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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of driving a display panel
composed of a first group of electrodes which are arranged in parallel to
one another, a second group of electrodes which cross the first group of
electrodes and which are arranged in parallel to one another, and display
cells which are respectively connected at the intersection points between
the first and second groups of electrodes and each of which has optical
characteristics being substantially symmetric for input electric
quantities of positive and negative polarities.
2. Description of the Prior Art
Where the panel is driven by the line progressive scanning, electric
quantities applied to the display cells during a period of the half
address state caused by nonaddress lines among the first group of
electrodes (hereinafter termed X-lines) and address lines among the second
group of electrodes (hereinbelow called Y-lines) and during a period of
the nonaddress state caused by nonaddress lines among the X-lines and
nonaddress lines among the Y-lines are respectively different in the
absolute value. Besides, the rate at which the half address state and the
nonaddress state arises varies in dependence on information
to-be-displayed. For this reason, there is the disadvantage that the
quality of display information differs in dependence on input information
as will be described in detail later. A further disadvantage is that the
electric quantities to be applied to addressed display cells are
restricted to small values.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a line progressive
scanning method capable of displaying pictures, numerals, etc., of good
quality.
Another object of the present invention is to provide a line progressive
scanning method capable of displaying pictures, numerals, etc., of good
contrast.
In order to accomplish such objects, the present invention carries out the
line progressive scanning by applying asymmetric voltages to address lines
in the X- and Y-directions and by applying voltages of equal absolute
value to nonaddress lines of the X-lines.
Hereunder the present invention will be explained in comparison with the
prior art with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a and 1b are explanatory diagrams which illustrate an example of
characteristics of a display cell of the type to which the present
invention is directed;
FIG. 2 is a connection diagram which shows an example of a panel of the
type to which the present invention is applied;
FIG. 3 is a connection diagram which shows an example of a segment type
display panel;
FIGS. 4, 5, and 6 are explanatory diagrams of prior art drive systems;
FIGS. 7 and 8 are explanatory diagrams of a drive system of the present
invention;
FIG. 8 is a waveform diagram which shows an example of waveform by a prior
art drive method;
FIG. 9 is an explanatory diagram which illustrates states in which two
levels of a picture are displayed;
FIGS. 10, 13, 14, 15, 19, 20, and 21 are waveform diagrams which show
examples of drive waveforms by the present invention;
FIG. 11 is a curve diagram which illustrates the color display
characteristic of a liquid crystal;
FIG. 12 is an explanatory diagram which shows a display picture at various
levels;
FIG. 16 is a block diagram which shows the construction of a character
display device;
FIGS. 17 and 18 are a block diagram and a circuit diagram, respectively,
which illustrate the constructions of various parts of the character
display device;
FIG. 22 is a waveform diagram of an applied voltage;
FIG. 23 is a characteristic diagram of a liquid crystal;
FIG. 24 is an explanatory diagram of a display picture; and
FIG. 25 is a waveform diagram of drive voltages.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As illustrated by way of example in FIG. 1a and FIG. 1b, a display cell for
use in the present invention has optical characteristics, such as
transmission factor T, reflection factor P and luminous intensity I, which
are substantially symmetric for positive and negative input electric
quantities. As display cells having such characteristics, there are liquid
crystal cells, electroluminescent cells, cells in which a ferroelectric
substance or a nonlinear resistance is added to the liquid crystal or
electroluminescent materials, and so forth. In order to simplify the
explanation, the case of a liquid crystal will be referred to. While the
input electric quantities include voltages, currents, charges, etc., the
following description will be made of only the case of voltages. The
symmetry of the display cell need not be especially strict, but it is
meant that display cells having clear asymmetry, such as found in the
diode characteristic are excluded.
Shown in FIG. 2 is an example of the equivalent circuit of a panel of the
type to which the present invention is applied. The figure illustrates the
case of a 3 .times. 3 arrangement of picture elements. The number of
picture elements may be two or larger, and the illustrated case is cited
in order to facilitate the description.
In the figure display cells arranged in the form of a matrix as an example
are connected at one end to a first group of electrodes X.sub.1, X.sub.2
and X.sub.3 at every row, and are connected at the other end to a second
group of electrodes Y.sub.1, Y.sub.2 and Y.sub.3 at every column. The
panel to which the present invention is applied may be any panel insofar
as its equivalent circuit has the form of FIG. 2. Of course, the invention
is applicable to a segment type display panel of the type shown in FIG. 3.
Examples of voltages V.sub.X1, V.sub.X2 and V.sub.X3, and voltages
V.sub.Y1, V.sub.Y2 and V.sub.Y3 to be applied in the prior art to the
respective electrodes X.sub.1, X.sub.2 and X.sub.3 and Y.sub.1, Y.sub.2
and Y.sub.3 of the panel of this sort are shown in FIGS. 4 to 6. These
figures illustrate a case where only the display cell a.sub.11 in FIG. 2
is addressed.
The prior art method of applying the voltages as illustrated in FIG. 4 is
the most fundamental. With this method, a voltage impressed on a
nonaddress display cell is 0 or 1/3 V.sub.0. On the other hand, a voltage
impressed on the address display cell a.sub.11 in FIG. 2 is double the
maximum value of the voltage impressed on the nonaddress display cell.
That is, when the value 1/3 V.sub.0 is taken as the threshold voltage
(hereinbelow denoted by V.sub.th) of the display cell, the voltage applied
to the address display cell a.sub.11 becomes 2V.sub.th.
In case of driving the panel as shown in FIG. 2, however, the drive need be
in the time division. In that case, the period of time during which the
address display cell is selected is naturally short. Therefore, the value
2V.sub.th is often insufficient for the voltage applied to the address
display cell (particularly in case where the number of display cells is
large). For this reason, the prior art has the disadvantage that a
satisfactory contrast is not achieved.
As a method which displays information at high speed by the use of the
panel in FIG. 2, the line progressive scanning method has been known. As
illustrated in FIG. 9, this method applies a scanning voltage to the first
group of electrodes X.sub.1, X.sub.2 and X.sub.3 from an X-axis electrode
drive circuit 12, and simultaneously applies a voltage of a picture
information for display to a second group of electrodes Y.sub.1, Y.sub.2
and Y.sub.3 from a Y-axis electrode drive circuit 10.
The following description is fully devoted to the case of the line
progressive scanning method.
A waveform in the case where the line progressive scanning is performed by
the driving method of FIG. 4 is shown in FIG. 8. The picture to be
displayed at this time is such that, as shown in FIG. 9, the display cells
a.sub.11, a.sub.21, a.sub.22, a.sub.31, a.sub.32, a.sub.33 are "on" while
those a.sub.12, a.sub.13 and a.sub.23 (indicated in black in the figure)
are "off". The voltage in FIG. 4 corresponds to that at t = t.sub.1 in
FIG. 8.
In case of performing such line progressive scanning, a period in which the
half address (parts surrounded by broken lines in FIG. 4) by a nonaddress
line among the X-lines and address lines among the Y-lines and the
nonaddress (a part surrounded by a one-dot chain line in FIG. 4) by the
nonaddress line among the X-lines and a nonaddress line among the Y-lines
arise differs in dependence on the picture to-be-displayed. It is 1 F (F:
frame) -- 1 H (H: horizontal scanning period) within 1 F at the maximum.
Where 1 F is composed of a large number of periods H, for example, 100 H,
the value (1 F - 1 H) is approximately equal to 1 F.
On the other hand, a period in which the half address (parts surrounded by
solid lines in FIG. 4) by address lines among the X-lines and the
nonaddress line among the Y-lines arises is only 1 H within 1 F. Where 1 F
is composed of a large number of periods H (for example, 100 H), the rate
at which this half address occurs is very small.
In consequence, the degradation of the display picture due to the half
address by the address lines among the X-lines and the nonaddress line
among the Y-lines is less than the degradation of the picture due to the
half address or nonaddress by the nonaddress line among the X-lines and
the address lines or nonaddress line among the Y-lines. It is therefore
possible that the voltage to be applied to the X-lines is made greater
than the voltage to be applied to the Y-lines, in other words, that the
voltages are made asymmetric.
In consideration of this fact, the prior art in FIG. 5 has improved the
system in FIG. 4. In this case, the voltage applied to the address display
cell is greater than in the case of FIG. 4, and a better contrast can be
expected.
According to the voltage applying methods in FIG. 4 and FIG. 5, however,
the absolute values of the voltages applied to the display cells during
the period of the half address by the nonaddress line among the X-lines
and the address line among the Y-lines and during the period of the
nonaddress by the nonaddress line among the X-lines and the nonaddress
line among the Y-lines are respectively different.
As previously stated, the rate at which the half address and the nonaddress
occur changes in dependence on the picture information to-be-displayed.
For this reason, in the first place, there is the disadvantage that the
quality of the display picture changes in dependence on input information.
The second disadvantage is that, even when the asymmetric applied voltages
illustrated in FIG. 5 are used, the voltage impressed on the address
display cell is still restricted to a small value.
As a method for obviating the disadvantages, the method in FIG. 6 has
heretofore been proposed. By appropriate selection of voltages to be
applied to the X-lines and the Y-lines, the absolute values of the
voltages of all the nonaddress display cells can be made equal, and the
voltage of the address display cell can be made at most three times as
great as the voltage of the nonaddress display cell.
Thus, in comparison with the case of FIG. 4, the change of the quality of
the display picture depending on the change of the input picture is
eliminated, and the voltage impressed on the address display cell
increases. Even the method in FIG. 6, however, has the disadvantage that
the voltage applied to the address display cell is still restricted to a
small value.
The present invention provides a new driving system which enjoys both the
increase of the address voltage owing to the application of the asymmetric
voltages as illustrated in FIG. 5 and the uniformalization of the absolute
values of the voltages applied to the display cells connected to the
nonaddress line among the X-lines as illustrated in FIG. 6, and makes the
picture display of good quality possible.
An embodiment of the driving system of the present invention is illustrated
in FIG. 7 in comparison with the prior art which has thus far been
described. In this case, unlike the case of FIG. 6, the amplitude of a
pulse to be impressed on the X-lines is made larger than the amplitude of
a pulse to be impressed on the Y-lines. Using this system, the change of
the picture quality dependent upon the input picture information is
eliminated, and besides, the voltage impressed on the address display cell
is great, so that the picture display of good contrast is made possible
for the first time.
An example of various waveform in the system of the present invention in
the case of performing the line progressive scanning is shown in FIG. 10.
The figure corresponds to the case where the display cells a.sub.11,
a.sub.21, a.sub.22, a.sub.31, a.sub.32 and a.sub.33 are "on" while those
a.sub.12, a.sub.13 and a.sub.23 are "off". The D.C. bias voltage V.sub.DC
can take an arbitrary value. The absolute values of voltages V.sub.Ya and
V.sub.Yb in the figure should preferably be approximately equal, and
actual measurements have revealed that they are substantially satisfactory
if they meet the conditions of the following equations:
##EQU1##
Regarding a voltage to be impressed on the X-lines, it has been revealed by
actual measurements that a range fulfilling the following equation is
preferable:
##EQU2##
(In the prior art in FIG. 6, the value of Equation (3) becomes 1.)
A case where the present invention stated above is applied to a liquid
crystal panel will now be explained in comparison with the case of the
prior art.
Panel employed: 10 .times. 50 picture elements
Display contents:
alphabetic letters and numerals
number of characters: 7
Operating mode of liquid crystal employed: dynamic scattering mode
Prior Art Drive Method (FIG. 6)
Contrast 5 : 1
(voltage applied to address point = 18 volts)
Drive Method of Present Invention
Contrast 12 : 1
(V.sub.X = 22.5 volts, V.sub.Ya = V.sub.Yb = 5.5 volts, voltage applied to
address point = 28 volts)
The above-mentioned values are those at the time when the identical panel
was used and was set at the best states for the respective methods. FIG.
26 illustrates an embodiment of the driving system of the present
invention utilized to obtain the voltage of 28 volts at the address point.
An embodiment of a liquid crystal display panel-driving device for
performing the drive system according to the present invention is shown as
a block diagram in FIG. 16. The figure shows a case of displaying
characters.
Referring to the figure, a coded character signal S.sub.b and a coded
display position signal S.sub.p of a character are transmitted from a
keyboard 1. On the other side, a scanning position signal S.sub.s is
transmitted from a scanning signal generator 6 repeatedly at all times.
When the coded display position signal S.sub.p and the scanning position
signal S.sub.s become coincident, a pulse is transmitted from a
coincidence circuit 5 and is impressed on a gate 2.
In the absence of the pulse from the coincidence circuit 5, the gate 2
supplies the output of a refresh memory 3 as the input of the same without
any change, to repeatedly supply the previously applied character signal
to a character generator 4. In contrast, where the pulse from the
coincidence circuit 5 is applied, the gate 2 inputs the character signal
S.sub.b of the keyboard 1 to the refresh memory 3.
A scanning circuit 7 supplies a scanning pulse to the character generator 4
and a gate and one-line memory 9 by the output signal of the signal
generator 6. By the coded character signal transmitted from the refresh
memory 3 acting as a delay circuit and the scanning signal transmitted
from the scanning circuit 7, the character generator 4 inputs to the gate
and one-line memory 9 a signal which corresponds to the shape of the
actual character. That is, the input applied to the character generator 4
is a coded signal of, for example, 6 bits or 8 bits, which is converted
into a signal representative of the actual character in the character
generator 4.
In the gate and one-line memory 9, the signals representative of the actual
characters which correspond to one line are held by the outputs of the
scanning circuit 7 and the character generator 4 for a period of 1 H or a
period close to 1 H. The output of the gate and one-line memory 9 and the
output of a gating signal generator 8 are applied to a Y-axis electrode
drive circuit 10, in which signals to be supplied to Y-axis electrodes are
prepared. They are applied to the Y-axis electrodes Y.sub.1 - Y.sub.11 of
a liquid crystal display panel 13.
On the other hand, an X-axis electrode scanning circuit 11 is actuated by
the signal from the scanning signal generator 6, and its output and the
output of the gating signal generator 8 are inputted to an X-axis
electrode drive circuit 12. Here signals to be supplied to X-axis
electrodes are prepared, and they are applied to the X-axis electrodes
X.sub.1 - X.sub.m of the liquid crystal display panel 13. As will be
described later, the gating signal generator 8 sets a polarity inversion
period for an output voltage as is necessary in case of applying the A.C.
drive to the present invention.
It is known that the color display is made possible by operating the liquid
crystal panel in the field effect mode. The color change of the liquid
crystal depends substantially on the effective value of the applied
voltage of each liquid crystal cell. An example of the color changes of
transmitted light relative to the applied voltages in this case is shown
in FIG. 11. In the figure, the abscissa represents the applied voltage (in
the effective value) and the ordinate the transmission factor T. As shown
in FIG. 11, the color changes in the order of white, whitish green,
whitish yellow, orange, purple, royal purple, bluish green, green and
yellow as indicated by black circles according to the magnitudes of the
applied voltages.
Here, consider a case of character display where the background and a
character portion are displayed by different colors. In order to prevent
the color of the background from changing even when the information of the
character to be displayed changes, it is necessary to employ the prior art
method in FIG. 6 or the method of the present invention. In the case of
FIG. 6, however, the difference between the effective value of the applied
voltage of the character portion and the effective value of the applied
voltage of the background portion cannot be set to be sufficiently large.
On the other hand, with the present invention, the difference can be made
large, and accordingly, the range of colors which can be selected widens.
In the present invention, the effective values E.sub.1 and E.sub.2 of
voltages applied to the character portion and the background portion are
as follows:
##EQU3##
where V.sub.Ya = - V.sub.Yb, and N denotes the number of the X-lines.
The ratio between E.sub.1 and E .sub.2 becomes as follows:
##EQU4##
where N >>1.
Here, if V.sub.X = 2 V.sub.Ya, such case falls into the prior art
illustrated in FIG. 6. That is, the ratio of the effective values E.sub.1
/E.sub.2, obtained by the prior art is restricted as follows:
##EQU5##
On the contrary, in the case of the present invention, V.sub.X can be
arbitrarily made large in comparison with V.sub.Ya, so that the ratio
E.sub.1 /E.sub.2 is not restricted. Therefore, the range of the colors
which can be selected expands.
Study will now be made of the construction of a device which readily
provides the drive waveform as shown in FIG. 10. Consider the case where
the potential of the ground line of the whole device (the first reference
potential) is held at (V.sub.Ya + V.sub.DC) or around (V.sub.Yb +
V.sub.DC). In this case, the Y-axis electrode drive circuit 10 may effect
the switching between the ground line level and another level, and can be
made of a simple circuit arrangement (for example, a grounded-emitter
circuit is constructed of one transistor and one resistor).
On the other hand, the X-axis electrode drive circuit 12 switches the
potential of the ground line of the whole device (the first reference
potential) and two different levels. A ground line in a portion enclosed
with one-dot chain line in FIG. 16, i.e., in the X-axis electrode scanning
circuit 11 and the X-axis electrode driving circuit 12, has its potential
(the second reference potential) made one with a bias D.C.-wise added to
the potential of the ground line of the whole device. Then, the X-axis
electrode drive circuit 12 may effect the switching between the ground
line level (the second reference potential) and one different level
likewise to the foregoing Y-axis electrode drive circuit 10, and can be
made of a simple circuit arrangement (of, for example, one transistor and
one resistor).
In this case, however, a signal to be inputted to the X-axis electrode
scanning circuit 11 and the X-axis electrode driving circuit 12 from
another circuit need be passed through a level shift circuit 14, in FIG.
17, for executing the level shift. The level shift circuit 14 can be
constructed of a capacitor and a resistor as shown in FIG. 18, or of a
diode, etc. Alternatively, it can be substituted by an amplifier. The
amount of the level shift has the optimum value determined by a signal
waveform to-be-inputted, etc., and the optimum value is close to
.vertline. V.sub.Ya .vertline. or .vertline. V.sub.Yb .vertline..
Here the signals which are inputted from another circuit to the circuits 11
and 12 surrounded by the one-dot chain line in FIG. 16 are several sorts
of clock signal, reset signal, etc. They are of a small number, and are
digital signals repeated at fixed periods (they do not change in
dependence on input picture information). For this reason, the number of
the required level shift circuits 14 is small, and their construction is
simple. By thus subjecting to the level shift the signals which are
inputted to the X-axis electrode scanning circuit 11 and the X-axis
electrode driving circuit 12 from another circuit, it becomes possible to
simply obtain the waveform of the present invention.
Although the above description has been made of the display of two states,
such as "bright" and "dark", the system of the present invention can be
used for the display of three or more states (the half tone display,
multicolor display, etc.). As one means therefor, the pulse width
modulation is conducted. As stated previously, the absolute value of the
voltage at the half address (the broken-line part in FIG. 7) by the
nonaddress line among the X-lines and the address line among the Y-lines
and the absolute value of the voltage at the nonaddress (the one-dot chain
line part in FIG. 7) are substantially equal. Consequently, even when the
pulse width modulation is carried out, the quality of the display picture
does not change in dependence on the change of input picture information,
and a good display is possible.
FIG. 13 shows an example of various waveforms according to the present
invention as is used in case of displaying a pattern shown in FIG. 12 or a
picture in which, among the display cells, a.sub.11 is the brightest,
a.sub.21 and a.sub.22 are the second in brightness, a.sub.31, a.sub.32 and
a.sub.33 are the third in brightness and a.sub.12 and a.sub.13 are the
darkest. V.sub.X, V.sub.Ya and V.sub.Yb in the figure satisfy the
respective equations (1), (2) and (3).
FIG. 14 shows another example of various waveforms for use in the display
of many states according to the present invention. The waveform in the
figure is of the case of displaying the pattern in FIG. 12. In FIG. 14, 1
F is composed of three fields, and the brightest display cell is addressed
at every field, in other words, three times within 1 F.
On the other hand, the display cells of the second and third brightnesses
are addressed twice and once within 1 F, respectively. In this manner, a
number of states can be displayed by constructing 1 F of a plurality of
fields and changing the number of times of the addressing. V.sub.X,
V.sub.Ya and V.sub.Yb fulfill the respective equations (1), (2) and (3).
As the method of displaying a number of states of three or more levels, the
two systems have been explained above. A system with the two systems
combined is, of course, possible. Similarly to the two systems, such
combined system, of course, has the features of the present invention that
the voltage applied to the address display cell is great and that the
change of the picture quality dependent upon the change of input picture
information does not occur.
When the waveform with the D.C. bias voltage added as in FIG. 10 is used,
the D.C. component is applied to the display cells and changes in
dependence on the contents of a picture to-be-displayed. The presence of
the D.C. component sometimes exerts a bad influence on the operation of
the panel. For example, the liquid crystal panel is subjected to such
influence, which will be described later.
Even where the expected operation is not achieved due to the presence of
the D.C. component, the present invention becomes applicable by the use of
the following method. In order to remove the D.C. component, the polarity
of the voltage applied to the display cell may be inverted at every fixed
period (this operation will be hereinafter termed the A.C. drive). By
bringing the waveform shown in FIG. 10 into the A.C. drive, great strides
of improvements in characteristics are also possible as will be stated
later.
FIG. 19 shows an example of various waveforms of the A.C. drive in which a
plurality of polarity inversions are performed within 1 H. In the figure,
V'.sub.DC can take an arbitrary value. The waveform illustrated in the
figure, however, is relatively complicated and is not practical.
Therefore, V.sub.DC in the figure is made approximately zero, that is, the
A.C. amplitude to be impressed during the nonaddress period of the X-lines
is made approximately zero. Thus, the application waveform is simplified
as shown in FIG. 20. Although the A.C. amplitude in the nonaddress period
of the X-lines should desirably be zero, it may be below 10% of the A.C.
amplitude in the address period of the X-lines.
A waveform in the case where the period of the polarity inversions is 1 F
and where V.sub.DC = V'.sub.DC = 0, is shown in FIG. 21. In FIGS. 20 and
21, V.sub.Xa and - V.sub.Xb, and V.sub.Ya and - V.sub.Yb should desirably
be equal to each other, but they may satisfy the following ranges:
##EQU6##
As in the case of D.C., the relation between V.sub.Xa, V.sub.Xb and
V.sub.Ya, V.sub.Yb may fulfill the following value:
##EQU7##
A waveform shown in FIG. 22, in which the absolute values of the positive
and negative applied voltages are equal, the negative polarity period of
time is T.sub.W and the recurrence period is T.sub.R, is applied to a
liquid crystal cell. As to this case, FIG. 23 illustrates the output
intensity OI (the luminous intensity at the time when the liquid crystal
cell is illuminated by a light source) at the time when the absolute
values of the applied voltages are constant, and the relationship between
the threshold voltage V.sub.th and T.sub.W /T.sub.R at the time when the
output intensity OI is constant. In FIG. 23 a solid line indicates the
output intensity OI, and a dotted line the threshold voltage V.sub.th.
Here, at T.sub.W /T.sub.R = 0.5, the D.C. component becomes zero. As
illustrated in the figure, even when the absolute values of the applied
voltages are constant, changes of characteristics are caused by the D.C.
component contained. It will now be described that the same changes of
characteristics arise when the waveform in FIG. 10 is employed.
In the description of this phenomenon, reference is had to FIG. 24. The
figure illustrates by way of example a case where a matrix panel whose
Y-lines consist of Y.sub.1, Y.sub.2, Y.sub.3 . . . . and Y.sub.10 and
whose X-lines consist of X.sub.1, X.sub.2, X.sub.3 . . . . and X.sub.10 is
so driven that the numbers of lit display cells represented by white
circles A and non-lit display cells represented by black circles B differ
for all the Y-lines. With note taken of the display cells a.sub.10,
.sub.1, a.sub.10 , .sub.2 , a.sub.10, .sub.3 . . . . and a.sub.10, .sub.10
at the intersection points between the line X.sub.10 and the respective
Y-lines, the relationship between the applied voltage waveform and the
intensity level at this time will be explained.
FIG. 25 shows voltage waveforms which are applied to X.sub.10 and Y.sub.1,
Y.sub.2, Y.sub.3 . . . . and Y.sub.10, and voltage waveforms V.sub.X10 -
V.sub.Y1, V.sub.X10 - V.sub.Y2, V.sub.X10 - V.sub.Y3 . . . . and V.sub.X10
- V.sub.Y10 which are applied to the respective display cells. In the
figure, T.sub.A indicates the address period of each electrode, V.sub.X
the voltage of the address period of the X-line, V.sub.Ya the voltage of
the nonaddress period of the Y-line, and V.sub.Yb the voltage of the
address period of the Y-line. In this case, the rate of the negative
polarity of the voltages V.sub.X10 V.sub.Y1, V.sub.X10 - V.sub.Y2,
V.sub.X10 - V.sub.Y3 . . . . and V.sub.X10 - V.sub.Y10 applied to the
respective display cells changes largely from 0 to nearly 90% in
dependence on a picture to-be-displayed as understood from the ratio of
the areas of parts indicated by oblique lines above and below a base line
in the figure.
As stated with reference to FIG. 23, even when the absolute values of the
applied voltages are equal, the characteristics of the liquid crystal
cells change in dependence on the rate of the D.C. component. In this
manner, with the waveform in FIG. 10, the rate of the D.C. component
changes in dependence on the picture to-be-displayed and accordingly the
characteristics of the respective liquid crystal cells change, so that a
satisfactory picture display is difficult.
In order to solve this problem, the rate of the D.C. component may be
prevented from changing in dependence on the picture to-be-displayed. The
prevention is accomplished by keeping the D.C. component zero or in the
vicinity thereof. To this end, the foregoing A.C. drive may be carried
out. In the previously-stated case of the character display by the liquid
crystal (10 .times. 50 picture elements, the transmission type, the
dynamic scattering mode), the contrast ratio was improved from 12 : 1 (the
drive system of applying the D.C. bias as illustrated in FIG. 10) to 20 :
1 (the A.C. drive system) by performing the A.C. drive. In addition, the
liquid crystal cells can be made to have a long life by the A.C. drive.
As described above, by applying the present invention, the change of the
picture quality dependent upon the input pictures is reduced, and the
electric quantity to be applied to the address display cell increases, so
that pictures of good contrast can be acquired. The invention is greatly
effective as the display panel driving system.
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