Display control device - Patent Review 5642128

Claims

What is claimed is:

1. A display control method for controlling a display device provided with a group of scanning electrodes and a group of signal electrodes and a display element placed therebetween, said display control method comprising the steps of:

applying, in accordance with information to be displayed, a non-zero first driving selection signal to a selected one of the group of scanning electrodes during a first period, and applying a non-zero second driving selection signal to the selected scanning electrode in a second period after the first period, each of the first and second driving selection signals being applied to the selected scanning electrode for enabling a writing operation while data signals are being applied to the group of signal electrodes,

wherein the first and second periods both occur during a single horizontal scan period of a frame,

wherein polarities of portions of the first driving signal are opposite to polarities of corresponding portions of the second driving signal, and

wherein polarities of the first and second driving selection signals applied to a subsequent scanning electrode are inverted from polarities of the first and second driving selection signals, respectively, applied to at least one previous scanning electrode.

2. A display control device for controlling a display device having a group of scanning electrodes and a group of signal electrodes so positioned in a matrix form as to define a group of pixels, and a display element positioned between said group of scanning electrodes and said group of signal electrodes, said display control device comprising:

a signal generator for applying, in accordance with information to be displayed, a non-zero first driving selection signal to a selected one of the group of scanning electrodes during a first period, and for applying a non-zero second driving selection signal to the selected scanning electrode in a second period after the first period, each of the first and second driving signals being applied to the selected scanning electrode for enabling a writing operation while data signals are being applied to the group of signal electrodes,

wherein the first and second periods both occur during a single horizontal scan period of a frame,

wherein the first and second periods are separated by a zero-voltage rest period,

wherein polarities of portions of the first driving selection signal are opposite to polarities of corresponding portions of the second driving selection signal, and

wherein said signal generator inverts the polarities of the first and second driving selection signals after all the scanning electrodes in the group have been selected at least once.

3. A display apparatus comprising:

a display device provided with a group of scanning electrodes and a group of signal electrodes and a display element placed therebetween;

display control means for controlling said display device; and

a driving selection signal generator for applying, in accordance with information to be displayed, a non-zero first driving selection signal to a selected one of the group of scanning electrodes during a first period, and for applying a non-zero second driving selection signal to the selected scanning electrode in a second period after the first period, each of the first and second driving selection signals being applied to the selected scanning electrode for enabling a writing operation while data signals are being applied to the group of signal electrodes,

wherein the first and second periods both occur during a single horizontal scan period of a frame,

wherein polarities of portions of the first driving selection signal are opposite to polarities of corresponding portions of the second driving selection signal, and

wherein polarities of the driving selection signals applied to a subsequent scanning electrode are inverted from polarities of the first and second driving selection signals, respectively, applied to at least one previous scanning electrode.

4. A display control device for controlling a display device provided with a group of scanning electrodes and a group of signal electrodes and a display element placed therebetween, said display control device comprising:

a driving selection signal generator for applying, in accordance with information to be displayed, a non-zero first driving selection signal to a selected one of the group of scanning electrodes during a first period, and for applying a non-zero second driving selection signal to the selected scanning electrode in a second period after the first period, each of the first and second driving selection signals being applied to the selected scanning electrode for enabling a writing operation while data signals are being applied to the group of signal electrodes; and

a data supply means for supplying the information to be displayed,

wherein the first and second periods both occur during a single horizontal scan period of a frame,

wherein polarities of portions of the first driving selection signal are opposite to polarities of corresponding portions of the second driving selection signal, and

wherein polarities of the first and second driving selection signals applied to a subsequent scanning electrode are inverted from polarities of the first and second driving selection signals, respectively, applied to at least one previous scanning electrode.

5. A display control device according to claim 4, wherein the display element comprises a ferroelectric liquid crystal.

6. A display control device for controlling a display device provided with a group of scanning electrodes and a group of signal electrodes and a display element placed therebetween, said display control device comprising:

a driving selection signal generator for applying, in accordance with information to be displayed, a non-zero first driving selection signal to a selected one of the group of scanning electrodes during a first period, and for applying a non-zero second driving selection signal to the selected scanning electrode in a second period after the first period, each of the first and second driving selection signals being applied to the selected scanning electrode for enabling a writing operation while data signals are being applied to the group of signal electrodes,

wherein the first and second periods both occur during a single horizontal scan period of a frame,

wherein polarities of portions of the first driving selection signal are opposite to polarities of corresponding portions of the second driving selection signal,

wherein the first and second driving selection signals are separated by a zero-voltage rest period,

wherein the first driving selection signal comprises a bi-polar pulse, and wherein the second driving selection signal comprises a bi-polar pulse, and

wherein a first pulse of the bi-polar pulse of the first driving selection signal has a longer duration than that of a second pulse of the bi-polar pulse of the first driving selection signal, and wherein a first pulse of the bi-polar pulse of the second driving selection signal has a longer duration than that of a second pulse of the bi-polar pulse of the second driving selection signal.

7. A display control device according to claim 6, wherein polarities of the first and second driving selection signals applied to a subsequent scanning electrode are inverted from polarities of the first and second driving selection signals, respectively, applied to at least one previous scanning electrode.

8. A display control device according to claim 6, wherein the display element comprises a ferroelectric liquid crystal.

9. A display control device for controlling a display device provided with a group of scanning electrodes and a group of signal electrodes and a display element placed therebetween, said display control device comprising:

a driving selection signal generator for applying, in accordance with information to be displayed, a non-zero first driving selection signal to a selected one of the group of scanning electrodes during a first period, and for applying a non-zero second driving selection signal to the selected scanning electrode in a second period after the first period, each of the first and second driving selection signals being applied to the selected scanning electrode for enabling a writing operation while data signals are being applied to the group of signal electrodes,

wherein the first and second periods both occur during a single horizontal scan period of a frame,

wherein polarities of portions of the first driving selection signal are opposite to a polarities of corresponding portions of the second driving selection signal, and

wherein polarities of the first and second driving selection signals applied to a subsequent scanning electrode are inverted from polarities of the first and second driving selection signals, respectively, applied to at least one previous scanning electrode.

10. A display control device according to claim 9, wherein the display element comprises a ferroelectric liquid crystal.

11. A display control device according to claim 9, wherein the first driving selection signal comprises a bi-polar pulse, and wherein the second driving selection signal comprises a bi-polar pulse.

12. A display control device according to claim 9, wherein the first and second driving selection signals separated by a zero-voltage rest period.

13. A display control device according to claim 12, wherein the data signal includes a zero-voltage rest period which starts simultaneously with the zero-voltage rest period between the two driving selection signals.

14. A display control device according to claim 12, wherein a section of the first driving selection signal immediately before the zero-voltage rest period has the same polarity as a section of the second driving selection signal immediately after the zero-voltage rest period.

15. A display control device according to claim 12, wherein the first driving selection signal comprises a bi-polar pulse, and wherein the second driving selection signal comprises a bi-polar pulse.

16. A display control device according to claim 15, wherein a first pulse of the bi-polar pulse of the first driving selection signal has a longer duration than that of a second pulse of the bi-polar pulse of the first driving selection signal, and wherein a first pulse of the bi-polar pulse of the second driving selection signal has a longer duration than that of a second pulse of the bi-polar pulse of the second driving selection signal.

17. A display control device for controlling a display device having a group of scanning electrodes and a group of signal electrodes so positioned in a matrix form as to define a group of pixels, and a display element positioned between said group of scanning electrodes and said group of signal electrodes, said display control device comprising:

a signal generator for applying, in accordance with information to be displayed, a non-zero first driving selection signal to a selected one of the group of scanning electrodes during a first period, and for applying a non-zero second driving selection signal to the selected scanning electrode in a second period after the first period, each of the first and second driving signals being applied to the selected scanning electrode for enabling a writing operation while data signals are being applied to the group of signal electrodes,

wherein the first and second periods both occur during a single horizontal scan period of a frame,

wherein the first and second periods are separated by a zero-voltage rest period,

wherein polarities of portions of the first driving selection signal are opposite to polarities of corresponding portions of the second driving selection signal, and

wherein said signal generator inverts the polarities of the first and second driving selection signals applied to a subsequent scanning electrode from polarities of the first and second driving selection signals, respectively applied to at least one previous scanning electrode.

18. A display control device according to claim 17, wherein the display element comprises an optical modulating element having bistability characteristics when an electric field is applied.

19. A display control device according to claims 17 or 18, wherein the display element comprises a ferroelectric liquid crystal.

20. A display control device according to claim 17, wherein said signal generator inverts the polarity of the first and second driving selection signals during driving of the scanning electrodes.

21. A display control device according to claim 18, wherein said signal generator inverts the polarity of the first and second driving selection signals during driving of the scanning electrodes.

22. A display control device according to claim 19, wherein said signal generator inverts the polarity of the first and second driving selection signals during driving of the scanning electrodes.

23. A display control device according to claim 18, wherein said signal generator inverts the polarity of the driving selection signals after at least one scanning electrode has been selected.

24. A display control device according to claim 19, wherein said signal generator inverts the polarity of the driving selection signals applied to subsequent electrodes from a polarity of a driving selection signal applied to at least one previous scanning electrode.

25. A display control device according to claim 18, wherein said signal generator inverts the polarity of the driving selection signals after all the scanning electrodes in the group have been selected at least once.

26. A display control device according to claim 19, wherein said signal generator inverts the polarity of the driving selection signals after all the scanning electrodes in the group have been selected at least once.

27. A display control device according to claim 17, wherein the first driving selection signal comprises a bi-polar pulse, and wherein the second driving selection signal comprises a bi-polar pulse.

28. A display control device, for controlling a display device having a group of scanning electrodes and a group of signal electrodes, and a display element placed therebetween, said control device comprising:

a selecting signal generator for applying a non-zero first selecting signal to an n-th scanning electrode, when that electrode is selected from the group of scanning electrodes during a first frame, simultaneously with an application of a data signal to the group of signal electrodes, and for applying a non-zero second selecting signal to the n-th scanning electrode when that electrode is selected in a second frame after the first frame,

wherein the second selecting signal is applied to an (n+1)th scanning electrode during the first frame and the first driving signal is applied to the (n+1)th scanning electrode during the second frame,

wherein n is a predetermined positive integer,

wherein each of the selecting signals is a two-phase signal in which each phase is a selection signal for enabling a writing operation, and

wherein a a polarity of a portion of the first selecting signal before the rest period is opposite to a polarity of a corresponding portion of the second selecting signal after the rest period.

29. A display control device according to claim 28, wherein the display element comprises an optical modulating element having bistability characteristics when an electrical field is applied.

30. A display control device according to claims 28 or 29, wherein said optical modulating element comprises a ferroelectric liquid crystal element.

31. A display control device according to claim 28, wherein the first selecting signal comprises a bi-polar pulse, and wherein the second selecting signal comprises a bi-polar pulse.

32. A display control device according to claim 28, wherein the data signal comprises first and second portions separated by a zero-voltage rest period, and wherein the first and second selecting signals are separated by a zero-voltage rest period.

33. A display control device according to claim 32, wherein the zero-voltage rest period of the data signal and the zero-voltage rest period between the first and second selecting signals start simultaneously.

34. A display control device according to claim 32, wherein a section of the first selecting signal immediately before the zero-voltage rest period has the same polarity as a section of the second selecting signal immediately after the zero-voltage rest period.

35. A display control device according to claim 32, wherein the first selecting signal comprises a bi-polar pulse, and wherein the second selecting signal comprises a bi-polar pulse.

36. A display control device according to claim 35, wherein a first pulse of the bi-polar pulse of the first selecting signal has a longer duration than that of a second pulse of the bi-polar pulse of the first selecting signal, and wherein a first pulse of the bi-polar pulse of the second selecting signal has a longer duration than that of a second pulse of the bi-polar pulse of the second selecting signal.

37. A display control device for controlling a display device having a group of scanning electrodes and a group of signal electrodes so positioned in a matrix form as to define a group of pixels, and a display element positioned between said group of scanning electrodes and said group of signal electrodes, said display control device comprising:

a signal generator for applying, in accordance with information to be displayed, a non-zero first driving selection signal to a selected one of the group of scanning electrodes during a first period, and for applying a non-zero second driving selection signal to the selected scanning electrode in a second period after the first period, each of the first and second driving signals being applied to the selected scanning electrode for enabling a writing operation while data signals are being applied to the group of signal electrodes,

wherein the first and second periods both occur during a single horizontal scan period of a frame,

wherein the first and second periods are separated by a zero-voltage rest period,

wherein polarities of portions of the first driving selection signals are opposite to polarities of corresponding portions of the second driving selection signals, and

wherein the first and second driving selection signals are separated by a zero-voltage rest period.

38. A display control device according to claim 37, wherein the data signal includes a zero-voltage rest period which starts simultaneously with the zero-voltage rest period between the first and second driving selection signals.

39. A display control device according to claim 37, wherein a section of the first driving selection signal immediately before the zero-voltage rest period has the same polarity as a section of the second driving selection signal immediately after the zero-voltage rest period.

40. A display control device according to claim 37, wherein the first driving selection signal comprises a bi-polar pulse, and wherein the second driving selection signal comprises a bi-polar pulse.

41. A display control device according to claim 40, wherein a first pulse of the bi-polar pulse of the first driving selection signal has a longer duration than that of a second pulse of the bi-polar pulse of the first driving selection signal, and wherein a first pulse of the bi-polar pulse of the second driving selection signal has a longer duration than that of a second pulse of the bi-polar pulse of the second driving selection signal.

42. A display control device for controlling a display device provided with a group of scanning electrodes and a group of signal electrodes and a display element placed therebetween, said display control device comprising:

a scanning signal generator for applying a non-zero first scanning signal to a selected one of the group of scanning electrodes during a first period, and for applying a non-zero second scanning signal to the selected scanning electrode in a second period after the first period, each of the first and second scanning signals being applied to the selected scanning electrode for enabling a writing operation while a driving signal is being applied to a predetermined number of signal electrodes of the group of signal electrodes,

wherein the first and second periods both occur during a single horizontal scan period of a frame,

wherein the first and second periods are separated by a rest period,

wherein polarities of portions of the first scanning signal are opposite to polarities of corresponding portions of the second scanning signal, and

wherein polarities of the first and second scanning signals applied to a subsequent scanning electrode are inverted from polarities of the first and second scanning signals, respectively, applied to at least one previous scanning electrode.

43. A display control device according to claim 42, wherein the display element comprises an optical modulating element having bistability characteristics when an electric field is applied.

44. A display control device according to claim 42, wherein the display element comprises a ferroelectric liquid crystal.

45. A display control device according to claim 42, wherein said scanning signal generator inverts the polarities of the first and second scanning signals during driving of the scanning electrodes.

46. A display control device according to claim 42, wherein said scanning signal generator inverts the polarities of the first and second scanning signals applied to a subsequent scanning electrode from the respective polarities of the first and second scanning signals applied to at least one previous scanning electrode.

47. A display control device according to claim 42, wherein said scanning signal generator inverts the polarity of the scanning signals after all the scanning electrodes in the group have been selected at least once.

48. A display control device according to claim 43, wherein said scanning signal generator inverts the polarities of the first and second scanning signals during driving of the scanning electrodes.

49. A display control device according to claim 44, wherein said scanning signal generator inverts the polarity of the first and second scanning signals during driving of the scanning electrodes.

50. A display control device according to claim 43, wherein said scanning signal generator inverts the polarities of the first and second scanning signals after at least one scanning electrode has been selected.

51. A display control device according to claim 44, wherein said scanning signal generator inverts the polarities of the first and second scanning signals applied to subsequent electrodes from polarities of the first and second scanning signals, respectively, applied to at least one previous scanning electrode.

52. A display control device according to claim 43, wherein said scanning signal generator inverts the polarities of the first and second scanning signals after all the scanning electrodes in the group have been selected at least once.

53. A display control device according to claim 44, wherein said scanning signal generator inverts the polarities of the first and second scanning signals after all the scanning electrodes in the group have been selected at least once.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display control device, and more particularly to a display control device adapted for use in a display device employing a display element exhibiting bistability to the electric field, such as ferroelectric liquid crystal display device.

2. Related Background Art

Among display devices employing liquid crystal compound, there is already known a device in which a group of scanning electrodes and a group of signal electrodes are positioned in the form of a matrix and a liquid crystal compound is filled therebetween for forming plural pixels thereby displaying image information.

For such a display device there has been employed so-called time-division driving method in which voltage signals are cyclically applied to the scanning electrodes and information signals are applied in parallel manner to the signal electrodes, in synchronization with the voltage signals supplied to the scanning electrodes. Such a display device and such driving method are associated with a difficulty in increasing the density of the pixels, or in increasing the image size.

In various liquid crystal compounds, there has almost solely been employed, in the display devices, the twisted nematic (TN) liquid crystal because of a relatively high response speed and a low power consumption. In the liquid crystal of this type, the nematic liquid crystal molecule with positive dielectric anitropy assumes a twisted (helical) structure in the absence of electric field, as shown in FIG. 50A, in the direction of thickness of the liquid crystal layer, and the liquid crystal molecules constitute, between the electrodes, twisted structures which are parallel mutually and in each layer. On the other hand, under an electric field, as shown in FIG. 50B, the nematic liquid crystal molecules with positive dielectric anisotropy are aligned in the direction of electric field, thus inducing optical modulation. If a display device is formed by employing such liquid crystal in combination with electrodes of a matrix structure, in an area where a scanning electrode and a signal electrode are both selected (selected point), there is applied a voltage exceeding the threshold value required for orienting the liquid crystal molecules perpendicularly to the electrodes, but, in an area where both of the scanning electrode and the signal electrode are not selected (unselected point), the above-mentioned voltage is not applied so that the liquid crystal molecules retain the twisted stable orientation parallel to the electrodes. By placing linear polarizers in mutually crossing relationship on both sides of such liquid crystal cell, the light is intercepted in the selected point but is transmitted in the unselected point because of the light-rotating property of the twisted structure of the liquid crystal. In this manner an image display device can be obtained.

However, in such a matrix electrode structure, a certain electric field is applied also to so-called half-selected point where the scanning electrode is selected but the signal electrode is not selected, or, the scanning electrode is not selected but the signal electrode is selected. The display device functions in normal manner as long as the voltage supplied to the selected point is sufficiently different from that supplied to the half-selected point, and the threshold voltage required for orienting the liquid crystal molecules perpendicularly to the electrodes is present between the above-mentioned voltages.

However, when the number N of the scanning lines is increased in such structure, the duty ratio, or the ratio of duration of effective electric field on a selected point to the period of scanning of the entire frame decreases as 1/N. Consequently the voltage difference, in the effective value, between the selected point and the unselected point in the repeated scanning operations decreases with the increase in the number of scanning lines, thus giving rise to lowered image contrast and crosstalk.

Such phenomena are fundamentally unavoidable in driving conventional liquid crystal lacking bistability (in which liquid crystal molecules are stable when oriented parallel to the electrodes and are perpendicularly oriented only during effective application of the electric field) by means of time accumulating effect (namely by repetitive scanning). In order to overcome such difficulties there have been proposed various methods such as voltage averaging method, two-frequency driving method, multiple matrix method etc., but these methods are still insufficient and the image size and the pixel density of the display devices have been limited by the limitation in the number of scanning lines.

Also for overcoming the above-mentioned drawbacks, the present applicant has already proposed driving methods for liquid crystal exhibiting bistability to the electric field, for example in the U.S. Pat. No. 4,655,561 issued on Apr. 7, 1987. For use in such driving methods, there is preferred chiral smectic liquid crystal with ferroelectricity, particularly that of C-phase (SmC* or H-phase (SmH*).

In the SmC* phase, as shown in FIG. 51, the liquid crystal molecules have parallel layered structure, in which the longer axis of the molecule is inclined to the layer. The molecules constitute a spiral structure as the direction of inclination of molecules is different amount different layers.

In the SmH* phase, as shown in FIG. 52, the molecules show parallel layered structure, with an inclination of the longer axis of the molecule to the layer, exhibiting a hexagonal packed structure in a plane perpendicular to the longer axis of the molecule.

In the SmC* or SmH* phase, the liquid crystal molecules assume a spiral structure, as schematically shown in FIG. 53.

In FIG. 53, e3 indicates a liquid crystal molecule; e4 an electric dipole moment; and e5 a boundary of layers. Each liquid crystal molecule e3 has a dipole moment in a direction perpendicular to the longer axis of the molecule, and moves with a fixed angle .theta. to the Z-axis perpendicular to the boundary plane e5 of the layers, thus constituting a spiral structure. The illustrated state exists in the absence of an applied voltage, but, in the presence of a voltage exceeding a certain threshold value in the direction of X-axis, the liquid crystal molecule is oriented in such a manner that the electric dipole moment e4 becomes parallel to the X-axis.

As the SmC* or SmH* phase can be realized in the course of phase transition by temperature, it is desirable, in case of using such liquid crystal compound, to select the display device according to the temperature range of use of the display device.

FIG. 54 schematically illustrates a cell utilizing the ferroelectric liquid crystal (FLC) explained above. Substrates (glass plates)el, el' respectively have transparent electrodes composed for example of In.sub.2 O.sub.2, SnO.sub.2 or indium tin oxide (ITO), and the liquid crystal of SmC* phase is sealed therebetween in such a manner that the layers e2 of the liquid crystal molecules become perpendicular to the glass plate surfaces. The liquid crystal molecule e3, represented by a thick line, has a dipole moment e4 in a direction perpendicular thereto. When a voltage, exceeding a fixed threshold value, is applied between the electrodes of the substrates el and el', the spiral structure of the liquid crystal molecule e3 is unwound and the orientation of the molecules e3 is changed in such a manner that the dipole moments e4 are all aligned in the direction of electric field. Because of the oblong shape, the liquid crystal molecule e3 shows anisotropy in the refractive index between the longer and shorter axis. It will therefore be easily understood that a liquid crystal optical modulating device in which the optical properties vary according to the polarity of applied voltage can be obtained by placing mutually crossing polarizers on both sides of the glass plates.

If the liquid crystal cell is made sufficiently thin (for example 1 .mu.m), the spiral structure of the liquid crystal molecule becomes unwound even in the absence of the electric field, as shown in FIG. 55, and the dipole moment p or p' thereof assumes an upward or downward position. If an electric field E or E' exceeding a threshold value is applied for a predetermined period, as shown in FIG. 55, the dipole moment is changed upwards or downwards according to the field vector of the electric field E or E', and the liquid crystal molecules are correspondingly oriented in a first stable state f3 or a second stable state f3'.

The use of such ferroelectric liquid crystal in the optical modulating device provides following two advantages: first, a very high response speed (1 .mu.sec-100 .mu.sec), and, second, bistable nature of the orientation of the liquid crystal molecules.

The above-mentioned second advantage will be further explained with reference to FIG. 55. Under the application of an electric field E, the liquid crystal molecules e3 are oriented in the first stable state f3, which remains stable even after the application of the electric field is discontinued. Under the application of the inverse electric field E', the liquid crystal molecules e3 are reoriented into the second stable state f3', which again remains stable even after the application of the electric field is terminated. Thus the liquid crystal molecules have a memory property, and retain their oriented state unless the applied electric field exceeds a certain threshold value.

In order to effectively exploit such high response speed and memory property, the cell is preferably as thin as possible, generally in a range of 0.5 to 20 .mu.m, particularly 1 to 5 .mu.m.

Now reference is made to FIGS. 47 to 49D for outlining the driving method for the ferroelectric liquid crystal.

FIG. 56 is a schematic view of a cell having matrix electrodes, between which a ferroelectric liquid crystal compound (not shogun) is sandwiched. There are illustrated common scanning electrodes com and signal electrodes sig. At first there will be explained a case in which a scanning electrode com1 is selected.

FIGS. 57A and 57B respectively show an example of an electrical scanning signal supplied to the selected scanning electrode com1 and an electrical scanning signal supplied to other (unselected) scanning electrodes com2, co