of interest is controlled to be 0 during the first sub-frame and 255 during the second sub-frame in the third and subsequent frames. However, in reality, the display elements have a response speed slower than a speed corresponding to one sub-frame cycle, and therefore brightness of the pixel of interest only reaches 238 during the second sub-frame in the third frame, and 6 and 242 during-the first and second sub-frames, respectively, in the fourth and subsequent frames. Therefore, pixels around the boundary may vary from other pixels in terms of the minimum brightness during the first sub-frame and the maximum brightness during the second sub-frame. 
     The viewer visually perceives the boundary which exhibits the brightness response waveform shown in  FIG. 28 , while temporally integrating display brightness along movement of the line of sight. As a result, the viewer sees brightness around the boundary as shown in  FIG. 29 . The time integration value for brightness changes in accordance with the horizontal display position as shown in  FIG. 29 , but there is a point (inflection point) with a small amount of change in time integration value for brightness, and therefore, there appear before and after the inflection point, two points with a large amount of change in time integration value for brightness. The points with a large amount of change in time integration value for brightness are recognized as contours, and in this case, the viewer visually perceives two contours (first and second contours) which are true and false contours, respectively. 
     Therefore, an objective of the present invention is to provide an image display device capable of improving moving image display performance, while making up for a lack of response speed of display elements. 
     Solution to the Problems 
     A first aspect of the present invention is directed to an image display device for providing a gradation display based on a video signal, comprising: a plurality of display elements; a drive circuit for driving the display elements; a video conversion circuit for obtaining a video signal in units of sub-frames based on a video signal inputted in units of frames; a time-division gradation process circuit for performing gradation level conversion on a video signal outputted from the video conversion circuit to distribute brightness for one frame cycle to a plurality of sub-frame cycles; and an overshoot process circuit for performing gradation level conversion on a video signal outputted from the time-division gradation process circuit to emphasize a temporal change of the signal, and outputting a resultant video signal to the drive circuit. 
     In a second aspect of the present invention, based on the first aspect of the invention, the video conversion circuit includes a predetermined-times higher speed process circuit for outputting the input video signal a plurality of times in units of frames. 
     In a third aspect of the present invention, based on the first aspect of the invention, the video conversion circuit includes: a frame interpolation process circuit for performing an interpolation process on the input video signal in units of frames; and a predetermined-times higher speed process circuit for outputting a video signal outputted from the frame interpolation process circuit a plurality of times in units of frames. 
     In a fourth aspect of the present invention, based on the first aspect of the invention, the video conversion circuit includes a frame interpolation process circuit for performing an interpolation process on the input video signal in units of frames. 
     In a fifth aspect of the present invention, based on the first aspect of the invention, the video conversion circuit includes one or more video processing circuits for processing the video signal, the video processing circuit and the time-division gradation process circuit switch between processing and not processing the video signal in accordance with a control signal, and the overshoot process circuit performs a different type of gradation level conversion in accordance with the control signal. 
     In a sixth aspect of the present invention, based on the first aspect of the invention, the overshoot process circuit performs gradation level conversion on a video signal outputted from the time-division gradation process circuit, such that brightness of the display elements after one sub-frame cycle corresponds to a pre-conversion gradation level of the video signal. 
     In a seventh aspect of the present invention, based on the first aspect of the invention, the overshoot process circuit performs the gradation level conversion only when a video signal outputted from the time-division gradation process circuit changes by a predetermined value or more from an immediately previous sub-frame. 
     In an eighth aspect of the present invention, based on the first aspect of the invention, where maximum reachable brightness value signifies brightness of the display elements after one sub-frame cycle since a gradation level of a video signal provided to the display elements changed from minimum to maximum, and minimum reachable brightness value signifies brightness of the display elements after one sub-frame cycle since the gradation level of the video signal provided to the display elements changed from maximum to minimum, the time-division gradation process circuit outputs a value between the minimum reachable brightness value and the maximum reachable brightness value as a post-conversion gradation level when a gradation level of the video signal outputted from the video conversion circuit is between the minimum reachable brightness value and the maximum reachable brightness value. 
     In a ninth aspect of the present invention, based on the eighth aspect of the invention, the time-division gradation process circuit outputs a pre-conversion gradation level as the post-conversion gradation level when the gradation level of the video signal outputted from the video conversion circuit is less than the minimum reachable brightness value or greater than the maximum reachable brightness value. 
     In a tenth aspect of the present invention, based on the first aspect of the invention, the display elements have a response speed slower than a speed corresponding to one sub-frame cycle. 
     In an eleventh aspect of the present invention, based on the tenth aspect of the invention, the display elements are liquid crystal display elements. 
     In a twelfth aspect of the present invention, based on the tenth aspect of the invention, the display elements are electroluminescence elements. 
     Effect of the Invention 
     According to the first aspect of the present invention, the overshoot process circuit is provided at the subsequent stage of the time-division gradation process circuit, and time-division gradation drive and overshoot drive are performed. Time-division gradation drive makes it possible to suppress occurrence of any moving image blur, and overshoot drive makes is possible to make up for a lack of response speed of the display elements. Accordingly, it is possible to improve moving image display performance, while making up for the lack of response speed of the display elements. 
     According to the second aspect of the present invention, a predetermined-times higher speed process is performed on an input video signal to obtain a video signal in units of sub-frames. Accordingly, by suppressing occurrence of any moving image blur via time-division gradation drive, and making up for a lack of response speed of the display elements via overshoot drive, it becomes possible to improve moving image display performance, while making up for the lack of response speed of the display elements. 
     According to the third aspect of the present invention, a frame interpolation process and a predetermined-times higher speed process are performed on an input video signal to obtain a video signal in units of sub-frames. Accordingly, by suppressing occurrence of any moving image blur via the frame interpolation process and time-division gradation drive, and making up for a lack of response speed of the display elements via overshoot drive, it becomes possible to improve moving image display performance, while making up for the lack of response speed of the display elements. 
     According to the fourth aspect of the present invention, a frame interpolation process is performed on an input video signal to obtain a video signal in units of sub-frames. Accordingly, by suppressing occurrence of any moving image blur via the frame interpolation process and time-division gradation drive, and making up for a lack of response speed of the display elements via overshoot drive, it becomes possible to improve moving image display performance, while making up for the lack of response speed of the display elements. 
     According to the fifth aspect of the present invention, by selectively operating the video processing circuit and the time-division gradation process circuit using a control signal, it is possible to switch between modes of driving the display elements. In addition, the overshoot process circuit is provided for performing a different type of gradation level conversion in accordance with the control signal, and therefore it is possible to eliminate the need to provide an overshoot process circuit for each drive mode, resulting in a reduction in the cost of the image display device. 
     According to the sixth aspect of the present invention, by performing gradation level conversion after the time-division gradation process, such that brightness of the display elements after one sub-frame cycle corresponds to the pre-conversion gradation level of the video signal, it becomes possible to control the brightness of the display elements to be at a desired level after one sub-frame cycle, thereby making up for a lack of response speed of the display elements. 
     According to the seventh aspect of the present invention, by performing gradation level conversion via the overshoot process circuit only when a video signal outputted from the time-division gradation process circuit changes by a predetermined value or more from the immediately previous sub-frame, it becomes possible to prevent any false operation due to noise contamination. 
     According to the eighth aspect of the present invention, when the gradation level of a video signal outputted from the video conversion circuit lies between the minimum reachable brightness value and the maximum reachable brightness value, it is possible to allow the brightness of the display elements to reach a desired level within one sub-frame cycle. 
     According to the ninth aspect of the present invention, even when the gradation level of a video signal outputted from the video conversion circuit does not lie between the minimum reachable brightness value and the maximum reachable brightness value, it is possible to allow the brightness of the display elements to reach a desired level within a short period of time. 
     According to the tenth aspect of the present invention, even when the display elements have a response speed slower than a speed corresponding to one sub-frame cycle, by suppressing occurrence of any moving image blur via time-division gradation drive and making up for a lack of response speed of the display elements via overshoot drive, it becomes possible to improve moving image display performance, while making up for the lack of response speed of the display elements. 
     According to the eleventh aspect of the present invention, it is possible to provide a liquid crystal display device capable of improving moving image display performance, while making up for a lack of response speed of the display elements. 
     According to the twelfth aspect of the present invention, it is possible to provide an electroluminescence display device capable of improving moving image display performance, while making up for a lack of response speed of the display elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating the configuration of a liquid crystal display device according to a first embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a frame-by-frame process flow in the liquid crystal display device shown in  FIG. 1 . 
         FIG. 3  is a diagram illustrating an exemplary display screen of the liquid crystal display device shown in  FIG. 1 . 
         FIG. 4  is a diagram illustrating an exemplary change of a video signal inputted to the liquid crystal display device shown in  FIG. 1 . 
         FIG. 5  is a diagram illustrating an exemplary change of an output signal of a simple double-speed process circuit in the liquid crystal display device shown in  FIG. 1 . 
         FIG. 6  is a diagram illustrating an exemplary change of an output signal of a time-division gradation process circuit in the liquid crystal display device shown in  FIG. 1 . 
         FIG. 7  is a diagram illustrating an exemplary change of an output signal of an overshoot process circuit in the liquid crystal display device shown in  FIG. 1 . 
         FIG. 8  is a diagram illustrating exemplary changes in gradation level of a video signal and in brightness of a liquid crystal display element, in relation to a pixel in the liquid crystal display device shown in  FIG. 1 . 
         FIG. 9  is a diagram illustrating exemplary changes in gradation level of a video signal provided to liquid crystal display elements and in brightness of the liquid crystal display elements in the liquid crystal display device shown in  FIG. 1 . 
         FIG. 10  is a graph illustrating characteristics of gradation level conversion by the time-division gradation process circuit in the liquid crystal display device shown in  FIG. 1 . 
         FIG. 11  is a graph illustrating a change in brightness around a boundary, as seen by the viewer, in the liquid crystal display device shown in  FIG. 1 . 
         FIG. 12  is a block diagram illustrating the configuration of a liquid crystal display device according to a second embodiment of the present invention. 
         FIG. 13  is a diagram for explaining a frame interpolation process in the liquid crystal display device shown in  FIG. 12 . 
         FIG. 14  is a diagram illustrating a frame-by-frame process flow in the liquid crystal display device shown in  FIG. 12 . 
         FIG. 15  is a diagram illustrating an exemplary change of a video signal inputted to the liquid crystal display device shown in  FIG. 12 . 
         FIG. 16  is a diagram illustrating an exemplary change of an output signal of a frame interpolation process circuit in the liquid crystal display device shown in  FIG. 12 . 
         FIG. 17  is a diagram illustrating exemplary changes in gradation level of a video signal and in brightness of a liquid crystal display element, in relation to a pixel in the liquid crystal display device&#39;shown in  FIG. 12 . 
         FIG. 18  is a block diagram illustrating the configuration of a liquid crystal display device according to a third embodiment of the present invention. 
         FIG. 19  is a diagram illustrating a frame-by-frame process flow in the liquid crystal display device shown in  FIG. 18 . 
         FIG. 20  is a diagram illustrating an exemplary change of a video signal inputted to the liquid crystal display device shown in  FIG. 18 . 
         FIG. 21  is a diagram illustrating-an exemplary change of an output signal of a frame interpolation process circuit in the liquid crystal display device shown in  FIG. 18 . 
         FIG. 22  is a diagram illustrating an exemplary change of an output signal of a time-division gradation process circuit in the liquid crystal display device shown in  FIG. 18 . 
         FIG. 23  is a diagram illustrating an exemplary change of an output signal of an overshoot process circuit in the liquid crystal display device shown in  FIG. 18 . 
         FIG. 24  is a diagram illustrating exemplary changes in gradation level of a video signal and in brightness of a liquid crystal display element, in relation to a pixel in the liquid crystal display device shown in  FIG. 18 . 
         FIG. 25  is a diagram illustrating exemplary changes in gradation level of a video signal and in brightness of a liquid crystal display element, in relation to another pixel in the liquid crystal display device shown in  FIG. 18 . 
         FIG. 26  is a block diagram illustrating the configuration of a liquid crystal display device according to a fourth embodiment of the present invention. 
         FIG. 27  is a diagram illustrating an exemplary change of a video signal inputted to a conventional display device. 
         FIG. 28  is a diagram illustrating exemplary changes in gradation level of a video signal provided to a display element and in brightness of the display element in the conventional display device. 
         FIG. 29  is a diagram illustrating a change in brightness around a boundary, as seen by the viewer, in the conventional display device. 
     
    
    
     DESCRIPTION OF THE REFERENCE CHARACTERS 
       1 ,  2 ,  3 ,  4  liquid crystal display device 
       11 ,  12 ,  13  timing control circuit 
       20  drive circuit 
       30  pixel array 
       31  liquid crystal display element 
       40 ,  45  overshoot process circuit 
       50 ,  55  time-division gradation process circuit 
       60 ,  65  simple double-speed process circuit 
       70 ,  75  frame interpolation process circuit 
       41 ,  46 ,  51 ,  56 ,  61 ,  66 ,  71 ,  76  processing portion 
       42 ,  62 ,  72  frame memory 
       43 ,  53  LUT 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     First Embodiment 
       FIG. 1  is a block diagram illustrating the configuration of a liquid crystal display device according to a first embodiment of the present invention. The liquid crystal display device  1  shown in  FIG. 1  includes a timing control circuit  11 , a drive circuit  20 , a pixel array  30 , an overshoot process circuit  40 , a time-division gradation process circuit  50 , and a simple double-speed process circuit  60 . The liquid crystal display device  1  performs three processes (a simple double-speed process, a time-division gradation process, and an overshoot process) on a video signal V 1 , and provides a gradation display using a resultant video signal V 2 . In the present embodiment, the simple double-speed process circuit  60  constitutes a video conversion circuit for obtaining a video signal in units of sub-frames based on a video signal inputted in units of frames. 
     An input signal X to be supplied to the liquid crystal display device  1  includes the video signal V 1  representing image data, and a synchronization signal T 0  for setting display timing. The video signal V 1  is inputted to the simple double-speed process circuit  60 , whereas the synchronization signal T 0  is inputted to the timing control circuit  11 . Based on the synchronization signal T 0 , the timing control circuit  11  outputs a synchronization signal T 1  to the simple double-speed process circuit  60 , the time-division gradation process circuit  50 , and the overshoot process circuit  40 , while outputting a synchronization signal T 2  to the drive circuit  20 . The pixel array  30  includes a plurality of liquid crystal display elements  31 , which are arranged two-dimensionally. The drive circuit  20  drives the liquid crystal display elements  31  based on the synchronization signal T 2 , and the video signal V 2  outputted from the overshoot process circuit  40 . As a result, the liquid crystal display device  1  displays a screen. 
     Hereinafter, it is assumed that the video signals V 1  and V 2  are eight-bit signals each representing a gradation level from 0 to 255, and brightness of the liquid crystal display elements  31  is minimized when the gradation level is 0, and maximized when the gradation level is 255. Also, in the liquid crystal display device  1 , the video signal V 1  has a refresh rate of 60 Hz. 
     The simple double-speed process circuit  60  includes a processing portion  61  and a frame memory  62 , and outputs the video signal V 1  twice at a double speed in units of frames (hereinafter, this process will be referred to as a “simple double-speed process”). More specifically, the frame memory  62  has a capacity capable of memorizing a video signal for at least one frame, and the video signal V 1  inputted to the simple double-speed process circuit  60  is written onto the frame memory  62 . The processing portion  61  reads the video signal written on the frame memory  62  twice at double the writing speed in units of frames. As a result, a video signal to be outputted from the simple double-speed process circuit  60  has twice (120 Hz) the refresh rate of the video signal V 1 . 
     The time-division gradation process circuit  50  includes a processing portion  51  and a lookup table (hereinafter, referred to as an “LUT”)  53 , and performs gradation level conversion on the video signal outputted from the simple double-speed process circuit  60  in order to distribute brightness for one frame cycle to two sub-frame cycles, i.e., the first and second sub-frame cycles. In the time-division gradation process circuit  50 , the gradation level conversion is performed for distributing the brightness to the two sub-frames such that the sum of time integration values for brightness of the first and second sub-frames is equalized with the brightness for one frame cycle. At this time, the brightness is preferentially distributed to the second sub-frame, and the first sub-frame has the brightness distributed thereto only when the brightness is distributed to the second sub-frame to a certain great extent. As such, by preferentially distributing the brightness to the second sub-frame, it becomes possible to perform pseudo-impulse drive. 
     The LUT  53  has previously stored therein gradation levels for the first and second sub-frames in association with pre-distribution gradation levels. The gradation levels for the first and second sub-frames are determined so as to satisfy the above conditions (i.e., the sum of the time integration values for brightness of the first and second sub-frames is equalized with brightness for one frame cycle, and the brightness is preferentially distributed to the second sub-frame). The processing portion  51  treats the gradation level of the video signal outputted from the simple double-speed process circuit  60  as a pre-distribution gradation level, and uses this value and information indicating either the first or second sub-frame to refer the LUT  53 . In this manner, the processing portion  51   30  uses the LUT  53  to perform gradation level conversion for distributing the brightness for one frame cycle to the two sub-frame cycles, i.e., the first and second sub-frame cycles. 
     The overshoot process circuit  40  includes a processing portion  41 , a frame memory  42 , and an LUT  43 , and subjects the video signal outputted from the time-division gradation process circuit  50  to gradation level conversion for emphasizing a temporal change of the signal. More specifically, the frame memory  42  has a capacity capable of memorizing a video signal for at least one frame, and a video signal inputted to the overshoot process circuit  40  is written onto the frame memory  42 . The LUT  43  has previously stored therein post-conversion gradation levels in association with combinations of pre-conversion gradation levels and immediately previous sub-frame gradation levels. The processing portion  41  refers the LUT  43  using the gradation level of the video signal outputted from the time-division gradation process circuit  50  as a pre-conversion gradation level, and the gradation level of the video signal memorized in the frame memory  42  as an immediately previous sub-frame gradation level. In this manner, the processing portion  41  uses the frame memory  42  and the LUT  43  to perform the gradation level conversion for emphasizing a temporal change of the signal. Note that the LUTs  43  and  53  are each configured by, for example, a ROM or suchlike. 
       FIG. 2  is a diagram illustrating a frame-by-frame process flow in the liquid crystal display device  1 . When the video signal V 1  has a refresh rate of 60 Hz, the simple double-speed process circuit  60  receives frames F 1 , F 2 , . . . , each being inputted every 16.67 ms. The frame F 1  is simply doubled in speed by the simple double-speed process circuit  60  to become a new frame NF 1  consisting of two sub-frames having the same content. The two sub-frames included in the frame NF 1  are converted to first and second sub-frames S 1 A and S 1 B in the time-division gradation process circuit  50 . The first sub-frame S 1 A is converted to a first sub-frame S 1 A′ by the overshoot process circuit  40  referring the second sub-frame immediately previous thereto (not shown). The second sub-frame S 1 B is converted to a second sub-frame S 1 B′ by the overshoot process circuit  40  referring the first sub-frame S 1 A immediately previous thereto. Similarly, frames F 2 , F 3 , . . . , included in the video signal V 1  are converted to sub-frames S 2 A′, S 2 B′, S 3 A′, S 3 B′, . . . , respectively. 
     The change in gradation level for each pixel in the liquid crystal display device  1  will be described below. Here, an example as shown in  FIG. 3  is considered where a region at a gradation level of  158  and a region at a gradation level of 0 are displayed on the screen, and-the boundary between the two regions moves horizontally rightward at a rate of two pixels per frame. In addition, a region of 1 pixel high×15 pixels wide within the screen is referred to as a “line of interest”, and a pixel in the line of interest (the seventh pixel from the left) is referred to as a “pixel of interest”. 
       FIGS. 4 to 7  are diagrams illustrating exemplary changes over six frame cycles of the video signal V 1 , an output signal of the simple double-speed process circuit  60 , an output signal of the time-division gradation process circuit  50 , and the video signal V 2 , in relation to pixels in the line of interest. In  FIGS. 4 to 7 , the horizontal axis denotes the horizontal display positions of the pixels, whereas the vertical axis denotes time in units of frame cycles or sub-frame cycles. 
       FIG. 8  is a diagram illustrating changes over six frame cycles in gradation level of the video signal and in brightness of the liquid crystal display element  31 , in relation to the pixel of interest. Note that brightness shown in  FIG. 8  is the value converted to the gradation level in hold-type display (the same applies to the following figures). In the simple double-speed process by the simple double-speed process circuit  60 , the refresh rate of the video signal is doubled, but the gradation level of the video signal does not change. Accordingly, when the input gradation level (the gradation level of the video signal V 1 ) is 0, the gradation level after the simple double-speed process is also 0, and when the input gradation level is  158 , the gradation level after the simple double-speed process is also  158 . 
     In the time-division gradation process circuit  50 , the video signal during the first sub-frame and the video signal during the second sub-frame are subjected to different types of gradation level conversion. Accordingly, in general, the post-conversion gradation level varies between the first and second sub-frames. For example, in the example shown in  FIG. 8 , when the input gradation level is 0, the post-conversion gradation level is 0 during both the first and second sub-frames, but when the input gradation level is 158, the post-conversion gradation level is 8 during the first sub-frame, and 238 during the second sub-frame. 
     Note that when performing time-division gradation drive, it is desirable to distribute brightness to the second sub-frame as much as possible, and therefore the first sub-frame might have brightness distributed thereto only when the maximum brightness is distributed to the second sub-frame. However, in the time-division gradation process circuit  50 , even when the maximum brightness is not distributed to the second sub-frame, the first sub-frame has brightness distributed thereto. For example, in the example shown in  FIG. 8 , when the input gradation level is 158, the post-conversion gradation level is 8 during the first sub-frame (the minimum is not 0), and 238 during the second sub-frame (the maximum is not 255). The reason why the time-division gradation process circuit  50  performs such gradation level conversion will be described later. 
     In the overshoot process circuit  40 , gradation level conversion is performed in order to emphasize a temporal change of the signal. More specifically, in the overshoot process circuit  40 , the video signal outputted from the time-division gradation process circuit  50  is subjected to a gradation level such that brightness (the value converted to the gradation level in hold-type display) of the liquid crystal display elements  31  after one sub-frame cycle corresponds to the pre-conversion gradation level of the video signal. Concretely, when the gradation level of the immediately previous sub-frame is lower than that of the current sub-frame, the gradation-level of the video signal is converted such that the maximum brightness of the liquid crystal display elements  31  after one sub-frame cycle corresponds to the pre-conversion gradation level of the video signal. When the gradation level of the immediately previous sub-frame is higher than that of the current sub-frame, the gradation level of the video signal is converted such that the minimum brightness of the liquid crystal display elements  31  after one sub-frame cycle corresponds to the pre-conversion gradation level of the video signal. By performing such gradation level conversion, it becomes possible to make up for a lack of response speed of the liquid crystal display elements  31 . 
     Hereinafter, the overshoot process circuit  40  will be described in detail with specific examples.  FIG. 9  is a diagram illustrating changes in gradation level of the video signal V 2  provided to the liquid crystal display elements  31  and in brightness of the liquid crystal display elements  31 . It should be noted that the video signal V 2  has a refresh rate of 120 Hz, and the liquid crystal display elements  31  have a response speed slower than a speed corresponding to a sub-frame cycle (8.3 ms). 
     In  FIG. 9 , the gradation level of the video signal V 2  changes from the minimum 0 to the maximum 255 during the sub-frame A, but the brightness of the liquid crystal display elements  31  does not reach the maximum 255 at the end of the sub-frame A, only rising to a smaller value of 238. In addition, the gradation level of the video signal V 2  changes from the maximum 255 to the minimum 0 during the sub-frame B, but the brightness of the liquid crystal display elements  31  does not reach the minimum 0 at the end of the sub-frame B, only falling to a greater value of 8. 
     Such brightness of the display elements after one sub-frame cycle since the gradation level of the video signal provided to the display elements changed from minimum to maximum is referred to as “maximum reachable brightness value”, whereas the brightness of the display elements after one sub-frame cycle since the gradation level of the video signal provided to the display elements changed from maximum to minimum is referred to as “minimum reachable brightness value”. In the example shown in  FIG. 9 , the maximum reachable brightness value is 238, and the minimum reachable brightness value is 8. 
     The post-conversion gradation level in the overshoot process circuit  40  is determined by, for example, the following method based on an actual measurement for the brightness response waveform of the liquid crystal display elements  31 . Specifically, after supplying a video signal at a given gradation level to the liquid crystal display elements  31  over a sufficiently long period of time, and setting the liquid crystal display elements  31  at a predetermined brightness level, the liquid crystal display elements  31  are measured for brightness after one sub-frame cycle since the gradation level of the video signal changed, so that the post-conversion gradation level can be obtained based on the actual measurement. 
     For example, during the sub-frame A shown in  FIG. 9 , the gradation level of the video signal changes from 0 to 255, and correspondingly, the brightness of the liquid crystal display elements  31  changes from 0 to238. Accordingly, when the gradation level of the immediately previous sub-frame is 0, and the gradation level of the current sub-frame is 238, the post-conversion gradation level is determined to be 255. In addition, during the sub-frame B shown in  FIG. 9 , the gradation level of the video signal changes from 255 to 0, and correspondingly, the brightness of the liquid crystal display elements  31  changes from 255 to 8. Accordingly, when the gradation level of the immediately previous sub-frame is 255, and the gradation level of the current sub-frame is 8, the post-conversion gradation level is determined to be 0. 
     With this method, the post-conversion gradation level can be determined for most combinations of the gradation levels of the immediately previous sub-frame and the current sub-frame. However, with this method alone, it is not possible to determine the post-conversion gradation level when the amount of change in gradation level is large. Therefore, when the gradation level of the immediately previous sub-frame is smaller than that of the current sub-frame, and the amount of change in gradation level is large, the post-conversion gradation level is determined to be the maximum 255, and when the gradation level of the immediately previous sub-frame is greater than that of the current sub-frame, and the amount of change in gradation level is large, the post-conversion gradation level is determined to be the minimum 0. In the example shown in  FIG. 9 , when the gradation level changes from 7 to 239, or when the gradation level changes from 0 to 255, the post-conversion gradation level is determined to be the maximum 255, and when the gradation level changes from 239 to 7, or when the gradation level changes from 255 to 0, the post-conversion gradation level is determined to be the minimum 0. In this manner, the post-conversion gradation level can be determined for all combinations of the gradation levels of the immediately previous sub-frame and the current sub-frame. 
     Note that the post-conversion gradation level in the overshoot process circuit  40  may be determined as well by any method other than the above. For example, when the difference in gradation level between the immediately previous sub-frame and the current sub-frame is less than a predetermined value, the post-conversion gradation level may be determined to be the same value as the pre-conversion gradation level. In this case, the overshoot process circuit  40  performs gradation level conversion only when the video signal outputted from the time-division gradation process circuit  50  changes by a predetermined value or more from the immediately previous sub-frame. This makes it possible to prevent any false operation due to noise contamination. 
     Hereinafter, the time-division gradation process circuit  50  will be described in detail with specific examples.  FIG. 10  is a graph illustrating characteristics of the gradation level conversion by the time-division gradation process circuit  50 . In  FIG. 10 , the horizontal axis denotes the pre-conversion gradation level, and the vertical axis denotes the post-conversion gradation level. In  FIG. 10 , the post-conversion gradation level is indicated by the dashed line for the first sub-frame and by the broken line for the second sub-frame. 
     As shown in  FIG. 10 , when the pre-conversion gradation level is either 7 or less, or 239 or more, the post-conversion gradation level is the same as the pre-conversion gradation level during both the first and second sub-frames. In addition, when the pre-conversion gradation level is from 8 to less than 158, the post-conversion gradation level is 7 during the first sub-frame, and a value corresponding to the pre-conversion gradation level during the second sub-frame. In addition, when the pre-conversion gradation level is from 158 to 238, the post-conversion gradation level is 239 during the second sub-frame, and a value corresponding to the pre-conversion gradation level during the first sub-frame. 
     The reason for determining in such a way the post-conversion gradation level in the time-division gradation process circuit  50  is as follows. In the example shown in  FIG. 9 , the maximum reachable brightness value of the liquid crystal display elements  31  is 238, and the minimum reachable brightness value is 8. Accordingly, when the gradation level of the video signal V 2  is between the minimum reachable brightness value and the maximum reachable brightness value, the brightness of the liquid crystal display elements  31  after one sub-frame cycle since the gradation level of the video signal V 2  changed is always at a desired level. Therefore, when the pre-conversion gradation level is between the minimum reachable brightness value 8 and the maximum reachable brightness value 238, the time-division gradation process circuit  50  outputs a value between the minimum reachable brightness value 8 and the maximum reachable brightness value 238 as the post-conversion gradation level. This makes it possible to allow the brightness of the liquid crystal display elements  31  to reach a desired level within one sub-frame cycle. 
     In addition, when the pre-conversion gradation level is less than the minimum reachable brightness value 8 or greater than the maximum reachable brightness value 238, the time-division gradation process circuit  50  outputs the pre-conversion gradation level as the post-conversion gradation level during both the first and second sub-frames. As a result, the video signal is not converted into impulse form, and therefore the effect of suppressing occurrence of any moving image blur is slightly reduced, but the brightness of the liquid crystal display elements  31  reaches a desired level within a short period of time (e.g., within two sub-frame cycles). 
     When using the time-division gradation process circuit  50  and the overshoot process circuit  40  thus configured, the brightness of the pixel of interest is 8 during the first sub-frame and 238 during the second sub-frame in the third and subsequent frames, as shown in  FIG. 8 , and pixels around the boundary coincide with other-pixels in terms of the minimum brightness during the first sub-frame and the maximum brightness during the second sub-frame. 
     The viewer visually perceives the boundary exhibiting the brightness response waveform shown in  FIG. 8 , while temporally integrating display brightness along movement of the line of sight. Therefore, the viewer sees brightness around the boundary as shown in  FIG. 11 . The time integration value for brightness changes in accordance with the horizontal display position, as shown in  FIG. 11 , but unlike in conventional display devices (see  FIG. 29 ), there is no point (inflection point) with a small amount of change in time integration value for brightness, and therefore there appears only one point with a large amount of change in time integration value for brightness. The point with a large amount of change in time integration value for brightness is perceived as a contour, and therefore in this case, the viewer visually perceives only one contour. In this manner, the liquid crystal display device  1  makes it possible to suppress occurrence of any false contour different from a true contour. 
     As described above, in the liquid crystal display device  1  according to the present embodiment, the overshoot process circuit  40  is provided at the subsequent stage of the time-division gradation process circuit  50 , and time-division gradation drive and overshoot drive are performed. Time-division gradation drive makes it possible to suppress occurrence of any moving image blur, and overshoot drive makes it possible to make up for a lack of response speed of the liquid crystal display elements  31 . Accordingly, the liquid crystal display device  1  makes it possible to improve moving image display performance, while making up for the lack of response speed of the display elements. 
     Second Embodiment 
       FIG. 12  is a block diagram illustrating the configuration of a liquid crystal display device according to a second embodiment of the present invention. The liquid crystal display device  2  shown in  FIG. 12  includes a timing control circuit  12 , a drive circuit  20 , a pixel array  30 , an overshoot process circuit  40 , a time-division gradation process circuit  50 , a simple double-speed process circuit  60 , and a frame interpolation process circuit  70 . The liquid crystal display device  2  performs four processes (a frame interpolation process, a simple double-speed process, a time-division gradation process, and an overshoot process) on a video signal V 1 , and provides a gradation display using a resultant video signal V 2 . In the present embodiment, the simple double-speed process circuit  60  and the frame interpolation process circuit  70  constitute a video conversion circuit for obtaining a video signal in units of sub-frames based on a video signal inputted in units of frames. In the present embodiment, the same elements as those in the first embodiment are denoted by the same reference characters, and any descriptions thereof will be omitted. 
     An input signal X to be supplied to the liquid crystal display device  2  includes the video signal V 1  and a synchronization signal T 0 . The video signal V 1  is inputted to the frame interpolation process circuit  70 , and the synchronization signal T 0  is inputted to the timing control circuit  12 . It should be noted that in the liquid crystal display device  2 , the video signal V 1  has a refresh rate of 50 Hz. Based on the synchronization signal T 0 , the timing control circuit  12  outputs a synchronization signal T 1  to the frame interpolation process circuit  70 , the simple double-speed process circuit  60 , the time-division gradation process circuit  50 , and the overshoot process circuit  40 , while outputting a synchronization signal T 2  to the drive circuit  20 . 
     The frame interpolation process circuit  70  includes a processing portion  71  and a frame memory  72 , and performs an interpolation process on the video signal V 1  in units of frames. More specifically, the video signal V 1  inputted to the frame interpolation process circuit  70  is written onto the frame memory  72 . The processing portion  71  detects a moving image portion from two frames based on the video signal V 1  for the current frame and the video signal memorized in the frame memory  72  for the previous frame. Next, the processing portion  71  obtains the position of the moving image portion at a time point between the previous frame and the current frame, and interposes an interpolation frame, which is a frame (motion-compensated frame) in which the moving image portion has been moved to the obtained position, between the previous frame and the subsequent frame. As a result, a video signal outputted from the frame interpolation process circuit  70  has a refresh rate (100 Hz) which is twice the refresh rate of the video signal V 1 . 
       FIG. 13  is a diagram for explaining the frame interpolation process by the frame interpolation process circuit  70 . When the video signal V 1  includes a series of frames (N−1) and (N), the frame interpolation process circuit  70  performs the following process to create a frame (hereinafter, referred to as a frame (N−0.5)) at a time point between the two frames. The frame interpolation process circuit  70  first detects a moving image portion (in  FIG. 13 , an image of a car) from the frames (N−1) and (N). The frame interpolation process circuit  70  then obtains the position of the moving image portion in the frame (N−0.5) based on the positions of the moving image portion in the frames (N−1) and (N). Next, the frame interpolation process circuit  70  creates the frame (N−0.5) by moving the moving image portion to the obtained position. The obtained frame (N−0.5) is interposed between the frames (N-1) and (N). 
     The video signal outputted from the frame interpolation process circuit  70  is inputted to the simple double-speed process circuit  60 . As in the first embodiment, the simple double-speed process circuit  60 , the time-division gradation process circuit  50 , and the overshoot process circuit  40  respectively perform the simple double-speed process, the time-division gradation process, and the overshoot process on the video signal outputted from the frame interpolation process circuit  70 . 
       FIG. 14  is a diagram illustrating a frame-by-frame process flow in the liquid crystal display device  2 . When the video signal V 1  has a refresh rate of 50 Hz, the frame interpolation process circuit  70  receives frames F 1 , F 2 , . . . , each being inputted every 20 ms. In the frame interpolation process circuit  70 , an interpolation frame P 1  is created based on the frames F 1  and F 2 , and interposed between the frames F 1  and F 2 . Similar processing is performed for interpolation frames P 2 , P 3 , and so on. As a result, a video signal is obtained having a refresh rate (100 Hz) which is twice the refresh rate of the video signal V 1 . As in the first embodiment, the video signal outputted from the frame interpolation process circuit  70  is subjected to the simple double-speed process, the time-division gradation process, and the overshoot process. 
     The change in gradation level for each pixel in the liquid crystal display device  2  will be described with respect to the example shown in  FIG. 3  as in the first embodiment.  FIGS. 15 and 16  are diagrams each illustrating changes over three frame cycles of the video signal V 1  and an output signal of the frame interpolation process circuit  70 , in relation to pixels in the line of interest. Note that in relation to the pixels in the line of interest, changes over three frame cycles of an output signal of the simple double-speed process circuit  60 , an output signal of the time-division gradation process circuit  50 , and the video signal V 2  are respectively the same as those as shown in  FIGS. 5 to 7 . 
       FIG. 17  is a diagram illustrating changes over three frame cycles in gradation level of the video signal and in brightness of the liquid crystal display element  31 , in relation to the pixel of interest. It should be noted that in the frame interpolation process by the frame interpolation process circuit  70 , for the pixel of interest, the input gradation level for the previous frame is outputted as the gradation level after the frame interpolation process. In this case, when the input gradation level changes at a rate of 50 Hz in the order: 0, 158, 158, and soon, the gradation level after the frame interpolation process changes at a rate of 100 Hz in the order: 0, 0, 158, 158, 158, 158, and so on. Processing after the frame interpolation process is the same as in the first embodiment, and therefore any description thereof will be omitted herein. 
     When using the time-division gradation process circuit  50  and the overshoot process circuit  40  that have been described in the first embodiment, as shown in  FIG. 17 , the brightness of the pixel of interest in the liquid crystal display device  2  is 8 during the first sub-frame, and 238 during the second sub-frame in the third and subsequent frames, as in the first embodiment. In addition, pixels around the boundary coincide with other pixels in terms of the minimum brightness during the first sub-frame and the maximum brightness during the second sub-frame. Accordingly, for the same reason as in the first embodiment, it is possible to suppress occurrence of any false contour different from a true contour. 
     As described above, in the liquid crystal display device  2  according to the present embodiment, the frame interpolation process circuit  70  and the overshoot process circuit  40  at the subsequent stage of the time-division gradation process circuit  50  are provided, and frame interpolation drive, time-division gradation drive, and overshoot drive are performed. Frame interpolation drive and time-division gradation drive make it possible to suppress occurrence of any moving image blur, and overshoot drive makes it possible to make up for a lack of response speed of the liquid crystal display elements  31 . Accordingly, the liquid crystal display device  2  makes it possible to improve moving image display performance, while making up for the lack of response speed of the display elements. 
     Third Embodiment 
       FIG. 18  is a block diagram illustrating the configuration of a liquid crystal display device according to a third embodiment of the present invention. The liquid crystal display device  3  shown in  FIG. 18  includes a timing control circuit  13 , a drive circuit  20 , a pixel array  30 , an overshoot process circuit  40 , a time-division gradation process circuit  50 , and a frame interpolation process circuit  70 . The liquid crystal display device  3  performs three processes (a frame interpolation process, a time-division gradation process, and an overshoot process) on a video signal V 1 , and displays a screen based on a resultant video signal V 2 . In the present embodiment, the frame interpolation process circuit  70  constitutes a video conversion circuit for obtaining a video signal in units of sub-frames based on a video signal inputted in units of frames. In the present embodiment, the same elements as those in the above-described embodiments are denoted by the same reference characters, and any descriptions thereof will be omitted. 
     An input signal X to be supplied to the liquid crystal display device  3  includes the video signal V 1  and a synchronization signal T 0 . The video signal V 1  is inputted to the frame interpolation process circuit  70 , and the synchronization signal T 0  is inputted to the timing control circuit  13 . It should be noted that in the liquid crystal display device  3 , the video signal V 1  has a refresh rate of 60 Hz. Based on the synchronization signal T 0 , the timing control circuit  13  outputs a synchronization signal T 1  to the frame interpolation process circuit  70 , the time-division gradation process circuit  50 , and the overshoot process circuit  40 , while outputting a synchronization signal T 2  to the drive circuit  20 . 
       FIG. 19  is a diagram illustrating a frame-by-frame process flow in the liquid crystal display device  3 . When the video signal V 1  has a refresh rate of 60 Hz, the frame interpolation process circuit  70  receives frames F 1 , F 2 , . . . , each being inputted every 16.67 ms. In the frame interpolation process circuit  70 , an interpolation frame P 1  is created based on the frames F 1  and F 2 , and interposed between the frames F 1  and F 2 . Similar processing is performed for interpolation frames P 2 , P 3 , and so on. As a result, a video signal is obtained having a refresh rate (120 Hz) which is twice the refresh rate of the video signal V 1 . The video signal outputted from the frame interpolation process circuit  70  is subjected to the time-division gradation process and the overshoot process as in the first embodiment. 
     The change in gradation level for each pixel in the liquid crystal display device  3  will be described with respect to the example shown in  FIG. 3  as in the first embodiment. Note that in the present embodiment, the eighth pixel from the left in the line of interest is taken as a pixel of interest.  FIGS. 20 to 23  are diagrams each illustrating changes over six frame cycles of the video signal V 1 , an output signal of the frame interpolation process circuit  70 , an output signal of the time-division gradation process circuit  50 , and the video signal V 2 , in relation to pixels in the line of interest. 
       FIG. 24  is a diagram illustrating changes over six frame cycles in gradation level of the video signal and in brightness of the liquid crystal display element  31 , in relation to the pixel of interest. It should be noted that in the frame interpolation process by the frame interpolation process circuit  70 , for the pixel of interest, the input gradation level for the previous frame is outputted as the gradation level after the frame interpolation process. In this case, when the input gradation level changes at a rate of 60 Hz in the order: 0, 0, 158, 158, . . . , the gradation level after the frame interpolation process changes at a rate of 120 Hz in the order: 0, 0, 0, 0, 158, 158, 158, 158, and so on. Processing after the frame interpolation process is the same as in the first embodiment, and therefore any description thereof will be omitted herein. Note that the same contents as in  FIG. 24  applied to a pixel to the right of the pixel of interest are shown in  FIG. 25 . 
     When using the time-division gradation process circuit  50  and the overshoot process circuit  40  that have been described in the first embodiment, as shown in  FIG. 24 , the brightness of the pixel of interest in the liquid crystal display device  3  is 8 during the first sub-frame, and 238 during the second sub-frame in the third and subsequent frames, as in the first embodiment. In addition, pixels around the boundary coincide with other pixels in terms of the minimum brightness during the first sub-frame and the maximum brightness during the second sub-frame. Therefore, for the same reason as in the first embodiment, it is possible to suppress occurrence of any false contour different from a true contour. 
     As described above, in the liquid crystal display device  3  according to the present embodiment, the frame interpolation process circuit  70  and the overshoot process circuit  40  at the subsequent stage of the time-division gradation process circuit  50  are provided, and frame interpolation drive, time-division gradation drive, and overshoot drive are performed. Frame interpolation drive and time-division gradation drive make it possible to suppress occurrence of any moving image blur, and overshoot drive makes it possible to make up for a lack of response speed of the liquid crystal display elements  31 . Accordingly, the liquid crystal display device  3  makes it possible to improve moving image display performance, while making up for the lack of response speed of the display elements. 
     Fourth Embodiment 
       FIG. 26  is a block diagram illustrating the configuration of a liquid crystal display device according to a fourth embodiment of the present invention. The liquid crystal display device  4  shown in  FIG. 26  includes a timing control circuit  12 , a drive circuit  20 , a liquid crystal display element  30 , an overshoot process circuit  45 , a time-division gradation process circuit  55 , a simple double-speed process circuit  65 , and a frame interpolation process circuit  75 . The liquid crystal display device  4  performs selected one or more of three processes (a frame interpolation process, a simple double-speed process, and a time-division gradation process), along with an overshoot process, on a video signal V 1 , and displays a screen based on a resultant video signal V 2 . In the present embodiment, the simple double-speed process circuit  65  and the frame interpolation process circuit  75  constitute a video conversion circuit for obtaining a video signal in units of sub-frames based on a video signal inputted in units of frames. In the present embodiment, the same elements as those in the above-described embodiments are denoted by the same reference characters, and any descriptions thereof will be omitted. 
     The liquid crystal display device  4  is provided with a mode switching signal M, along with an input signal X including the video signal V 1  and a synchronization signal T 0 . The mode switching signal M is a control signal for independently switching between performing and not performing the three processes on the video signal. It should be noted that in the liquid crystal display device  4 , the video signal V 1  has a refresh rate of 60 Hz. 
     The frame interpolation process circuit  75  includes a processing portion  76  and a frame memory  72 . The processing portion  76  is obtained by providing the processing portion  71  according to the second embodiment with an additional function of switching between performing and not performing the frame interpolation process in accordance with the mode switching signal M. The simple double-speed process circuit  65  includes a processing portion  66  and a frame memory  62 . The processing portion  66  is obtained by providing the processing portion  61  according to the first embodiment with an additional function of switching between performing and not performing the simple double-speed process in accordance with the mode switching signal M. The time-division gradation process circuit  55  includes a processing portion  56  and an LUT  53 . The processing portion  56  is obtained by providing the processing portion  51  according to the first embodiment with an additional function of switching between performing and not performing the time-division gradation process in accordance with the mode switching signal M. When performing no process on the video signal, the processing portions  56 ,  66 , and  76  output the video signal inputted thereto without modification. 
     The overshoot process circuit  45  includes a processing portion  46 , a frame memory  42 , and a plurality of LUTs  43 . The processing portion  46  is obtained by providing the processing portion  41  according to the first embodiment with an additional function of selecting an LUT to be used for gradation level conversion from among the LUTs  43 , in accordance with the mode switching signal M. 
     The frame interpolation process circuit  75 , the simple double-speed process circuit  65 , and the time-division gradation process circuit  55  each switch between processing and not processing the video signal in accordance with the mode switching signal M, and the overshoot process circuit  45  changes the contents of the gradation level conversion in accordance with the mode switching signal M. 
     For example, when the mode switching signal M takes a first value, the simple double-speed process circuit  65  and the time-division gradation process circuit  55  perform their processes, but the frame interpolation process circuit  75  does not perform its process. In this case, the liquid crystal display device  4  operates in the same manner as the liquid crystal display device  1  according to the first embodiment. In addition, when the mode switching signal M takes a second value, the frame interpolation process circuit  75 , the simple double-speed process circuit  65 , and the time-division gradation process circuit  55  perform their processes. In this case, the liquid crystal display device  4  operates in the same manner as the liquid crystal display device  2  according to the second embodiment. In addition, when the mode switching signal M takes a third value, the frame interpolation process circuit  75  and the time-division gradation process circuit  55  perform their processes, but the simple double-speed process circuit  65  does not perform its process. In this case, the liquid crystal display device  4  operates in the same manner as the liquid crystal display device  3  according to the third embodiment. In addition, when the mode switching signal M takes a fourth value, the frame interpolation process circuit  75  performs its process, but the simple double-speed process circuit  65  and the time-division gradation process circuit  55  do not perform their processes. In this case, the liquid crystal display device  4  performs frame interpolation drive and overshoot drive. 
     As described above, the liquid crystal display device  4  according to the present embodiment switches between performing and not performing any of the processes on the video signal, and between changing and not changing the contents of the gradation level conversion in the overshoot process circuit  45 , in accordance with a control signal. Accordingly, it is possible to change the mode of driving the display elements based on the control signal. In addition, the overshoot process circuit is provided for performing a different type of gradation level conversion in accordance with the control signal, and therefore it is possible to eliminate the need to provide an overshoot process circuit for each drive mode, resulting in a reduction in the cost of the image display device. 
     Note that the liquid crystal display devices according to the first to fourth embodiments have been described as including the simple double-speed process circuit and the frame interpolation process circuit for doubling the refresh rate of the video signal, but instead of including them, they may include a simple M-times higher speed process circuit (a predetermined-times higher speed process circuit) for increasing the refresh rate of the video signal by M-fold, and a frame interpolation process circuit for increasing the refresh rate of the video signal by N-fold. 
     Also, the LUT included in the overshoot drive circuit may have post-conversion gradation levels stored therein in association with all or part of combinations of pre-conversion gradation levels and gradation levels of immediately previous sub-frames, or the overshoot drive circuit may include no LUT. When no post-conversion gradation is stored in the LUT, the processing portion included in the overshoot drive circuit may perform arithmetic operations based on the pre-conversion gradation levels and the gradation levels of the immediately previous sub-frame, thereby obtaining the post-conversion gradation levels. 
     Also, the frame interpolation process circuit, the simple M-times higher speed process circuit, and the overshoot drive circuit may independently include an individual frame memory, or they may share one frame memory. Moreover, the frame interpolation process circuit may obtain a motion vector based on previous and subsequent frames, and create an interpolation frame based on the obtained motion vector, or it may create an interpolation frame by any other method. 
     Also, the display mode of the liquid crystal display device may be a VA (vertically aligned) mode, an IPS (in-plane switching) mode, an OCB (optically-compensated birefringence) mode, a TN (twisted nematic) mode, or any other mode. Moreover, electroluminescence elements may be used as display elements to configure an electroluminescence display device. 
     As with the liquid crystal display devices according to the first to fourth embodiments, these liquid crystal display device and the image display device also make it possible to improve moving image display performance, while making up for a lack of response speed of the display elements. 
     INDUSTRIAL APPLICABILITY 
     The image display device of the present invention makes it possible to improve moving image display performance, while making up for a lack of response speed of display elements, and therefore can be used as any of various image display devices, such as a liquid crystal display device or an electroluminescence display device.