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Description  |
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FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a liquid crystal apparatus using a
ferroelectric liquid crystal capable of providing a discriminable contrast
depending on the direction of an electric field applied thereto.
The use of a liquid crystal device showing bistability has been proposed by
Clark and Lagerwall in U.S. Pat. No. 4,367,924; Japanese Patent
Application (Kokai) 56-107216. As the bistable liquid crystal, a
ferroelectric liquid crystal showing chiral smectic C phase (SmC*) or H
phase (SmH*) is generally used. The ferroelectric liquid crystal assumes
either a first optically stable state or a second optically stable state
in response to an electric field applied thereto and retains the resultant
state in the absence of an electric field, this showing a stability.
Further, the ferroelectric liquid crystal quickly responds to a charge in
electric field, and thus the ferroelectric liquid crystal device is
expected to be widely used in the field of a high-speed and memory-type
display apparatus, etc.
In case where a pair of substrates constituting the ferroelectric liquid
crystal device are respectively provided with groups of stripe electrodes
crossing each other on their inside surfaces to provide a matrix display
apparatus, the matrix display apparatus can be driven by a multiplex
driving method as disclosed in U.S. Pat. Nos. 4,548,476; 4,655,561; U.S.
patent application Ser. No. 691,761 and 701,765; etc.
However, a ferroelectric liquid crystal device as mentioned above involves
a problem that it causes flickering when subjected to multiplexing drive.
For example, European patent publication EP-A 149899 discloses a multiplex
driving method wherein an AC voltage which reverses its phases for each
writing frame is applied, selective writing of "white" (with cross nicols
arrange to provide a bright state) is effected in a frame, and selective
writing of "black" (with cross nicols arranged to provide a dark state) is
effected in a subsequent frame.
In such a driving method, at the time of selective writing of "black" after
the selective writing of "white", a pixel selectively written in "white"
in a preceding frame is half-selected and is supplied with a voltage which
is smaller than the writing voltage but is effective. Accordingly, at the
time of selective writing of "black" in the multiplex driving method,
selected pixels of "white" forming the background of, e.g., a black
letter, are uniformly supplied with a half-selection voltage for each
cycle of 1/2 frame (a half of a vertical scanning period, and the "white"
selected pixels change their optical characteristics for a cycle of 1/2)
frame. For this reason, in the case of a display of a black letter in the
white background, white selected pixels which are for more than black
selected pixels cause flickering. On the other hand, in the case of a
display of a white letter in the black background, similar flickering is
observed. In the case of an ordinary frame frequency of 30 Hz, the above
half-selected voltage is applied at a half frame frequency of 15 Hz, so
that the flickering is noticeable to an observer and results in a
remarkably degraded display quality.
Another problem of such a multiplexing drive method wherein one picture is
formed-through a plurality of writing frame scans is occurrence of an
awkward image called "tailing" on the display picture, which is observable
when the drive method is applied to a motion picture display as in a
television display or letter-scrolling on a screen of a word processor.
This problem will be further discussed hereinafter.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a liquid crystal apparatus
having an increased voltage margin.
Another object of the present invention is to provide driving apparatus for
a display panel having solved a problem of flickering on a display.
Another object of the present invention is to provide a driving apparatus
for affording normal motion picture display or scroll display.
According to the present invention, there is provided a liquid crystal
apparatus, comprising scanning electrodes and data electrodes intersecting
with each other to form a pixel at each intersection, and a ferroelectric
liquid crystal disposed between the scanning electrodes and data
electrodes; the improvement comprising:
first means for applying to the scanning electrodes at least two scanning
selection signals in at least two vertical scanning periods, said at least
two scanning selection signals comprising mutually different waveforms and
each comprising a pulse of one or the other voltage polarity with respects
to the level of a voltage applied a scanning electrode when it is not
selected; and
second means for applying data pulses to the data electrodes in phase with
said pulse of one or the other voltage polarity;
said first means and second means in combination applying a fore voltage
pulse to a pixel on a scanning electrode selected by application of said
pulse of one or the other voltage polarity prior to each application of a
writing voltage formed by combination of said pulse of one or the other
polarity and an information pulse, said fore voltage pulse having a
polarity opposite to that of the writing voltage and an amplitude which is
1/2 or less of that of the writing voltage.
According to a second aspect of the present invention, there is provided a
driving apparatus, comprising scanning electrodes, scanning-side drive
means connected to the scanning electrodes, data electordes intersecting
with the scanning electrodes and data-side drive means connected to the
data electrodes; the improvement wherein
said scanning-side drive means includes means for supplying a first
scanning selection signal and a second scanning selection signal having
mutually different voltage waveforms, which are supplied to the scanning
electrodes in one vertical scanning period and supplied to one scanning
electrode in at least two vertical scanning periods.
According to a third aspect of the present invention, there is provided a
driving apparatus, comprising scanning electrodes, scanning-side drive
means connected to the scanning electrodes, data electrodes intersecting
with the scanning electrodes and data-side drive means connected to the
data electrodes; the improvement wherein
said scanning-side drive means includes means for subjecting the scanning
electrodes to frame scanning respectively with a first scanning selection
signal having a voltage of one polarity and a second scanning selection
signal having a voltage of the other polarity at the same phase,
respectively with respect to the level of a voltage applied to a scanning
nonselection line, at least one of the first and second scanning selection
signals being used for at least two times of frame scanning to form one
picture; and
said data side drive means includes means for supplying data signals to the
data electrodes in phase with said first and second scanning selection
signals.
These and other objects, features and advantages of the present invention
will become more apparent upon a consideration of the following
description of the preferred embodiments of the present invention taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a characteristic view showing the influence of a reverse-polarity
fore pulse on the threshold voltage of ferroelectric liquid crystals;
FIGS. 2 and 3 are waveform diagrams of driving voltages used in
multiplexing drive according to the present invention;
FIG. 4 is a plan view showing a display example;
FIG. 5 is a characteristic view showing the dependence of the threshold
voltage of a ferroelectric liquid crystal device on the pulse duration;
FIG. 6 is a waveform diagram showing another preferred set of driving
waveforms;
FIG. 7 is a waveform diagram showing another set of driving waveforms used
in the invention, FIG. 8 is a time-serial waveform diagram using the same;
FIG. 9 is a waveform diagram showing still another set of driving waveforms
used in the invention, FIG. 10 is a time-serial waveform diagram using the
same;
FIGS. 11 and 12 are waveform diagrams each showing another set of driving
waveforms used in the invention;
FIG. 13 is a plan view showing a display example;
FIGS. 14 and 15 are waveform diagrams each showing another set of driving
waveforms used in the invention;
FIG. 16 is a waveform diagram showing still another set of driving
waveforms used in the invention; FIG. 17 is a time-serial waveform diagram
using the same;
FIGS. 18, 19, 20 and 21 are waveform diagrams each showing another set of
driving waveforms used in the invention;
FIG. 22A is an explanatory view illustrating voltage application states at
pixels (on scanning selection lines) on a picture according to the
invention; FIG. 22B is an explanatory view illustrating the corresponding
display states;
FIG. 23 is a waveform diagram showing still another set of driving
waveforms used in the invention;
FIG. 24A is an explanatory view illustrating voltage application states at
pixels on a picture outside the scope of the present invention; FIG. 24B
is an explanatory view showing the corresponding display states;
FIGS. 25 and 26 are waveform diagrams each showing still anotehr set of
driving waveforms used in the invention;
FIG. 27A is an explanatory view illustrating another set of voltage
application states at pixels on a picture according to the invention; FIG.
27B is an explanatory view showing the corresponding display states;
FIGS. 28 and 29 are schematic perspective views illustrating a
ferroelectric liquid crystal device used in the invention; and
FIG. 30 is a block diagram of a display panel according to the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
According to our experiments, it has been found that in case where a
voltage of one polarity is applied to a particular pixel prior to
application of a writing voltage of the other polarity to the pixel, the
threshold voltage for the writing of the ferroelectric liquid crystal
constituting the pixel is changed depending on the amplitude of the
voltage of one polarity (hereinafter sometimes referred to as
"reverse-polarity fore pulse" or "reverse-polarity fore voltage").
FIG. 1 shows the dependency of the threshold voltage Vth of ferroelectric
liquid crystal cells on the reverse-polarity fore pulse. The curve 11
represents the threshold characteristic of a ferroelectric liquid crystal
cell used in Example 1 described hereinafter, and the curve 12 represents
the threshold characteristic of a cell used in Example 2. In FIG. 1, Vb
denotes the amplitude of the reverse-polarity fore pulse (This voltage
corresponds to a clearing voltage); Vw denotes the amplitude of the
writing pulse; and t.sub.1 and t.sub.2 (t.sub.1 =t.sub.2 =30 .mu.sec)
denote the durations of the respective pulses.
FIG. 1 shows that the threshold voltage steeply increases as the amplitude
Vb of the reverse-polarity fore pulse is increased.
As a result of further experiments of ours, it has been found that the
influence of the reverse-polarity fore pulse at the time of writing can be
minimized if the amplitude thereof (Vb) is set to 1/2 or below, preferably
1/3 or below, of the amplitude of the writing pulse (Vw).
FIG. 2 shows a set of driving signal waveforms used in a preferred driving
embodiment of the invention, and FIG. 3 is a time-serial waveform diagram
using the driving signals. In FIG. 2 and similar figures described
hereinafter, a signal followed by (n) is one applied in an n-th frame and
a signal followed by (n+1) is one applied in an (n+1) th frame. A picture
is formed in two frames. S.sub.S denotes a scanning selection signal;
S.sub.NS, a scanning nonselection signal; I.sub.W, a "white"-writing
signal, and I.sub.B, a "black"-writing signal.
In this driving embodiment, in order to prevent the above-mentioned
influence of the fore pulse, a reset operation of preliminarily bringing
all the pixels on a selected scanning line uniformly to, e.g., the "white"
(or "bright") state is not effected, but one picture is displayed in two
frames wherein, for example, the white state is written in desired pixels
in the first frame and pixels to be written in "black" are then written as
such in the subsequent second frame while the polarity of the scanning
signal is reversed. In this driving embodiment, "white" is written in an
n-th frame (n is an integer) and "black" is written in the subsequent
(n+1)th frame. The waveforms of driving signals and voltages applied to
pixels in the respective frames are as shown in the figures. By
selectively applying the information signals in the respective frames,
crosstalk based on the influence of a reverse-polarity fore pulse can be
obviated.
FIG. 3 show time-serial waveforms of scanning signals S.sub.1, S.sub.2, . .
. , S.sub.5, an information or data signal I.sub.1, a voltage (I.sub.1
-S.sub.2) applied to a pixel A and a voltage (I.sub.1 -S.sub.3) applied to
a pixel B for providing a display pixel pattern shown in FIG. 4.
In this instance, the voltage levels of the respective signals may be set
to satisfy the following relationship:
##EQU1##
The above-mentioned liquid crystal cells were driven under the following
conditions to provide very good images:
Environmental temperature: 30.degree. C.
Driving pulse duration: .DELTA.t=(t.sub.1 =t.sub.2)=30 .mu.sec
Driving voltage: .vertline.V.sub.S +V.sub.I .vertline.=24 volts
Bias ratio: .vertline.V.sub.S +V.sub.I .vertline./.vertline.V.sub.I
.vertline.=3
FIG. 5 shows the dependence of the threshold voltage on the pulse duration
when a single pulse with a pulse duration .DELTA.t was applied a
ferroelectric liquid crystal cell used in Example 1 described hereinafter.
Herein,
##EQU2##
denotes a voltage causing an inversion at a part of a pixel (250
.mu.m.times.250 .mu.m), and
##EQU3##
denotes a voltage causing an inversion over the entire region of a pixel.
FIG. 6 shows a set of driving waveforms in another driving embodiment. In
FIG. 6, (n) and (n+1), etc., have the same meanings as in FIG. 2. In this
driving embodiment, different information or data signals are used for the
same data in two successive scans. Further, in the two successive scans,
the information signals providing the same data are applied at different
instants or phases in a scanning selection period or have mutually
opposite polarities.
In the liquid crystal apparatus of the present invention, a particular
pixel showing the same display state is supplied with DC voltage
components of mutually opposite polarities in an n-th frame period and in
an (n+1)th frame period, and the voltages applied to the pixel assume zero
on a time-average, i.e., as a time-weighted average, during the period of
two frames.
FIG. 7 shows another set of driving waveforms used in the invention. More
specificaly, FIG. 7 shows a scanning selection signal S.sub.2n-1 (n=1,2,3.
. . ) applied to an odd-numbered scanning electrode and a scanning
selection signal. S.sub.2n applied to an even-numbered scanning electrode
in both an odd-numbered frame F.sub.2M-1 and an even-numbered frame
F.sub.2M. In FIG. 7 and subsequent similar figures; "W" denotes a white
signal, "B" denotes a black signal, and "H" denotes a hold signal for
retaining the previous state. According to FIG. 7, the scanning selection
signal S.sub.2n-1 has mutually opposite voltage polarities (i.e., voltage
polarities with respect to the voltage of the scanning nonselection
signal) in the odd frame F.sub.2M-1 and the even frame F.sub.2M. This also
holds true with the scanning selection signal S.sub.2n. Further, the
scanning selection signals S.sub.2n-1 and S.sub.2n applied in one frame
period have mutually different voltage waveforms and have mutually
opposite voltage polarities in a single phase.
Further, in the driving embodiment shown in FIG. 7, a third phase for
having the whole picture pose (e.g., by applying a zero voltage to all the
pixels constituting the picture) is provided and the third phase for each
scanning selection signal is set to a zero voltage (the same voltage level
as the scanning nonselection signal).
Further, in the embodiment of FIG. 7, as for the information signals
applied to signal electrodes in the odd frame F.sub.2M-1, a white signal
("W", providing a voltage 3V.sub.0 exceeding the threshold voltage of the
ferroelectric liquid crystal at the second phase in combination with the
scanning selection signal S.sub.2n-1 to form a white pixel) and a hold
signal ("H", providing a pixel with voltages .+-.V.sub.0 below the
threshold voltage of the ferroelectric liquid crystal in combination with
the scanning selection signal S.sub.2n-1) are selectively applied in phase
with the scanning signal S.sub.2n-1 ; and a black signal ("B", providing a
voltage -3V.sub.0 exceeding the threshold voltage of the ferroelectric
liquid crystal at the second phase in combination with the scanning
selection signal S.sub.2n to form a black pixel) and a hold signal ("H",
providing a pixel with voltages .+-.V.sub.0 below the threshold voltage of
the ferroelectric liquid crystal) are selectively applied in phase with
the scanning selection signal S.sub.2n.
In the even frame F.sub.2M subsequent to writing in the above-mentioned odd
frame F.sub.2M-1, the above-mentioned black signal ("B") and hold signal
("M") are selectively applied in phase with the scanning selection signal
S.sub.2n-1, and the above mentioned white signal ("W") and hold signal
("H") are selectively applied in phase with the scanning selection signal
S.sub.2n.
FIG. 8 is a time chart for providing a display state shown in FIG. 13
(wherein .smallcircle. denotes a white pixel and denotes a black pixel)
by using the unit signals shown in FIG. 8. In FIG. 8, at I.sub.1 -S.sub.1
is shown a time-sectional voltage waveform applied to the intersection of
a scanning electrode S.sub.1 and a signal electrode or data electrode
I.sub.1, and at I.sub.2 -S.sub.1 is shown a time-serial voltage waveform
applied to the intersection of the scanning electrode S.sub.1 and a signal
electrode I.sub.2.
FIG. 9 shows another set of driving signal waveforms used in the invention.
Scanning Selection Signals S.sub.2n-1 and S.sub.2n used in the embodiment
of FIG. 9 respectively have two voltage pulses of mutually opposite
polarities with respect the voltage level of the scanning nonselection
signal, and the former voltage pulses have durations twice those of the
latter pulses of the opposite polarities. Further, each of the information
signals has a zero voltage (the same voltage level as the scanning
nonselection signal) at the first phase and has an alternating voltage
with voltages of mutually opposite polarities with respect to the voltage
level of the scanning nonselection signal at the second and third phases.
FIG. 10 is a time chart for providing a display state shown in FIG. 13 by
using the unit signals shown in FIG. 9.
FIGS. 11 and 12 respectively show another set of the driving signal
waveforms used in the invention. In the embodiments shown in FIGS. 11 and
12, each of the scanning selections and information or data signals is set
to have two levels, so that the designing of the drive circuit is
simplified.
In the above driving embodiments, the amplitude of the scanning selection
signals is set to 2.vertline..+-.V.sub.0 .vertline., and the amplitude of
the information signals is set to .vertline.IV.sub.0 .vertline.. In the
present invention, the amplitude of the scanning selection signal may be
set to .vertline.S.sub.ap .vertline.and the amplitude of the information
signals may be set to .vertline.I.sub.ap .vertline.so as to satisfy the
relationship of .vertline.I.sub.ap .vertline./.vertline.S.sub.ap
.vertline..ltoreq.1, preferably .vertline.I.sub.ap
.vertline./.vertline.S.sub.ap .vertline.<1/1.2.
Further, in the present invention, when a ferroelectric liquid crystal
shows two threshold voltages, Vth.sub.1 and -Vth.sub.2 (Vth.sub.1,
Vth.sub.2 >0), the above-mentioned voltage V.sub.0 may be set to satisfy:
V.sub.0 <Vth.sub.1 <3V.sub.0 and -3V.sub.0 <-Vth.sub.2 <-V.sub.0.
The following Table 1 shows a time table for applying a white selection
voltage Sw and a half-selection voltage H at that time for forming white
selection pixels in frames F.sub.1, F.sub.2, F.sub.3, F.sub.4, . . . .
TABLE 1
______________________________________
##STR1##
______________________________________
In contrast, the following Table 2 shows a similar time table for writing
white selection pixels outside this aspect of the present invention.
TABLE 2
______________________________________
##STR2##
______________________________________
According to the time table 1 of the present invention, a half-selection
voltage is applied to pixels (white selection pixels) on the odd-numbered
scanning lines S.sub.1, S.sub.3, . . . in the even-numbered frames
F.sub.2, F.sub.4, . . . . In contrast, according to the time table 2
outside the present invention, such a half-selection voltage is applied to
pixels (white selection pixels) on all the scanning lines in the
even-numbered frames F.sub.2, F.sub.4, . . . . Accordingly, in the driving
embodiment outside the present invention shown in Table 2, flickering
occurs at a half of the frame frequency. In contrast thereto, according to
time table 1 of the present invention, the number of pixels supplied with
a half selection voltage during one frame period is decreased to a half of
that according to the time table 2, so that flickering is effectively
prevented or alleviated.
FIGS. 14 and 15 respectively show another set of driving signal waveforms
used in the invention. More specifically, in the driving embodiment shown
in FIG. 14, the scanning selection signal applied to 1st, 2nd, 5th, 6th, .
. . (4N-3) th and 4 (N-2) th scanning electrodes (N=1 , 2, 3, . . .), and
the scanning selection signal applied to 3rd, 4th, 7th, 8th, . . . (4N-1)
th and 4N-th scanning electrodes, are respectively changed depending on
whether they are applied in an odd frame or an even frame. Further, in the
embodiment shown in FIG. 15, the scanning selection signal applied to 1st,
2nd, 3rd, . . . (6N-5)th, (6N-4)th and (6N-3)th scanning electrodes (N=1,
2, . . .), and the scanning selection signal applied to 4th, 5th, 6th, . .
. (6N-2)th, (6N-1)th and 6N-th scanning electrodes, are respectively
changed depending on whether they are applied in an odd frame or in an
even frame. The above-mentioned number "N" refers to the number of blocks
when the scalling lines are divided into the blocks in a plurality. In the
embodiments of FIGS. 14 and 15, the number of scanning lines in each block
has been 2 and 3, respectively, but is not generally restricted to these
numbers.
As a preferred embodiment of the present invention, there is provided a
driving apparatus, comprising s | | |
