Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1997-04-04
2001-05-15
Chow, Dennis-Doon (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S094000, C345S096000, C345S058000
Reexamination Certificate
active
06232944
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of driving a liquid crystal display (LCD), especially a simple matrix type LCD, a drive IC for the method and a drive circuit using the drive IC.
An LCD has been widely used in a personal computer, a word processor, and other electronic equipment for its thin and light features, while its display capacity has been increased rapidly. Especially, a super twisted nematic (STN) type LCD is widely used in inexpensive equipment since its cost is lower than a thin film transistor (TFT) type LCD.
An STN type LCD increases its display capacity by increasing a twist angle of a liquid crystal molecule more than two hundred degrees so as to sharpen electro-optical properties of a threshold characteristic of the LCD. The STN type LCD can be manufactured at a low cost compared to a TFT type LCD that has an active matrix structure with a switching element for each pixel.
A multiplex drive method is generally used for driving a simple matrix type LCD including the STN type LCD. The simple matrix LCD has no switching element for each pixel, so that a display intensity of a pixel depends on a root mean square (rms) value voltage including a state in which the scanning electrode of the pixel is not selected. This multiplex drive method keeps display uniformity by making rms voltages equal between enabled pixels as well as disabled pixels.
FIG. 47
shows the above mentioned drive method. Numeral
503
is an LCD panel,
504
-
507
are scanning electrodes, and
508
-
511
are signal electrodes. A scanning voltage pulse (+Vs)
501
is applied to the scanning electrodes in order, and signal voltage
502
is applied to the signal electrode, where the signal voltage
502
corresponds to on/off states of the pixel on the selected scanning electrode. The signal voltage is −Vd for the on state and +Vd for the off state. The polarity of the voltage is reversed over a predetermined period to apply an alternating voltage to the liquid crystal.
In a real LCD panel, there is a switching distortion of the voltage wave form applied to the liquid crystal, due to a CR circuit made of an electrode resistance of the scanning electrode and/or the signal electrode, an output resistance of the drive IC and a capacitance of the liquid crystal. Therefore, the rms voltage applied to each pixel deviates from an ideal value, so that the intensity of the pixel, which should be constant, varies depending on a display pattern of other parts. This phenomenon is so-called “crosstalk”.
There are several causes of such a crosstalk. The most important and basic cause is a switching distortion of a data signal. In
FIG. 47
, though only four scanning electrodes
504
-
507
are shown, there are plural electrodes following the electrode
507
, and all pixels are supposed to be in the on state (i.e., white is displayed). For example, the signal voltage applied to the signal electrode
509
is switched three times between off and on states during scanning periods of the scanning electrodes
504
-
507
, while the signal voltage applied to the signal electrode
508
maintains the on state without switching. Therefore, pixels on the signal electrode
509
are provided with a lower rms voltage due to the switching distortion compared with the pixels on the signal electrodes
508
. As a result, the white level of the pixels on the signal electrode
509
is darker than that of the pixels on the signal electrode
508
, so that stripes are displayed even though the display data are all white. This crosstalk is called a character crosstalk.
In a liquid crystal display, a dc voltage is prevented from being applied to the liquid crystal by switching the polarity of the scanning voltage as well as the polarity of the signal voltage of the data signal in a predetermined period. A drive method for decreasing the character crosstalk is disclosed in Japanese laid open patent application (Tokukai-Sho) 60-19195 and the technical report of Japanese Television Gakkai, IPD82-4 (1983). In this drive method, the switching frequency of the driving voltage polarity is increased in a constant intensity display part by switching drive voltage polarity based on a period of plural horizontal scanning periods that is shorter than one frame. Currently, it is normal to switch the polarity every 10-30 horizontal scanning periods, that is one to several tens of switching frequency per one frame in an LCD having 200-500 scanning lines.
However, this drive method can not eliminate the character crosstalk completely. In addition, this drive method may create another crosstalk (vertical line crosstalk) when a vertical bar is displayed since the polarity switching generates a voltage distortion on the scanning electrode (refer the text of The Second Fine Process Technology Japan '92 Seminar R17).
Another drive method is explained in Japanese laid open patent application (Tokukai-Hei) 4-360192 or 8-292744. This method suppresses the crosstalk by shifting the output level of the signal voltage so as to compensate the switching distortion when the signal voltage switches its level with regard to the non-selected level of the scanning voltage. As shown in
FIG. 48
, when the output level of the signal voltage is switched, a compensating pulse
521
is added, which shifts the output level of the signal voltage for a predetermined period, so as to compensate for an rms voltage decrease due to the waveform distortion. In this Figure, the non-selected level of the scanning voltage is shifted from V1 to V4 when the polarity of the scanning voltage is switched, for controlling the output voltage of a scanning IC.
FIG. 49
shows a drive circuit for obtaining the wave form shown in
FIG. 48
as disclosed in Tokukai-Hei 4-360192. This drive circuit generates four additional voltage levels VDD, V2, V3, V5. An LCD driving voltage generator 525 generates ten voltage levels VDD, VDD′, V1-V5, V2′, V3′ and V5′, and eight levels of them are supplied to a signal drive circuit
523
. Numeral
522
is an LCD panel and
524
is a scan drive circuit.
If the non-selected level of the scanning voltage is a constant value V1, the signal voltage waveform is as shown in FIG.
50
. This is obtained by shifting the latter half of the signal voltage waveform in FIG.
48
. The scanning IC is required to output positive and negative pulses (+/−Vs), and the lower half of the voltage level generated by the LCD scan voltage generator
525
is not necessary.
In the drive method disclosed in Tokukai-Hei 8-292744, a compensating pulse is superimposed on the supplied voltage to the signal drive circuit for obtaining the waveform shown in
FIG. 48
or FIG.
50
. This drive method makes the output of the signal drive IC high impedance so that the compensating pulse does not reach the signal electrode when the signal voltage is not switched (is not inverted), and turn on the output of the signal drive IC so that the compensating pulse is applied to the signal electrode when the signal voltage is switched (is inverted).
Another drive method is disclosed in Tokukai-Hei 5-333315. This drive method adds a pulse voltage that decreases the rms signal voltage when the signal voltage is not inverted, opposite to the above mentioned method disclosed in Tokukai-Hei 4-360192 or 8-292744, so as to generate a waveform distortion that may occur when the level is inverted and makes both rms voltages equal. The non-selected level of the scanning electrode or the opposite level of the signal voltage (the off level when continuing on signal, and the on level when continuing off signal) is used as a compensation voltage level, so that the crosstalk is suppressed without additional voltage levels.
The above mentioned drive methods in the prior art have some disadvantages as explained below.
In the method of Tokukai-Hei 4-360192, the number of the voltage levels supplied to the LCD drive IC is increased, along with the numbers of bus wires and switches in the drive IC as well as the numbers of connections between t
Kawaji Akira
Kumagawa Katsuhiko
Matsunami Masahito
Okuno Takeshi
Suyama Tohru
Chow Dennis-Doon
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
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