This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-355104, filed on Dec. 8, 2005 and the prior Japanese Patent Application No. 2006-239546, filed on Sep. 4, 2006; the entire contents of which are incorporated herein by reference.
The present invention relates to a liquid crystal display device.
Conventionally, a TN (Twisted Nematic) liquid crystal display device is generally used as a liquid crystal display device, however, in order to improve moving image visibility, an OCB liquid crystal display device characterized by high speed response is proposed.
As shown in
In this liquid crystal display device 100, in a state before power is turned on, as shown in
In this OCB liquid crystal display device 100, in order to keep the bend orientation state, a reverse transition prevention voltage is applied in each frame for a period with a specific ratio or more, and the reverse transition to the spray orientation state is prevented. At this time, when the reverse transition prevention voltage is made equal to the optimum black display voltage, a high contrast is ensured, and moving image visibility can be improved.
In order to prevent the occurrence of the reverse transition from the bend orientation to the spray orientation, it is necessary to apply the reverse transition prevention voltage of a specific voltage or higher for a specific period or longer.
Accordingly, in the case where it is necessary to set the reverse transition prevention voltage to be higher than the optimum black display voltage, there is a problem that black display quality must be sacrificed.
Besides, in order to keep the black display quality, it is necessary to make the reverse transition prevention voltage substantially equal to the optimum black display voltage. However, in this case, it is necessary to prevent the reverse transition by setting an application period of the reverse transition prevention voltage to be long, that is, by setting the period (black insertion ratio) in which the reverse transition prevention voltage is applied in a frame to be long (high), or by setting a white display voltage to be high. However, in such a case, since the display time ratio (display time occupied in the frame period) is reduced, or the white brightness is reduced, there is a problem that the use efficiency of light is remarkably impaired.
In order to solve this problem, JP-A-2003-279931 proposes to change a black insertion ratio and a black insertion voltage (reverse transition prevention voltage) according to the peak brightness of a video signal.
However, in the method of JP-A-2003-279931, there remains a problem that the black display quality in a video signal in which white and black are mixed in a selected line is sacrificed.
Then, in view of the above problems, the invention provides an OCB liquid crystal display device in which the black display quality is not sacrificed even in a video signal in which black and white are mixed in a selected line, and a driving method of the same.
According to embodiments of the present invention, in a driving method of a liquid crystal display device to perform an image display by combining a video image displayed by a gradation voltage based on a video signal and a reverse transition prevention image displayed by a reverse transition prevention voltage for keeping a bend orientation state of liquid crystal molecules, the reverse transition prevention voltage is determined based on the gradation voltage in display pixel unit, and the reverse transition prevention image is displayed based on the corresponding reverse transition prevention voltage in the display pixel unit.
According to the invention, even in the video signal in which black and white are mixed in the selected line, the black display quality is not sacrificed.
Hereinafter, a liquid crystal display device 10 of a first embodiment of the invention will be described with reference to
(1) Structure of the Liquid Crystal Display Device 10
The structure of the liquid crystal display device 10 will be described based on the block diagram of
A liquid crystal panel 12 of the liquid crystal display device 10 has an effective display area of 9 inches in diagonal size, and as described above, a liquid crystal is sandwiched between an array substrate and a counter substrate. On the array substrate, 800 signal lines and 480 scanning lines are disposed to be orthogonal to each other, and a polysilicon thin film transistor (TFT) is formed in the vicinity of each of intersections of the signal lines and the scanning lines. A source electrode of the TFT is connected to the signal line, a gate electrode is connected to the scanning line, and a drain electrode is connected to a pixel electrode. A source driver 14 and a gate driver 16 are connected to the liquid crystal panel 12. The plural signal lines are connected to the source driver 14, and the plural scanning lines are connected to the gate driver 16.
The liquid crystal display device 10 includes also a control unit 18. The control unit 18 includes a memory 20, a signal conversion unit 22, a drive control unit 24, and a counter control unit 26. A video signal inputted from the outside is once stored in the memory 20, and based on a synchronizing signal inputted from the outside, the drive control unit 24 outputs the analog video signal stored in the memory 20 to the signal conversion unit 22. The outputted analog video signal is AD-converted in synchronization with a horizontal synchronizing signal from the drive control unit 24 and is outputted as a digital video signal to the source driver 14. The drive control unit 24 outputs a horizontal synchronizing signal, a horizontal start signal and the like based on the synchronizing signal. Besides, the drive control unit 24 outputs a vertical synchronizing signal, a vertical start signal and the like to the gate driver 16 based on the synchronizing signal. Further, the drive control unit 24 controls a voltage to be applied to the counter substrate through the counter control unit 26.
Further, the drive control unit 24, the memory 20 and the signal conversion unit 22 perform a control to output a reverse transition prevention voltage typically applied in the OCB type.
(2) Output Method of Reverse Transition Prevention Voltage
Next, an output method of a reverse transition prevention voltage will be described with reference to
As described in the Background of the Invention section, in the OCB liquid crystal display device, it is necessary that a video image based on a video signal in one frame and a reverse transition prevention voltage image displayed by a reverse transition prevention voltage for keeping a bend orientation state of liquid crystal molecules are alternately displayed in one frame.
In a conventional OCB liquid crystal display device, only the area of γ1 is used, and a voltage applied in the black insertion period is made constant. That is, the gradation voltage for performing the video display is in the range of γ1, and the reverse transition prevention voltage (black insertion voltage) for keeping the bend orientation is made equal to the optimum black display voltage. In the method as stated above, excellent black display quality can be obtained, however, in the case where the optimum black display voltage is lower than the reverse transition prevention voltage, there is a fear that the reverse transition occurs according to the display image. Thus, in order to prevent the reverse transition, it becomes necessary to prolong an application period of the reverse transition prevention voltage, that is, the black insertion ratio, or to set the white display voltage to be high, and sufficient light use efficiency can not be obtained. Besides, in the method of patent document 1, although the light use efficiency can be improved without causing the reverse transition, the black display quality in, for example, a video signal in which black and white are mixed in a selected line is sacrificed.
Then, in this embodiment, in order to solve this problem, in the case where video display is performed, the gradation voltage is applied in the range of γ1, and the reverse transition prevention voltage is applied in a range including the range of γ2, more specifically, the range of γ2. Then, the relation between the gradation voltage of the digital video signal and the reverse transition prevention voltage is made such that as the gradation voltage of the digital video signal becomes low (transmittance becomes high), the reverse transition prevention voltage is made high (transmittance is high). Incidentally, a maximum value of the reverse transition prevention voltage is made a maximum applied voltage in the liquid crystal panel 12. For example, the range of γ2 in this embodiment is set to be from the optimum black voltage of 4.5 V to the maximum voltage of 6 V, and this range is used as the black insertion voltage.
The relation between the gradation voltage of the video signal and the value of the reverse transition prevention voltage is stored in the memory 20. The inputted video signal is once stored in the memory 20 in each pixel unit, and in each frame, the reverse transition prevention voltage is calculated from the correspondence table stored in the memory 20 based on the gradation voltage of the video signal. In
(3) Description of Operation State
Next, a specific example in a case where an image display is performed will be described with reference to
As shown in
As stated above, in this embodiment, the reverse transition prevention voltage image corresponding to each transmittance in each video display is inserted. That is, in the case where the picture is dark, the black reverse transition prevention voltage image is inserted, and in the case where the picture is bright, the gray or white reverse transition prevention voltage image is inserted, and the reverse transition is effectively prevented without impairing the display quality. Besides, the use efficiency of light can also be improved by the above structure. Further, even in the picture in which black and white are mixed in a selected line, the black display quality is not sacrificed.
As shown in
An OCB liquid crystal display device 10 of a second embodiment will be described with reference to
A different point between this embodiment and the first embodiment is that a memory 20 in which the foregoing correspondence table is stored and a signal conversion unit 22 are separate from a control unit 18, and a processing is performed as a set circuit 28 at the stage of a signal processing of an image receiving circuit. That is, in a recent video apparatus, especially in a digital display video apparatus, a frame memory is generally prepared, and display data for a signal and for a reverse transition prevention are created at the stage of a signal processing here. By this, redundancy of memories is avoided, and the signal processing is unified, so that the cost of the liquid crystal display device 10 can be reduced.
Next, a third embodiment will be described with reference to
In the first embodiment, the number of gradations of the reverse transition prevention voltage and that of the gradation voltage for the video display are 128 gradations and are equal to each other. However, in order to enrich the expression of the video display, the number of gradations of the video signal is increased, and the number of gradations of the reverse transition prevention voltage is reduced by that. For example, in the case where a 8-bit source driver 14 is used, when 128 gradations are allocated to the reverse transition prevention voltage, only the remaining 128 gradations are used for the video signal. However, a very larger number of gradations are not required for the reverse transition prevention voltage, and a suitable number of gradations (for example, 32 gradations) are sufficient for the reverse transition prevention and for the improvement of light use efficiency.
Then, in the voltage outputs of 0 to 255 gradations of the 8-bit source driver 14, setting is made such that 32 gradations are for the reverse transition prevention voltage, that is, 32 to 255 gradations are for the video signal, and 0 to 32 gradations are for the reverse transition prevention voltage.
By this, an enriched image can be displayed.
In addition to this, for example, with respect to an 8-bit input signal, the source driver is made to deal with 10 bits, 8 bits is made to be used for the video signal, and 2 bits is made to be used for the reverse transition prevention voltage. By this, the representation of the image is not sacrificed.
Besides, a driver for the reverse transition prevention voltage may be provided separately from, for example, the source driver.
Number | Date | Country | Kind |
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2005-355104 | Dec 2005 | JP | national |
2006-239546 | Sep 2006 | JP | national |
Number | Name | Date | Kind |
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6909412 | Son | Jun 2005 | B2 |
7126573 | Park et al. | Oct 2006 | B2 |
7161574 | Kimura et al. | Jan 2007 | B2 |
7450101 | Arimoto et al. | Nov 2008 | B2 |
Number | Date | Country |
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2003-279931 | Oct 2003 | JP |
Number | Date | Country | |
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20070132688 A1 | Jun 2007 | US |