Abstract:
A display apparatus includes a drive correction unit that compares image data items on a frame basis and changes a driving voltage on the frame basis for image data to be displayed, in accordance with the result of the comparison, and a magnification unit that raises at N times a frame rate of the image data to be displayed by the driving voltage changed by the drive correction unit, wherein N is an even number. In addition, a driving signal generation unit changes the polarity of the driving voltage for the image data to be displayed at the raised frame rate raised by the magnification unit and generates a display driving signal.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a display apparatus such as a liquid crystal and the like and to a display control method therefor. 
   BACKGROUND OF THE INVENTION 
   In recent years, a liquid-crystal display apparatus has been utilized as a display apparatus of a TV receiver or a personal computer. Because a liquid-crystal display apparatus such as this can be formed in a thin shape, whereby it is space-saving, and is power-saving, it has widely been utilized. However, a liquid-crystal display apparatus such as this has a problem in that response time between input of image data and actual display is long. In this regard, as a drive method to improve the response speed of a liquid-crystal display apparatus, it has been proposed that image data to be displayed is compared with the immediate previous image data and over-voltage driving with raised voltage is implemented in accordance with the result of the comparison (refer to Japanese Patent Laid-Open No. 11-126050). 
     FIG. 17  is a graph for explaining an example of the over-voltage driving signal; the ordinate denotes the voltage applied to a liquid crystal, and the abscissa denotes the elapse of time. Reference numeral  1700  denotes the positive voltage of the driving signal in the case of normal driving, and reference numeral  1701  denotes the positive voltage in the case of the over-voltage driving, and reference numeral  1702  denotes an optical response to the over-voltage driving, and reference numeral  1703  denotes an expected optical response in the case where the over-voltage driving is recurrently implemented. In addition, reference numeral  1704  denotes an optical response of the liquid crystal to the normal driving. 
   As shown in  FIG. 17 , due to the over-voltage driving being implemented at only one side of the AC driving (at the positive side in  FIG. 17 ), a DC component remains; therefore, depending on the structure or the configuration of the liquid-crystal display apparatus, reliability of the liquid-crystal display apparatus may be deteriorated. Therefore, for a liquid-crystal display apparatus in which a pixel electrode and an opposite electrode are arranged on the same substrate and a voltage is generated in parallel with the substrate, a method has been proposed (refer to Japanese Patent Laid-Open No. 2001-34238) in which by, as a countermeasure, forming at least one of the pixel electrode and the opposite electrode with an ITO film, the deterioration of the reliability, due to the DC component, is prevented. 
   In the foregoing liquid-crystal display apparatus in which a pixel electrode and an opposite electrode are arranged on the same substrate and a voltage is generated in parallel with the substrate, by employing a structure in which at least one of the pixel electrode and the opposite electrode is formed with an ITO film, the deterioration of the reliability, due to a DC component caused by over-voltage driving, can be prevented. However, for that purpose, it is necessary to change the panel structure of a liquid-crystal display apparatus, whereby it has been a problem that the structure cannot universally be applied to various kinds of liquid-crystal display apparatuses. 
   SUMMARY OF THE INVENTION 
   The present invention is to solve the foregoing disadvantages of conventional techniques. 
   In addition, the feature of the present invention is to provide a display apparatus in which imbalance, in DC components, due to over-voltage driving is eliminated, whereby the response speed and the reliability are improved, and a display control method therefor. 
   According to the present invention, there is provided with a display apparatus comprising: 
   drive correction means for comparing image data items on a frame basis and changing a driving voltage for image data to be displayed, in accordance with the result of the comparison; 
   magnification means for raising at N times a frame rate of the image data to be displayed by the driving voltage changed by said drive correction means; and 
   driving signal generation means for changing the polarity of the driving voltage for the image data to be displayed at the frame rate raised by said magnification means and generating a display driving signal. 
   Further, according to the present invention, there is provided with a display control method for a display apparatus, comprising: 
   a drive correction step of comparing image data items on a frame basis and changing a driving voltage for image data to be displayed in accordance with the result of the comparison; 
   a magnification step of raising at N times a frame rate of the image data to be displayed, that is displayed by the driving voltage changed in said drive correction step; and 
   a driving signal generation step of changing the polarity of the driving voltage for the image data to be displayed at the frame rate raised in said drive correction step and generating a display driving signal. 
   The feature can be achieved by the combination of features described in the independent claims, and the dependent claims are to specify further advantageous illustrative embodiments of the present invention. 
   In addition, the summary of the present invention does not enumerate all necessary features; therefore, a subcombination of these features may be an invention. 
   Other features, objects and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  depicts a side view of a rear-projection-type display apparatus according to the present embodiment; 
       FIG. 2  depicts a view for explaining the structure of the projection-type display engine D 1  according to the present embodiment; 
       FIG. 3  is a block diagram illustrating the configuration of a display engine of a display apparatus according to a first embodiment of the present invention; 
       FIG. 4  is a chart representing a signal that is outputted from the resolution converter according to the first embodiment and drives a specific pixel; 
       FIG. 5  is a chart representing an example, in the first embodiment, in which display driving is implemented, with an over-driving amount added, in Frame ( 3 ) where the pixel data changes from the black-color to the halftone-gradation data; 
       FIG. 6  depicts a view representing a state of display driving in the case where, in the example of display driving in  FIG. 5 , the frame rate is doubled; 
       FIG. 7  is a chart representing a state in which the polarity of the signal, represented in  FIG. 6 , to which the double-speed conversion has been applied is reversed on a frame basis, in a polarity inverter according to the first embodiment; 
       FIG. 8  is a block diagram for explaining signal processing methods including a response speed corrector according to a second embodiment of the present invention; 
       FIG. 9  is a chart representing a signal that is inputted to a resolution converter according to the second embodiment and drives a specific pixel of a liquid-crystal panel; 
       FIG. 10  is a chart representing image data outputted from the resolution converter according to the second embodiment; 
       FIG. 11  is a chart representing image data outputted from the response speed corrector according to the second embodiment; 
       FIG. 12  is a chart representing a state in which the polarity of the signal, shown in  FIG. 11 , to which the double-speed conversion has been applied is reversed on a frame basis, in a polarity inverter according to the second embodiment; 
       FIG. 13  is a chart representing, as a reference example, a signal that is inputted to the resolution converter and drives a specific pixel of a liquid-crystal panel; 
       FIG. 14  is a chart representing image data outputted from the resolution converter as a reference example; 
       FIG. 15  is a chart representing, as a reference example, image data outputted from the response speed corrector; 
       FIG. 16  is a chart representing, as a reference example, a state in which, in a polarity inverter, the polarity of a signal to which the double-speed conversion has been applied is reversed on a frame basis; 
       FIG. 17  is a chart for explaining a conventional correction method; 
       FIG. 18  is a flowchart for explaining processing according to the first embodiment of the present invention; and 
       FIG. 19  is a flowchart for explaining processing according to the second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, preferred embodiments of the present invention will be explained in detail, with reference to the accompanying drawings. The embodiments described below do not limit thereto the invention related to the claims, and all the combinations of features described in the embodiments are not necessarily requisite to solution means of the present invention. 
   First Embodiment 
     FIG. 1  depicts a side view of a rear-projection-type display apparatus  200  according to the present embodiment. 
   In  FIG. 1 , an image projected from a projection-type display engine D 1  is reflected by a reflecting mirror  201  and projected onto a screen  6  from the rear side thereof. A digitizer  202  is mounted on the front face of the screen  6 . By designating by using a pen  203  for the digitizer a position on the front face of the screen  6  of the digitizer  202 , the coordinates of the designated position can be inputted to the display apparatus  200 . As the digitizer, various kinds of devices, such as an optical device, a pressure-sensitive device, or an ultrasonic device, can be utilized. A brightness adjustment switch  204  is a device for adjusting brightness of an image displayed on the screen  6 . 
     FIG. 2  depicts a view for explaining the structure of the projection-type display engine D 1  according to the present embodiment. 
   In the projection-type display engine D 1 , three respective liquid-crystal panels  2 R,  2 G, and  2 B corresponding to colors R, G, and B are utilized as light modulation devices, and are arranged at respective positions opposing a cross prism. In addition, in the present embodiment, as the liquid-crystal panels  2 R,  2 G, and  2 B, TN-type liquid-crystal panels driven by means of TFTs are utilized. Additionally, a pair of polarization plates  8  is arranged at both sides of each of the liquid-crystal panels  2 R,  2 G, and  2 B in such a way as to flank each liquid-crystal panel; at the light-emitting side of the cross prism  7 , a projection lens  9  and the screen  6  (a member onto which light is projected) are arranged. 
   Meanwhile, by arranging a paraboloidal reflector  10  in such a way as to surround a lamp (light source)  1 , exiting light L 1  from the lamp  1  is converted into parallelized light beams L 2 . In addition, instead of being paraboloidal, the reflector  10  may be an ellipsoidal device so that light is converted into condensed light beams. As the lamp  1 , a metal halide lamp, a xenon lamp, or the like can be utilized. Fly&#39;s eye type integrators  40  and  41  are arranged in the light path of light emitted from the lamp  1 , in such a way as to be in a conjugate relation ship with the liquid-crystal panels  2 R,  2 G, and  2 B, whereby nonuniformity is improved. Additionally, at the light-emitting side of the fly&#39;s eye type integrator  40  and  41 , a relay lens  11  and a mirror  12  are arranged in that order. Additionally, two dichroic mirrors  13  and  14  are arranged so that exiting light from the lamp  1  is split into three light beams, and a relay lens  15  and mirrors  16 ,  17 , and  18  are arranged so that the respective split light beams are led to the liquid-crystal panels  2 R,  2 G, and  2 B. In addition, reference numeral  19  denotes field lenses. 
   Meanwhile, a video signal processor  3  as illustrated in  FIG. 3  and the like are connected to the liquid-crystal panels  2 R,  2 G, and  2 B. 
   Next, electric-signal processing in a projection-type display engine D 1  according to the present embodiment will be explained. 
     FIG. 3  is a block diagram illustrating the configuration of a display engine of a display apparatus according to the first embodiment of the present invention. 
   In the video signal processor  3 , a switch  30  switches a video signal inputted through a terminal  50  from a PC  300  and an NTSC signal inputted from a terminal  51 . A signal processing circuit  52  applies to an NTSC signal inputted through the terminal  51  signal processing such as decoding of the NTSC signal, noise-reduction processing, bandwidth-limiting filtering, and signal-level adjustment. An A/D converter  31  converts an inputted analogue video signal into a digital signal. A DSP (Digital Signal Processor)  32  receives and applies signal processing to the A/D-converted digital image data and outputs the resultant data. A resolution converter  101  implements conversion of the resolution of the inputted image data. A memory  33  stores image data of a current frame, image data to be displayed in the following frame, and the like. A timing generator  34  outputs a timing signal that specifies operation timing for respective sections. A memory  102  stores image data items that have been displayed in the previous frames, in order to correct response speed. A response speed corrector  103  compares image data (to be displayed) outputted from the resolution converter  101  with image data (displayed already) that has passed through the memory  102 , and then corrects response speed. A double speed converter  104  creates image data for double-speed conversion, by means of a memory  105 . A polarity inverter  106  reverses the polarity of the image signal based on the image data. A D/A converter  35  converts the digital image data into an analogue image signal. A panel driver  36  generates video signals based on the analogue image signal and supplies the video signals to the liquid-crystal panels  2 R,  2 G,  2 B. A driving power supply of the liquid-crystal panels  2 R,  2 G,  2 B is also supplied by the panel driver  36 . In addition, in the DSP  32 , image processing such as contrast adjustment, brightness adjustment, color conversion, and the like are implemented. 
   In this situation, in the block diagram, only analogue input signals are illustrated, however, it goes without saying that, instead of those, provision of input terminals for digital signals such as an LVDS or a TMDS and a D 4  terminal for a digital TV is also effective. 
   A ballast  57  is an electric power source for a lamp  1 , connected to the lamp  1 . Reference numerals  58  and  60  denote a system electric power source and an AC inlet, respectively. A remote controller  61  instructs various operation items for the display apparatus. A control panel  62  receives a signal from the remote controller  62 . 
   Moreover, reference numeral  204  denotes the brightness adjustment switch, and a brightness adjustment switch detector  109  detects the state of the brightness adjustment switch  204 . A digitizer detector  118  detects the coordinates indicated by the digitizer  202 . Reference numeral  107  denotes a USB/IF. Still moreover, and reference numerals  63 ,  64 , and  65  denote a CPU, a ROM, and a RAM, respectively. The CPU  63  is connected to the video signal processor  3 , the control panel  62  the ballast  57 , the brightness adjustment switch detector  109 , the digitizer detector  118 , the USBI/F  107 , and the like, and implements drive control of liquid-crystal panels  2 R,  2 G, and  2 B and the lamp  1 , and enlargement, diminution, and shifting of an display image. 
   In the present embodiment, it has been explained that the brightness adjustment switch detector  109 , the digitizer detector  118 , the USB-I/F  107 , and the like are connected to the CPU  63 , however, the display engine may be configured in such a way that the CPU  63  and a program therefor realize the functions of the foregoing constituent elements. 
   Next, the configuration of a PC (personal computer) connected to the display apparatus will be explained. 
   The PC  300  has a CPU  301 , an HD (hard disc)  302 , a RAM  303 , a ROM  304 , a video memory  305 , a graphic controller  306 , a mouse I/F  307 , a USB I/F  308 , and the like, and is equipped with a video output terminal  309 , a USB input terminal  310 , and a mouse input terminal  311 . A mouse  312  functions as a pointing device and is connected to the mouse input terminal  311 . 
   Next, the operation of the display device according to the first embodiment will be explained with reference to  FIGS. 3 and 4  to  7 . 
     FIGS. 4-7  are charts for explaining contents of processing in a display device according to the first embodiment, and depict respective signal waveforms at the processing sections. 
   Either one of a video signal inputted through the PC input terminal  50  and a video signal inputted through the NTSC input terminal  51  is selected by the switch  30 . The A/D converter  31  converts the selected signal from an analogue signal into a digital signal. Subsequently, the DSP  32  applies to the digital image processing such as contrast adjustment, brightness adjustment, color conversion, and the like. The image data outputted from the DSP  32  is further converted by the resolution converter  101  into image data having a desired resolution and a desired frame rate. In the present embodiment, a case will be explained where the outputted image data is converted into image data having a frame rate of 60 Hz. On that occasion, by storing, in the memory  33 , inputted image data for at least one frame and changing the speed of reading the image data from the memory  33 , the inputted image data can be converted into an image signal having a frame rate different from that of the inputted image signal. 
     FIGS. 4 to 7  are charts representing respective signals that, in signal processing sections, drive a specific pixel in a liquid-crystal panel in the display apparatus according to the first embodiment. 
     FIG. 4  is a chart representing a signal that is outputted from the resolution converter  101  and drives the specific pixel to display at a frame rate of 60 Hz. 
   In Frames ( 1 ) and ( 2 ), black-color pixel data (voltage V 0 ) is applied to the pixel, and the pixel data is changed in Frame ( 3 ) to halftone-gradation pixel data (voltage V 1 ). In this situation, in the response speed corrector  103 , so-called over-voltage driving is implemented so as to improve the response speed of the liquid-crystal display apparatus. The voltage in the case of the over-voltage driving is defined V 2  (V 2 &gt;V 1 ). In the response speed corrector  103 , one-frame-prior pixel data that has been displayed and stored in the memory  102  is compared with pixel data of a current frame. In this situation, if any pixel-data change exists, the over-voltage driving is implemented in response to the result. In the example in  FIG. 4 , because the pixel data at the timing of displaying Frame ( 3 ) is different from the pixel data for the prior Frame ( 2 ), the over-voltage driving is implemented.  FIG. 5  represents the foregoing situation. 
     FIG. 5  is a chart representing an example in which, by adding an over-driving amount to the pixel data, in Frame ( 3 ) where the pixel data changes from the black color to the halftone gradation, the driving of the pixel is implemented with the voltage V 2 . 
   With regard to processing in and after Frame ( 4 ) in  FIG. 5 , because the pixel data in Frame ( 3 ) in  FIG. 4  and the pixel data in Frames ( 4 ) and ( 5 ) in  FIG. 4  are equal to each other, the over-voltage driving is not implemented. 
   Correction methods, for the response speed, implemented in the response speed corrector  103  include, for example, a method in which, by means of an LUT (look-up table), the correction amount is changed, in accordance with the difference between the level of one-frame-prior pixel data and the level of pixel data to be displayed in the current frame, and a method in which the correction amount is decided, in accordance with the result of the subtraction between one-frame-prior pixel data and pixel data to be displayed in the current frame. With the former method in which the LUT is utilized, it is made possible to implement optimal over-voltage driving in accordance with change in the pixel-data level, for example, enhancement of the correction amount for an image area, having the halftone gradation, in which the response speed is low. 
   In the double speed converter  104 , the image data outputted from the response speed corrector  103  is stored in the memory  105  and then read at a double frame rate. Accordingly, the frame rate is converted into a double rate. In consequence, as represented in  FIG. 6 , the frame rate of the outputted image data is converted into 120 Hz. 
     FIG. 6  depicts a view representing a state of display driving in the case where, in the example of display driving in  FIG. 5 , the frame rate is doubled. 
   In  FIG. 6 , each one frame represented in  FIG. 5  is designated by two frames, for example, Frames ( 1 ) and ( 1 )′ for Frame ( 1 ) and Frames ( 2 ) and ( 2 )′ for Frame ( 2 ) (and so forth). 
   Next, in the polarity inverter  106 , the polarity of the signal is reversed on one frame basis, after the frame where the double-speed conversion has been implemented, and then the resultant signal is outputted. 
     FIG. 7  is a chart representing a state in which the polarity of the signal, represented in  FIG. 6 , to which the double-speed conversion has been applied is reversed on a frame basis by the polarity inverter  106  according to the first embodiment. In addition, Frames in  FIG. 7  are the same as those in  FIG. 6 ; the absolute values of the driving voltages V 1  and V 2  in  FIG. 7  are the same as those in  FIG. 6 . 
     FIG. 18  is a flowchart for explaining response-speed correction according to the first embodiment of the present invention, and the processing of the response-speed correction is implemented by the response speed corrector  103 , the double speed converter  104 , and the polarity inverter  106 , however, that processing may be implemented under the control of the CPU  63 , based on a program stored in the ROM  64 . 
   In the step S 1  in the first place, respective pixel data items for an image to be displayed in a current frame are inputted, and in the step S 2 , the respective pixel data items are compared with corresponding pixel data items, for a one-frame-prior image, that have been stored in the memory  102 . In the case where the respective pixel data items for the frames coincide to each other, the process advances to the step S 5 . On the other hand, in the case where the respective pixel data items for the frames do not coincide to each other, the process advances to the step S 4 , and in displaying a pixel where respective pixel data items do not coincide to each other, the driving voltage is raised to implement the over-voltage driving. Subsequently, in the step S 5 , the double speed converter  104  doubles the frame rate of the image data. Next, in the step S 6 , the polarity inverter  106  reverses on a frame basis the polarity of the driving signal and outputs to the D/A converter  35 . In the step S 7 , the liquid-crystal panel is driven to display. In the step S 8 , it is determined whether or not the image-display processing has been completed. If the image-display processing has not been completed, the process returns to the step S 1 , and the above mentioned processing is applied to the image data for the following frame. 
   Accordingly, the over-voltage driving is implemented equally in normal-polarity driving and in reversed-polarity driving. 
   As explained heretofore, according to the first embodiment, because a balance in the voltage for the over-voltage driving is ensured between normal-polarity driving and reversed-polarity driving, the reliability of display is raised. 
   Second Embodiment 
   Next, the second embodiment of the present invention will be explained. In the second embodiment, a method will be explained in which, compared with the first embodiment described above, the capacity of the frame memory  102  is reduced, whereby a low-cost display apparatus is realized. 
   In the first embodiment described above, in a display apparatus in which reversed-polarity driving is implemented, the resolution converter  101  that converts an input signal into a signal having a predetermined frame rate, the response speed corrector  103  that implements comparison of the output from the resolution converter  101  and correction of the response speed, and the double speed converter  104  that doubles frame rate for reverse-polarity driving are utilized. 
   In contrast, in the second embodiment, since the resolution converter  101  reads image data at a double speed (preliminarily at a frame rate required for reversed-polarity driving), the frame memory  105  for the double-speed driving in the first embodiment is no longer required. Further, a problem (the imbalance between the normal-polarity driving and the reversed-polarity driving at AC driving causes deterioration of a liquid crystal), in response-speed correction, that is caused by that the resolution converter  161  reads the image data at a double speed is prevented. 
   Hereinafter, the second embodiment will be explained in detail. 
     FIG. 8  is a block diagram for explaining signal processing methods at signal processing sections including a response speed corrector according to the second embodiment of the present invention. In addition, the configuration of the display engine of a display apparatus according to the second embodiment is the same as the block diagram illustrated in  FIG. 3 , except for the configuration illustrated in  FIG. 8 ; therefore, the explanation for the same configuration will be omitted. 
   In  FIG. 8 , a memory  502  stores image data displayed in the current frame and image data to be displayed in the following frame. A memory  503  stores image data in the previous frames, in order to correct the response speed. A response speed corrector  504  is the same as the response speed corrector  103  described above. In addition, a polarity inverter  505  is also the same as the polarity inverter  106 , therefore, explanations for them will be omitted. Image data outputted from the polarity inverter  505  is displayed on a liquid-crystal panel, through a D/A converter  35  and the like, as shown in  FIG. 3 . 
   Next, the operation of the second embodiment will be explained with reference to  FIGS. 8 and 9  to  12 . 
     FIGS. 9 to 12  are charts for explaining contents of processing according to the second embodiment, and represent respective signal waveforms at processing sections. 
   What the second embodiment differs from the first embodiment is that, after the resolution converter  501  converts the frame rate to 120 Hz, the response speed corrector  504  corrects the response speed. What the second embodiment differs from the first embodiment is also that image data to be referred to in the response speed corrector  504  is altered. 
   Image data inputted to the resolution converter  501  is converted by the resolution converter  501  into image data having a desired resolution and a desired frame rate. In the second embodiment, the image data is converted to image data having a frame rate of 120 Hz. On that occasion, by storing, in the memory  502 , inputted image data for at least one frame and changing the speed of reading the image data, the inputted image data is converted into image data having a frame rate different from that of the inputted image signal. 
     FIG. 9  is a chart representing a signal that is inputted to the resolution converter  501  and drives a specific pixel on the liquid-crystal panel. 
   In this situation, the frame rate for displaying an image is 60 Hz. In Frames ( 1 ) and ( 2 ), a black-color signal (voltage V 0 ) is applied to the pixel, and the black-color signal is changed in Frame ( 3 ) to halftone-gradation pixel data (voltage V 1 ). 
     FIG. 10  is a chart representing image data outputted from the resolution converter  501 . In  FIG. 10 , as is the case with  FIG. 6 , the frame rate is converted to 120 Hz. In other words, each one frame represented in  FIG. 9  is designated by two: frames, for example, Frames ( 1 ) and ( 1 )′ for Frame ( 1 ) and Frames ( 2 ) and ( 2 )′ for Frame ( 2 ) (and so forth). 
   In the second embodiment, the resolution converter  501  converts the frame rate of an image, from 60 Hz to 120 Hz. In the resolution converter  501 , by reading, twice in series, image data stored in the memory  502 , the frame rate is doubled. Next, in order to improve the response speed of the liquid-crystal display apparatus, the response speed corrector  504  implements so-called over-voltage driving. 
     FIG. 11  is a chart representing a signal that is outputted from the response speed corrector  504  according to the second embodiment and drives a specific pixel to display at a frame rate of 60 Hz. 
   The response speed corrector  504  compares pixel data, for a two-frame-prior image, that has been stored in the memory  503  and pixel data for an image to be displayed in a current frame. If a difference in the data items exists, the over-voltage driving is implemented in accordance with the result of the comparison. In the example in  FIG. 10 , pixel data items in Frames ( 3 ) and ( 3 )′ are different from those in respective two-frame-prior Frames ( 2 ) and ( 2 )′, therefore, as represented in  FIG. 11 , the over-voltage driving is implemented with a voltage V 2 , in the Frames ( 3 ) and ( 3 )′. Pixel data items in and after Frames ( 4 ) and ( 4 )′ subsequent to Frames ( 3 ) and ( 3 )′ in  FIG. 10  are the same as those in respective two-frame-prior Frames ( 3 ) and ( 3 )′ to ( 5 ) and ( 5 )′, therefore, the over-voltage driving is not implemented. 
   As discussed above, in the example in  FIG. 11 , driving is implemented, with an extra voltage for overdriving added, in Frames ( 3 ) and ( 3 )′ in which change occurs in the pixel data, from the black-color level to the halftone-gradation level. In this situation, in each of Frames ( 3 ) and ( 3 )′, the same amount of over-voltage driving is implemented. 
   Methods for the response-speed correction include, for example, a method in which, by means of an LUT, the correction amount is changed, in accordance with the input level of one-frame-prior image data and the input level of image data to be displayed in a current frame, and a method in which the correction amount is decided, in accordance with the result of the subtraction between one-frame-prior image data and the image data to be displayed in the current frame. With the method in which the LUT is utilized, it is made possible to implement optimal over-voltage driving in accordance with change in the image-data level, for example, enhancement of the correction amount for an image area, having the halftone gradation, in which the response speed is low. 
   In the polarity inverter  505 , the signal outputted from the response speed corrector  504  reverses its polarity every one frame whose rate has been converted to 120 Hz. Accordingly, as shown in  FIG. 12 , it is possible to implement the over-voltage driving equally in normal-polarity driving and in reversed-polarity driving. 
     FIG. 12  is a chart representing a state in which the polarity of the signal, as shown in  FIG. 11 , to which the double-speed conversion has been applied is reversed on a frame basis, in the polarity inverter  505  according to the second embodiment. In addition, Frames in  FIG. 12  are the same as those in  FIG. 11 , and the absolute values of the driving voltages V 1  and V 2  are the same as those in  FIG. 11 . 
     FIG. 19  is a flowchart for explaining response-speed correction according to the second embodiment of the present invention, and the processing of the response-speed correction is implemented by the response speed corrector  504  and the polarity inverter  505 . The processing may be implemented under the control of the CPU  63 , based on a program stored in the ROM  64 . 
   In the first place, in the step S 11 , image data to be displayed in a current frame is inputted. In the step S 12 , the frame rate of the image is doubled. Next, in the step S 13 , pixel data items for the image to be displayed in the current are compared with corresponding pixel data items, for a two-frame-prior image, that have been stored in the memory  503 . In the case where the respective pixel data items for the frames coincide to each other, the process advances to the step S 16 , but in the case where the respective pixel data items for the frames do not coincide to each other, the process advances to the step S 15 , and with the driving voltage corresponding to a pixel where the respective pixel data items do not coincide to each other raised, the over-voltage driving: is implemented. Next, in the step S 16 , the polarity inverter  106  reverses on a frame basis the polarity of the signal and outputs to the D/A converter  35 . Next, in the step S 17 , the liquid-crystal panel is driven to display. In the step S 18 , it is determined whether or not the display processing has been completed . . . . If the display processing has not been completed, the process returns to the step S 11  and the above mentioned processing is implemented for the following frame. 
   As explained heretofore, according to the second embodiment, because no imbalance in the over-voltage driving occurs between normal-polarity driving and reversed-polarity driving, the reliability of display is raised. 
   In addition, in the second embodiment, the 120 Hz double-speed conversion is implemented in the resolution converter  501 . Therefore, unlike the first embodiment described above, the second embodiment does not require the double speed converter and a memory for the double speed converter, whereby a great effect is demonstrated in terms of reduction of costs and mounting areas. 
   Additionally, in the second embodiment, the frame rate is doubled before the response speed is corrected (in the step S 12 ). For that reason, in order to correct the response speed, in a case where the image data items are compared, a present-frame image is compared with a two-frame-prior image. 
   It will be explained below what kind of problem occurs in the case where, as is the case with the first embodiment, image data for a current frame is compared with image data for a one-frame-prior. 
   Reference Example 
     FIGS. 13 to 16  are each a chart, representing a signal waveform in a corresponding signal processing block, for explaining a malfunction caused through comparison between image data for a current frame and image data for a one-frame-prior. 
   Image data inputted to the resolution converter  501  is converted by the resolution converter  501  into image data having a desired resolution and a desired frame rate. In Reference Example, the image data is converted to image data having a frame rate of 120 Hz. 
   On that occasion, by storing, in the memory  502 , inputted image data for at least one frame and changing the speed of reading the image data, the inputted image data can be converted into an output signal having a frame rate different from that of the inputted image signal. 
     FIG. 13  is a chart representing a signal that is inputted to the resolution converter  501  and drives a specific pixel of a liquid-crystal panel. In Frames ( 1 ) and ( 2 ), a black-color signal (voltage V 0 ) is applied to the pixel, and the black-color signal is changed in Frame ( 3 ) to halftone-gradation pixel data (voltage V 1 ). 
     FIG. 14  is a chart representing image data outputted from the resolution converter  501 . 
   In the resolution converter  501 , the frame rate is converted from 60 Hz to 120 Hz. In the resolution converter  501 , by reading, twice in series, image data stored in the memory  502 , the frame rate is doubled. Next, in order to improve the response speed of the liquid-crystal display apparatus, the response speed corrector  504  implements so-called over-voltage driving. 
     FIG. 15  is a chart representing image data outputted from the response speed corrector  504 . 
   In the response speed corrector  504 , pixel data, for a one-frame-prior image, that has been stored in the memory  503  is compared with corresponding pixel data to be displayed in a current frame. In this situation, if any pixel-data change exists, the over-voltage driving is implemented in response to the result. As represented in  FIG. 15 , driving (with a voltage V 2 ) is implemented, with an extra voltage for overdriving added, in Frame ( 3 ) in which, compared with one-frame-prior image, change occurs in the input signal, from the black-color level to the halftone-gradation level. In this case, the over-voltage driving is applied only to Frame ( 3 ), but the over-voltage driving is not implemented in Frame ( 3 )′. 
   In the polarity inverter  505 , the signal outputted from the response speed corrector  504  reverses its polarity every one frame whose rate has been converted to 120 Hz. Accordingly, as represented in Frames ( 3 ) and ( 3 )′ in  FIG. 16 , the over-voltage driving consists of normal-polarity driving (+V 2 ) and reversed-polarity driving (−V 1 ), thereby being not balanced, therefore, a DC component occurs. 
   As described above, a problem exists in which, when the frame rate is doubled before the response speed is corrected, and the image data for a one-frame-prior image is utilized as an image data to be compared, an imbalance, in the over-voltage driving, between the normal-polarity driving and the over-voltage driving occurs. 
   Therefore, in the case where, as in the second embodiment, the frame rate is doubled before the response speed is corrected, image data to be compared in correction of the response speed should be the image data for a two-frame-prior image. 
   In the second embodiment, a case has been explained in which the frame rate is doubled before the response speed is corrected. In the case where the frame rate is multiplied by N, image data to be compared in correction of the response speed may be the image data for an N-frame-prior image. 
   According to the second embodiment, an effect is demonstrated in which, in contrast to the first embodiment, in  FIG. 3 , that requires three memories  33 ,  102 , and  105 , only two memories  502  and  503  are required, as illustrated in  FIG. 8 . 
   As explained heretofore, according to a display apparatus related to any one of the first and the second embodiments, in a liquid-crystal display apparatus in which AC driving is implemented, the same image is recurrently outputted at the beginning of normal-polarity driving or reversed-polarity driving and an equal-amount correction signal is given to the respective driving waveforms in normal-polarity driving and reversed-polarity driving. As a result, deterioration, in a signal, due to a DC component caused by over-voltage driving can be improved. In particular, a configuration according to any one of the present embodiments can be applied to a commonly used display apparatus, without employing a special structure. Moreover, it is possible to provide a high-image-quality display apparatus, without deteriorating reliability. 
   Still moreover, in the present embodiments, a case has been explained in which the frame rate is doubled. The present invention is not limited to the doubled frame rate, and, for example, any frame rate may be accepted as long as it is an integer (N)-fold frame rate. 
   Furthermore, according to the second embodiment, by, prior to response-speed correction, implementing N-fold-speed driving and utilizing N-frame-prior data, as comparison data in accordance with which the response speed is corrected, the capacity of a frame memory can be reduced, whereby a low-cost display apparatus can be provided. 
   The present invention is not limited to the above embodiment, and various changes and modifications can be made thereto within the spirit and scope of the present invention. Therefore, to apprise the public of the scope of the present invention, the following claims are made. 
   This patent application claims the benefit of Japanese Patent Application No. 2005-107747, filed Apr. 4, 2005, which is hereby incorporated by reference herein in its entirety.