Patent Grant
8130187
The present invention relates to a liquid crystal display and, more particularly to a liquid crystal display capable of performing high speed drive.
Conventionally, a TN (twisted nematic) liquid crystal display element has been generally used as a liquid crystal display. In actuality, since a TN liquid crystal display has a low response speed, an OCB (Optically Compensated Bend) display has been studied as a high-speed responsive liquid crystal display. See “Syadan Hojin Denki Tsushin Gattsukai Shingakugihou EDI98-144 P199” to know the detail of the OCB liquid crystal display.
In this OCB liquid crystal display, a liquid crystal is sandwiched between substrates and transparent electrodes are formed on inner surfaces of the substrates. Before power is turned ON, the liquid crystal has a spray orientation state. Then, when the power of the liquid crystal display is turned ON or the like, a relatively high voltage is applied to the transparent electrodes for a short time period to cause the liquid crystal to transition from the spray orientation state to a bend orientation state. In OCB liquid crystal display mode, the bend orientation state is employed for display, thereby enabling high speed response. By the way, problems associated with a HOLD-type display were pointed out in “Jyouhoukagakuyou Yuuki Zairyou 142th Iinkai A bukai (liquid crystal material) 71th Kenkyukai Bkai (intelligent organic material) 62nd Kenkyukai Shiryou Nov. 20, 1988, Nihongakujyutsu Shikoukai P 1-5”, and techniques for displaying a moving picture in the liquid crystal display with performance equal to that of CRT (cathode ray tube) were suggested. The simplest one of these techniques is to write onto a picture at a high speed and insert a black picture on a periodic basis. Such a method for write onto the picture in a short time is generally referred to as “high speed drive” herein.
However, the OCB liquid crystal display is capable of performing high speed response but is unsatisfactorily performing high speed drive.
The present invention has been directed to solving the above-described problem and an object thereof is to provide a liquid crystal display capable of performing high speed drive.
To solve the above-described problem, there is provided a liquid crystal display comprising: a liquid crystal layer capable of bend orientation; a display screen on which an image is displayed by light transmitted through a bend-oriented liquid crystal layer; and liquid crystal voltage application means for applying a liquid crystal voltage to the liquid crystal layer according to luminance information for each field of image information composed of serial fields, the liquid crystal voltage being applied to cause transmittance of the light to change, thereby sequentially displaying the image corresponding to the fields of the image information on the display screen, wherein when the luminance information changes between current and subsequent fields, the liquid crystal voltage application means applies the liquid crystal voltage which changes so as to have a value according to the luminance information by the time the liquid crystal voltage is applied for the subsequent field.
With such a configuration, the voltage different from the voltage according to the luminance information of the image information is transiently applied to the liquid crystal. Thereby, the speed of change in the transmittance of the liquid crystal, i.e., the response speed, can be controlled in the OCB liquid crystal mode.
In this case, when the luminance information changes to cause the corresponding liquid crystal voltage to be increased, the liquid crystal voltage application means may apply the liquid crystal voltage which changes so as to have the value according to the luminance information after excessively increased, and when the luminance information changes to cause the corresponding liquid crystal voltage to be reduced, the liquid crystal voltage application means may apply the liquid crystal voltage which changes so as to have the value according to the luminance information after excessively reduced.
With such a configuration, since the transient voltage facilitates the change in the transmittance of the liquid crystal, the high speed response of the liquid crystal is achieved. In addition, since the variation of the amount of transmitted light with respect to the change in the dielectric constant of the liquid crystal is large in the OCB liquid crystal mode, the response speed is much more improved than that of the conventional OCB liquid crystal mode by synergism of this effect and the effect of the transient voltage application. Consequently, this liquid crystal display is capable of performing “high speed drive”.
The liquid crystal voltage may converge to the value according to the luminance information after excessively increased or reduced.
Thereby, the liquid crystal voltage easily transitions to the voltage according to th luminance information of the image information.
The display screen may be composed of a plurality of pixels and the liquid crystal display voltage application means may comprise pixel voltage application means for sequentially applying a pixel voltage to the liquid crystal layer of all the pixels according to the luminance information for each pixel in the field.
Thereby, in the liquid crystal display having the display screen composed of the plurality of pixels, the liquid crystal voltage can be changed.
The liquid crystal display may further comprise gate drive means for sequentially scanning the plurality of pixels through a gate electrode; source drive means for applying a base voltage based on the luminance information of the pixels of the image information to the liquid crystal layer of the pixels sequentially scanned, through a source electrode; and compensation voltage application means for applying a compensation voltage to the pixels through capacitive coupling after the pixels are scanned such that the compensation voltage is overlapped with the base voltage, and the source drive means and the compensation voltage application means may constitute the pixel voltage application means such that the base voltage and the compensation voltage change as the pixel voltage, according to change in a liquid crystal capacitor of the pixels.
Thereby, since the source drive means capable of applying the voltage only during the scanning by the gate drive means is adapted to apply a constant base voltage, the compensation voltage is overlapped with the base voltage by utilizing the capacitive coupling during the period after scanning in which the pixel voltage is to be changed, and the resulting overlapped voltage changes so as to have the value according to the luminance information of the pixels due to the change in a capacity for the liquid crystal capacitor, the transient voltage according to the luminance information of the pixels can be automatically applied. That is, the transient voltage can be applied in a simplified manner.
The capacitive coupling may be formed between the pixel electrode and a preceding gate electrode in the order in which the pixels are scanned.
Thereby, since the compensation voltage can be applied by using the gate electrode, the configuration of the compensation voltage application means can be simplified.
The gate drive means may be adapted to cause the preceding gate electrode to vary a potential thereof in order to apply the compensation voltage.
The capacitive coupling may be formed between the pixel electrode and a dedicated capacitor line.
The compensation voltage may be applied by varying a potential of the capacitor line.
The liquid crystal voltage application means may comprise a voltage supply source for supplying the liquid crystal voltage only through a signal line through which the voltage based on the luminance information for each field of the image information is applied to the liquid crystal layer.
Thereby, the waveform of the transient voltage can be easily controlled.
The voltage supply source may comprise means for storing the image information of the current and subsequent fields; means for deriving change in the luminance information between the fields of the stored image information; means for generating the compensation voltage according to change in the derived luminance information; and liquid crystal voltage supply means for generating the base voltage based on the luminance information of the subsequent field, overlapping the compensation voltage with the base voltage, and outputting the overlapped voltage as the liquid crystal voltage.
In this case, an image information write period during which the image information of one field is sequentially written to all the pixels may occupy less than 90% of a field period corresponding to a predetermined cycle in which the image information of one field is written.
Thereby, the sharpness of the displayed moving picture can be improved by the insertion of the black picture in the field period.
The image information write period may be less than 16.6 ms.
Thereby, in a moving picture display system of a field frequency of 60 Hz generally adopted, the liquid crystal display can improve the sharpness of the displayed moving picture by the insertion of the black picture.
In this case, the image information write period may occupy less than half of the field period.
Also, the image information write period may be less than 8 ms.
Thereby, since this liquid crystal display is capable of performing “double speed drive” and can display the sharp moving picture by the insertion of the black picture in the moving picture display system of the field frequency of 60 Hz generally adopted, the liquid crystal display can be practically used in the television, monitor, or the like in terms of the response speed.
The pixel voltage application means may be adapted to apply a pixel voltage to display a substantially black picture on the display screen during a period of the field period except the image information write period.
Thereby, the sharpness of the moving picture can be improved.
The liquid crystal display may further comprise: a lighting device including a light source for supplying light transmitted through the liquid crystal layer and control means for controlling the light source to be tuned on during the image information write period of the field period and to be turned off during the remaining period.
Thereby, since the display screen is dark while the light source is OFF, the sharpness of the moving picture can be improved.
In this case, a ratio of a capacity for the capacitive coupling to the capacity for the liquid crystal capacitor of the pixel may be 0.7 or more.
Thereby, since the change in the pixel voltage due to the change in the capacity for the liquid crystal capacitor is large, the transient voltage can be made higher. Consequently, the high speed response of the liquid crystal can be achieved.
In this case, the ratio of the capacity for the capacitive coupling to the capacity for the liquid crystal capacitor of the pixel may be 1 or more.
Thereby, since the transient voltage can be made higher, the high speed response of the liquid crystal can be achieved.
Also, in this case, a maximum level of the pixel voltage and a minimum level of the pixel voltage respectively may correspond to upper and lower limit levels of the luminance information of the image information and a ratio of dielectric constant of the liquid crystal layer under the minimum level to dielectric constant of the liquid crystal layer under the maximum level may be 1.2 or more.
Thereby, since the change in the capacity for the liquid crystal capacitor occurring when the luminance information of the image information changes is large, the high speed response of the liquid crystal can be achieved.
The ratio of dielectric constant may be 1.4 or more.
Thereby, the higher response speed of the liquid crystal can be achieved.
The dielectric constant anisotropy of the liquid crystal layer may be 6.5 or more.
Thereby, the change in the dielectric constant of the liquid crystal occurring when the luminance information of the image information changes is increased according to the dielectric constant anisotropy and “high speed drive” is possible when the dielectric constant anisotropy is 6.5 or more.
The dielectric constant anisotropy of the liquid crystal layer may be 7.7 or more.
Thereby, higher response speed of the liquid crystal can be achieved.
According to the present invention, there is also provided a liquid crystal display comprising: a liquid crystal layer capable of bend orientation; a display screen composed of a plurality of pixels on which an image is displayed by light transmitted through a bend-oriented liquid crystal layer; and pixel voltage application means for sequentially applying a pixel voltage to the liquid crystal layer of all the pixels according to luminance information for each pixel of image information, the pixel voltage being applied to cause transmittance of the light to change, thereby displaying the image corresponding to the image information on the display screen, and the pixel voltage application means is adapted to apply an offset voltage forming the pixel voltage together with a voltage applied to the liquid crystal layer of the pixels during the sequential application through capacitive coupling after the sequential application to prevent backward transition from bend orientation to spray orientation of the liquid crystal layer.
With this configuration, the offset voltage can be applied without limiting an available size of the liquid crystal panel depending on the charging capacity of the liquid crystal panel, although the application of the offset voltage by the change of the counter voltage limits the available size of the liquid crystal panel depending on the charging capacity of the liquid crystal panel. Also, since the pixel voltage transiently changes, the offset voltage can be applied by utilizing the CC drive. Therefore, the liquid crystal display can realize very high speed response and simplify the configuration to apply the offset voltage.
The liquid crystal display may further comprise: gate drive means for sequentially scanning the plurality of pixels through a gate electrode, and the pixel voltage application means may include source drive means for applying a base voltage based on the luminance information of the pixels of the image information to the liquid crystal layer of the pixels sequentially scanned, through a source electrode; and offset voltage application means for applying an offset voltage forming the pixel voltage together with the base voltage to the pixel through the capacitive coupling after the pixels are scanned, and the capacitive coupling may be formed between the pixel electrode and a preceding gate electrode in the order in which the pixels are scanned.
Thereby, since the offset voltage can be applied by utilizing the gate electrode, the configuration of the offset voltage application means can be simplified.
The capacitive coupling may be formed between a pixel electrode and a dedicated capacitor line.
The offset voltage may be 1 v or more.
Thereby, in the general OCB liquid crystal panel, the backward transition from the bend orientation to the spray orientation can be prevented.
The offset voltage may be greater than a voltage at which the liquid crystal layer transitions backward from the bend orientation to the spray orientation.
Thereby, the backward transition from the bend orientation to the spray orientation can be prevented.
In this case, a substantially black picture may be displayed on the display screen in a field period corresponding to a predetermined cycle in which the image information of one field is written.
Thereby, the required offset voltage can be reduced and the sharpness of the moving picture can be improved.
The display screen may be substantially rectangular and have a diagonal line having a length of 10 inches or more.
Thereby, in the liquid crystal display of this size, the offset voltage can be applied advantageously by the configuration of this embodiment.
The diagonal line may have a length of 15 inches or more.
Thereby, in the liquid crystal display of this size, the offset voltage can be applied only by using the configuration of the present invention.
According to the present invention, there is further provided liquid crystal display comprising: a liquid crystal layer capable of bend orientation; a display screen composed of a plurality of pixels on which an image is displayed by light transmitted through a bend-oriented liquid crystal layer; and a pixel voltage application means, the pixel voltage being applied to cause transmittance of the light to change, thereby displaying the image corresponding to the image information on the display screen, and the liquid crystal layer of the pixels transitions to bend orientation by using a voltage applied to the liquid crystal layer of the pixels through capacitive coupling.
With such configuration, in addition to the normal voltage applied by the pixel voltage application means, the voltage applied through the capacitive coupling can be used as the transition voltage. Therefore, the liquid crystal can transition in a short time.
The liquid crystal display may have an inactive period during which no voltage is applied to the liquid crystal layer of the pixels, prior to the transition.
Thereby, since no voltage is applied to the liquid crystal layer before transition, the preferable transition can take place.
The liquid crystal display may further comprise: gate drive means for sequentially scanning the plurality of pixels through a gate electrode; and the pixel voltage application means may comprise source drive means for applying a base voltage based on the luminance information of the pixels of the image information to the liquid crystal layer of the pixels sequentially scanned, through a source electrode, and a cumulated voltage application means for applying a cumulated voltage forming the pixel voltage together with the base voltage to the pixels through the capacitive coupling after the pixels are scanned, and the cumulated voltage may be used to cause the liquid crystal layer of the pixels to transition to bend orientation.
With this configuration, by transiently changing the pixel voltage, the cumulated voltage by the CC drive can be used as part of the transition voltage. Therefore, the liquid crystal display can realize very high speed response and reduce the transition time.
The capacitive coupling may be formed between the pixel electrode and a preceding gate electrode in the order in which the pixels are scanned.
Thereby, since the cumulated voltage can be applied by using the gate electrode, the configuration of the cumulated voltage application means can be simplified.
The capacitive coupling may be formed between a pixel electrode and a dedicated capacitor line.
The gate drive means as the cumulated voltage application means may be adapted to apply the cumulated voltage to the respective pixels while sequentially scanning all the pixels during the transition.
Thereby, the gate drive means can operate in the same mode during transition and during display.
The source drive means may be adapted to output an alternating current base voltage having a transition voltage value, and the gate drive means may be adapted to output a gate signal having two voltage levels at which a switching element provided for each pixel is placed in a conductive state when the pixel is scanned and is placed in a cut-off state when the pixel is not scanned, during the inactive period, and output a gate signal having two voltage levels at which the cumulated voltage having a polarity according to a polarity of the base voltage just after the pixel is scanned, in addition to the two voltage levels, during the transition period.
Thereby, the cumulated voltage can be applied to the liquid crystal of the pixels during transition, and is prevented from being generated during the inactive period. Consequently, transition can take place preferably and in a short time.
The source drive means may be adapted to output a direct current base voltage having a transition voltage value, the gate drive means may be adapted to output a gate signal having two voltage levels at which a switching element provided for each pixel is placed in a conductive state when the pixel is scanned and is placed in a cut-off state in which the pixel is not scanned, during the inactive period, and output a gate signal having one voltage level at which the cumulated voltage having a polarity identical to a polarity of the base voltage can be applied just after the pixel is scanned, in addition to the two voltage levels, during the transition period.
Thereby, since the cumulated voltage has one polarity, it can be generated with high efficiency.
According to the present invention, there is still further provided a liquid crystal display comprising: a twisted nematic mode liquid crystal layer; a display screen on which an image is displayed by light transmitted through the liquid crystal layer; and a liquid crystal voltage application means for applying a liquid crystal voltage to the liquid crystal layer according to luminance information for each field of image information composed of serial fields, the liquid crystal voltage being applied to cause transmittance of the light to change, thereby sequentially displaying the image corresponding to the fields of the image information, on the display screen, and the liquid crystal voltage application means may apply the liquid crystal voltage which changes so as to have a value according to the luminance information by the time the liquid crystal voltage is applied for the subsequent field when the luminance information changes between current and subsequent fields, the liquid crystal voltage changing so as to have a value according to the luminance information after excessively increased when the luminance information changes to cause the corresponding liquid crystal voltage to be increased, and the liquid crystal voltage changing so as to have a value according to the luminance information after excessively reduced when the luminance information changes to cause the corresponding liquid crystal voltage to be reduced, and the liquid crystal layer has a thickness of 3 μm or less.
Thereby, high speed response of the liquid crystal can be achieved because the large electric field is generated in the liquid crystal layer. As a result, since this liquid crystal display is capable of performing “double speed drive” and displays the sharp moving picture by the insertion of the black picture in the moving picture display system of the field frequency of 60 Hz generally adopted, this can be used practically in the television, monitor, or the like in terms of the response speed.
These objects as well as other objects, features and advantages of the invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawings.
a), 7(b) are graphs showing the relationship between change in the gate signal and change in the source signal, wherein
a)-9(e) are graphs for explaining change in transmittance of the pixel according to normal drive, wherein
a)-10(e) are graphs for explaining change in transmittance of the pixel according to the first embodiment of the present invention, wherein
a)-12(c) are tables showing Rise time and Decay time between gray scales, wherein
a), 13(b) are three-dimensional graphs visually showing Rise time and Decay time between gray scales, wherein
a), 19(b) are graphs each showing waveforms of the counter voltage, the gate signal, and the source signal at the activation of the liquid crystal display according to the third embodiment, wherein
Hereinafter, embodiments of the present invention will be described with reference to drawings.
Referring now to
The backlight 18 is adapted to supply the display light to the liquid crystal display element 106 via a light guide plate (not shown) from a light source 15 driven by a lighting circuit 16.
The display control circuit 19 comprises a display controller 13, a gate driver 11, a source driver 12, and a lighting controller 17. The display controller 13 is adapted to output control signals to the gate driver 11, the source driver 12, and the lighting controller 17, according to the video signal 14, respectively. In accordance with the control signal, the gate driver 11 is adapted to output a gate signal through a gate electrode 2, thereby sequentially scanning (selecting) a pixel of the liquid display element 106 for each gate electrode 2. In accordance with the control signal, the source driver 12 is adapted to output a source signal according to the timing of the gate signal, thereby sequentially writing the source signal to the scanned pixel through a source electrode 3. Thereby, transmittance of each pixel with respect to the display light is varied according to the source signal. Consequently, an image according to the video signal 14 is displayed on the liquid display element 106. The lighting controller 17 serves to control the lighting circuit 16 to drive the light source 15 in accordance with the control signal from the display controller 13.
Referring to
As shown in
Subsequently, an operation of the liquid crystal display 1 so structured will be explained.
As shown in
The gate driver 11 outputs a gate signal Sg described b low. The gate signal Sg has a voltage of Vgon in a write period Ta, Vge1 in the odd field and Vge2 in the even field in a cumulating period Tp subsequent to the write period Ta, and VgOff in the remaining period Tr other than the write period Ta and the cumulating period Tp. Vge1 is set higher than Vgoff by Vge(+) and Vge 2 is set lower than Vgoff by Vge(−). Vge1 as well as Vg2 is set to cause the switching element 5 to be placed in a cut-off state (high-resistance state). The cumulating period Tp is set more than twice as long as the write period Ta. In the gate signal Sg of this embodiment, Vgon is set to a predetermined positive value, Vgoff is set to −10V, Vge1 is set to −3V, Vge(+) is set to 7V, Vge 2 is set to −18V, and Vge(−) is set to −8V.
As shown in
In the even field, the pixel voltage Vp′ changes negative to positive, in which case, as shown in
Vcc=Cst/(Cst+Cgd+Clc)×(Vge(+) or Vge(−))
In general, the voltage including the compensation voltage Vcc and to be applied to the pixel electrode 6 is expressed as:
Vp′=Vs+Vcc
The CC drive is defined as the method for driving the liquid crystal element described above. It is known that the use of the CC drive permits a higher response speed in the TN liquid crystal. This is due to dielectric constant anisotropy.
Here it is assumed that the transmittance of the liquid crystal display element (hereinafter simply referred to as transmittance) changes from 100% to 0% in an arbitrary pixel and the display mode is a normally white mode. When the transmittance is 100%, the voltage applied to the liquid crystal is low and the dielectric constant of the liquid crystal is small. Conversely, when the transmittance is 0%, the voltage applied to the liquid crystal is high and the dielectric constant is large.
Since the response of the liquid crystal molecules requires time longer than that of charging of the pixel electrode (write of source signal), it is delayed with respect to the charging of the pixel electrode.
The voltage Vp′ applied to the pixel electrode (hereinafter referred to as pixel voltage) in an initial stage of charging of the pixel electrode, and more accurately, just after the end of the write period, is given by:
Vp′(initial value)=Vs+Cst/(Cst+Cgd+Clc(100))×Vge(+)
By the response of the liquid crystal, this changes as follows:
Vp′(saturation value)=Vs+Cst/(Cst+Cgd+Clc(0))×Vge(+)
Assuming that Clc(100) is a capacity for liquid crystal capacitor of transmittance=100% and Clc(0) is a capacity for liquid crystal capacitor of transmittance=0%, in this capacity for liquid crystal capacitor, the relationship between Clc(100) and Clc(0) is:
Clc(100)<Clc(0)
Therefore, the following relationship is established:
Vp′(initial value)>Vp′(saturation value)
In this case, Vp′ (saturation value) corresponds to the voltage to be applied to the pixel electrode 6, i.e., the set pixel voltage Vp, which corresponds to luminance information (gray scale) for each pixel of the video signal.
Since the transmittance changes 100% to 0%, the voltage being applied to the liquid crystal correspondingly changes from low to high. During this change, a high voltage such as Vp′ (initial value) is transiently applied to the liquid crystal in the initial stage of charging, thereby resulting in a higher response speed of the liquid crystal.
On the other hand, when a dark state with low transmittance changes to a relatively bright intermediate gray scale state with relatively high transmittance, the voltage being applied to the liquid crystal change from high to relatively low. In this case, since Vp′ (initial value)<Vp′ (saturation value), in the initial stage of charge, the low voltage of Vp′ (initial value) is transiently applied to the liquid crystal. Consequently, also in this case, a higher response speed of the liquid crystal is achieved.
Subsequently, to clarify the characteristic of the present invention, comparison between the present invention and a normal drive method (hereinafter referred to as normal drive) will be explained.
As shown in
An operation of the normal drive will be explained. Assume that the voltage being applied to the liquid crystal (pixel voltage Vp′) rapidly changes from high to low. As shown in
Vp′=(Cst+Clc(0))/(Cst+Clc(100))×Vp
In summary, the problem with the normal drive is that the change of the dielectric constant of the liquid crystal changes the pixel voltage Vp′ such that the pixel voltage Vp′ degrades the response of the liquid crystal.
Accordingly, in this embodiment, the change of the dielectric constant changes the pixel voltage Vp′ so that the pixel voltage Vp′ quickens the response speed of the liquid crystal. Specifically, a pulse gate signal is adopted in this embodiment like the normal drive as shown in
As should be appreciated, the present invention is characterized in that the compensation voltage is applied to permit a faster response of the liquid crystal, and the CC drive is defined as the drive carried out by automatically applying the compensation voltage according to the change in the capacity for the liquid crystal capacitor.
Subsequently, effects of the liquid crystal display according to this embodiment will be explained. In the normal drive, although the OCB liquid crystal mode permits high speed response, it was difficult to realize the response within one field regardless of the OCB liquid crystal mode. This is because the change of the dielectric constant impedes the high-speed response of the liquid crystal as described above. Accordingly, the OCB liquid crystal mode and the CC drive are combined to reliably achieve the response within one field period.
0.92(τr)+3.2(τd)=4.12 [ms]
In
In the OCB liquid crystal mode of the normal drive, the response speed between gray scales is 12.8 ms at maximum. The OCB mode of the normal drive is capable of performing “high speed drive” to some degree as well as writing of the video signal at a field frequency of 60 Hz but is incapable of writing of the video signal at 120 Hz enabling the display of the sharp moving picture”, i.e., “double speed drive”. Consequently, the OCB liquid crystal mode of the normal drive is impracticable for use in television, monitor, or the like.
The characteristic of the OCB liquid crystal mode of the CC drive is indicated by a curved line A in
The characteristic of the TN liquid crystal mode of the normal drive widely used currently is indicated by a curved line C in
a), 13(b) are three-dimensional graphs visually showing Rise time and Decay time between gray scales, wherein
a), 13(b) show measurement of Rise time and Decay time between gray scales which have levels more than those of the measurement of
As can be seen from
Subsequently, a temperature characteristic of the liquid crystal display according to the embodiment will be explained. In the OCB liquid crystal mode of the CC drive, the lower limit of temperature at which “double speed” was possible was 10° C. It should be remembered that 10° C. refers to the temperature of the liquid crystal display element warmed by the backlight or the like and an ambient temperature in this case was 10° C. This means that the liquid crystal display of this embodiment realized satisfactorily preferable “double speed drive” below the room temperature. On the other hand, in the OCB liquid crystal mode of the normal drive, the lower limit of temperature at which the drive at the field frequency of 60 Hz was possible was 25° C., and below 25° C., even the drive at 60 Hz was difficult.
Subsequently, preferable conditions of this embodiment will be described. The high speed response by the CC drive is brought about by the overlapped compensation voltage Vcc and the change in the pixel voltage Vp′ due to the dielectric anisotropy as described above. Therefore, it is preferable that anisotropy of the dielectric constant is high. This embodiment adopted a liquid crystal material with the dielectric constant which is 11 under a full voltage, 5 under non-voltage, 10 under a black display voltage, and 7 under a white display voltage. One important parameter in selecting the liquid crystal material is the ratio of the dielectric constant under the black display voltage and the dielectric constant under the white display voltage (hereinafter referred to as a dielectric ratio) and the higher ratio is effective. In this embodiment, the liquid crystal material with the dielectric ratio of 1.4 was used. When the dielectric ratio is 1.2 or more, the high speed response is achieved, and when the ratio is 1.4 or more, the material was applicable to the “double speed drive” at a frequency of 120 Hz during the image information write period. In general, the TN liquid crystal has the dielectric ratio of 2 or more, while the OCB liquid crystal has a slightly lower dielectric ratio because the liquid crystal is used in the state in which liquid crystal molecules thereof are substantially raised. This limits the degree of freedom at which the liquid crystal material is selected. Accordingly, in this embodiment, the liquid crystal material with high dielectric constant anisotropy was selected, thereby improving the dielectric ratio. The dielectric constant of the liquid crystal material used in this embodiment was ∈ vertical=3.7, ∈ parallel=11.5. Therefore, the dielectric constant anisotropy Δ∈=∈ parallel−∈ vertical=7.8 As for the selection of the liquid crystal material, when Δ∈>6.5, the dielectric ratio is 1.2 or more, and the high speed response is achieved, and when Δ∈>7.7, the dielectric ratio is 1.4 or more and the material was applicable to the “double speed drive”.
Another important parameter in the CC drive is the ratio of a capacity for the storage capacitor Cst to a capacity for the liquid crystal capacitor Clc and larger capacity for the storage capacitor Cst is effective. In this embodiment, the capacity ratio Cst/Clc is set to 1. To achieve high speed response, the capacity ratio is preferably set to 0.7 or more. To apply to the “double speed drive”, the capacity ratio is more preferably set to 1 or more.
As should be appreciated, according to this embodiment, the response time of the liquid crystal element can be reduced to ½ or less as compared to the conventional drive method. This is a very effective in view of empirical rule of the TN liquid crystal mode. It is considered that this effect is brought about by the characteristic of the OCB liquid crystal mode in which a variation in the amount of transmitted light with respect to the change of the dielectric constant of the liquid crystal is large. In other words, the effect of this embodiment is the synergism due to the compatibility of the configuration of the CC drive with the characteristic of the OCB liquid crystal mode rather than the sum of the high speed response effect by the CC drive and the high speed response effect of the OCB liquid crystal mode. Also, it was confirmed that the increase in the anisotropy of the dielectric constant further enhanced the effects of high speed response.
Subsequently, a modification of this embodiment will be described.
The method for supplying the compensation voltage to the pixel electrode is not limited to a preceding gate method. What is needed is the compensation voltage is supplied to the pixel electrode from an electrode capacitively coupled thereto.
In the above-described example, the compensation voltage is supplied from the capacitively coupled gate electrode to be automatically overlapped. The primary aim of the present invention is to apply the compensation voltage so as to accelerate the change in the transmittance of the liquid crystal display element, and is therefore achieved without the use of the capacitive coupling. Accordingly, in this modification, a compensation voltage application circuit for this purpose is embodied.
In the embodiment described above, the OCB liquid crystal mode of the CC drive is employed to realize a higher speed response, while in this modification, the insertion of the black picture within the field period is combined into the OCB liquid crystal mode of the CC drive. With such a configuration, the sharpness of the moving picture, i.e., viewability thereof is improved. Here, the field period is defined as a cycle in which image information (video signal) corresponding to one picture is written. Also, a period in the field period during which the image information corresponding to one picture is sequentially written to all the pixels is called an image information write period. Further, a period in the field period during which the black picture is written is called a black picture insertion period. In this modification, effects were provided when the image information write time was less than 90% of the field period. For example, when the black picture insertion period was set to 10% or more of the field period, the liquid crystal hardly returned to the spray orientation, that is, hardly transitioned backward. When the image information write period is set to less than half of the field period, the remaining period is used as the black picture insertion period. Therefore, viewability can be further improved. It should be noted that the voltage for black picture display may be a black level or substantially black level voltage, or a voltage higher than the black level.
In this modification, the backlight is turned off during the black picture insertion period within the field period. More specifically, in the configuration of
In this modification, in the liquid crystal display in the TN liquid crystal mode of the CC drive, a cell thickness is set to 3 μm or less. Thus reduced cell thickness provides large strength of an electric field generated in the liquid crystal. Thereby, the high speed response is achieved. When the cell thickness was 3 μm or less, “double speed drive” was achieved as in the case of the OCB liquid crystal mode of the CC drive. Off course, a higher response is obtained in this configuration by selecting the dielectric constant anisotropy and the dielectric ratio of the liquid crystal material as described above.
The CC drive employed in the first embodiment advantageously optimizes the drive voltage as well as permits the high speed response. In the second embodiment, the offset voltage is applied by utilizing the CC drive.
The whole configuration of this embodiment is identical to that of the first embodiment except that the compensation voltage Vcc of
The problem associated with the OCB liquid crystal display is that the spray orientation tends to be generated at a very low voltage. For this reason, in general, there has been used a drive method in which the pixel voltage is set to a fixed value or more. One preferable drive method may be that the potential of the counter electrode is changed in the form of the AC square waveform, and thereby the offset voltage is applied.
This drive method is suitable for a small-sized liquid crystal panel (liquid crystal display element) but is less suitable for a large-sized liquid crystal panel. This is because a CR time constant during charge is too large due to a too large capacity of the liquid crystal panel. According to the study by the inventor of this invention, in practice, it was impossible to apply the offset voltage to the liquid crystal panel of 10 inch type or more by the above drive method. Further, without the CC drive, it was impossible to apply the offset voltage to the liquid crystal panel of 15 inch type or more. Here, x type means that the length of a diagonal line of a substantially rectangular display screen of the liquid crystal panel is x inches.
Accordingly, in this embodiment, the offset voltage is applied by utilizing the CC drive.
By the way, in the OCB liquid crystal display, the voltage at which the liquid crystal transitions backward to the spray orientation depends on a pretilt angle. When the pretilt angle was 15 degrees, this backward transition voltage was 1 v. According to the study of the inventor, the general OCB liquid crystal panel required the offset voltage of 1 v or more. Also, when the black picture was inserted into one field, a lower offset voltage was satisfactorily used. That is, the bend orientation is kept by the insertion of the black picture even if the lower voltage is temporarily applied to the liquid crystal. In this case, however, it should be remembered that a critical voltage at which backward transition to the spray orientation takes place is just lowered, and therefore, the offset voltage needs to be always applied. The offset voltage in this case may be 1 v or less.
The third embodiment employs the CC drive in transition from the spray orientation to the bend orientation at the activation of the liquid crystal display.
The liquid crystal display of this embodiment has the configuration of the first embodiment and is adapted to output the counter voltage, the gate signal, and the source signal in waveforms described below when activated. The liquid crystal display is provided with a driver for driving the counter electrode.
As shown in
Referring to
On the other hand, the two-valued signal is output during the inactive period T1. The reason is as follows. For preferable transition, it is desirable that no voltage is applied to the liquid crystal during the inactive period T1. However, if the four-valued signal is output like during the transition voltage application period T1, the CC drive causes the cumulated voltage Vcc to be applied to the liquid crystal. Accordingly, the gate signal Sg during the inactive period T1 was set as the two-valued signal to prevent the generation of the cumulated voltage Vcc.
The source signal Ss has a voltage equal to the counter voltage Vcom during at least the inactive period T1 to prevent the voltage from being applied to the voltage during the inactive period T1. In this embodiment, in the transition period T3, the source signal Ss is set to a constant value of 3 v during the inactive period T1 and the transition voltage application period T2.
In this embodiment, with the above-described configuration, high speed transition was achieved. Specifically, the transition time, which was conventionally 3 seconds, was reduced to 2 seconds.
One example of prior arts is disclosed in Japanese Laid-Open Patent Application No. Hei. 9-185037. In this prior art, the gate voltage as the transition voltage being applied was always set to High level. In this embodiment, for the efficient transition, the gate electrode is scanned like the display state (after transition) and thereby, the cumulated voltage Vcc is efficiently utilized during transition.
Subsequently, a modification of this embodiment will be described.
In this modification, during the inactive period T1, the source signal Ss and the counter voltage Vcom are both set to 0 v and no voltage is therefore applied to the liquid crystal. During the transition voltage application period T2, the counter voltage Vcom is greatly swung to −20V, whereas the source signal Ss is swung to +7 v. The gate signal Sg is a three-valued signal as shown in an enlarged view of dot-lined portion of
While in the first to third embodiments, the layered electrode made of a conductive material is formed on the inner surface of the substrate as an electrode portion, this electrode portion is only illustrative. For example, between the electrode and the liquid crystal, there may be placed an electric characteristic variant in which its electric characteristic thereof switches between insulativity and conductivity by irradiation of light, and the electric characteristic variant and the electrode may constitute the electrode portion.
Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, the description is to be construed as illustrative only, and is provided for the purpose of teaching those kill d in the art the best mode of carrying out the invention. The details of the structure and/or function may be varied substantially without departing from the spirit of the invention and all modifications which come within the scope of the appended claims are reserved.
The liquid crystal display of the present invention is useful as a liquid crystal television, a liquid crystal monitor, or the like for displaying the moving picture requiring high speed response.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2000-218543 | Jul 2000 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP01/06203 | 7/18/2001 | WO | 00 | 7/16/2003 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO02/07142 | 1/24/2002 | WO | A |
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