Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-12-08
2003-09-09
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S092000, C345S087000, C345S088000, C345S089000, C345S090000
Reexamination Certificate
active
06618033
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a so-called active-matrix-driven liquid crystal display device which is driven by TFTs (Thin Film Transistors) and, more particularly, to a small-sized liquid crystal display device to be used for projection systems.
FIG. 13
shows an example of the structure of a driving system for a small-sized active-matrix-driven liquid crystal display (LCD) device which has conventionally been used in a projection system.
FIG. 17A
schematically shows an example of the drive method. Conventionally, because small-sized active-matrix-driven LCD devices used for projection systems require an extremely small pitch of connections of a driver LSI (Large Scale Integrated Circuit), a so-called driver-monolithic LCD device is generally used, which integrally incorporates a driver by using polysilicon TFTs.
As to the driving operation, as shown in
FIG. 13
, a video data signal to be displayed is fed as an analog signal, and after converted into a digital signal by an A/D converter
1
, the signal is subjected to processings by a processor
2
such as gamma control of the display data voltage to adjust the electro-optical response (i.e., V-T curve) of liquid crystals, scaling for format conversion of the screen, etc. The signal thus processed is converted back into an analog signal by a D/A converter
3
, and further converted into multiphase parallel signals by a plurality of sample-and-hold circuits
4
. After that, with the frequency lowered to “1/(the number of phases),” the signals are supplied to a data driver of an LCD panel through linear amplifiers (not shown).
In the data driver, these and other data signals are sequentially held in capacitances of the source bus line according to the opening/closing of an analog switch (not shown) controlled by an output of a horizontal scanning circuit. Then, these data signals held by the source bus line are transferred to the capacitances of individual pixels connected to the source bus line via the TFTs until one horizontal scanning period terminates. After one horizontal scanning period has terminated, the data signals are held in the capacitances of the pixels.
On the other hand, in the driving operation of liquid crystals, for prevention of an orientation film and liquid crystals from deterioration due to electrochemical reaction as well as prevention of sticking or persistence of image, it is necessary to use an alternating voltage as the voltage applied to the liquid crystals. Therefore, as shown in
FIG. 17B
, the polarity of the video data signal is alternated every frame so that the AC driving is effectuated. As a result, a signal voltage whose polarity is alternately switched over every frame is applied across a pixel electrode, which has potentials determined by written data, and a counter electrode, whose potential is set to the intermediate potential between the potentials of the pixel electrode.
Liquid crystals respond to root mean square voltages. Thus, if the alternately positive and negative voltage has a completely symmetrical waveform, the resulting optical response occurs at a frequency at which the frame is switched (i.e., frame frequency). However, when the waveform is asymmetrical, even a little, there would arise a sub-harmonic component whose frequency is ½ of the frame frequency. Further, because a TFT has a characteristic that is not completely symmetrical in the positive and negative senses, there would also arise an offset of the DC potential due to feedthrough by switching. Thus, the potential of the counter electrode is set so as to counterbalance any effects of these. However, even if the waveform is adjusted so as to be completely symmetrical in polarity to one data voltage, it is extremely difficult to achieve a completely symmetrical waveform for all data voltages, because of the nonlinearity of capacitances of the liquid crystals and TFTs and/or the asymmetry in polarity and offsets of the gain of a linear amplifier circuit. Moreover, even if the waveform can be made completely symmetrical, the waveform may change with time and shift, resulting in an asymmetrical one.
Generally, the frame frequency is 60 Hz-85 Hz, and its secondary sub-harmonic frequency component, which has a frequency of 30 Hz-43 Hz, is observed as a flicker to human eyes, causing the display quality to be considerably impaired. To avoid this, it has conventionally been practiced to exert the so-called line inversion drive that the frequency at which liquid crystals blink is artificially doubled as shown in
FIG. 17A
so as to make the flicker indiscernible.
However, the small-sized active-matrix-driven LCD device used in conventional projection systems has the following problems. That is, in the drive method for the LCD device (scan line inversion drive in the example shown in FIG.
17
A), voltages of opposite polarities are applied to adjoining pixel electrodes in order to avoid the flicker. Due to this, at electrode edge of a pixel electrode sandwiched by opposite-polarity voltages, a uniform electric field between the pixel electrode and the counter electrode (hereinafter, referred to as a “longitudinal electric field”) is disturbed, causing a component of an electric field in a transverse direction (hereinafter, referred to as a “transverse electric field”) to arise. Therefore, for example, in the TN (Twisted Nematic) mode, there arises an inverted tilt region in response to a transverse electric field and depending on a pre-tilt. As a result, in the normally white mode in which the polarizer is set cross-Nicol, at and around a pixel electrode edge between the pixels with different polarity data, where the pre-tilt region and the inverted tilt region appear depending on the surface unevenness, pre-tilt angle, and the transverse electric field, there arise a region where light leakage occurs in the black display state as well as a region where the electro-optical response (V-T curve) of liquid crystals to display data voltages is shifted toward the higher voltage side. This would lower the contrast considerably. On the other hand, in the normally black mode in which the polarizer is set parallel-Nicol, the transmittance for the white level would lower by the above effects caused by the transverse electric field and, in addition to this, a high contrast as well as a neutral black are hard to realize because the same rotation of polarization axis or optical rotatory dispersion is not achieved over the entire visible range. Consequently, the practical display in the TN mode is limited to the normally white mode in which the polarizer is set cross-Nicol.
In order to obtain a sufficient display quality in the normally white mode, the region where the light-leakage occurs as well as the region where the V-T curve is shifted toward the higher voltage side need to be shielded from light. The region where the light-leakage occurs and the region where the V-T curve is shifted toward the higher voltage side extend over a generally constant distance from either pixel end, and thus, have noticeable effects particularly for small-sized pixels. In the conventional small-sized active-matrix-driven LCD device or the like used in the projection systems, the source line inversion drive was early adopted because the problems involved in drive would less occur. However, as the pixel size was scaled down more and more, the region to be shielded from light went relatively larger, which has led to a considerably lowered aperture ratio.
In the display section, as shown in
FIG. 12
, while only source bus lines
5
are provided vertically, gate bus lines
6
and common lines
7
for storage capacitors are provided laterally. Therefore, originally, the laterally-extending region that does not transmit light is larger than the vertically-extending region that does not transmit light. Thus, the scan line inversion drive, although having some problem in terms of drive, has come to be adopted for the purpose of utilizing, as a light-shield region, the laterally extending region that originally does not tr
Nixon & Vanderhye P.C.
Shalwala Bipin
Shapiro Leonid
Sharp Kabushiki Kaisha
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