Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-07-21
2003-07-08
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S098000, C345S204000
Reexamination Certificate
active
06590553
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for driving the same, and in particular relates to a liquid crystal display device and a drive method, for increasing the performance of a display.
2. Background Art
At present, practically all liquid crystal display elements are of the twisted nematic (TN) type display form. Liquid crystal display elements of this TN type display form use a nematic liquid crystal composition, and are largely divided into two.
One of these is an active matrix form where a switching element is provided for each pixel. For example, one (a TN-TFT form) is known which uses a thin film transistor (TFT: Thin Film Transistor) for a TN type display form. The other one is an STN (Super Twisted Nematic) form.
With this STN form, contrast and visual angle dependency are improved compared to the simple matrix form which uses the conventional TN type, but since the response speed is low, this is not suitable for moving picture display. Furthermore, this has the defect in that, compared to the active matrix form which uses the TFT, the display quality is low. This result means that at present, the TN-TFT form has become the market standard.
On the other hand, due to the requirement for further high image quality, research has started into methods of improving the angle of visibility, and practical use has been reached. As a result, with the main stream of current high performance liquid crystals displays, there are three types of TFT form active matrix liquid crystal display devices, namely a form which uses a compensating film in the TN mode, or an in plane switching (IPS) mode, or a multi domain vertical aligned (MVA) mode. With these active matrix liquid crystal display devices, normally, since the image signal involves positive and negative writing at 30 Hz, this is rewritten at 60 Hz, and the time for one field is approximately 16.7 ms (milliseconds) (the total time for both the positive and negative fields is referred to as one frame, and is approximately 33.3 ms). On the other hand, the response speed of current liquid crystals, even in the fastest state, is only about this frame time. Therefore, in the case where a projection signal comprising a moving picture is displayed, or in the case where a high-speed computer image is displayed, or in the case where a high-speed game image is displayed, a response speed higher than the current frame time is required.
On the other hand, in order to target even higher resolution, a field sequential (time sharing) color liquid crystal display where a back light serving as the illumination light for the liquid crystal display, switches time-wise between red green and blue, is also being investigated. With this form, since it is not necessary to arrange color filters spatially, high resolution three times higher than heretofore is possible. With the field sequential liquid crystal display, since it is necessary to display one color in one third of the time for one field, then the time which can be used for the display is approximately 5 ms. Consequently, for the liquid crystal itself, a response faster than 5 ms is required. As a liquid crystal which can realize this high-speed response, liquid crystals having spontaneous polarization such as ferroelectric liquid crystals or antiferroelectric liquid crystal are being studied. Furthermore, with the nematic liquid crystals also, studies are being conducted for example to increase the dielectric anisotropy, to reduce the viscosity, to make the film thinner, and to change the liquid crystal molecular orientation to a &pgr; type orientation, or to increase speed by devising drive voltage wave forms.
Here, with an active matrix liquid crystal display element, the time where voltage or electric charge is actually written to the liquid crystal, is only the selection time (writing time) of each scanning line. This time, in the case of having 1000 lines with writing normally in one field time is 16.7 &mgr;s (microseconds), and in particular in the case where field sequential drive is performed, is approximately 5 &mgr;s. At present, a liquid crystal or a liquid crystal operation mode which completes the response within this time, is practically non existent. Even with a liquid crystal having the abovementioned spontaneous polarization, or a high-speed nematic liquid crystal, an element which can give this fast response is not known. As a result, the liquid crystal responds after completion of writing of the signal, causing the following problems. At first, with a liquid crystal having spontaneous polarization, a depolarization field is produced due to the rotation of the spontaneous polarization, so that the voltage at both ends of the liquid crystal layer suddenly drops. Therefore, the voltage which has been written to both ends of the liquid crystal changes markedly. On the other hand, also with the high-speed nematic liquid crystal, due to the anisotropy of the dielectric constant, the change in the capacity of the liquid crystal layer becomes very large. Hence a change occurs in the holding voltage for holding the writing in the liquid crystal layer. Such a drop in the holding voltage, that is to say a drop in the effective applied voltage, lowers the contrast due to insufficient write in. Furthermore, in the case where the same signal is written in repeatedly, luminance continues to change until the holding voltage ceases to drop. Hence in order to obtain stabilized luminance, several frames are required.
Furthermore, as shown in Japanese Applied Physics, chapter 36, part 1, number 2, pages 720 to 729, in the case where the same image signal continues to be written across several frames from the frame where there has been a change in the absolute value of the signal voltage for which the image signal has changed, a phenomena called a “step response” appears. This phenomena is a phenomena where the transmittance across several frames oscillates between light and dark with respect to the signal voltage of the AC drive at the same amplitude. Subsequently, this stabilizes to a constant transmitted light quantity. An example of this phenomena is shown by the schema in FIG.
24
. Part (a) shown in
FIG. 24
is a wave form diagram for data voltage, part (b) shown in
FIG. 24
is a wave form diagram for gate voltage, and part (c) shown in
FIG. 24
is a wave form diagram for transmittance at the time. At the time of AC drive, transmittance is stable after a step response. The transmittance when stabilized is shown by a two dot chain line, and the transmittance at the time of maximum darkness is shown by a single dot chain line.
Furthermore,
FIG. 25
is a timing chart for each scanning line, under drive of
FIG. 24
, schematically showing the luminance for the light and dark of a positive display period
102
and a negative display period
104
, based on the transmittance of part (c) shown in FIG.
24
. Moreover, in the figure, a time of 16.7 &mgr;s being the time for one field, is shown by the arrow. In this figure, six scanning lines are assumed. From the top scanning line in sequence, positive polarity writing
101
is performed, and after obtaining the positive display
102
, then again from the top scanning line in sequence, negative polarity writing
103
is performed to obtain the negative display
104
. For each scanning line, the field where the period of the positive polarity writing
101
and the positive display
102
are added, is the first field, and the field where the period of the negative polarity writing
103
and the negative display
104
are added for each scanning line, is the second field, and the total of the two fields becomes one frame. Therefore, when the data voltage of part (a) shown in
FIG. 24
is applied and the TFT switch comes on with the gate voltage of part (b) shown in
FIG. 24
, then as in part (c) shown in
FIG. 24
, the transmittance oscillates between light and dark for each field. Such oscillation of the transmittance is observed as a flicker, causing de
Asada Hideki
Kimura Kazunori
Osorio Ricardo
Shalwala Bipin
Sughrue & Mion, PLLC
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