Patent Publication Number: US-9418614-B2

Title: Liquid crystal display apparatus and method for controlling the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display (LCD) apparatus and a method for controlling the liquid crystal display apparatus. 
     2. Description of the Related Art 
     Conventionally, liquid crystal monitors and projectors are known as LCD apparatuses. These LCD apparatuses control the transmissivity of light from a light source by using a liquid crystal panel to display gradations corresponding to an image. To accomplish this control, the LCD apparatuses change a potential difference between a common electrode of the liquid crystal panel and a pixel electrode corresponding to each pixel (i.e., a voltage applied to a liquid crystal between electrodes) to control the polarization direction of the liquid crystal of each pixel of the liquid crystal panel, thus changing the transmissivity of light. 
     In recent years, some of such LCD apparatuses have employed a new technique for changing the transmissivity of each pixel of the liquid crystal panel. Specifically, the technique maintains a common electrode at a constant voltage, simultaneously inputs a ramp signal having a monotonically changing voltage to each pixel electrode, and turns OFF a switch corresponding to each pixel electrode at a timing according to a gradation displayed by each pixel. 
     Even this technique enables charging a voltage to each pixel electrode similar to conventional liquid crystal panels, making it possible to change the transmissivity of light of each pixel of the liquid crystal panel as discussed, for example, in Japanese Patent No. 3367808. 
     To display an image of one line in the horizontal direction, this technique simultaneously inputs a ramp signal having a monotonically increasing voltage to each pixel electrode in one line of the liquid crystal panel, and turns off a switch corresponding to each pixel electrode at a timing according to an image value (gradation to be displayed) of each pixel. In particular, in Japanese Patent No. 3367808, a monotonically increasing ramp signal generated by one reference voltage source  41  for charging pixel voltage is simultaneously applied to each pixel electrode for one line in the horizontal direction (see  FIGS. 1 and 2 ). That is, each pixel electrode in one line shares the monotonically increasing ramp signal generated by the one reference voltage source for charging pixel voltage. 
     However, when the liquid crystal panel is driven by the method as discussed in Japanese Patent No. 3367808, for example, if there are many pixels having the same gradation (gradation A) in one line as illustrated in  FIG. 8A , switches corresponding to many pixel electrodes are simultaneously turned off at a timing when the ramp signal reaches a voltage corresponding to gradation A. 
     In this case, a load on the one power supply for charging pixel voltage rapidly changes to result in a turbulent ramp signal. In the case illustrated in  FIG. 8A , for example, many switches turn off at the same time at a timing corresponding to many gradations A. As a result, the ramp signal corresponding to gradations after gradation A becomes turbulent, as illustrated in  FIG. 8B . 
     If pixel electrode switches corresponding to pixels for displaying gradation B are turned off in a state where the ramp signal corresponding to gradation B (gradation after gradation A) is turbulent, the charge voltage to the pixel electrodes of the pixels for displaying gradation B may become a voltage not corresponding to gradation B. 
     In this case, the transmissivity of the pixels which should display gradation B of the liquid crystal panel becomes different from the transmissivity of the pixels for displaying gradation B. This signal turbulence is an example, and may be larger for a longer time period. In the example illustrated in  FIG. 8A , there has been a problem that a degraded image is presented to a user since the image of gradation B may become brighter or darker than gradation B although the image of gradation A is correctly displayed. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a liquid crystal display apparatus and a method for controlling the liquid crystal display apparatus capable of preventing image degradation when driving a liquid crystal panel. 
     According to an aspect of the present invention, a liquid crystal display apparatus including a liquid crystal device, includes an acquisition unit configured to acquire an image, a gradation signal generation unit configured to generate a gradation signal which provides a value indicating a first gradation and then a value indicating a second gradation in synchronization with a horizontal synchronization signal of the image acquired by the acquisition unit, a voltage generation unit configured to generate a pixel voltage, based on the gradation signal, to be simultaneously supplied to a plurality of pixel electrodes for liquid crystal pixels of the liquid crystal device, and a control unit configured to, based on a pixel value of each pixel of the image acquired by the acquisition unit and a value of the pixel voltage, control a supply state of the pixel voltage to the plurality of pixel electrodes simultaneously supplied with the pixel voltage, wherein, when there are more pixels indicating the first gradation than the second gradation, the control unit controls the supply state of the pixel voltage to the plurality of pixel electrodes simultaneously supplied with the pixel voltage to prolong the time period between the timing of changing the supply state of the pixel voltage corresponding to the first gradation and the timing of changing the supply state of the pixel voltage corresponding to the second gradation so as to become longer than that when there are fewer pixels indicating the first gradation than the second gradation. 
     Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a block diagram illustrating an overall configuration of a liquid crystal projector. 
         FIG. 2  is a flowchart illustrating control of basic operations of a liquid crystal projector according to an exemplary embodiment of the present invention. 
         FIG. 3  is a block diagram illustrating a configuration of a liquid crystal control unit according to the present exemplary embodiment. 
         FIG. 4  illustrates a configuration of a liquid crystal device according to the present exemplary embodiment. 
         FIG. 5  is a diagram illustrating signals generated by a gradation signal generation unit and an image analysis unit according to the present exemplary embodiment. 
         FIGS. 6A to 6D  illustrate a result of analysis by the image analysis unit, and tables for determining a delay amount of a comparison signal. 
         FIG. 7  is a diagram illustrating signals generated by the gradation signal generation unit and the image analysis unit according to the present exemplary embodiment. 
         FIGS. 8A and 8B  illustrates operations of a conventional technique. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. 
     An exemplary embodiment will be described below centering on a liquid crystal projector as an example of an LCD apparatus. However, the present invention is not limited to a liquid crystal projector but applicable to any displays having a liquid crystal device, such as liquid crystal televisions, liquid crystal monitors, and electronic devices having a liquid crystal display unit. Although single-panel and 3-panel types are generally known as liquid crystal projectors, the present invention is applicable to both types. 
     The liquid crystal projector according to the present exemplary embodiment controls the transmissivity of light of a liquid crystal device according to an image to be displayed to project on a screen the light from a light source which has transmitted through the liquid crystal device, thus presenting the image to the user. In the present exemplary embodiment, a liquid crystal control unit simultaneously supplies a varying pixel voltage to a plurality of liquid crystal pixels of the liquid crystal device. The varying pixel voltage is used to control the transmissivity of the liquid crystal to be the transmissivity corresponding to a plurality of gradations to be displayed. 
     When the supplied pixel voltage becomes a voltage corresponding to a specific gradation, the liquid crystal control unit changes the supply state of the pixel voltage to the liquid crystal pixels which should display the specific gradation (in the present exemplary embodiment, from the supply state to the non-supply state), thus controlling the liquid crystal device. A liquid crystal projector based on such a method will be described below. 
     &lt;Overall Configuration&gt; 
     An overall configuration of the liquid crystal projector according to the present exemplary embodiment will be described below with reference to  FIG. 1 .  FIG. 1  illustrates the overall configuration of a liquid crystal projector  100  according to the present exemplary embodiment. 
     The liquid crystal projector  100  according to the present exemplary embodiment includes a control unit  110 , an operation unit  111 , an image input unit  130 , and an image processing unit  140 . The liquid crystal projector  100  further includes a liquid crystal control unit  150 , liquid crystal devices  151 R,  151 G, and  151 B, a light source control unit  160 , a light source  161 , a color separation unit  162 , a color combination unit  163 , an optical system control unit  170 , and a projection optical system  171 . The liquid crystal projector  100  may further include a recording and reproduction unit  191 , a recording medium  192 , a communication unit  193 , an image pickup unit  194 , a display control unit  195 , and a display unit  196 . 
     The control unit  110  controls each operation unit of the liquid crystal projector  100 . The control unit  110  includes, for example, a central processing unit (CPU), a memory, and a microprocessor. The recording and reproduction unit  191  can reproduce still and video images of still and moving image data read from the recording medium  192  and still and moving image data received via the communication unit  193 . 
     Still and video images acquired by the image pickup unit  194  can be converted into still and moving image data, respectively, and recorded in the recording medium  192 . The operation unit  111  receives a user instruction and transmits a relevant instruction signal to the control unit  110 . The operation unit  111  includes, for example, switches, dials, and a touch panel provided on the display unit  196 . 
     The operation unit  111  may transmit a predetermined instruction signal to the control unit  110  based on a signal received by a signal receiving unit (such as an infrared receiving unit) configured to receive a signal from a remote controller. The control unit  110  receives a control signal input from the operation unit  111  and the communication unit  193 , and controls each operation unit of the liquid crystal projector  100 . 
     The image input unit  130  receives a video signal from an external apparatus. The image input unit  130  includes a composite terminal, a separate (S) video terminal, a digital (D) terminal, a component terminal, an analog red/green/blue (RGB) terminal, a digital visual interface-A (DVI-A) terminal, a digital visual interface-D (DVI-D) terminal, and a high-definition multimedia interface (HDMA: registered trademark) terminal. Upon reception of an analog video signal, the image input unit  130  converts the received analog video signal into a digital video signal, and transmits the received video signal to the image processing unit  140 . 
     The external apparatus may be any apparatus capable of outputting a video signal, such as a personal computer, a camera, a mobile phone, a smart phone, a hard disk recorder, and a game machine. 
     The image processing unit  140  applies processing for changing the number of frames, number of pixels, and image shape to the video signal received from the video input unit  130 , and transmits the changed video signal to the liquid crystal control unit  150 . The image processing unit  140  includes, for example, a microprocessor for image processing. 
     The image processing unit  140  is not necessarily a dedicated microprocessor and may be, for example, a part of the processing function of the CPU and memory or the microprocessor operating the control unit  110 . The image processing unit  140  is capable of executing interframe thinning processing, frame interpolation processing, resolution conversion processing, and distortion correction processing (keystone correction process). 
     In addition to the video signal received from the video input unit  130 , the image processing unit  140  is further capable of applying the above-described change processing to still and video images reproduced by the control unit  110 . 
     The liquid crystal control unit  150  controls a voltage to be applied to the liquid crystal of a pixel composed of the liquid crystal devices  151 R,  151 G, and  151 B based on the video signal processed by the image processing unit  140  to adjust the transmissivity of the liquid crystal devices  151 R,  151 G, and  151 B. The liquid crystal control unit  150  includes a microprocessor for control. 
     The liquid crystal control unit  150  is not necessarily a dedicated microprocessor and may be, for example, a part of the processing function of the CPU and memory or microprocessor operating the control unit  110 . For example, when a video signal is input to the image processing unit  140 , the liquid crystal control unit  150  controls, each time one-frame image is received from the image processing unit  140 , the liquid crystal devices  151 R,  151 G, and  151 B so as to provide the transmissivity corresponding to the image. 
     The liquid crystal device  151 R, which is a liquid crystal device for red, adjusts the transmissivity of red light out of red (R), green (G), and blue (B) color components separated by the color separation unit  162  out of the light output from the light source  161 . 
     The liquid crystal device  151 G, which is a liquid crystal device for green, adjusts the transmissivity of green light out of red (R), green (G), and blue (B) color components separated by the color separation unit  162  out of the light output from the light source  161 . The liquid crystal device  151 B, which is a liquid crystal device for blue, adjusts the transmissivity of blue light out of red (R), green (G), and blue (B) color components separated by the color separation unit  162  out of the light output from the light source  161 . 
     Operations performed by the liquid crystal control unit  150  to control the liquid crystal devices  151 R,  151 G, and  151 B, and the configurations of the liquid crystal devices  151 R,  151 G, and  151 B will be described below. 
     The light source control unit  160  controls the ON/OFF state and the light quantity of the light source  161 . The light source control unit  160  includes a microprocessor for control. The light source control unit  160  is not necessarily a dedicated microprocessor and may be, for example, a part of the processing function of the CPU and memory or microprocessor operating the control unit  110 . 
     The light source  161  outputs light for projecting an image on a screen (not illustrated), and may be a halogen lamp, a xenon lamp, or a high-pressure mercury vapor lamp. The color separation unit  162  separates the light output from the light source  161  into red (R), green (G), and blue (B) color components. The color separation unit  162  includes, for example, a dichroic mirror and a prism. When light emitting diodes (LEDs) corresponding to respective colors are used as the light source  161 , the color separation unit  162  is not required. 
     The color combination unit  163  combines red (R), green (G), and blue (B) color components which have transmitted through the liquid crystal devices  151 R,  151 G, and  151 B. The color combination unit  163  includes, for example, a dichroic mirror and a prism. The color combination unit  163  combines the red (R), green (G), and blue (B) color components and transmits the resultant light to the projection optical system  171 . 
     In this case, the liquid crystal devices  151 R,  151 G, and  151 B are controlled by the liquid crystal control unit  150  so as to provide the transmissivity of light corresponding to an image input by the image processing unit  140 . Therefore, when the light combined by the color combination unit  163  is projected on the screen by the projection optical system  171 , an image corresponding to the image input by the image processing unit  140  is displayed on the screen. 
     The optical system control unit  170  controls the projection optical system  171 . The optical system control unit  170  includes a microprocessor for control. The optical system control unit  170  is not necessarily a dedicated microprocessor and may be, for example, a part of the processing function of the CPU and memory or microprocessor operating the control unit  110 . 
     The projection optical system  171  projects on the screen the combined light output from the color combination unit  163 . The projection optical system  171  includes a plurality of lenses and an actuator for driving the lenses. Driving the lenses with the actuator enables enlarging and reducing the projected image and performing focal point adjustment therefor. 
     The recording and reproduction unit  191  reproduces still and moving image data from the recording medium  192 , receives still and moving image data for still and video images obtained by the image pickup unit  194  from the control unit  110 , and records these pieces of data on the recording medium  192 . The recording and reproduction unit  191  may record on the recording medium  192  the still and moving image data received from the communication unit  193 . 
     The recording and reproduction unit  191  includes, for example, an interface for electrically connecting with the recording medium  192  and a microprocessor for communicating with the recording medium  192 . The recording and reproduction unit  191  does not necessarily include a dedicated microprocessor. The function of communicating with the recording medium  192  may be achieved by a part of the processing function of the CPU and memory or microprocessor operating the control unit  110 . 
     The recording medium  192  is a medium capable of recording still and moving image data and control data required for the liquid crystal projector  100  according to the present exemplary embodiment. The recording medium  192  may be any type of a recording medium such as a magnetic disk, an optical disk, and a semiconductor memory, and may further be detachably attached to or included in the liquid crystal projector  100 . 
     The communication unit  193  receives a control signal, still image data, and moving image data from an external device. The communication method of the communication unit  193  may be, for example, a wireless local area network (LAN), a wired LAN, a universal serial bus (USB), or Bluetooth (registered trademark). 
     When the terminal of the image input unit  130  is, for example, an HDMA (registered trademark) terminal, consumer electronics control (CEC) communication may be performed via the HDMA terminal. The external apparatus may be any apparatus capable of communicating with the liquid crystal projector  100 , such as a personal computer, a camera, a mobile phone, a smart phone, a hard disk recorder, a game machine, and a remote control. 
     The image pickup unit  194  captures an image of a subject and acquires an image signal. For example, the image pickup unit  194  is capable of capturing an image (screen direction) projected via the projection optical system  171 . The image pickup unit  194  transmits the acquired still and moving images to the control unit  110 . The control unit  110  converts the still and moving images into still and moving image data, respectively. 
     The image pickup unit  194  includes a lens for acquiring an optical image of a subject, an actuator for driving the lens, a microprocessor for controlling the actuator, an image sensor for converting the optical image acquired via the lens into an image signal, and an analog-to-digital (A/D) converter for converting the image signal acquired by the image sensor into a digital signal. The image-capturing direction of the image pickup unit  194  is not limited to the screen direction and may be, for example, the viewer side opposite to the screen. 
     The display control unit  195  performs control to display on the display unit  196  provided on the liquid crystal projector  100  an operation screen and images of switch icons for operating the liquid crystal projector  100 . The display control unit  195  includes a microprocessor for display control. 
     This microprocessor is not necessarily a dedicated microprocessor for the display control unit  195  and may be, for example, a part of the processing function of the CPU and memory or microprocessor operating the control unit  110 . The display unit  196  displays an operation screen and switch icons for operating the liquid crystal projector  100 . The display unit  196  may be any type of display as long as it can display images. 
     The display unit  196  may be, for example, a liquid crystal display, a cathode ray tube (CRT) display, an organic electroluminescence (EL) display, or an LED display. An LED corresponding to each button may light up to enable a user to recognize a specific button. 
     The image processing unit  140 , the liquid crystal control unit  150 , the light source control unit  160 , the optical system control unit  170 , the recording and reproduction unit  191 , and the display control unit  195  according to the present exemplary embodiment may be a single or a plurality of microprocessors capable of performing similar processing to the processing of each of these blocks. Alternatively, the CPU and memory or microprocessor of the control unit  110  may perform similar processing. 
     &lt;Basic Operations&gt; 
     Basic operations performed by the liquid crystal projector  100  according to the present exemplary embodiment will be described below with reference to  FIGS. 1 and 2 . 
       FIG. 2  is a flowchart illustrating control of basic operations of the liquid crystal projector  100  according to the present exemplary embodiment. Basically, the operations illustrated in  FIG. 2  are implemented when the control unit  110  controls each of the above-described function blocks. The flowchart illustrated in  FIG. 2  starts at a time when the user instructs to turn ON the power of the liquid crystal projector  100  via the operation unit  111  or a remote control (not illustrated). 
     When the user instructs to turn ON the power of the liquid crystal projector  100  via the operation unit  111  or a remote control (not illustrated), the control unit  110  supplies the power from a power supply unit (not illustrated) to each unit of the liquid crystal projector  100  via a power supply circuit (not illustrated). 
     In step S 210 , the control unit  110  determines a display mode selected by the user by operating the operation unit  111  or the remote controller. The “input image display mode” is one of display modes of the liquid crystal projector  100  according to the present exemplary embodiment, in which an image input from the image input unit  130  is displayed. The “file reproduction and display mode” is one of display modes of the liquid crystal projector  100  according to the present exemplary embodiment, in which still and video images of the still and moving image data read from the recording medium  192  by the recording and reproduction unit  191  are displayed. 
     The “file reception and display mode” is one of display modes of the liquid crystal projector  100  according to the present exemplary embodiment, in which still and video images of the still and moving image data received via the communication unit  193  are displayed. Although the present exemplary embodiment will be described below based on a case where a display mode is selected by the user, the display mode when the power is turned ON may be the display mode when the liquid crystal projector  100  was last turned off, and any one of the above-described display modes may be a default display mode. In this case, the processing in step S 210  can be omitted. 
     The present exemplary embodiment will be described below on the premise that the user selects the “input image display mode” in step S 210 . When the user selects the “input image display mode” in step S 210 , then in step S 220 , the control unit  110  determines whether an image is input from the image input unit  130 . When an image is not input (NO in step S 220 ), the processing waits until image input is detected. When an image is input (YES in step S 220 ), then in step S 230 , the control unit  110  performs projection processing. 
     The control unit  110  transmits to the image processing unit  140  the image input from the image input unit  130  as projection processing, instructs the image processing unit  140  to change the number of pixels of a video image and the frame rate, and perform shape change (for example, trapezoidal correction), and transmits a processed image for one screen to the liquid crystal control unit  150 . 
     Then, the control unit  110  instructs the liquid crystal control unit  150  to control the transmissivity of the liquid crystal panels  151 R,  151 G, and  151 B to provide the transmissivity according to the gradation level of each of the red (R), green (G), and blue (B) color components of the received image for one screen. Then, the control unit  110  instructs the light source control unit  160  to control the output of light from the light source  161 . The color separation unit  162  separates the light output from the light source  161  into red (R), green (G), and blue (B) color components, and supplies respective light components to the liquid crystal panels  151 R,  151 G, and  151 B. 
     With the light components supplied to the liquid crystal panels  151 R,  151 G, and  151 B, the transmitting light quantity is limited for each pixel of each liquid crystal panel. The red (R), green (G), and blue (B) color components which have transmitted through the liquid crystal panels  151 R,  151 G, and  151 B, respectively, are supplied to the color combination unit  163  for recombination. Then, the light combined by the color combination unit  163  is projected on a screen (not illustrated) via the projection optical system  171 . 
     During image projection, this projection processing is sequentially performed for each image for one frame. When the user input an instruction for operating the projection optical system  171  from the operation unit  111 , the control unit  110  instructs the optical system control unit  170  to control the actuator of the projection optical system  171  so as to change the focus of the projected image and the magnification of the projection optical system  171 . 
     During execution of display processing, in step S 240 , the control unit  110  determines whether the user inputs an instruction for changing the display mode from the operation unit  111 . When the user inputs the instruction for changing the display mode from the operation unit  111  (YES in step S 240 ), the processing returns to step S 210  to determine the display mode. 
     In this case, the control unit  110  transmits to the image processing unit  140  a menu screen for display mode selection as an on-screen display (OSD) image, and controls the image processing unit  140  to superimpose the OSD screen on the projected image. The user selects a display mode while monitoring the projected OSD screen. 
     Otherwise, when the user does not input the instruction for changing the display mode from the operation unit  111  during execution of display processing (NO in step S 240 ), then in step S 250 , the control unit  110  determines whether the user inputs an instruction for terminating projection from the operation unit  111 . 
     When the user inputs the instruction for terminating projection from the operation unit  111  (YES in step S 250 ), the control unit  110  stops the power supply to each block of the liquid crystal projector  100  to terminate image projection. On the other hand, when the user does not input the instruction for terminating projection from the operation unit  111  (NO in step S 250 ), the control unit  110  returns the processing to step S 220 . Subsequently, the control unit  110  repeats the processing in steps S 220  to S 250  until the user inputs the instruction for terminating projection from the operation unit  111 . 
     As described above, the liquid crystal projector  100  according to the present exemplary embodiment projects an image on the screen. 
     In the “file reproduction and display mode”, the control unit  110  instructs the recording and reproduction unit  191  to read from the recording medium  192  a file list of still and moving image data and thumbnail data for each file. 
     Then, the control unit  110  generates texts and images based on the read file list and images based on the thumbnail data for each file, and transmits the images to the image processing unit  140 . Then, the control unit  110  controls the image processing unit  140 , the liquid crystal control unit  150 , and the light source control unit  160 , similar to the regular projection processing performed in step S 230 . 
     Then, on the projected image, the user inputs an instruction for selecting text and image corresponding to still and moving image data recorded in the recording medium  192  via the operation unit  111 . Then, the control unit  110  controls the recording and reproduction unit  191  to read the selected still and moving image data from the recording medium  192 . The recording and reproduction unit  191  reproduces still and video images of the read still and moving image data. 
     Then, the control unit  110  sequentially transmits, for example, video images of the reproduced moving image data to the image processing unit  140 , and controls the image processing unit  140 , the liquid crystal control unit  150 , and the light source control unit  160 , similar to the regular projection processing performed in step S 230 . When the still image data is reproduced, the control unit  110  transmits the reproduced images to the image processing unit  140 , and controls the image processing unit  140 , the liquid crystal control unit  150 , and the light source control unit  160 , similar to the regular projection processing performed in step S 230 . 
     In the “file reception and display mode”, the control unit  110  reproduces still and video images of the still and moving image data received from the communication unit  193 . Then, the control unit  110  sequentially transmits, for example, video images of the reproduced moving image data to the image processing unit  140 , and controls the image processing unit  140 , the liquid crystal control unit  150 , and the light source control unit  160 , similar to the regular projection processing performed in step S 230 . 
     When the still image data is reproduced, the control unit  110  transmits the reproduced images to the image processing unit  140 , and controls the image processing unit  140 , the liquid crystal control unit  150 , and the light source control unit  160 , similar to the regular projection processing performed in step S 230 . 
     &lt;Configurations of Liquid Crystal Control Unit and Liquid Crystal Devices&gt; 
     Operations of the liquid crystal control unit  150  of the liquid crystal projector  100  according to the present exemplary embodiment, and operations of the liquid crystal devices  151 R,  151 G, and  151 B will be described below with reference to  FIGS. 3, 4, and 5 . Since operations of the liquid crystal devices  151 G and  151 B are controlled in a similar way to the liquid crystal device  151 R, control operations of only the liquid crystal device  151 R will be described below. 
     To simplify the descriptions, the present exemplary embodiment will be described below on the premise that the transmissivity of each pixel of each of the liquid crystal device  151 R can be the transmissivity corresponding to 8 gradations (“gradation  0 ” for the lightest gradation and “gradation  7 ” for the deepest gradation). In the present exemplary embodiment, the image processing unit  140  transmits a vertical synchronization signal, a horizontal synchronization signal, a data signal of pixel value (indicating gradation) for each color component, and a data clock signal. 
       FIG. 3  is a block diagram illustrating a configuration of the liquid crystal control unit  150  according to the present exemplary embodiment.  FIG. 4  is a diagram illustrating a configuration of the liquid crystal device  151 R according to the present exemplary embodiment.  FIG. 5  is a diagram illustrating signals generated by the gradation signal generation unit  350  and the image analysis unit  370  according to the present exemplary embodiment. 
     In the following descriptions, each pixel of the liquid crystal on the liquid crystal device  151 R is referred to as “a liquid crystal pixel”, and each pixel of an input image signal (data signal) is referred to as “an image pixel.” 
     A regular configuration of the liquid crystal device  151 R of the liquid crystal projector  100  will be described below. The liquid crystal device  151 R generally includes a first polarizing filter, a first glass substrate, a first transparent electrode, a first oriented film, a liquid crystal layer, a second oriented film, a second transparent electrode, a second glass substrate, and a second polarizing filter. Light with polarization direction polarized by the first polarizing filter is supplied to the liquid crystal layer. Only light with a polarization direction that can pass through the second polarizing filter, out of light with polarization directions adjusted by the liquid crystal layer, passes through the second polarizing filter. Thus, the transmissivity (transmitted light quantity) is controlled. 
     The liquid crystal layer is composed of a liquid crystal sandwiched between the first and second oriented films, with which molecules are arranged in a fixed direction. The liquid crystal layer is sandwiched by the first and second transparent electrodes. Applying different voltages to the first and second transparent electrodes produces a potential difference at a position corresponding to each pixel of the liquid crystal layer, thus applying a voltage to the liquid crystal. 
     Since the liquid crystal has characteristics that the molecular arrangement changes according to the applied voltage, the degree of light polarization of the liquid crystal corresponding to each pixel changes accordingly. Thus, changing the polarization direction of light with polarization direction polarized by the first polarizing filter enables adjusting the transmissivity (transmitted light quantity) of the second polarizing filter. 
     Referring to  FIG. 3 , the liquid crystal control unit  150  includes a reference voltage generation unit  310 , a pixel voltage generation unit  320 , a vertical drive unit  330 , a horizontal drive unit  340 , a gradation signal generation unit  350 , a switch group  360 , and an image analysis unit  370 . The horizontal drive unit  340  includes a shift register  341 , a data memory  342 , a latch unit  343 , and a comparison unit  344 . 
     As illustrated in  FIGS. 3 and 4 , the liquid crystal device  151 R includes M gate row lines Y 1  to Ym and N data column lines X 1  to Xn corresponding to pixels of M rows×N columns arranged on a substrate. A thin film transistor T (m, n), which is a switching device corresponding to each pixel, is arranged at each point of intersection of these gate row and data column lines. A signal from a gate row line Y is supplied to the gate electrode of the thin film transistor T (m, n), and a signal from a data column line Xis supplied to the source electrode thereof. 
     A pixel electrode P (m, n) is connected to the drain electrode of the thin film transistor T (m, n). A signal from a source column line X is supplied to the pixel electrode P (m, n) only while a signal is being input from the gate row line Y. 
     Meanwhile, the other side of the pixel electrode P is a common electrode to which a Vcom voltage  311  (constant voltage) generated by the reference voltage generation unit  310  is supplied. With the thus-configured liquid crystal device  151 R according to the present exemplary embodiment, the voltage applied to the liquid crystal between the common electrode and the pixel electrode P is adjusted by the difference between the voltage (Vcom) of the common electrode and the voltage of the pixel electrode P (a voltage applied from the data column lines X). 
     Referring to  FIG. 3 , the reference voltage generation unit  310  generates the Vcom voltage  311  (constant voltage) to be supplied to the common electrode of the liquid crystal device  151 R. 
     The pixel voltage generation unit  320  generates a pixel voltage to be applied to the data column lines X. A pixel voltage generated by the pixel voltage generation unit  320  is supplied to the data lines X 1  to Xn of the liquid crystal device  151 R via the switch group  360 . 
     In the present exemplary embodiment, a pixel voltage is generated based on a gradation signal  352  (a ramp signal in the present exemplary embodiment) corresponding to each gradation (gradations  0  to  7 ) generated by the gradation signal generation unit  350 . 
     When the gradation signal  352  has a value corresponding to gradation  3 , for example, the pixel voltage generation unit  320  generates a voltage required to provide the transmissivity of each liquid crystal pixel of the liquid crystal device  151 R corresponding to gradation  3 . 
     Specifically, the pixel voltage generation unit  320  generates a voltage required to provide the transmissivity of each liquid crystal pixel of the liquid crystal device  151 R corresponding to each gradation, according to the input value (indicating a gradation) of the gradation signal  352 . 
     With this configuration, the pixel voltage supplied from the data column lines X is supplied to the pixel electrode for each liquid crystal pixel of the liquid crystal device  151 R according to the present exemplary embodiment, enabling changing the transmissivity of the liquid crystal by the potential difference between the voltage of the pixel electrode and the voltage Vcom of the common electrode. 
     Turning off each switch corresponding to pixel electrodes by using the switch group  360  at a timing when the voltage corresponding to the gradation of each pixel is supplied from the pixel voltage generation unit  320  enables adjusting the transmissivity of the liquid crystal to the transmissivity according to the pixel value of each pixel of an input image. The horizontal drive unit  340  controls the ON/OFF state of each switch of the switch group  360 . 
     When a low voltage is applied to the liquid crystal, the transmissivity of the liquid crystal device  151 R according to the present exemplary embodiment becomes low. When a high voltage is applied to the liquid crystal, the transmissivity thereof becomes high. However, the relation between the applied voltage and the transmissivity may be reversed. 
     As long as the pixel voltage generation unit  320  generates a voltage required to provide the transmissivity of each liquid crystal pixel of the liquid crystal device  151 R corresponding to each gradation, according to the input value (indicating a gradation) of the gradation signal  352 , the voltage may change in any way. 
     In the present exemplary embodiment, when a monotonically increasing ramp signal is input from the gradation signal generation unit  350  as the gradation signal  352 , the pixel voltage generation unit  320  generates a monotonically increasing pixel voltage. 
     However, a monotonically decreasing pixel voltage may be generated based on the monotonically increasing ramp signal as the gradation signal  352 . This is based on the characteristics that, regardless of the polarity of a voltage from the pixel electrode P to the common electrode (positive or negative voltage), applying the voltage having the same absolute value to the liquid crystal enables similarly controlling the orientation of the liquid crystal. Specifically, even if a ramp signal monotonically increasing for each period of the horizontal synchronization signal is input, the pixel voltage generation unit  320  may generate a voltage which increases centering on the Vcom voltage  311  or a voltage which decreases centering thereon. 
     Referring to  FIG. 3 , the vertical drive unit  330  acquires a horizontal synchronization signal of an image transmitted from the image processing unit  140 , and sequentially supplies a gate signal to the gate row lines Y 1  to Ym of the liquid crystal device  151 R at a timing according to the horizontal synchronization signal. The vertical drive unit  330  may acquire a vertical synchronization signal. For example, if the vertical drive unit  330  detects a change in the horizontal synchronization signal twice since it has detected a change in the vertical synchronization signal, the vertical drive unit  330  supplies a signal to the gate row line Y 1  and does not supply the relevant signal to other gate row lines Y 2  to Ym. 
     Then, when the vertical drive unit  330  detects a change in the horizontal synchronization signal, it supplies a signal to the gate row line Y 2  and does not supply the relevant signal to other gate row lines Y 1  and Y 3  to Ym. Thus, each time the vertical drive unit  330  according to the present exemplary embodiment detects a change in the horizontal synchronization signal, it sequentially changes the destination of signal supply (gate row lines Y 1  to Ym). 
     When the vertical drive unit  330  detects a change in the vertical synchronization signal, it supplies a signal to the gate row line Y 1  if it detects a change in the horizontal synchronization signal twice after detecting a change in the vertical synchronization signal even if signal supply up to the gate row line Ym is not completed. 
     The vertical drive unit  330  supplies a signal to the gate row line Y 1  when it detects the horizontal synchronization signal for the second time so that the vertical drive unit  330  waits at least until a data signal for one row has been input to the level control unit  340 . 
     When the vertical drive unit  330  supplies a signal to the gate row line Y 1 , in the liquid crystal device  151 R according to the present exemplary embodiment, thin film transistors T( 1 ,  1 ) to T( 1 , n) conduct to enable applying to pixel electrodes P ( 1 ,  1 ) to P ( 1 , n) the voltage supplied by the data column lines X 1  to Xn, respectively. In this case, since thin film transistors other than the thin film transistors T ( 1 ,  1 ) to T ( 1 , n) do not conduct, the voltage supplied by the data column lines X 1  to Xn is not applied to any pixel electrodes other than the pixel electrodes P ( 1 ,  1 ) to P ( 1 , n). 
     Likewise, when the vertical drive unit  330  supplies a signal to the gate row line Y 1 , thin film transistors T ( 2 ,  1 ) to T ( 2 , n) conduct to enable applying to pixel electrodes P ( 2 ,  1 ) to P ( 2 , n) the voltage supplied by the data column lines X 1  to Xn, respectively. Subsequently, this operation is repeated for up to the gate row line Ym. 
     The horizontal drive unit  340  acquires the horizontal synchronization signal, the data signal, and the data clock transmitted from the image processing unit  140 , and further acquires the gradation signal  352  (a ramp signal in the present exemplary embodiment) transmitted from the gradation signal generation unit  350  and a comparison signal transmitted from the image analysis unit  370 . Then, based on the acquired signals, the horizontal drive unit  340  controls the ON/OFF state of the switch group  360 . 
     As described above, in the present exemplary embodiment, the pixel voltage generated by the pixel voltage generation unit  320  is supplied to the data column lines X 1  to Xn of the liquid crystal device  151 R via the switch group  360 , based on the gradation signal  352  from the gradation signal generation unit  350 . 
     The horizontal drive unit  340  maintains the ON state of each switch of the switch group  360  until the gradation signal  352  transmitted from the gradation signal generation unit  350  coincides with the pixel value of each image pixel corresponding to the data column lines X 1  to Xn. When the gradation signal  352  coincides with the pixel value of each pixel corresponding to the data column lines X 1  to Xn, the horizontal drive unit  340  turns off each switch of the switch group  360  for the matched data column lines. 
     Thus, the horizontal drive unit  340  controls the ON/OFF state of each switch of the switch group  360  to control the supply state of the pixel voltage to the pixel electrode for each liquid crystal pixel of the liquid crystal device  151 R. Operations of the horizontal drive unit  340  will be described below. 
     The gradation signal generation unit  350  generates the gradation signal  352  and a gradation clock signal  351  (a synchronization signal of the gradation signal  352 ) in synchronization with the horizontal synchronization signal of the image signal. Then, the gradation signal generation unit  350  supplies the gradation signal  352  to the pixel voltage generation unit  320  and the horizontal drive circuit  340 , and supplies the gradation clock signal  351  to the image analysis unit  370 . 
     As illustrated in  FIG. 5 , the gradation signal  352 , which is a monotonically increasing ramp signal, is a digital signal indicating values corresponding to respective gradations (gradations  0  to  7 ) in a period of the horizontal synchronization signal. The gradation signal  352  increases the signal value by one gradation for each four clocks of the gradation clock signal  351 . 
     The switch group  360  selects whether the pixel voltage from the pixel voltage generation unit  320  is to be supplied to the data column lines X 1  to Xn of the liquid crystal device  151 R. For example, the switch group  360  includes a plurality of physical switches, a plurality of switching devices, or a plurality of logical switches. 
     When a change in the horizontal synchronization signal is detected, the switch group  360  turns on each switch. On the other hand, when a signal for turning off each switch is input from the horizontal drive unit  340 , the switch group  360  turns off each switch specified to be turned off. The image analysis unit  370  acquires the horizontal synchronization signal and the data signal transmitted from the image processing unit  140  and the gradation clock signal  351  transmitted from the gradation signal generation unit  350 , and transmits a comparison signal  371  to the horizontal drive unit  340 . 
     In the present exemplary embodiment, the comparison signal  371  is used to determine a timing when the comparison unit  344  of the horizontal drive unit  340  compares the data of the latch unit  343  with the value of the gradation signal  352 . Then, the image analysis unit  370  transmits to the horizontal drive unit  340  the comparison signal  371  at a timing based on the value of the data signal of the image signal transmitted from the image processing unit  140 . 
     Operations of the image analysis unit  370  will be described below. The comparison unit  344  of the horizontal drive unit  340  may not determine based on the comparison signal  371  a timing of comparing the data of the latch unit  343  with the value of the gradation signal  352 . Instead, the comparison unit  344  may change an output timing of the signal for turning off each switch of the switch group  360 . 
     &lt;Liquid Crystal Device Control Operations by Liquid Crystal Control Unit&gt; 
     Operations performed by the liquid crystal control unit  150  according to the present exemplary embodiment to control the liquid crystal device  151 R will be described below with reference to  FIGS. 3, 5, 6A, 6B, 6C, and 6D . 
       FIGS. 6A to 6D  illustrate a result of analysis by the image analysis unit  370 , and tables for determining a delay amount of the comparison signal  371 .  FIG. 6A  illustrates for each row a result of analysis by the image analysis unit  370 .  FIG. 6B  illustrates the number of image pixels for each gradation, and a delay amount of transmission timing of the comparison signal  371  corresponding to a gradation following the relevant gradation. The unit of delay amount is the number of clocks of the gradation clock signal  351 . 
       FIG. 6C  illustrates for each row a delay amount of transmission timing of the comparison signal  371  corresponding to a gradation following each gradation based on the analysis result (see  FIG. 6A ) and the delay amount (see  FIG. 6B ).  FIG. 6D  is a table, converted from the table in  FIG. 6C , illustrating for each row a delay amount of transmission timing of the comparison signal  371  corresponding to each gradation. 
     As illustrated in  FIGS. 6C and 6D , the transmission timing of the comparison signal  371  is delayed from the regular timing by 1 to 3 clocks of the gradation clock signal  351 . As a result, the output timing of the signal for turning on/off each switch of the switch group  360  can be delayed from the regular timing. 
     When an image signal is input from the image processing unit  140 , the vertical synchronization signal, the horizontal synchronization signal, the data signal, and the data clock signal are provided to each block of the liquid crystal control unit  150 , as described referring to  FIG. 3 . 
     In the horizontal drive unit  340 , the data clock signal is supplied to the shift register  341 , the data signal is supplied to the data memory  342 , and the horizontal synchronization signal is supplied to the latch unit  343 . The gradation signal  352  output from the gradation signal generation unit  350  and the comparison signal  371  output from the image analysis unit  370  are supplied to the comparison unit  344 . 
     In the horizontal drive unit  340 , based on the data clock signal transmitted from the image processing unit  140 , the shift register  341  controls operations for recording in the data memory  342  the data signal transmitted from the image processing unit  140 . The data memory  342  stores data (pixel value) corresponding to each image pixel for one row of the data signal transmitted from the image processing unit  140 . 
     The latch unit  343  reads and latches data (pixel value of each image pixel) of the data memory  342  at a timing of detection of a change in the horizontal synchronization signal transmitted from the image processing unit  140 . More specifically, after the data signal for the  1 st row is stored in the data memory  342 , the latch unit  343  reads and latches the pixel value of each image pixel of the  1 st row stored in the data memory  342 , at a timing when the horizontal synchronization signal for the  2 nd row is input. 
     Then, the comparison unit  344  compares the gradation signal  352  (ramp signal) indicating each gradation supplied from the gradation signal generation unit  350  with the pixel value of each image pixel latched by the latch unit  343  at a timing when the comparison signal  371  transmitted from the image analysis unit  370  is input. When the pixel value of each image pixel of the latch unit  343  corresponding to each image pixel coincides with the gradation signal  352 , the comparison unit  344  transmits control signals SW 1  to SWn for turning off each switch of the switch group  360  corresponding to the matched image pixels. 
     The comparison unit  344  may compare in advance the gradation signal  352  (ramp signal) indicating each gradation supplied from the gradation signal generation unit  350  with the data (pixel value) of the image pixel latched by the latch unit  343 . 
     In this case, when the value of the latch unit  343  corresponding to each image pixel coincides with the gradation signal  352  at a timing when the comparison signal  371  transmitted from the image analysis unit  370  is input, the comparison unit  344  transmits the control signals SW 1  to SWn for turning off each switch of the switch group  360  corresponding to the matched pixels. 
     As described above, according to the input of the gradation signal  352 , the switch group  360  is supplied with a voltage for providing the transmissivity of each liquid crystal pixel corresponding to the value indicated by the input gradation signal  352 . Then, as described above, when the gradation indicated by the gradation signal  352  coincides with the pixel value of each image pixel, the horizontal drive unit  340  turns off each switch of the switch group  360 . Therefore, a pixel voltage according to the pixel value of each image pixel is supplied from the data column lines X via the switch group  360 . 
     The above-described operations allows inputting a signal to the gate row line Y 1  from the vertical drive unit  330 , and supplying a pixel voltage according to the pixel value of each image pixel from the data column lines X 1  to Xn respectively to the pixel electrodes P ( 1 ,  1 ) to P ( 1 , n) for each liquid crystal pixel of the 1st row of the liquid crystal device  151 R. 
     Thus, the transmissivity of each liquid crystal pixel of the 1st row is adjusted to the transmissivity according to the pixel value of each image pixel. The liquid crystal control unit  150  according to the present exemplary embodiment sequentially repeats the above-described operations from the 1st to M-th rows to enable sequentially controlling the transmissivity of each liquid crystal pixel of each row of the liquid crystal device  151 R. 
     Then, the liquid crystal control unit  150  enables transmission of the light of red (R) from the color separation unit  162  through the liquid crystal device  151 R, thus supplying to the color combination unit  163  the red light corresponding to the red gradation of the input image. Then, the color combination unit  163  combines the green (G) and blue (B) light with the red (R) light, and the projection optical system  171  projects the combined light. Thus, an image corresponding to the input image can be projected on the screen. 
     Operations of the image analysis unit  370  according to the present exemplary embodiment will be described below. To simplify descriptions, a case where image data including 60 pixels vertically and 120 pixels horizontally is displayed is exemplified. In this case, one horizontal row includes 120 image pixels. 
     In the present exemplary embodiment, when there are not so many pixels displaying each gradation out of image pixels in one row (regular case), the comparison signal  371  is transmitted once for each 4 clocks of the gradation clock  352 , like the comparison signal for the A-th row illustrated in  FIG. 5 . Further, like the comparison signal for the A-th row illustrated in  FIG. 5 , the comparison signal  371  is transmitted within one clock of the gradation clock  352  after the value of the gradation signal  352  has increased. Although, in the present exemplary embodiment, the comparison signal  371  is regularly transmitted in this way, the present invention is not limited thereto. 
     As described above, based on input image data for one row, the image analysis unit  370  transmits to the comparison unit  344  of the horizontal drive unit  340  the pixel value latched by the latch unit  343  and the comparison signal  371  for determining a timing of comparison with the gradation signal  352 . 
     The comparison unit  344  may not perform the comparison at the timing of reception of the comparison signal  371 . Instead, the comparison unit  344  may transmit a signal for turning off each switch corresponding to each data column line X of the switch group  360  according to the timing of reception of the comparison signal  371 . 
     In this case, the comparison signal  371  is not a signal for determining a timing of comparing the pixel value latched by the latch unit  343  with the gradation signal  352  but a signal for determining an output timing of a signal for controlling each switch of the switch group  360 . 
     Therefore, the image analysis unit  370  performs image analysis of image data for one row input as a data signal. Specifically, the image analysis unit  370  analyzes input image data for one row and counts the number of pixels for each gradation. 
     Although, in the present exemplary embodiment, the image analysis unit  370  performs image analysis each time image data for one row is input, it may perform image analysis for each row of image data for one frame as long as the liquid crystal control unit  150  includes a memory for buffering image data for one frame. 
       FIG. 6A  illustrates a result of image analysis. According to the analysis result, the 1st row includes 15 pixels for each of gradations  0  to  7 . The A-th row includes 15 pixels for gradation  0 , 25 pixel for gradation  1 , 15 pixels for gradation  2 , 5 pixels for gradation  3 , 15 pixels for gradation  4 , 5 pixels for gradation  5 , 20 pixels for gradation  6 , and 20 pixels for gradation  7 . 
     The B-th row includes zero pixels for gradation  0 , 0 pixels for gradation  1 , 30 pixels for gradation  2 , 60 pixels for gradation  3 , 30 pixels for gradation  4 , 0 pixels for gradation  5 , 0 pixels for gradation  6 , and 0 pixels for gradation  7 . 
     To change the timing of turning off the pixel voltage supply to each liquid crystal pixel corresponding to the image pixel of a gradation next to the gradation with many pixel counts, the image analysis unit  370  changes the timing of transmitting the comparison signal  371  to the comparison unit  344 . As described above, the horizontal drive unit  340  compares the gradation signal  352  with the pixel value of the latch unit  343  at a timing of reception of the comparison signal  371 . 
     Therefore, changing the transmission timing of the comparison signal  371  enables changing the timing of executing the comparison by the comparison unit  344 , thus changing the timing of turning off each switch of the switch group  360 . The transmission timing of the comparison signal  371  is determined by the image analysis unit  370  based on the table illustrated in  FIG. 6B . 
     More specifically, with the image signal for the 1st row illustrated in  FIG. 6A , the number of pixels is  29  or less for all gradations according to the table illustrated in  FIG. 6B . Therefore, the transmission timing of the comparison signal  371  corresponding to a gradation next to each gradation is not delayed. Therefore, with the image signal for the 1st row, the delay amount of the comparison signal  371  corresponding to a gradation next to the relevant gradation is 0, as illustrated in  FIG. 6C . 
     With the image signal for the 1st row illustrated in  FIG. 6A , the number of pixels is  29  or less for all gradations according to the table illustrated in  FIG. 6B . Therefore, the transmission timing of the comparison signal  371  corresponding to a gradation next to each gradation is not delayed. Therefore, with image signal of the A-th row, the delay amount of the comparison signal  371  corresponding to a gradation next to the relevant gradation is 0, as illustrated in  FIG. 6C . 
     With the image signal of the B-th row illustrated in  FIG. 6A , the transmission timing of the comparison signal  371  corresponding a gradation next to each of gradations  2 ,  3 , and  4  according to the table illustrated in  FIG. 6B . Specifically, since the number of pixels for gradation  2  is 30, the delay amount of the comparison signal  371  corresponding to the next gradation (gradation  3 ) is set as one clock of the gradation clock signal  351  according to the table illustrated in  FIG. 6B . 
     Since the number of pixels for gradation  3  is 60, the delay amount of the comparison signal  371  corresponding to the next gradation (gradation  4 ) is set as two clocks of the gradation clock signal  351  according to the table illustrated in  FIG. 6B . Since the number of pixels for gradation  4  is 30, the delay amount of the comparison signal  371  corresponding to the next gradation (gradation  5 ) is set as one clock of the gradation clock signal  351  according to the table illustrated in  FIG. 6B . 
     Therefore, with the image signal of the B-th row, the delay amount of the comparison signal corresponding to a gradation next to each of the gradations  2 ,  3 , and  4  is 1 clock, 2 clocks, and 1 clock, respectively, as illustrated in  FIG. 6C . Referring to the table in  FIG. 6D , which is converted from the table in  FIG. 6C , the delay amount of the comparison signal  371  corresponding to gradation  3  is 1 clock, the delay amount of the comparison signal  371  corresponding to gradation  4  is 2 clocks, and the delay amount of the comparison signal  371  corresponding to gradation  5  is 1 clock. 
     According to the present exemplary embodiment, for example, at a moment  60  switches of the switch group  360  are turned off at the same time to display gradation  3  in the B-th row, a turbulence arises in the pixel voltage supplied from the pixel voltage generation unit  320 . Accordingly, the pixel voltage corresponding to a gradation (gradation  4 ) next to gradation  3  is also affected by turbulence. 
     This turbulence decreases with time. In the present exemplary embodiment, a switch of the pixel corresponding to the next gradation (gradation  4 ) is turned off at a timing later than the regular timing so that the switch group  360  may not be turned off to display the next gradation (gradation  4 ) in a time period when the turbulent pixel voltage becomes turbulent. Thus, it is possible to prolong the time period between the timing of changing the supply state of the pixel voltage corresponding to gradation  3  and the timing of changing the supply state of the pixel voltage corresponding to gradation  4 . 
     Therefore, since the switch of each pixel of the next gradation (gradation  4 ) is turned off after turbulence in the pixel voltage has been reduced to some extent, the degradation of pixels for gradation  4  can be reduced. 
     According to the present exemplary embodiment, the comparison signal  371  is transmitted once for each 4 clocks of the gradation clock  352  like the comparison signal for the A-th row illustrated in  FIG. 5 . Further, when there are not so many pixels displaying each gradation, the comparison signal  371  is transmitted within 1 clock of the gradation clock  352  after the value of the gradation signal  352  has increased, like the comparison signal for the A-th row illustrated in  FIG. 5 . 
     Although, in the present exemplary embodiment, the comparison signal  371  is normally transmitted in this way, the present invention is not limited thereto. For example, unlike the example illustrated in  FIG. 5 , the comparison signal  371  may be transmitted at the fourth clock of the gradation clock  352  after the value of the gradation signal  352  has increased. In this case, the transmission timing of the comparison signal  371  corresponding to a gradation itself involving many pixels to be displayed is brought forward, for example, by 1 to 3 clocks. 
     Specifically, the present invention is not limited to delaying the transmission timing of the comparison signal  371 . The comparison signal  371  may be transmitted at the third clock of the gradation clock  352  after the value of the gradation signal  352  has increased. In this case, the transmission timing of the comparison signal  371  corresponding to a gradation itself involving many pixels to be displayed may be brought forward, for example, by 1 to 2 clocks, or the transmission timing of the comparison signal  371  corresponding to a gradation next to the gradation involving many pixels to be displayed may be delayed by 1 clock. 
     Specifically, the LCD apparatus according to the present exemplary embodiment may preferably control the transmission timing of the comparison signal  371  corresponding to the gradation itself involving many pixels to be displayed and/or the transmission timing of the comparison signal  371  corresponding to a gradation following the gradation involving many pixels to be displayed. 
     As described above, the LCD apparatus according to the present exemplary embodiment compares the gradation signal  352  with the pixel value in one row of the image to be displayed to control the supply state of the pixel voltage based on the gradation signal  352  supplied to each liquid crystal pixel. 
     Then, in the gradation signal  352 , when displaying the second gradation next to the first gradation, it is possible to change the time period between the timing of changing the supply state of the pixel voltage corresponding to the first gradation and the timing of changing the supply state of the pixel voltage corresponding to the second gradation. 
     When there are many pixels for the first gradation of the image data in one row, this time period is controlled to be longer than that when there are fewer pixels for the first gradation. Therefore, the LCD apparatus according to the present exemplary embodiment controls the timing of changing the supply state of the pixel voltage corresponding to the first gradation and/or the timing of changing the supply state of the pixel voltage corresponding to the second gradation. 
     As described above, according to the LCD apparatus of the present exemplary embodiment, it is possible to prolong the release timing of each switch of the switch group  360  corresponding to the first gradation involving many pixels to be displayed and the release timing of each switch of the switch group  360  corresponding to the next gradation (second gradation) so that it becomes longer than that when there are a small number of pixels for the first gradation. 
     The above-described configuration enables turning off each switch of the switch group  360  corresponding to the second gradation next to the first gradation after the influence of turbulence in the pixel voltage (generated when each switch of the switch group  360  corresponding to the first gradation involving many pixels to be displayed) has been reduced, thus preventing image degradation. 
     Although the LCD apparatus according to the present exemplary embodiment controls the timing of changing the supply state of the pixel voltage corresponding to the first gradation and/or the timing of changing the supply state of the pixel voltage corresponding to the second gradation without changing the waveform of the gradation signal  352 , the configuration is not limited thereto. 
     For example, when there are many pixels for a specific gradation, the image analysis unit  370  may control the gradation signal generation unit  350  to prolong the time period during which a signal indicating the first gradation and/or a signal indicating the second gradation are output. An example of a waveform of the gradation signal  352  and an example of an output waveform of the comparison signal  371  are illustrated in  FIG. 7 . 
     Referring to these examples, when displaying the image of the B-th row according to the present exemplary embodiment, the image analysis unit  370  controls the gradation signal gene ration unit  350  to generate the gradation signal  352  not containing a signal of a gradation not to be displayed to prolong the output timing of the comparison signal  371  so that it becomes longer than that when there are many pixels indicating each gradation. The output timing of the gradation signal  352  may be in any timing as long as it is within one period of the horizontal synchronization signal. 
     These examples also enable turning off each switch of the switch group  360  corresponding to the second gradation (a gradation next to the first gradation) after waiting sufficient time period until the influence of turbulence in the pixel voltage (generated when each switch of the switch group  360  corresponding to the first gradation involving many pixels is turned off) has been reduced. 
     In the present exemplary embodiment, a plurality of liquid crystal devices is provided to a liquid crystal projector of 3-panel type. However, a liquid crystal projector of single-panel type is provided with one liquid crystal device  151 , and, instead of the color separation unit  162  and the color combination unit  163 , a color wheel for enabling transmission of only light of each color component on a time sharing basis is arranged before or after the liquid crystal device. 
     Then, the liquid crystal control unit  150  controls the liquid crystal device  151  to provide the transmissivity corresponding to each color component at a timing of displaying each color component. Even with such a liquid crystal projector of single-panel type, it is also possible to analyze the number of pixels for each gradation to be displayed by each of the plurality of pixel electrodes to which a voltage is charged at the same time to prolong the time period between the timing of changing a voltage to the pixel electrode for a frequently displayed gradation and the time of charging a voltage to the pixel electrode for a gradation following the frequently displayed gradation. This method enables reducing image quality degradation similar to the liquid crystal projector of 3-panel type according to the present exemplary embodiment. 
     As described above, the LCD apparatus according to the present exemplary embodiment drives the liquid crystal device with a method for controlling, based on the gradation value (pixel value) of each image pixel corresponding to each of liquid crystal pixels, the supply state of the pixel voltage to be supplied to the pixel electrode of each liquid crystal pixel of the liquid crystal device generated based on a signal indicating gradations. When the value of the signal indicating the gradation indicates the value of the first gradation and then the value of the second gradation, when there are more pixels for the first gradation, it is possible to prolong the time period between the timing of changing the supply state of the pixel voltage corresponding to the first gradation and the timing of changing the supply state of the pixel voltage corresponding to the second gradation so that it becomes longer than that when there are a small number of pixels for the first gradation. With the above-described configuration, the present invention enables supplying the pixel voltage corresponding to the second gradation to the liquid crystal device in a time period during which there is a small influence of turbulence in the pixel voltage generated when there are a number of pixels for the first gradation, thus reducing degradation in an image for the second gradation to be displayed. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures, and functions. 
     This application claims priority from Japanese Patent Application No. 2011-224185 filed Oct. 11, 2011, which is hereby incorporated by reference herein in its entirety.