Display apparatus, image display system, and terminal using the same

In a display, the reduction in brightness which is a problem of the mobile picture improving method in a black reset scheme is reduced to a minimum value and a clear image is displayed.The display includes a movement amount extracting unit 42 to extract an amount of movement from a time series image signal and a ratio setting unit 43 to set a first period (image gradation display period) to conduct image display on the display element and a second period (black reset period) to display black image thereon according to the extracted amount of movement.

TECHNICAL FIELD

The present invention relates to a display device, an image display system, and a terminal device using the same, and in particular, to improvement of mobile picture quality in a hold type display device continuously emitting light during a frame period.

RELATED ART

Recently, in a liquid crystal display of active matrix type, its size has been increased, an image displayed thereon has become finer, and colors has become purer to obtain a still picture having fully high picture quality. On the other hand, to display a mobile picture, although picture quality is improved by increasing a response speed of liquid crystal, the picture quality thus obtained is not sufficient when compared with that of a cathode ray tube (CRT).

In an operation to display a mobile picture on a hold type display such as a liquid crystal display, when a human eyes track a moving object on a screen, there is obtained an image of the moving object having a blurred periphery (this phenomenon will be referred to as “edge blur” hereinbelow), and hence the mobile picture quality is deteriorated.

An article (Technical Report of IEICE EID96-4 (1996)) describes in detail the cause of deterioration in the mobile picture quality of such a hold type display. The article describes that the cause of deterioration in the mobile picture quality of a hold type display is a principle event associated with 0-order hold (continuously displaying the same gradation during one frame period) by an active element such as a thin film transistor (TFT).

This means that the deterioration in the mobile picture quality cannot be removed only by increasing the response speed of liquid crystal of the liquid crystal display.

To solve the problem described above, several conventional methods have been proposed to improve picture quality by conducting black reset in a frame using liquid crystal having a high-speed response characteristic.

As black reset methods,

(1) a method of writing in liquid crystal a reset voltage corresponding to a black output, and

(2) a method of conducting black reset by making a backlight blink at timing synchronized with a frame period

can be considered.

For the method (1) above, reference is to be made to, for example, the Japanese Patent Laid-Open No. 2000-122596, and for the method (2) above, reference is to be made to, for example, the Japanese Patent Laid-Open No. 2000-275604.

The display described in the Japanese Patent Laid-Open No. 2000-122596 has a configuration including a display screen including a plurality of lines of pixels, and during a period of time in which an image is written in at least one of the pixel lines, a black color is written in other pixel lines to output a black color to liquid crystal to improve the mobile picture quality.

The liquid crystal display described in the Japanese Patent Laid-Open No. 2000-275604 has a configuration in which a lighting device including a plurality of lamps are divided into subareas, and when liquid crystal display sections corresponding to such subareas make a response, a lighting driver starts an operation after a predetermined period of time to turn lamps on in areas of the lighting device corresponding to the sub-areas and turns the lamps off after a predetermined period of time to reduce edge blurs due to the 0-order hold to improve the mobile picture quality.

PROBLEM TO BE SOLVED BY THE INVENTION

However, although the deterioration in the mobile picture quality due to the 0-order hold is suppressed in the conventional black reset insertion scheme, there arises a new problem that the black reset insertion reduces display luminance and contrast.

In the liquid crystal display described in the Japanese Patent Laid-Open No. 2000-275604, although the reduction in the display luminance is suppressed when displaying a still picture by turning all light sources of the lighting device on to display the still picture, a picture becomes darker in an operation to display a mobile picture when compared with a case in which the black reset is not conducted.

Moreover, the deterioration in the mobile picture quality in the hold type display is proportional to a moving speed of the object; however, there has not been a method to improve the mobile picture quality by fully considering the difference in the moving speed.

It is therefore an primary object of the present invention, which has been devised to remove the problem, to provide a display device in which by fully considering the moving speed of the object and the gradation of output signals with respect to the deterioration in the mobile picture quality of the hold type display, the dynamic range is widened by lowering the luminance, particularly, in a black display operation to thereby improve the mobile picture quality.

It is also an object of the present invention is to provide an image display system in which the dynamic range is widened to improve the mobile picture quality in a hold type display.

Moreover, an object of the present invention is to provide a terminal device using the display device and the image display system.

DISCLOSURE OF THE INVENTION

To achieve the objects, there is provided a display device in accordance with the present invention including a display element for sequentially converting a time series image signal into image display light and for displaying an image, movement amount extracting means for extracting an amount of movement from the time series image signal, and ratio setting means for setting a first period to conduct image display on the display element and a second period to display black (image) thereon according to the amount of movement thus extracted.

Moreover, a display device in accordance with the present invention includes movement amount extracting means for extracting an amount of movement from a time series image signal and ratio setting means for setting a ratio between the first period to conduct image display on a display element and the second period to display black image thereon according to the amount of movement thus extracted.

Moreover, a terminal device in accordance with the present invention uses the display device and the image display system.

Additionally, in the display device and the image processing system in accordance with the present invention, by adding a gradation correcting section for input image signals, an image can be displayed according to a feature of brightness of the image.

The display device in accordance with the present invention includes a backlight subdivided into plurality of areas in a scanning direction of the display device, a control circuit capable of controlling on and off of the areas in an independent fashion, a unit to extract features (an amount of movement and a feature of brightness) of an input video signal, and a unit to control on and off of the backlight according to the features. Thanks to the configuration described above, there is obtained a display device having a wide dynamic range and improved mobile picture quality.

Furthermore, the display device in accordance with the present invention includes an optical shutter subdivided into plurality of areas in a scanning direction of the display device, a control circuit capable of controlling transmission and interruption of light for the areas in an independent fashion, a unit to extract features (an amount of movement and a feature of brightness) of an input video signal, and a unit to control the transmission and interruption of light of the optical shutter according to the features. Thanks to the configuration described above, there is obtained a display device having a wide dynamic range and improved mobile picture quality.

Also, the terminal device in accordance with the present invention is characterized by using the display device and the image processing system.

In this connection, numeral1indicates a display panel. Numeral2indicates a signal line driver. Numeral3denotes a scanning line driver. Numeral4indicates a gradation correcting section. Numeral5is a control signal generating section. Numeral6designates a light driving section. Numeral7indicates a lighting section (backlight). Numeral8indicates a scanning line. Numeral9is a signal line. Numeral10indicates a pixel. Numeral11denotes a thin film transistor. Numeral12indicates an auxiliary capacitor. Numeral13designates an image memory. Numeral14indicates a decoding circuit section. Numeral15designates an optical shutter. Numeral16indicates an optical shutter control section. Numeral20is a liquid crystal display section. Numeral21denotes a liquid crystal panel. Numeral22indicates a driving circuit. Numeral30designates a variable-length decoding section. Numeral32is a dequantizing section. Numeral33indicates an inverse DCT section. Numeral34denotes a movement compensating circuit. Numeral35indicates a movement amount extracting section. Numeral36is a black reset period setting section. Numeral41indicates an image memory. Numeral42designates a movement amount extracting section. Numeral43indicates a black reset period setting section. Numeral50is a backlight section. Numeral71indicates an image memory. Numeral72denotes a movement amount extracting section. Numeral73indicates a brightness feature extracting section. Numeral74is a black reset period setting section. Numeral81indicates an image memory. Numeral82denotes a movement amount extracting section. Numeral83indicates a PGB→Y converting section. Numeral84designates a mean luminance calculating section.

Numeral85indicates a black reset period setting section. Numeral101is an image memory. Numeral102denotes a movement amount extracting section. Numeral103designates a PGB→Y converting section. Numeral104indicates a mean luminance calculating section. Numeral105is a black reset period setting section. Numeral106indicates a gradation correcting section. Numeral121denotes an image memory. Numeral122indicates a movement amount extracting section. Numeral123indicates a PGB→Y converting section. Numeral124is a mean luminance calculating section. Numeral125denotes a black reset period setting section. Numeral126indicates a gradation correcting section. Numeral131is luminance conversion (PGB→Y) histogram processing. Numeral132indicates correction1. Numeral133denotes correction2. Numeral134designates black reset width setting. Numeral141indicates an optical shutter. Numeral142is a light transmitting section. Numeral143indicates a light interrupting section. Numeral144denotes an optical shutter control section. Numeral151indicates an optical shutter. Numeral152is an integrator. Numeral153indicates a light reflecting section. Numeral261denotes an image signal converting section.

BEST MODE FOR CARRYING OUT THE INVENTION

Description will be given of an embodiment of the present invention. First, the principle and operation of the present invention will be described for a hold type display, primarily, a liquid crystal display as an example.

As described above, a liquid crystal display using the black reset scheme to improve mobile picture quality is accompanied by a problem that the maximum luminance and the brightness of the overall screen become lower in proportion to the black reset ratio.

To solve the problem in this situation, there is provided a configuration in accordance with the present invention in which the amount of movement of an image signal is extracted and the black reset ratio is changed according to the amount of movement.

In this connection, “amount of movement” indicates a distance moved by a rigid body during one frame period. This corresponds to a component of magnitude of a movement vector included in a signal encoded according to the moving picture experts group (MPEG) standard. Moreover, when mutually different movements exist for the entire screen, the amount of movement varies between the respective positions. In this case, it is assumed that a representative value thereof indicates the amount of movement.

In accordance with the present invention, the black reset ratio is changed according to the amount of movement because the edge blur has magnitude proportional to the amount of movement of an object and the black reset ratio at least required to improve mobile picture quality varies depending on the amount of movement. This is because a human (a user, an evaluating person) evaluates the mobile picture quality using the width of the edge blur.

In consequence, by setting the black reset ratio to a minimum ratio required to improve the edge blur, the lowing of the maximum luminance and the brightness of the overall screen can be reduced to a minimum value.

FIG. 1is a diagram to explain an operation principle of the present invention.FIG. 1schematically shows a time-luminance curve of a liquid crystal element when the black reset ratio is changed according to the amount of movement. As shown inFIG. 1, the liquid crystal element displays an image for each frame, and one frame period includes a black reset period and an image gradation display period. In this situation, an amount of movement is extracted using an image signal. When it is determined as a result that the amount of movement is relatively small, the black reset period can be relatively short, and hence the ratio of the image gradation display period to one frame period is set to a larger value to suppress the reduction in the screen brightness and the maximum luminance. On the other hand, when it is determined that the amount of movement is relatively large, the ratio of the image gradation display period is set to a smaller value to reduce the edge blur width. An example of a specific relationship between the amount of movement and the black reset period will be described below.

FIG. 2is a diagram showing an example of the relationship between the amount of movement and the black reset period in which the abscissa represents the amount of movement and the ordinate represents the ratio of the black reset period to one frame period. When the amount of movement is too large, the human eyes cannot track the movement and hence the track movement as one principle to cause the edge blur does not take place. Therefore, as shown inFIG. 2, when the amount of movement is equal to or more than a certain amount of movement, the black reset period need only be fixed to suppress the reduction in brightness. Specifically, assuming that one pixel corresponds to one minute of the visual angle of the eyes in the observing environment, the limit speed for occurrence of the track movement is indicated as a value ranging from 20 to 40 pixels per frame in page 854 of “New Edition Sense•Perception Psychology Handbook”. Therefore, when the amount of movement is large, the black reset period is fixed. As can be seen fromFIG. 2, when the amount of movement is more than 20 pixels/frame (B shown inFIG. 2), the black reset period is set to be equal to 20 pixels/frame. Moreover, since it is considered that the edge blur width is not so problematic until the amount of movement becomes equal to a certain amount, the black reset period is set to be equal to that used when the amount of movement is zero. Since the edge blur is rarely observed in a natural picture when the amount of movement is equal to or less than, for example, three pixels/frame (A shown inFIG. 2), the black reset period is kept unchanged for three pixels/frame or less. The black reset period may be zero in this case; however, if the response time of liquid crystal or the edge blur width is improved by inserting the black reset, a black reset period of about ten percent can be used. Furthermore, for the amount of movement equal to or more than 20 pixels/frame, the black reset period is set to 75% of one frame. It is further favorable that this can be adaptively changed according to sharpness of the video source. That is, when the edge blur width is appropriately reduced in the configuration, the relationship between the amount of movement and the black reset period is not restricted by the relationship shown inFIG. 2, but there may be used another configuration. In accordance with the present invention, the black reset period is relatively enlarged or reduced to improve the edge blur width. That the black reset period is relatively large (or relatively small) is associated with a quantity determined according to human engineering and the quantity need not necessarily conform to the values shown in accordance with the present invention. In other words, in accordance with the present invention, the black reset period can be appropriately increased or decreased to reduce the edge blur width.

The edge blur width changes also according to the response time of liquid crystal in addition to the amount of movement. The response time of liquid crystal is favorably as small as possible, but it is desirable that the response time is at least equal to or less than one frame and is equal to or less than eight milliseconds (ms) if possible.

Moreover, in a panel having a long response time of liquid crystal, by generally elongating the ratio of the black reset period, the edge blur width can be improved as much.

In this case, various methods can be considered to extract the amount of movement depending on types of the image signal inputted to the system. When the input image signal is an encoded digital signal including movement vector information such as the MPEG2 signal, it is possible to extract the amount of movement from the movement vector information.

When the input image signal does not include information regarding the amount of movement such as the RGB signal, it is possible to extract the amount of movement from a plurality of frame images.

To explain in more detail an embodiment of the present invention, description will be given thereof by referring to the drawings.

FIG. 3is a diagram showing a configuration of an embodiment in accordance with the present invention.FIG. 3shows an example of a display in a configuration in which the amount of movement is extracted when an MPEG2 encoded digital signal is inputted as an image signal. Referring toFIG. 3, the MPEG2 signal inputted in the display is decoded by the MPEG2 decoding circuit30and is inputted to the liquid crystal display section20, and a decoded image is displayed on the liquid crystal panel21. In the operation, the variable-length decoding section31of the MPEG2 decoder30extracts movement vector information contained in a signal obtained by conducting a variable-length decoding operation for the digital signal encoded according to the MPEG2 standard. The decoding section31produces an output signal, and the output signal is dequantized by the dequantizing section32and is subjected to an inverse discrete cosine transform in the inverse DCT section33and is fed to the movement compensating circuit34, which produces a video signal to be supplied to the driving circuit22.

The movement vector information (movement vector) from the variable-length decoding section31is inputted to the movement amount extracting section35, and the section extracts it as an amount of movement; the black reset period setting section36sets a black reset period according to the extracted amount of movement and sends it to the liquid crystal display section20.

FIG. 4is a diagram showing a configuration of a second embodiment in accordance with the present invention and showing an example of extraction of an amount of movement using an image signal. As shown inFIG. 4, using images of frames preceding a current frame accumulated in the image memory41and the image of the current frame, the movement amount extracting section42extracts an amount of movement. As a method of detecting the amount of movement, there is used a known method, for example, a block matching method. In the block matching method, a block most similar to a pixel block for which an amount of movement is to be estimated is retrieved from a reference frame (the image stored in the image memory41in this case). A representative amount of movement is extracted in the method described above and is fed to the liquid crystal display section20. According to the amount of movement, the black reset period setting section43sets the black reset period to the display section20.

Incidentally, timing to change the black reset width need not be necessarily set such that the width is changed at an interval of one frame period. It is also possible in the configuration that the black reset width is changed according to the amount of movement, for example, at occurrence of an abrupt change in the amount of movement such a change which takes place when a video scene is greatly changed. In such a configuration, it is possible to suppress in the same video scene a change in the maximum luminance and a variation in the brightness.

FIG. 5is a diagram showing timing to change the movement amount setting. A check is made to determine presence or absence of a change in a video scene for each frame, and when the video scene is greatly changed, the width of the black reset period is set or changed according to the amount of movement.

In this case, the change in the video scene can be detected using a method of determining, for example, a difference between image frames. Or, the change can be detected using a method in which by determining a difference in RGB histograms of an input signal, the change in the video scene is assumed when a sum of difference is equal to or more than a fixed value.

Although the change timing of the black reset is when the video scene is changed, the amount of movement may also be set or changed when the amount of movement is greatly changed.

Description has been given of an operation in which by setting the black reset width according to the amount of movement, the reduction in the maximum luminance and the brightness of the entire screen can be reduced to a minimum value.

Next, description will be given of a fact that a liquid crystal display in which the reduction in the brightness is less annoying can be obtained by setting the black reset period using a feature of brightness of an image in addition to the amount of movement.

The input image signal includes video images of various colors as can be seen from gradation histograms of images such as whitish video images (namely, bright video images) and blackish video images (namely, dark video images). As already described, the edge blur width of a mobile picture is basically proportional to the moving speed of a mobile object although a slight difference exists depending on the brightness. However, even if the mobile picture quality is improved, when the same black reset period is used for a bright video image and a dark video image, the image generally becomes dark.

In this situation, to display a bright video image as bright as possible and to display a dark video image as a darker video image, it is desirable to change the black reset period according to the brightness.

This is comprehensively shown inFIG. 6.

(1) When the amount of movement is large and the screen brightness is low, the black reset period is set to a large value.

(2) When the amount of movement is large and the screen brightness is high, the black reset period is set to an intermediate value.

(3) When the amount of movement is small and the screen brightness is low, the black reset period is set to an intermediate value.

(4) When the amount of movement is small and the screen brightness is high, the black reset period is set to a small value.

In accordance with the present invention, by setting the black reset period using a decision table shown inFIG. 6according to a relationship between the amount of movement and the screen brightness, the mobile picture quality is improved and there can be obtained a clear image according to a video scene and an image having a wide dynamic range.

FIG. 7is a diagram showing a configuration of a third embodiment of the present invention.FIG. 7shows an example of a liquid crystal display setting a black reset period according to an amount of movement and a feature of brightness. The difference between the configuration of this embodiment and that of the embodiment shown inFIG. 4setting a black reset period according to an amount of movement resides in that this embodiment includes a new unit, i.e., the brightness feature extracting section73to extract a brightness feature using a time series image signal. The black reset period setting section74sets a black reset period using the decision conditions shown inFIG. 6according to the amount of movement extracted by the movement amount extracting section72and the brightness feature extracted by the brightness feature extracting section73.FIG. 7shows a case using a time series image signal as input signal; however, by disposing a similar brightness feature extracting section73, a similar advantage of operation can be obtained even when an encoded digital signal such as an encoded signal of MPEG2 is used as input signal.

In this embodiment, several values are used for the brightness feature. When an RGB signal is inputted as the time series image signal, mean luminance of a frame image can be obtained as the brightness feature. Since a Y signal indicating luminance can be represented as a linear combination of RGB signals, the mean luminance is calculated by easily by conducting a color conversion for each pixel.

FIG. 8is a diagram showing a configuration of a fourth embodiment of the present invention, namely, is a diagram showing an example of a liquid crystal display in which an RGB signal is inputted as a time series image signal. Referring toFIG. 8, the configuration includes, as a feature extracting section which inputs a time series image signal to extract a brightness feature thereof, an RGB→Y converting section83to convert an RGB signal into a Y signal and a mean luminance calculating section84. The output (mean luminance) from the mean luminance calculating section84and the amount of movement delivered from the movement amount extracting section82are fed to the black reset period setting section85in which the black reset period is determined according to the amount of movement and the mean luminance.

Moreover, a signal including a luminance signal as a component (for example, an NTSC composite signal) is used as the time series image signal, the mean luminance can be calculated without executing the color conversion processing.

Additionally, in the setting of the ratio of the black reset period, a more effective setting operation can be conducted by using maximum luminance and a brightness feature such as an areal ratio of a component having higher luminance.

FIG. 9shows a frame image and its histogram (a graph showing a gradation level and an appearance frequency thereof) and a setting operation of the black reset ratio according to the histogram. In this connection, the same amount of movement is set to

InFIG. 9(a), a portion of the most bright gradation level occupies a half of the screen, but the mean luminance is just at an intermediate level.

InFIG. 9(b), the entire screen is at about the mean luminance level.

In this case, the ratio of the portion including a higher luminance component is higher inFIG. 9(a) and is lower inFIG. 9(b).

Consequently, when the ratio of the portion including a higher luminance component (FIG. 9(a)) becomes higher, the ratio of the black reset period is reduced to suppress the reduction of luminance in the portion.

As a result, a liquid crystal display having a good balance of brightness and moving picture quality can be obtained by setting the black reset period using the amount of movement and brightness features such as mean luminance, maximum luminance, and an areal ratio of a component having higher luminance.

Although the setting of the black reset period in each embodiment of the present invention has been discussed, the gradation output is directly conducted regardless of a histogram of an image displayed on the liquid crystal display.

Next, description will be given of a fact that a liquid crystal display having a wider dynamic range can be obtained by further conducting a gradation correction.

FIG. 10shows a configuration of a fifth embodiment of the present invention.FIG. 10shows a configuration of a liquid crystal display setting a black reset period according to an amount of movement and mean luminance and conducting correction of gradation according to the amount of movement. Referring toFIG. 10, an RGB→Y converting section103, an image memory101, a movement amount extracting section102, a mean luminance calculating section104, and a black reset period setting section105are the same as the RGB→Y converting section83, the image memory81, a movement amount extracting section82, the mean luminance calculating section84, and the black reset period setting section85ofFIG. 8. This embodiment includes a gradation correcting circuit106which inputs a time series image signal and an amount of movement from the movement amount extracting section102to correct gradation of the time series image signal according to the amount of movement. The black reset period is set in almost the same setting method as for the liquid crystal display ofFIG. 8.

In this embodiment, the black reset period is set according to the amount of movement and the mean luminance, and even when the mean luminance is the same, a different value is set to the period if the amount of movement varies depending on cases.

That is, depending on timing to set or to change the black reset period, the brightness of the display image from the liquid crystal display section20may generally varied.

In this situation, to suppress the variation, the gradation correcting section106corrects the time series image signal so that the mean luminance of the image is not varied according to the amount of movement.

FIG. 11is a diagram to explain an example of a specific correction by the gradation correcting section106in this embodiment. Since the black reset period becomes long when the amount of movement is large in an image, the mean luminance is increased using gradation as much in the correction. A relationship between input gradation (abscissa) and output gradation (ordinate) is represented by an upwards convex curve.

Since the black reset period becomes short when the amount of movement is small, the mean luminance is decreased using gradation. A relationship between input gradation (abscissa) and output gradation (ordinate) is represented by a downwards convex curve.

In this way, the mean luminance of the liquid crystal display can be kept fixed. Moreover, to remove this problem, a light source having controllable brightness is used such that when the black reset period becomes long according to the amount of movement on a screen, the light source is made to be brighter; and when the period becomes short according to the amount of movement, the light source is made to be darker. It is also possible to thereby obtain a similar advantage.

On the other hand, as an advantage to conduct the gradation correction, a clear image can be obtained by emphasizing gradation on a brighter side for a bright image and by emphasizing gradation on a darker side for a dark image.

FIG. 12is a diagram showing a configuration of a sixth embodiment of the present invention. Referring toFIG. 12, a liquid crystal display in accordance with the embodiment includes a gradation correcting section126to conduct correction of gradation according to the amount of movement and mean luminance. As distinct from the configuration shown inFIG. 10, the gradation correcting section126receives as inputs a time series image signal, an amount of movement, and mean luminance.

The gradation correcting section126checks brightness of the overall image using mean luminance of the image produced from the mean luminance calculating section124and conducts the gradation correction to emphasize a gradation difference on a bright side when the image is bright. On the other hand, the section126conducts the gradation correction to emphasize a gradation difference on a dark side when the image is dark.

FIG. 13is a diagram showing processing of gradation correcting section126in functional blocks. The section126correct gradation in two stages of steps (corrections1and2).

The section126receives as an input an input RGB signal and executes luminance conversion (RGB→Y) histogram processing to produce mean luminance and an amount of movement (131). In correction1(132), the section126receives as inputs the mean luminance and the input RGB signal and checks the mean luminance to increase input gradation when the image is dark and to decrease input gradation when the image is bright. In correction1(132), when the black reset is beforehand changed according to the movement, the section126conducts the gradation correction according to the change and produces an output RGB signal. InFIG. 13, the gradation correcting section126includes two stages for easy understanding thereof, however, these stages may be combined with each other to execute the processing in one stage of the step.

Next, description will be given of an embodiment of the present invention according to a method to actually set a black reset period.

FIG. 14is a diagram schematically showing a liquid crystal display in the first embodiment of the present invention. The pixel section (a thin film transistor (TFT)11to serve as a pixel switch, an auxiliary capacitor12, and a liquid crystal layer) is partly magnified and shown inFIG. 14.

Referring toFIG. 14, the liquid crystal display of this embodiment includes a liquid crystal display section20including a display panel1at least including a plurality of scanning lines8and a plurality of signal lines9mutually intersecting each other, a plurality of pixels10disposed at the respective intersections in a matrix form via respective thin film transistors11, and auxiliary capacitors12connected in parallel, a scanning line driver3to control the scanning lines8, and a signal line driver2to control the signal lines9; a backlight section50including a plurality of lighting devices7, a light driving section6to control on and off of the lighting devices7in an independent way, a control signal generating section5to send a control signal to the light driving section6according to an input video signal and a control signal, and an image memory13to store a video signal of a frame immediately before a current frame. The backlight section50is placed on a rear surface of the liquid crystal display section20and is arranged in the liquid crystal display device.

Description will be given of operation of the respective sections ofFIG. 14. To display an image on the liquid crystal display section20, a converted input video signal and control signals such as a horizontal synchronizing signal HSync, a vertical synchronizing signal VSync, and a clock signal CLK are inputted. The input video signal and the control signals are directly fed to the liquid crystal display section20. For the input video signal inputted to the signal line driver2, data rearranging and a conversion from a digital signal to an analog signal (D/A conversion) are conducted such that an analog signal is outputted to the signal lines9. On the other hand, for the scanning line driver3, one line or row of pixels are selected by the scanning line8, the transistors11on the selected row turn on, and signals from the signal lines9are written in selected pixels. Since the liquid crystal display conducts “line sequential scanning”, the signal is written in pixels for each scanning line8.

The signal written via the transistor11from the signal line9is supplied to the pixel10and the auxiliary capacitor12to be charged to a signal line voltage (selection period). Thereafter, even when the transistor11enters an off state, the signal voltage is kept in the pixel10and the auxiliary capacitor12and is kept retained until a subsequent selection period (retention period). In this regard, since the response time of liquid crystal ranges from several milliseconds (ms) to several tens of milliseconds (ms) and is long as compared with the selection period, the orientation of liquid crystal changes and the transmissivity varies also during the retention period.

The backlights7is subdivided at least in a direction parallel to the scanning lines8of the liquid crystal display section20and are sequentially turned on and off by the light driving section6like the line sequential writing in pixels.

The control signal generator5generates control signals (a signal to turn the backlight7on, a signal to control turn off timing) and sends the signals to the light driving section6.

The control signal generator5produces signals to control on and off timing of the backlight7according to an input signal video signal, a video signal of a frame immediately before a current frame accumulated in the image memory13, and control signals.

Next, an operation principle of the liquid crystal display will be described by referring to a signal timing chart.FIG. 15is a diagram showing driving timing of the liquid crystal display ofFIG. 14. The vertical synchronizing signal VSync is a pulse signal turned on at an interval of a vertical period.FIG. 15shows a relationship between luminance of respective backlights A, B, C, and D of the backlight section50and the transmissivity of liquid crystal pixels of lines of the liquid crystal display section20corresponding to the respective backlights A, B, C, and D.

In the liquid crystal display section20, a voltage is sequentially applied to the scanning lines8beginning at an upper scanning line8at a period of one frame to turn the transistors11located in the row of the scanning line8on to write a video signal in pixels10. The transmissivity of the liquid crystal changes several milliseconds after the writing operation.

In this diagram, the scanning lines8A-1and8A-2are respectively a line on which the pixel writing operation is first conducted and a line on which the pixel writing operation is last conducted among the lines included in a backlight area A. As can be seen from a time-transmissivity characteristic of the lines8A-1and8A-2, from when the transmissivity change of pixels in the line8A-1starts to when the transmissivity change of pixels in the line8A-1ends, all pixels10of the liquid crystal display section20included in the backlight area A do not become stable.

Resultantly, to improve the mobile picture quality, it is desirable that the backlight A turns off (arrow x in the diagram) when the writing starts in the pixels corresponding to scanning line8A-1in the upper-end of the zone A and turns on (arrow Y in the diagram) when the response of the pixels corresponding to scanning line8A-2in the lower-end of the zone A finished. This is also the case with the backlights B, C, and D.

Thanks to the configuration, by inserting the turn off period, namely, the “black” display in a hold type display, i.e., an LCD, the mobile picture quality can be improved.

The improvement in the mobile picture quality up to this point can also be similarly obtained also using the black reset driving method.

The difference with respect to the black reset insertion by the driving of this embodiment resides in that the light source is actually turned off and hence luminance in the “black” display is further lowered in this embodiment.

In the embodiment, by referring to a type of video signal and a gradation histogram of a display image, black reproducibility is regarded as important, and when the video image is generally dark, the mobile picture quality is improved using the gradation correction and the change of on time of the backlight, and there can be obtained a liquid crystal display superior in gradation reproducibility.

In this connection, a combination with the black reset driving is also applicable to the present invention, and also when the black reset driving is applied to the present invention, there can be naturally obtained a remarkable advantage.

Since the liquid crystal driving circuit conducts the line sequential scanning, it is desirable that the backlight is also divided into subareas according thereto to conduct the line sequential scanning.

However, since the cost and the power consumption greatly increase, the number of backlight subareas is limited. As described above, the timing to turn the backlight on and off need only be controlled.

However, when the number of backlight subareas is too small or when lighting devices are turned on before the complete response of liquid crystal to increase luminance, for example, when a black rectangle160(crosshatched for convenience of drawing) moves in a direction from the left to the right on a white background, gradation areas161(hatched for convenience of drawing) appear in ghost zones as shown inFIG. 16. Therefore, it is required to set an optimal number of backlight subareas.

As above, description has been given of the backlight control method and timing and it has been described that the black reset period can be set. By setting the ratio of the black reset period according to the amount of movement by use of the control signal generator5, there can be implemented a liquid crystal display in which the reduction in brightness is suppressed and the mobile picture quality is improved.

It is naturally possible that the first embodiment is configured such that the movement amount extracting section, brightness feature extracting section, the mean luminance calculating section, and the gradation correcting section described by respectively referring toFIGS. 4,7,8,10, and12are arranged in the control signal generator5or outside the generator5.

The input video signal is an RGB signal in the description; however, any encoded digital signal including vector information such as an MPEG2 signal can be used without necessity of the image memory13such that movement vector information is extracted from the signal and is fed to the control signal generator5.

FIG. 17is a diagram schematically showing a configuration of a liquid crystal display in the second embodiment of the present invention. The embodiment differs from the liquid crystal display in the first embodiment in that this embodiment includes a decoding circuit14to decode the encoded video signal and the control signal generator5sets the black reset period using movement vector information obtained by the decoding circuit14. The configuration of this embodiment does not require the image memory13of the first embodiment. The other configuration is the same as that of the first embodiment.

FIG. 18is a diagram schematically showing a liquid crystal display in the third embodiment of the present invention. The embodiment differs from the liquid crystal display in the first embodiment ofFIG. 14in that the control signal generator5sets the black reset period and there is disposed a gradation correcting section4to conduct gradation correction for an input video signal using a control signal sent from the control signal generator5. The other configuration is the same as that of the first embodiment. According to the embodiment in this configuration, a clearer image can be displayed.

In this regard, in addition to the backlight including cold-cathode discharge tubes arranged in the scanning direction, there can be used areal light emitting devices such as electro luminance elements and light-emission diodes to obtain a similar advantage.

Next, description will be given of a fourth embodiment of the present invention.FIG. 19is a diagram schematically showing a configuration of a liquid crystal display in the fourth embodiment of the present invention. The configuration is different from that of the liquid crystal display in the first embodiment of the present invention shown inFIG. 14in that although the backlight7is of a full area on type, an optical shutter15having high contrast is arranged between the backlight7and the liquid crystal display section20or in front of the display section20.

The optical shutter15is divided into subareas in the scanning direction of the liquid crystal display section20and the optical shutter controller16can control the subareas in an independent way in this configuration. As the optical shutter15, ferrodielectric liquid crystal having a high-speed response characteristic is used.

The optical shutter15is controlled to interrupt light during the black reset period and to transmit light during the image gradation display period. Timing to control the shutter15and a quantity of control are determined in a method equal to that described for the first embodiment. The control signal generator5generates control signals to control transmission and interruption timing of the optical shutter15according to a video signal inputted thereto, a video signal of a frame before a current frame stored in the image memory, and control signals (VSync, HSync, etc.) and delivers the control signals to the optical shutter controller16.

Also in this embodiment, like in the first to third embodiments, it is possible to improve the mobile picture quality and to reduce the black luminance.

Incidentally, it is not required to divide the backlight in this embodiment; therefore, the embodiment is applicable not only to a display of direct viewing type but also to a projection type display including a single light source such as a liquid crystal projector. By arranging a gradation correcting section also in the embodiment, a clearer image can be naturally obtained.

FIG. 20is a diagram schematically showing a configuration of a liquid crystal display in the fifth embodiment of the present invention. This display differs from the liquid crystal displays in the first to third embodiments in that the backlight7is of a type of “full area on at a time”, and a black reset period setting signal produced from the control signal generator5is fed to the scanning line driver3of the liquid crystal panel.

Description will be given of a driving principle of the liquid crystal display ofFIG. 20by referring toFIG. 21.FIG. 21shows an image gradation signal to each scanning line, timing to write a black reset signal, and a time-luminance curve associated therewith.

InFIG. 21(a), one image gradation display pulse and one black reset display pulse are supplied to each scanning line once during one frame period. The system is set such that when the image gradation display pulse starts at an upper position at a start point of one frame period, the black reset display pulse starts at a position apart in the scanning direction by a distance to insert the black reset from the start point of the image gradation display pulse. Thereafter, as time lapses, the image gradation display pulse and the black reset display pulse respectively shift in the scanning direction at the same speed to drive a subsequent line.

In this situation, since one signal line9is disposed for each pixel10in the configuration of the liquid crystal panel in the embodiment, the image gradation writing and the black reset writing are conducted for each line during one line selection period.

If two signal lines are disposed for each pixel, it is only necessary to apply a voltage corresponding to the image gradation signal and a voltage corresponding to the black reset voltage to the respective signal lines to select either one thereof. In this way, the black reset writing can be conducted.

FIG. 21(b) is a diagram showing timing to write the image gradation display pulse and the black reset display pulse when the black reset period is elongated as compared withFIG. 21(a). The scanning start line of the black reset signal is lower than that ofFIG. 21(a). In this case, the black reset period is elongated in the time-luminance curve ofFIG. 21(b).

The start position of the black reset display pulse is set according to an output signal from the control signal generator5. In the black reset driving operation, the black reset width can also be changed according to an amount of movement. Also in this embodiment, the advantage described above is naturally enhanced by adding a gradation correcting section and a brightness feature extracting section.

As above, although the embodiment has been described using an example of a liquid crystal display, the present invention is not restricted by the liquid crystal display, but is naturally applicable to a hold type display.

FIG. 22is a diagram schematically showing a configuration of a projection liquid crystal display in the sixth embodiment of the present invention. The display differs from the projection liquid crystal display including a single light source in the fourth embodiment in that a plurality of rotary optical shutters141are arranged between the backlight7and the liquid crystal display section20or in front of the display section20. Moreover, there is disposed an optical shutter control section144to control rotation and a phase of each optical shutter. The rotary optical shutter includes a light transmitting zone142and a light interrupting zone143which are formed alternately with a fixed interval therebetween, and the shutter rotates in association with the scanning and writing operation for pixels of the liquid crystal display section20. The optical shutter controller144controls the rotation. As a result, until the writing operation is finished for the pixels, light for the associated pixels is not projected thanks to the light interrupting zone143, and only after the writing operation is finished, the display light is projected through the light transmitting zone141.

Moreover, a plurality of optical shutters disposed in an overlapping fashion can set the respective rotary phases to arbitrary values, and hence the black reset period can be dynamically changed according to the amount of movement and the brightness feature. Assuming, for example, that two optical shutters are used and a ratio between the light transmitting zone and the light interrupting zone is two to one for each shutter, when the light interrupting zones of the shutters completely overlap each other, the ratio between the light transmitting zone and the light interrupting zone is kept as two to one. However, when the light interrupting zones of the shutters do not completely overlap each other and the shutters rotate at the same speed, the ratio becomes one to two. Therefore, when two optical shutters are used and the ratio between the light transmitting zone and the light interrupting zone is two to one, the black reset period can be freely set to a value between ⅓ to ⅔ by changing the rotary phase between the shutters. The optical shutter controller144also controls the phase.

Also in this embodiment, the mobile picture quality can be improved and the black luminance can be reduced like in the first to fourth embodiments.

In this connection, by arranging a gradation correcting section in this embodiment, a clearer image can also be naturally obtained.

FIG. 23is a diagram schematically showing a configuration of a projection liquid crystal display in the seventh embodiment of the present invention. This display differs from the projection liquid crystal display of the sixth embodiment in that an integrator152is arranged at a position of a path of light incident to the optical shutters151on the light incident side of the optical shutters151rotating together in an overlapped state. The configuration of the optical shutters151is the same as that of the optical shutters151shown in the sixth embodiment, or a light reflecting section153is disposed in place of the light interrupting zone143of the light shutter141.

The integrator152has a rod shape and receives light from a light source as shown inFIG. 24, and the light totally reflects in the rod and is emitted from a surface opposing a surface which the incident light enters. The optical shutter151is disposed on the light emitting surface such that the light is directly emitted through the light transmitting zone142, but is reflected on the light reflecting zone153and returns again to the integrator. The light repeatedly conducts total reflection to reach the light transmitting zone142.

As above, by disposing the integrator, the utilization ratio of light from the light source is increased, and hence even when an optical shutter is arranged to set a black reset period, the mobile picture quality can be improved while suppressing the reduction in the maximum luminance.

Moreover, also in this embodiment, it is natural that a clearer image can be obtained by disposing a gradation correcting section.

FIG. 25is a diagram showing a configuration of an image processing system in an eighth embodiment of the present invention. This configuration is obtained by removing the display section from the sixth embodiment shown inFIG. 12.

Thanks to the configuration, in a case in which data is transmitted to, for example, a plurality of liquid crystal displays, it is possible by only using this image processing system to provide a liquid crystal display in which the mobile picture quality is improved while suppressing the reduction in the maximum luminance. Furthermore, the transmission destination may be not only the liquid crystal display but also a terminal or a unit such as a portable terminal on which a liquid crystal display is mounted. Since it is only required that a method to set a black reset period is included in the liquid crystal display and the portable terminal, the cost of each liquid crystal display can be lowered without installing a complex algorithm therein.

Additionally, although the configuration of the image processing system is obtained by removing the display section from the sixth embodiment in the description, the configuration does not restrict the image processing system such that the advantage as the image processing system can also be naturally obtained in a configuration implemented by removing the display section from the configurations of each of the first to fifth embodiments.

FIG. 26is a diagram schematically showing a terminal device in a ninth embodiment of the present invention. The terminal includes a data receiving section, an image processing system section, and a liquid crystal display section. The configurations of the image processing system section and the liquid crystal display section20are the same as those of the sixth embodiment shown inFIG. 12.

The data receiving section receives a signal from an external device and converts the received signal by the image signal converter261into a time series image signal.

By using this configuration, the mobile picture quality can be improved and the black luminance can be reduced in the liquid crystal display section arranged in the terminal.

Moreover, the configuration of the terminal is based on that of the sixth embodiment in the description; however, this does not restrict the image processing system such that the advantage as the terminal can also be naturally obtained using a configuration based on the configuration of each of the first to fifth embodiments.

INDUSTRIAL APPLICABILITY

As described above, in accordance with the present invention, by dynamically changing the black reset period according to the amount of movement and the brightness feature, there can be provided a display device in which the mobile picture quality is improved while removing the problem of the black reset scheme, namely, while lowering the reduction in the brightness to a minimum value.

Moreover, in accordance with the present invention, by additionally conducting the gradation correction for the input image signal according to the amount of movement and the brightness feature, there can be provided a display device having improved mobile picture quality and displaying a clear image.

Furthermore, in accordance with the present invention, by dynamically changing the black reset period according to the amount of movement and the brightness feature, there can be provided an image display system in which the dynamic range is widened to improve the mobile picture quality of a hold type display device.

Moreover, in accordance with the present invention, by using the display device and the image display system, there can be provided a terminal device in which the dynamic range is enlarged to improve the mobile picture quality.