This invention relates to a method and apparatus for driving a liquid crystal display device and, in particular, for a method and apparatus for driving a liquid crystal display device which provides a uniform display and reduces unevenness of display. While liquid crystal display devices have taken many forms, simple matrix type liquid crystal display devices are generally driven by a voltage averaging method. The liquid crystal is driven by applying a selective voltage sequentially to each scanning electrode, and applying a lighting or non-lighting voltage to each signal electrode in sync with the scanning electrode that receives the selective voltage.
The liquid crystal panel is provided with scanning and signal electrodes each having a resistance which is greater then zero (0) and a liquid crystal layer which acts as a dielectric. Therefore, the effective voltage at the display elements or dots formed by the intersection of each scanning electrode and signal electrode changes depending upon the nature of the characters and images displayed by the liquid crystal panel. This results in unevenness of display on the display device.
This is a problem that has been well known in the art. Many problem solving techniques have been used in the past, such as the line inverse driving method. The line inverse driving method is a method whereby the polarity of the voltage applied to the liquid crystal panel is inverted a multiplicity of times within each frame. This method is disclosed in Japanese Patent Laid-Open Publication Nos. 31825/1987, 19195/1985 and 19196/1985.
The line inverse driving method has the effect of decreasing the unevenness of display. The unevenness of display is caused by changes in the optical characteristics of the liquid crystal display device, which are caused by changes in the frequency components of the voltages applied to the scanning and signal electrodes. Thus, the line inverse driving method cannot completely eliminate the unevenness of display.
Referring to FIG. 1 the unevenness of display caused by the above referenced factors is explained. FIG. 1 is a schematic perspective view of a liquid crystal panel showing an example of a display pattern. FIG. 1 depicts a liquid crystal panel generally indicated as 1, composed of a liquid crystal layer 5, a first substrate 2 and a second substrate 3 for sandwiching the liquid crystal layer 5 therebetween. A plurality of scanning electrodes Y1-Y6 are formed on substrate 2 in the horizontal direction and a plurality of signal electrodes X1-X6 are formed on substrate 3 in substantially the vertical direction. Each intersection of a scanning electrode Y1-Y6 and a signal electrode X1-X6 forms a display element (dot) 7. Display elements 7 marked with crosshatching represent the lighting or illuminated state and display elements 7 without crosshatching represent the non-lighting or non-illuminated state. The display panel of FIG. 1 is shown as a 6.times.6 matrix or 36 display elements for simplicity, however, in exemplary embodiments the number of display elements of liquid crystal panel 1 maybe greater.
The liquid crystal panel is driven by the sequential application of the selective voltage to scanning electrodes Y1-Y6. This procedure of applying the selective voltage sequential to scanning electrodes Y1-Y6 is continually repeated. Simultaneously with the application of the selective voltage to scanning electrodes Y1-Y6, lighting and non-lighting voltages are applied to signal electrodes X1-X6. The result of a signal electrode being applied with the lighting voltage intersecting a scanning electrode being applied with the selective voltage, is a display element 7 being in the lighting condition. Alternatively, if the non-lighting voltage is applied to the signal electrode then all display elements formed by the intersection of that signal electrode with a scanning electrode will be non-lighting display elements. When the effective voltage applied to the display element increases beyond a threshold voltage, positive display results. Positive display is the term used to represent the lighting condition when the display element is dark (rendered visible).
In order to prevent direct current from being applied to the liquid crystal panel 1, after the application of the selective voltage to scanning electrodes Y1-Y6 (which is called one frame, indicated by F1 in FIG. 3) , the next frame is driven by a selective voltage which has its polarity inverted from the previous frame (this period is indicated as F2 of FIG. 3).
Even if the resistances of the scanning electrodes Y1-Y6 were set ideally to zero (0), which is not possible, a low pass filter would nevertheless be formed by the resistances of signal electrodes X1-X6 and the capacitances resulting from the display elements, the liquid crystal material serving as the dielectric substance. FIG. 2 is a schematic diagram illustrating the low-pass filter. In FIG. 2, R represents the resistance of each individual signal electrode X1-X6, and C represents the capacitance formed by each display element. The ground represents the scanning electrodes having a resistance of zero. Referring to FIG. 2, the low pass filter causes the voltage waveform applied to the signal electrodes to become attenuated with respect to the ground (wherein the ground is represented by the scanning electrode). Thus, the effective voltage between the signal electrode and the scanning electrode is reduced when the frequency of changes from the lighted to the non-lighted state increases.
With reference to FIG. 1, the display elements depicted by the intersection of signal electrode X2 and scanning electrodes Y1-Y6 frequently change in state from the lighting condition to the non-lighting condition. The sequence of voltages of the data driving waveform applied to signal electrode X2 in FIG. 1 is: non-lighting, lighting, non-lighting, lighting, non-lighting and lighting voltage. There is much larger attenuation of the signal electrode voltage in the case of signal electrode X2 then in the case of signal electrodes X4, because in the case of signal electrodes X4 the signal electrode voltage is infrequently switched from the lighting to non-lighting voltage. In the case of signal electrode X4 the sequence of voltages of the data driving waveform applied to signal electrode X4 are as follows: non-lighting, lighting, lighting, lighting, lighting and non-lighting voltage. Thus, the voltage waveforms applied to signal electrodes X2 and X4 and the scanning electrodes Y1-Y6 are represented by FIGS. 3(a)-3(c) and FIGS. 4(a)-4(c).
FIGS. 3(a)-3(c) and 4(a)-4(c) are voltage waveform diagrams depicting the voltage waveform applied to the signal electrodes and scanning electrodes over time. The vertical axis represents the voltage applied and the horizontal axis represents the unit of time for which that voltage is applied. The periods T1, T2, T3 . . . , T6 represent the first frame, F1, in which T1 is the period in which the selective voltage is applied to scanning electrode Y1, T2 is the period in which the selective voltage is applied to scanning electrode Y2, T3 is the period in which the selective voltage is applied to scanning electrode Y3, . . . , and T6 is the period in which the selective voltage is applied to scanning electrode Y6. In frame F2 the period t1 is the period in which the selective voltage is applied to scanning electrode Y1, t2 is the period in which the selective voltage is applied to scanning electrode Y2, t3 is the period in which the selective voltage is applied to scanning electrode Y3, . . . , and t6 is the period in which the selective voltage is applied to scanning electrode Y6.
During frame F1, the voltages V0, V4, V5 and V3 are selective, non-selective, lighting and non-lighting voltages, respectively. During frame F2 the voltages V5, V1, V0 and V2 are selective, non-selective, lighting and non-lighting voltages, respectively.
FIG. 3(a) depicts the voltage waveform of signal electrode X2. FIG. 3(b) illustrates the voltage waveform of scanning electrode Y4. FIG. 3(c) depicts the difference between the voltage waveform of signal electrode X2, and scanning electrode Y4 or the difference between FIG. 3(a) and 3(b). Similarly, FIG. 4(a) depicts the voltage waveform of signal electrode X4. FIG. 4(b) depicts the voltage waveform of scanning electrode Y4. FIG. 4(c) illustrates the difference between the voltage waveforms of signal electrode X4, and scanning electrode Y4 or the difference between FIG. 4(a) and FIG. 4(b).
In the waveform diagrams, the hatched portions represent the deficiencies from the ideal voltage waveforms. When FIG. 3(c) and FIG. 4(c) are compared it can be seen that in the waveform diagram of FIG. 3(c) there are more deficiencies than in the waveform of FIG. 4(c). Therefore, the display element on signal electrode X2 is considerably pale, and the display element on signal electrode X4 is slightly pale.
The line inverse driving method discussed above can help to decrease the non-uniformity of display. However, the line inverse driving method cannot decrease the unevenness of display in all cases. Therefore, to decrease unevenness of display in all cases, caused by the circumstances described above, other methods of decreasing unevenness of display must be employed.