Patent Publication Number: US-8111225-B2

Title: Liquid crystal display device having a plurality of subfields

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. nonprovisional application is a divisional of U.S. application Ser. No. 11/441,224, filed May 26, 2006 now abandoned, which claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2005-155751, filed on May 27, 2005, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display device. 
     2. Description of the Related Art 
     In recent years, liquid crystal display devices are in use as a unit of a digital television set, however, they are devaluated due to responses inferior to those of CRTs in displaying moving images. As a main cause thereof, it is known that a displacement is caused during one frame between an image on the same screen continuously displayed and an eye movement. As methods to obtain a display of the same level as of CRT, there have been presented a method in which a black screen is inserted and a method in which the flashing cycle (or a repeating cycle of a light state and a dark state) of a back light is made to coincide with a display cycle. However, both the methods are forced to lower the display luminance of completely white, having not yet reached to a wide practical use. 
     Further, in Japanese Patent Application Laid-Open No. 2000-338464 (patent document 1), there is described that, in a display element displaying images of plural frames in a second, a single frame F 0  is displayed by being divided into at least two fields F 1 , F 2 , in which at least in a sub field  1 F of the single field F 1 , a desired image is displayed at a first luminance T x  and, in a remaining single sub-field  2 F, an image being practically the same as the image displayed at the first luminance is displayed at a second luminance T y  being smaller than the first luminance and larger than 0 (zero). 
     As an art realizing a higher luminance and a moving-image display performance together, it is conceivable to perform halftone display by driving the panel at a double speed. However, in the liquid crystal display device adopting the art, there exist two problems as will be described below. First, an insufficient resolution in the halftone display. Second, a ghost image at a luminance change is not resolved especially between low-tones. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to prevent resolution insufficiency in halftone display, or to prevent a ghost image at a luminance change between low tones. 
     According to an aspect of the present invention, a liquid crystal display device including: a liquid crystal panel provided with plural gate lines to select a pixel and plural data lines to supply pixel data; and a data driver dividing a single frame into plural fields and converting frame data into field data to supply the field data to the data line is provided. When the frame data has a tone change, the data driver performs correction to data of an odd-number field of the frame in a same direction as an increase/decrease direction of the tone change of the frame data, and performs correction to data of an even-number field of the frame in an opposite direction to the increase/decrease direction of the tone change of the frame data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing an example configuration of a liquid crystal display device according to first to fourth embodiments of the present invention; 
         FIG. 2  is a view showing a field tone signal, a transmissivity in the vicinity of a bank structure, a transmissivity at a center portion of a pixel, and a transmissivity of a liquid crystal unit in the case where no response compensation is performed; 
         FIG. 3  is a view showing the field tone signal, the transmissivity in the vicinity of the bank structure, the transmissivity at the center portion of the pixel, and the transmissivity of the liquid crystal unit in the case where only the first field FD 1  is compensated in response; 
         FIG. 4  is a view showing the field tone signal, the transmissivity in the vicinity of the bank structure, the transmissivity at the center portion of the pixel, and the transmissivity of the liquid crystal unit in the case where the first field FD 1  and the second field FD 2  are compensated in response; 
         FIG. 5  is a view to explain a time proportion of divided fields according to the second embodiment of the present invention; 
         FIG. 6A  is a view showing tones of the first field FD 1  and the second field FD 2  according to the second embodiment of the present invention and  FIG. 6B  is an enlarged view of a low-tone region of the  FIG. 6A ; 
         FIG. 7  is a view showing frame tone (input into a liquid crystal unit), field tone, liquid crystal unit luminance and display in the case where a back light is lighted continuously; 
         FIG. 8  is a view showing a back-light luminance, the liquid crystal unit luminance and display in the case where the back light is driven to/from light and dark; 
         FIG. 9A  is a view showing a connection example of the back light and a liquid crystal panel, and  FIG. 9B  is a sectional view of the back light and the liquid crystal panel; 
         FIG. 10  is a view showing a relation between the frame tone and the field tone; 
         FIG. 11  is a view showing a relation between luminance of a frontal view and the luminance of an oblique view; 
         FIG. 12A  is a view showing the connection example of the back light and the liquid crystal panel according to a fourth embodiment of the present invention, and  FIG. 12B  is a sectional view of the back light and the liquid crystal panel; 
         FIG. 13  is a timing chart showing a signal timing and a driving of the panel and the back light; and 
         FIG. 14  is a view showing an example where a single frame is divided into two fields. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a view showing an example configuration of a liquid crystal display device according to first to fourth embodiments of the present invention. A timing controller  104  includes a data converter  105  and is capable of writing/reading into/from a memory  106 . The data converter  105  divides a single frame into plural fields in view of time to convert frame data into field data. The plural fields display respectively at a tone different from each other. A gate driver  102  supplies gate pulse voltage to a gate line (scanning line) in a liquid crystal panel  101  for each field thereof under the control of the timing controller  104 . The gate line is a line to select a pixel. A data driver  103  supplies data voltage a data line (signal line) in the liquid crystal panel  101  for each field thereof under the control of the timing controller  104 . The data line is a line to supply pixel data. The liquid crystal panel  101  includes an array substrate in which plural gate lines cross over plural data lines and active elements (TFTs: thin-film transistors) are provided at their crossover points, and an opposite substrate having at least ITO formed. The array substrate and the opposite substrate sandwich a liquid crystal layer therebetween. The above described TFT is arranged for each pixel. The TFT is, partly or wholly, formed by polysilicon. Also, the TFT is connected to the gate line and the data line via its gate and drain, respectively. When the gate line is supplied with gate pulse, the corresponding TFT turns ON, allowing the pixel of the TFT to be selected. In the pixel of the selected TFT, the alignment direction of liquid crystal molecules are determined in accordance with the data voltage supplied to the data line whereby the amount of transmitted light is determined, allowing the tone value of the pixel to be controlled. 
       FIG. 14  is a view showing an example in which a single frame is divided into two fields. The horizontal axis indicates tones of the inputted frame data and the vertical axis indicates tones of a first field FD 1  and a second field FD 2 . The first field FD 1  is a first field and the second field FD 2  is a last field. 
     As a data voltage, on a lower tone side, a data voltage V 1  is applied to the first field FD 1  and a data voltage Vd of black (lowest tone value) is applied to the second field FD 2 . Further, on a higher tone side, a data voltage Vw of white (highest tone value) is applied to the first field FD 1  and a data voltage V 3  is applied to the second field FD 2 . Respective voltages are selected so that the respective luminance originally aimed by the respective frames can be achieved on the basis of time quadrature of the data voltages V 1  and Vb at the lower tone and the data voltages Vw and V 3  at the higher tone. 
     A tone in which the voltage of the first field FD 1  changes from V 1  to Vw is a tone requiring 255 tone=Vw as V 1  to achieve the frame luminance, being around 200 tone as an example. For instance, it is set so that a sum of the tone of the first field FD 1  and the second field FD 2 −luminance characteristics meets γ=2.4. 
     A first object of such a tone setting is to improve response speed. The response characteristic of liquid crystal of vertical alignment (VA) type is known for its worse response when changing from a halftone to a halftone. In order to improve the response characteristic, there are two approaches as will be described below. (1) an approach that applies a voltage of a tone close to black in advance by which the liquid crystal molecules are given a pretilt angle, so that the response characteristic to a next tone is improved. (2) an approach that increases the voltage value of the tone to be achieved in that the response characteristic is better as the achieved tone becomes higher. 
     The later approach corresponds to a principle of overdrive. The voltage except the black voltage according to the former approach is because there is sometimes a case where the response speed is faster when an appropriate halftone is applied than when the black voltage is applied in advance. 
     The tone selection in the present embodiment has effects of a response speed improvement by enabling to apply a higher voltage to the first field FD 1  than the conventional tone voltage on the lower tone side, and of a response characteristic improvement for the first field FD 1  of the next frame by fixing the voltage applied to the second field FD 2  to the black voltage on the lower tone side. Further, by fixing the voltage applied to the first field FD 1  on the higher tone side to the white voltage, an effect of improving the response characteristic from the second field FD 2  of the previous frame can be obtained. 
     A second object is to improve a moving-image characteristic. By applying the black voltage to the second field FD 2  on the lower tone side, it is limited only to the first field FD 1  that contributes as luminance when the liquid crystals have completely responded. This degrades the moving-image characteristic, in which an impulse-type display is realized from a hold-type display. 
     A third object is to improve a viewing-angle characteristic. In order to improve the viewing angle characteristic of tones, it is required to keep plural tone characteristics, namely plural luminance characteristics, in the pixel or for a time period, and this tone selection method is exactly the one that performs that for each field of the frames. In other words, a halftone driving effect can be obtained by a double-speed driving. 
     In the present embodiment, the halftone driving at an n-times speed can be performed. The halftone driving at the n-times speed is a driving that achieves the aimed luminance on a time-average basis by performing plural tone displays, which are different in each field of the plural fields, by a unit of pixel. 
     Also, in this improvement on the viewing angle characteristic of tones, it is known that a larger difference between two tones, namely luminance characteristics, obtains a larger effect. Accordingly, the second field FD 2  is fixed at the black voltage Vb on the lower tone side, and the first field FD 1  is fixed at the white voltage Vw on the higher tone side. 
     However, in order to obtain desired improvement effects in the moving-image characteristic and in the viewing angle characteristic of tones by this data voltage application, the response characteristic of the second field FD 2  to black becomes important. In the first field FD 1 , the response characteristic is not regarded as a major problem backed by the application of overdrive (OD), however, when the black voltage is applied to the second field FD 2 , the response characteristic of liquid crystals to black becomes important since it is impossible to apply a voltage lower than the black voltage thereto. When the liquid crystals having a slower response speed are used here, the luminance cannot fall to black completely, so that the improvement effect in view of the moving-image characteristic degrades. 
     In other words, in the case of the liquid crystals having a slower response speed, it is required to use the overdrive (OD) for the second field FD 2  as well. In this case, the voltage set in the second field FD 2  at the lower tone is preferably the voltage of around 4 to 16 tone. When the voltage higher than the above is applied, the effect as an impulse-type display degrades and also the improvement effect in the viewing angle characteristic of tones degrades. 
     Further, when the response of the first field FD 1  to the white voltage is slow, the overdrive (OD) is applicable by intentionally lower the white voltage beforehand. As methods to lower, a method of lowering the white voltage and a method of using a driver that can apply higher voltage while maintaining the white voltage can be cited. 
     In the former case, the luminance lowers as well, in which the voltage cannot be lowered extremely, and that when a higher voltage is applied when the white voltage has lowered, an overshoot of a response waveform arises to affect adversely to the moving-image characteristic. Accordingly, as an example target to lower, a voltage of a tone of approximately 240 is appropriate from a practical usage viewpoint. 
     It is possible to use overdrive when responding from black to white as well as from white to black by assigning the white voltage to the black voltage within the voltage range excluding the maximum and minimum voltages without using the applicable maximum or minimum voltage as the white voltage or the black voltage. 
     As described above, in the last field of the plural fields, a first constant voltage (black (smallest tone value) voltage or a voltage near the black voltage) is applied to the data line when the frame data is the smallest tone value to a first tone value. Further, in the first field of the plural fields, a second constant voltage (white (highest tone value) voltage or a voltage near the white voltage), which is higher than the first constant voltage, is applied to the data line when the frame data is a second tone value to the highest tone value. 
     The first problem is resolution insufficiency in the halftone display. As shown in  FIG. 14 , the first problem is a phenomenon caused because a characteristic (γ characteristic) between a tone of an input signal and a displayed luminance is based on a power function, in which the luminance at a mean value of the tone is largely shifted from a half (½) of white luminance. In a low tone region  1401  and a high tone region  1403 , the field tone is excessive as compared to the displayed tone. In a halftone region  1402 , the field tone is insufficient as compared to the displayed tone, causing tone collapse. 
     The above-described problems can be solved with any of approaches described below. A first approach is an approach that adjusts the tone, namely luminance, characteristic at a stage of being inputted into the liquid crystal panel, which will be described later in a first embodiment. A second approach is an approach that reduces the time of the field performing a light display among the fields displaying different tones by time-dividing the frame, which will be described later in a second embodiment. 
     The second problem is that the ghost image in the luminance change is not yet resolved especially between the low tones (the response compensation method at the time of the displayed tone change is not unspecified). The second problem can be solved by matching the form of a luminance—time waveform in the frame (a portion becoming a boundary between the moving images) just after the tone change with the form of a luminance—time waveform after the tone change. The approach will be described later in third and forth embodiments. 
     First Embodiment 
     As a first embodiment according to the present invention, a case where a single frame cycle is divided into two fields will be described. A drive circuit includes the memory  106  and the data converter  105  to correct the data voltage as shown in  FIG. 1 . The data converter  105  compares the data of the previous frame with the data of the current frame, reads out a correction value in a data conversion table in the memory  106 , and adds it to the data of the field of the current frame to obtain a compensated tone data. The compensated tone data is designed to be applied to a pixel through the timing controller  104  and the data driver  103 . Further, this conversion is performed to the data of the two fields in the single frame. 
     When it is driven at a frame frequency of approximately 60 Hz, in the liquid crystal panel of VA type of which response time (time from 10% to 90% of the achieved luminance) is approximately 12 ms, the responses from black to halftone and from white to halftone have a response characteristic as shown in  FIG. 2  as a result of the combination of the elements. 
       FIG. 2  is a view showing a field tone signal, a transmissivity (luminance) in the vicinity of a bank structure, a transmissivity at a center portion of a pixel, and a transmissivity of a liquid crystal unit in the case where no response compensation is performed. At the center portion of the pixel, a delay by a phase  201  exists. In the vicinity of the bank in the pixel, the response is made at a time constant of 5 ms or below for the voltage change in each field, however, the response time constant delays as far it is away from the bank structure. In addition, the phase delay also arises with respect to the voltage change caused by repeating light and dark. As a result, in the response from black to halftone, the luminance change as shown in  FIG. 2  can be seen. 
       FIG. 3  is a view showing the field tone signal, the transmissivity in the vicinity of the bank structure, the transmissivity at the center portion of the pixel and the transmissivity of the liquid crystal unit in the case where only the first field FD 1  is compensated in response. The field tone is made to change with a compensation value  301  so that the luminance of an end of the first field FD 1  is made to be the same as of an end after the tone change (after stabilized) from the moment when the input tone signal changes. This compensation value  301  is the same numerical reference as of the input signal change, of which absolute value is approximately a third (⅓) or below as compared to the tone of completely white. However, only with this compensation, the luminance of the second field FD 2  tends to increase under the influence of such an element of the above-described response that has the phase delay. 
       FIG. 4  is a view showing the field tone signal, the transmissivity in the vicinity of the bank structure, the transmissivity at the center portion of the pixel and the transmissivity of the liquid crystal unit in the case where the first field FD 1  and the second field FD 2  are compensated in response. In addition to the above-described compensation value  301 , the second field FD 2  is reduced by a compensation value  402  being approximately a tenth ( 1/10) of an effective voltage of the tone of completely white. Therefore, in the second field (field on the lower luminance side) FD 2 , a larger effective voltage is set with respect to the black voltage (by the voltage for several tones). With this compensation value  402 , the luminance increase in the second field FD 2  in  FIG. 3  can be prevented. 
     As described above, when a frame data has a tone change, the correction is made with the compensation value  301  to the data of the first field FD 1  of the frame in the same direction as the increase/decrease direction of the tone change of the frame data, and the correction is made with the compensation value  402  to the data of the second field FD 2  of the frame in the opposite direction to the increase/decrease direction of the tone change of the frame data. 
     Furthermore, in the above-described setting, a third field tends to cause luminance insufficiency; the addition within the range of 10 tone at maximum or below is therefore made to the field tone. When the frame tone (input) changes from light to dark, the tone correction for the above-described response compensation adopts opposite numerical references for the compensation value. 
     Second Embodiment 
       FIG. 6A  is a view showing tones of the first field FD 1  and the second field FD 2  according to a second embodiment of the present invention and  FIG. 6B  is an enlarged view of a low-tone region of  FIG. 6A . The first field FD 1  and second field FD 2  have a function of displaying gray at an accuracy of 8-bit tone, respectively, realizing a gray display of an accuracy of 10-bit tone in combination. 
       FIG. 5  is a view to explain how to determine a time proportion of the fields according to the present embodiment. The horizontal axis indicates time and the vertical axis indicates luminance. T indicates a frame cycle. DB indicates the luminance level of completely black and DW indicates the luminance level of completely white. A reference number  511  indicates a response curve from completely white to completely black and a reference number  512  indicates a response curve from completely black to completely white. A reference number  501  indicates a light amount of the first field FD 1 , a reference number  502  indicates a light amount of the second field FD 2 , and a hatching portion denoted by a reference number  503  indicates a light amount of completely white (steady-state). A time T 3  is represented by τ 2 /Tτ×L. A luminance level D 1  is represented by L/1.25τ 1 . 
     For the liquid crystal panel in which the response time of the response curve  512  from completely black to completely white is 8 ms and the response time of the response curve  511  from completely white to completely black is 6 ms, the division proportion to divide a single frame into two fields is defined as one to two (1:2). Of the plural fields divided, the time of the first field FD 1  on the higher luminance side is shorter than the time of the other field FD 2 . 
     When the single frame is divided into two fields and when a frame time is defined as T, the time of the first field FD 1  on the higher luminance side is defined as L, and a response time from 10% to 90% of the finally achieved luminance when a change is made from a black display to a white display is defined as (2, then a field time proportion is set to meet an inequality shown below:
 
0.15 ×T&lt;L   2 /(2×τ1×1.25)+τ2/(2×τ1)× L&lt; 0.25 ×T   (1)
 
     What the inequality (1) means will be described with reference to  FIG. 5 . In the drawing, a case where the frame tone is expressed by approximating the light amounts, which are integrated with respect to time, to triangles as shown in the drawing by assuming the first field FD 1  as the tone of completely white and the second field FD 2  as the tone of completely black, since response times τ 1  and τ 2  of the liquid crystal panel are substantially at the same level as compared to the frame cycle T, is shown. The light amount  501  emitted during the first field FD 1  is represented by the following formula.
 
I0×L 2 /(1.25τ1)/2
 
     The light amount of the second field FD 2  is represented by the following formula.
 
I0×L×τ2/(2×τ1)
 
     When the combination of this field tones (255 tone/0 (zero) tone) corresponds to the halftone (128 tone) in the frame tone (input), it is possible to say that the combination of the field tones is assigned most effectively. 
     However, only with this art, it is possible only to set a 9-bit gray tone at most. Therefore, the second field FD 2  is defined as not black but 4 tone (8-bit expression) at maximum. By changing a tone of the second field FD 2  within the range from 0 (zero) tone to 4 tone, the rising speed of the luminance in the first field FD 1  shows a subtle change. As a result, the fineness in the tone expression can be improved. The respective field tones on the basis of 10-bit expression are shown in  FIGS. 6A and 6B . 
     When the frame tone data (input into the liquid crystal display device) is 5 tone or more and approximately 128 tone or below on the basis of 8-bit tone expression, the tone of the second field FD 2  on the lower luminance side can adopt the value of 2 tone to 5 tone on the basis of the 8-bit tone expression as the maximum value, and when the frame tone data (input into the liquid crystal display device) is approximately 128 tone or more on the basis of the 8-bit tone expression, the tone of the second field FD 2  on the lower luminance side can adopt the value of 250 tone to 253 tone on the basis of the 8-bit tone expression as the minimum value. 
     Third Embodiment 
     A third embodiment according to the present invention has a function in which the frame tone data of the previous frame stored in the memory  106  is compared with the tone data of the current frame and when there is a difference between them, the field tone change is performed only to the first field. 
       FIG. 9A  is a view showing a connection example of a back light  901  and a liquid crystal panel  902 , and  FIG. 9B  is a sectional view of the back light  901  and the liquid crystal panel  902 . The back light  901  emits light to the liquid crystal panel  902 . The liquid crystal panel  902  controls transmissivity of the light of the back light  901  to carry out the tone expression. 
       FIG. 7  is a view showing frame tone (input into a liquid crystal unit), field tone, liquid crystal luminance and display in the case where the back light  901  is lighted continuously. When the frame tone is constant, a tone display is performed by being divided into two fields so that they should be in order of dark tone/light tone. A relation between the input tone into the liquid crystal unit=the frame tone and the luminance of the liquid crystal unit (hereinafter referred to as “γ setting of a display section” is set to γ=2.4, and a relation between the frame tone and the respective field tones is set as shown in  FIG. 10 . The 8-bit frame tone is converted into an 8-bit tone of the first field (field on the lower luminance side) FD  1  and an 8-bit tone of the second field (fired on the higher luminance side) FD 2 . The time proportion of the first field FD 1  and the second field FD 2  is 1:1. A region  1003  is a region in which the tone of the second field FD 2  is saturated. 
       FIG. 8  corresponds to  FIG. 7  and is a view showing the frame tone (input into the liquid crystal), the field tone, the liquid crystal luminance and the display in the case where the back light  901  is driven to/from light and dark. As shown in  FIGS. 9A and 9B , the back light  901  is divided into four portions in the vertical direction of the screen, in which a light/dark state at a duty ratio of 40% is repeatedly lighted at the same frequency as of the frame frequency. The setting as to the back light is as shown below. 
     fluorescent tube: a cold-cathode tube of an outer diameter of φ3.0 mm and an inside diameter of φ2.4 mm. 
     tube current: a light state 7 mA, a dark state 3.5 mA (instantaneous luminance ratio: light state 5:dark state 2) 
     For synchronizing the driving of the back light  901  with the driving of the liquid crystal panel  902 , the following method is adopted. Around the time period of the writing performed into the gate line and assumed by the gate driver  102 , a binary constant voltage signal (3.5 V/0 (zero) V) is sent to the driving portion of the back light  901  at the timings shown in the drawing. The driving portion of the back light can be light at a phase being independent in each block of the back light  901 , and is placed under the control of the previously-described constant voltage signal to light. It is set to turn OFF at a signal voltage of 3 V or more and to turn ON at a signal voltage of 0.5 V or below. 
     When the frame tone has a change, a compensation value  702  is set to the first field FD 1  so that the finally achieved luminance of the first field FD 1  comes to the luminance of the first field FD 1  of which frame tone is stabilized after the change to thereby activate overdrive. The compensation value is as shown in  FIG. 10 . With only this response compensation function, the time change of panel transmissivity becomes as shown in  FIG. 7 , in which a gradation of blur can be viewed in a period of a half (½) frame  701  from the moment that the frame tone changes. 
     Backed by the above-described configuration of the backlight drive circuit, between the light/dark driving of the back light, the panel driving shifts by a half (½) cycle at a center of each block with respect to the panel driving. Therefore, around the moment where the second field FD 2  ends, the back light emits light while it is in the light state, and as a result, the unit luminance changes as shown in  FIG. 8 . After the frame tone change, a luminance waveform  802  of the first field FD 1  becomes substantially the same as of the latest luminance (light amount) waveform, so that a clear contour appears. The display in an intermediate state appears only in a period  801  being shorter than the half (½) of the period  701 . 
     As described above, the back light  901  increases/decreases luminance at the same frequency as the frame frequency. The back light  901  is divided into plural blocks in a line direction (in the direction toward which a line extends). In each block, for a pixel at the center portion of each block, the back light becomes the light state in a period around the end time of the second field FD 2  having the highest tone out of the plural divided fields. The pixel at the center portion has a delay of a phase  201  as shown in  FIGS. 2 to 4 . 
     When a single frame is divided into two fields, between a tone I 1  of the previous first field FD 1  and a tone I 2  of the following field, a relation of I 2 &gt;=I 1  is established all the time. 
     Incidentally, as a merit of placing the fields in the order of a dark field/light field in the single frame, a wider tone range applicable to the response compensation can be cited.  FIG. 11  shows: an ideal state being a luminance rising state of 0 (zero) by a characteristic line  1001 ; a display unit according to the present embodiment by a characteristic line  1002 ; a display unit driven only at a double speed in which a single frame is divided into two fields by a characteristic line  1003 ; and a display unit with no field division by a characteristic line  1004 . The horizontal axis shows luminance of a frontal view and the vertical axis shows luminance of an oblique view. In the case where it is in the order of the light field/dark field, no room is allowed to overdrive at a tone lighter than the halftone of the high tone (when the time proportion of the field division is 1:1, around 200 tone, and when the time proportion is 1:2, around 128 tone, both on the basis of 8-bit expression); meanwhile, in the case where it is in the order of the dark field/light field, the response compensation can be performed other than the case where a change is made to the tone of completely black or completely white. A completely black  1111  and a completely white  1112  have no effect in principle. The characteristic line  1002  according to the present embodiment comes close to the ideal characteristic line  1001 , allowing a black floating caused when viewing obliquely to be eliminated, so that an oblique-view characteristic can be improved. 
     Fourth Embodiment 
       FIG. 12A  is a view showing a connection example of a back light  1201  and a liquid crystal panel  1202  according to a fourth embodiment of the present invention, and  FIG. 12B  is a sectional view of the back light  1201  and the liquid crystal panel  1202 . The back light  1201  emits light to the liquid crystal panel  1202 . The liquid crystal panel  1202  controls transmissivity of the light of the back light  1201  to carry out the tone expression. 
     The present embodiment shown in  FIGS. 12A and 12B  is that improves the moving-image display with a configuration being simpler than that of the third embodiment shown in  FIGS. 9A and 9B . The back light  1201  is a back light of a direct-underlying type in which the entire screen is defined as a single section. 
       FIG. 13  is a timing chart showing a signal timing and a driving of the panel and the back light. The light/dark state at a duty ratio of 50% is repeatedly lighted at the same frequency as of the frame frequency. The setting as to the back light  1201  is as shown below. 
     fluorescent tube: a cold-cathode tube of an outer diameter of φ3.0 mm and an inside diameter of 2.4 mm. 
     tube current: a light state 7 mA, a dark state 3.5 mA (instantaneous luminance ratio: light state 5:dark state 2) 
     Around the time period of the writing performed into the gate line and assumed by the gate driver  102 , a binary constant voltage signal (3.5 V/0 (zero) V) S 1  is sent to the driving portion of the back light  1201  at the timing shown in the drawing. The back-light driving portion is controlled to light by the constant voltage signal S 1 . It is set to turn OFF at a signal voltage of 3 V or more, and turn ON at a signal voltage of 0.5 V or below. 
     The first and second fields have the first to the L-th lines, respectively, in which respective data writing timings are shown in  FIG. 13 . The transmissivity of the pixel of the panel is shown as to a pixel on a line of the L-th line/fourth column and a pixel on a line of the third line/fourth column. Also, the unit luminance is shown as to the pixel on the line of the L-th/fourth column and the pixel on the line of the third line/fourth column, corresponding thereto. The present embodiment can obtain the same effect as of the third embodiment as well. 
     As described above, according to the first and second embodiments, in the technology that divides a single frame time into plural fields and utilizes converted data with respect to data voltage of every field, it is possible to improve the fineness in the gray tone and the moving-image characteristic by adding the appropriate and minimum drive circuit technology thereto. Further, according to the third and fourth embodiments, by interlocking with the flashes of the back light, the degradation in the viewing-angle characteristic (luminance floating in the halftone, color shift) can be reduced. 
     According to the first and second embodiments, it is possible to prevent resolution insufficiency in the halftone display. Further, according to the third and fourth embodiments, the response compensation in the case where the frame tone data changes is enabled, so that a ghost image can be prevented especially in the luminance change between low tones. 
     Further, the display method of the halftone-driving method and the display method of displaying a constant tone for each frame (normal driving method) can be used appropriately by settings. At that time, a setting of tone-applied voltage is different between the halftone driving method and the normal driving method. 
     The resolution insufficiency in the halftone display can be prevented by dividing the single frame into the plural fields and performing correction to the field data when the tone change arises in the frame data. 
     The present embodiments are to be considered in all respects as illustrative and no restrictive, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof.