Liquid crystal display device and method for driving display device

A method for driving a simple matrix type display device includes the steps of: applying a data voltage corresponding to values obtained by an orthogonal transform of input data to the data electrodes; applying a scanning voltage to the scanning electrodes, the scanning voltage corresponding to an orthogonal function used for the orthogonal transform; and reproducing the input data by an orthogonal inverse transform of the data voltage on the display panel, wherein the step of applying the scanning voltage includes the steps of: applying a scanning selection pulse signal having at least two levels to the scanning electrodes as a scanning voltage; and fixing the scanning selection pulse signal to an unselected level during a first period, a second period, or both of the first and second periods, the first period being defined as a period from the beginning of the data output until a predetermined time later in a data voltage output period, and the second period being defined as a period from a predetermined short time before the completion of the data output until the completion of the data output in the data voltage output period.

BACKGROUND OF THE INVENTION 
1. FIELD OF THE INVENTION 
The present invention relates to a liquid crystal display device and to a 
method for driving a display device. In particular, the present invention 
relates to a driving circuit for generating a driving waveform that 
provides uniform display quality and peripheral circuitry thereof. 
2. DESCRIPTION OF THE RELATED ART 
In recent years, there has been increasing demand for display devices 
capable of displaying a large amount of information at the same time due 
to the rise of a highly information-oriented society. CRT (Cathode Ray 
Tubes) displays have conventionally been used for such purposes. However, 
CRTs are generally large and tend to consume a large amount of power, 
making them unsuitable for use other than as desk-top devices. On the 
other hand, flat display devices such as LC (liquid crystal) display 
devices are attracting much attention because of their thinness and light 
weight. 
LC display devices were originally developed as display devices for 
calculators, watches, etc. However, current LC display devices typically 
include a matrix of scanning electrodes and data electrodes, and are 
capable of displaying images on a large screen owing to progress in 
technology concerning STN (Super-Twisted Nematic) liquid crystal and TFT 
(Thin Film Transistor) elements. 
Such matrix type LC display devices can be classified into simple matrix 
type display devices and active matrix type display devices in terms of 
their driving methods. 
Active matrix type LC display devices, which are typically driven by using 
TFT elements or MIM (Metal Insulator Metal) elements, include a matrix of 
scanning electrodes and data electrodes with switching elements of TFTs, 
diodes, and the like located at the respective intersections of the 
scanning and data electrodes. A display is realized by controlling such 
switching elements so as to apply a voltage independently to portions of 
liquid crystal corresponding to the respective pixels. In such active 
matrix type LC display devices, the LC is usually driven in its TN 
(Twisted Nematic) mode, thereby achieving high contrast and a quick 
response at the same time. Since the voltage to be applied to each portion 
of LC corresponding to a pixel can be independently controlled, it is 
relatively easy to display intermediate gray scale tones. 
On the other hand, a typical simple matrix type LC display device driven in 
a STN mode includes an LC layer interposed between glass substrates having 
a matrix of electrodes formed on the surface thereof so as to conduct 
display by utilizing the steep characteristics of the electrooptical 
effects of LC, i.e., the change in the optical characteristics of LC when 
an electric field is applied thereto. As a result, simple matrix type LC 
display devices require a relatively simple panel structure and production 
process, and therefore are more preferable in terms of cost than active 
matrix type LC display devices. 
Simple matrix type STN LC display panels have conventionally been driven by 
a time-divided method (or "duty driving") which is also referred to as a 
linearly sequential driving method. Since a plurality of pixels are 
coupled to one electrode in an active matrix type LC display device, the 
applied voltage has time-divided pulses. Generally, scanning electrodes 
are linearly sequentially scanned at a frame cycle of 20 ms or less. A 
large selection pulse is applied to each scanning electrode once per 
frame, in synchronization with which a data signal is applied via a data 
electrode. 
Since conventional STN LC display devices have a relatively low LC response 
speed, e.g., 300 ms, the LC can respond in accordance with the ON/OFF 
ratio of the effective voltage applied in linearly sequential driving, 
thereby achieving a practical contrast level. However, once quick response 
is realized in STN LC display devices (such that moving images can be 
displayed thereby) by reducing the viscosity of LC and/or reducing the 
thickness of the LC layer, etc., the linearly sequential driving results 
in a noticeable degradation of contrast due to a so-called frame response 
phenomenon described below. 
Liquid crystal is generally considered to respond to the effective values 
(rms) of the driving waveform. Assuming that an effective voltage of 
V.sub.on (rms) is applied to a selected electrode and an effective voltage 
of V.sub.off (rms) is applied to an unselected electrode, a driving margin 
(V.sub.on (rms) / V.sub.off (rms)) takes the maximum value: 
##EQU1## 
based on the voltage averaging method. In the above equation, N represents 
the number of scanning lines, and 1/N represents the duty ratio. Usually 
V.sub.off is set equal to a threshold voltage V.sub.th of the LC. 
A liquid crystal panel having very quick response tends to deviate from 
such an inherent response mode (i.e., responding to effective values(rms)) 
and instead responds to the driving waveform itself, so that the 
transmittance value fluctuates corresponding to each frame. This 
phenomenon is referred to as a "frame response phenomenon". 
Because of the frame response phenomenon, the off-transmittance increases 
even if the V.sub.off (for the unselected pixels) is set equal to 
V.sub.th. In the selected pixels, the actual transmittance is reduced 
although the optimum effective voltage of V.sub.on (rms) is being applied. 
Thus, the conventional linearly sequential driving method, when applied to 
a high-speed STN LC panel, can remarkably deteriorate the display contrast 
thereof. 
Therefore, in order to maintain the optical contrast in a high-speed and 
high-resolution STN LC panel, it is necessary to drive the LC so as to 
suppress the frame response phenomenon. 
On the other hand, a driving method called a multiple scanning line 
simultaneous selection driving method (also referred to as "active 
driving") has been proposed, which generates scanning selection pulses 
from an orthogonal matrix. By the active driving method, a plurality of 
scanning lines are simultaneously selected during one frame period in 
order to control the frame response phenomenon, thereby supplying a number 
of small scanning selection pulses for one scanning electrode during each 
frame period. Thus, the active driving method utilizes the cumulative 
response effect of LC so as to reconcile rapid response and high contrast. 
According to the active driving method, input image data is subjected to an 
orthogonal transform process using an orthogonal matrix, and a signal 
corresponding to the transformed data is supplied from the data electrode 
side. From the scanning electrode side, scanning voltage pulses are 
applied corresponding to the elements of column vectors of the orthogonal 
matrix used for the transform. An orthogonal inverse transform performed 
on the panel side for the input image data reproduces the input image. 
Active driving methods can be generally classified into an active 
addressing method (hereinafter referred to as the "AA method") and a 
multiline selection method (hereinafter referred to as the "MLS method"), 
although both are based on the same principle. For detailed descriptions 
of the AA method, see T. J. Scheffer, et al., SID' 92, Digest, p.228; 
Japanese Laid-Open Patent Publication No. 5-100642; and the like. For 
detailed descriptions of the MLS method, see T. N. Ruchmongathan et al., 
Japan Display 92, Digest, pp.65-68, Japanese Laid-Open Patent Publication 
No. 5-46127, and the like. 
FIGS. 1A to 1C show examples of respective orthogonal functions used for 
the AA method and two variants of the MLS method. 
The AA method uses an orthogonal function such as the WALSH function shown 
in FIG. 1A. Positive or negative voltages (i.e., voltages corresponding to 
the elements 1! or -1! of the orthogonal matrix) are simultaneously 
applied to all of the scanning electrodes. 
The MLS method, as in the conventional duty driving method, has unselected 
periods of scanning pulses. The elements 0! in the orthogonal matrices 
shown in FIGS. 1B and 1C correspond to the unselected periods. The MLS 
method has an advantage of using mathematical operations of a much smaller 
scale than the AA method because when an element of the matrix is 0!, the 
result of an orthogonal transform with given data (i.e., 
multiplication/addition) always becomes 0. 
The MLS method is further classified into a dispersion MLS method (FIG. 1B) 
in which the selection pulses of the orthogonal function are dispersed 
through-out one frame period, and a non-dispersion MLS method (FIG. 1C) in 
which selection pulses of the orthogonal function are grouped into blocks. 
An example of the dispersion MLS method is a SAT (Sequency Addressing 
Technique) disclosed in Japanese Laid-Open Patent Publication No. 6-4049. 
An example of the non-dispersion MLS method is an IHAT (Improved Hybrid 
Addressing Technique) disclosed in T. N. Ruchmongathan et al., IDRC 1988 
pp.80-85. 
An intrablock dispersion MLS method (Japanese Patent Application No. 
6-291848), in which the selection pulses are dispersed within each of a 
plurality of blocks into which one frame is divided, is classified as a 
non-dispersion MLS method in terms of its fundamental operation sequence, 
and therefore requires a smaller memory capacity than does the dispersion 
MLS method. However, hereinafter the intrablock dispersion MLS method and 
the dispersion MLS method will be collectively referred to as "the 
dispersion MLS method" because the intrablock dispersion MLS method is 
capable of reducing the number of simultaneously selected lines to that 
required by the dispersion MLS method. 
In general, the dispersion MLS method is considered to provide the same 
effect, by using a smaller number of selected lines, as that of the 
non-dispersion MLS method. In fact, an experiment in which a VGA-class LC 
panel having a response speed of 150 ms was driven while being split into 
upper and lower halves so as to display an image at a frame frequency of 
60 Hz showed that the dispersion MLS method only requires 7-15 lines to be 
simultaneously selected in order to attain the same contrast level as that 
attained by the AA method, which selects all of the 240 scanning lines. On 
the other hand, the non-dispersion MLS method required 60 or more lines to 
be simultaneously selected in order to attain the above-mentioned contrast 
level. 
However, the memory capacity required for the orthogonal transform 
operation depends on the calculation order of the orthogonal transform 
operation, i.e., the specific orthogonal transform matrix chosen. Thus, 
the non-dispersion MLS method has an advantage in that it only requires a 
memory capacity corresponding to the number of selected lines, whereas the 
AA method and the dispersion MLS method fundamentally require a memory 
capacity for storing data corresponding to at least one entire frame. 
Therefore, neither the dispersion MLS method nor the non-dispersion MLS 
method is superior. 
However, when contemplating a system which primarily aims to maintain a 
satisfactory contrast level, a smaller operation scale is desirable 
because it leads to lower power consumption. Therefore, the dispersion MLS 
method is considered the most practical among the various active driving 
methods for rapid STN LC panels. 
As described above, among the various active driving methods for high-speed 
STN LC panels, the dispersion MLS method is considered to have the optimum 
balance between the contrast level and circuit scale. 
However, the inventors discovered upon driving a high-speed STN LC panel by 
the dispersion MLS method, that the dispersion MLS method has problems 
unique to itself, e.g., degradation in display quality such as a 
double-image (ghost) phenomenon and display unevenness occurring in a 
horizontal zone as described below. These problems do not belong to the 
duty driving method. 
The above-mentioned problems are ascribed to nothing but the operation 
principle of the dispersion MLS method, i.e., all the scanning lines are 
divided by the number of selected lines into a plurality of subgroups in 
such a manner that the scanning selection waveform is dispersed within 
each subgroup, as described below. 
FIG. 2 shows an exemplary orthogonal function matrix used for the 
dispersion MLS method. In this case, there is a total of 8 scanning lines 
to be selected, two of which are simultaneously selected, and there are 8 
data electrodes. In theory, the elements +1! and -1! of the orthogonal 
matrix correspond to scanning selection pulse potentials +V.sub.r and 
-V.sub.r, respectively, and the element 0! of the orthogonal matrix 
corresponds to a unselected potential V.sub.com (=0). Data shown in FIG. 3 
is to be displayed by using the orthogonal function in FIG. 2. FIG. 4 
shows the waveform of pulses to be applied to the scanning electrodes by a 
common driver IC on the scanning side for driving the LC. 
In an actual LC panel module, the electrode resistance of the scanning 
electrodes e.g., those of ITO (Indium Tin Oxide), the ON resistance of the 
scanning-side driver IC, and the capacitance component of the LC itself 
form a low-pass filter, which cuts off the harmonics components contained 
in the steep rises and steep falls of the scanning pulses. As a result, 
the waveform of the voltage to be applied to the scanning electrodes is 
distorted (or blunted) as shown in FIG. 5 in actual operation. 
Among the distortions of the waveform of scanning selection pulses, the 
distortion occurring at the foot of the falling edge of each pulse, which 
causes some degradation in the display quality, will be first described. 
When such distortion occurs, the fall of the +V.sub.r pulse (or the rise of 
the -V.sub.r pulse) has some delay so that each scanning selection pulse 
is applied to the same scanning electrode for a period slightly longer 
than the intended period, as shown in FIG. 5. 
With respect to the scanning electrodes S1 and S2, the first selection 
pulse in one frame is to be applied during a period t1. However, the 
above-mentioned distortion of the scanning selection pulse waveform is 
applied as a secondary selection pulse to the scanning electrodes S1 and 
S2 for a period of .DELTA.t in addition to the period t1. The period 
.DELTA.t exists within a period t2, during which a selection pulse is to 
be applied to the next scanning electrodes S3 and S4. 
In other words, a data signal from the segment side is applied (as ON 
voltage) to portions of the LC corresponding to the scanning electrodes S1 
and S2 during not only the intended period t1 but also the period .DELTA.t 
within the period t2, during which the selection pulse is to be applied to 
the scanning electrodes S3 and S4. As a result, the image data to be 
reproduced at positions corresponding to the scanning electrodes S3 and S4 
are reproduced so as to be slightly visible at positions corresponding to 
the scanning electrodes S1 and S2, thus creating a ghost image. In 
summary, any waveform distortion occurring at the falling edge of a pulse 
allows an image which should be reproduced only under a selected number of 
scanning electrodes to be also reproduced under adjoining scanning 
electrodes, thereby resulting in a ghost or a faint image of the same 
pattern appearing at a position slightly shifted from the original image. 
It may seem that the scanning electrodes S7 and S8 are free from the ghost 
phenomenon because they are located at the end of the 8 scanning 
electrodes, and also physically at an end of the LC panel. However, since 
the waveform distortion of the scanning selection pulse to be applied to 
the scanning electrodes S7 and S8 exists within the period during which 
the scanning electrodes S1 and S2 are selected, the image data to be 
reproduced at positions corresponding to the scanning electrodes S1 and S2 
appear as a ghost at positions corresponding to the scanning electrodes S7 
and S8. However, when the scanning goes back from the scanning electrodes 
S7 and S8 to the scanning electrodes S1 and S2, the function data (i.e., 
the orthogonal function) changes so that not just a simple ghost of the 
image to be reproduced at the scanning electrodes S1 and S2 but a reversed 
image (i.e., white portions appearing black and vice versa) of the ghost 
often appears at the scanning electrodes S7 and S8. 
As a result, the display device data of FIG. 3 is likely to appear as in 
FIG. 6. 
In the case of the duty driving method, scanning electrodes are 
sequentially selected one by one, so that the ghost of an image to be 
reproduced at the intended scanning electrodes, occurring due to waveform 
distortion at the falling edge of the scanning selection pulse, appears in 
principle at scanning electrodes next to the intended scanning electrodes, 
rather than at a position substantially away from the intended scanning 
electrodes as in the case of the active driving method. Moreover, the duty 
driving method selects a scanning electrode only once in every frame, so 
that any waveform distortion of a scanning selection pulse within one 
frame has a smaller influence than in the case of the active driving 
method, which selects a scanning electrode a plurality of times in every 
frame. Furthermore, the duty driving method is typically adopted for a 
low-speed panel, which has a thicker LC layer than that of a high-speed 
panel, that is, the capacitance component is smaller than in the case of a 
high-speed panel. Therefore, the influence of waveform distortion becomes 
even smaller. Thus, the double-image phenomenon of an original image being 
accompanied by a ghost image is not as prominent in the duty driving as in 
the active driving. 
Next, the degradation in display quality due to waveform distortion 
occurring at the rising edge of a pulse will be described. The following 
description illustrates a case where the data signal is intended for 
displaying an all-white image. 
When an orthogonal transform is performed for a normally-black LC panel by 
a binary digital system, white data corresponds to "1" (i.e., High) and 
black data corresponds to "0" (i.e., Low). Elements +1! and -1! 
correspond to "1" (i.e., High) and "0" (i.e., Low), respectively. 
An orthogonal operation by this system is performed by taking an Exclusive 
OR of each column vector of the data and the function, and adding the 
results of the Exclusive ORs by an adder, the result of the addition 
defining a data signal corresponding to display data (i.e., a signal to be 
applied to the data electrodes). Accordingly, it is presumable that the 
operation result has a large dependence on the function when the data is 
all-white, i.e., all "1" (High). 
Now a case will be contemplated where the orthogonal function matrix in 
FIG. 2 is used for an LC panel system composed of 8 scanning electrodes 
and 8 data electrodes (as in the above description of the double-image 
phenomenon). Herein, the display data is assumed to be all-white. The 
signal waveform on the data side of the circuitry in this case is constant 
irrespective of the data electrodes, as shown in FIG. 7. As seen from FIG. 
7, the data signal waveform drastically varies only at a boundary between 
the period t4 and the period t5, at which the orthogonal function changes. 
In the duty driving method and the MLS methods, unselected periods are 
predominant in the scanning signal waveform for every frame. Therefore, 
the change in the data signal on the segment side is induced to the common 
side, thereby appearing as an induction distortion in the waveform of the 
scanning signal. 
In this exemplary case, the data signal changes only once in one frame, 
i.e., at the boundary between the periods t4 and t5 as shown in FIG. 8, 
and therefore does not cause induction distortion in any other periods in 
the frame. In other words, among scanning selection pulses, only the rise 
of the selection pulse applied to the scanning electrode S1 during the 
period t5 and the fall of the selection pulse applied to the scanning 
electrode S2 during the period t5 are influenced by the induction from the 
segment side (data electrodes). 
Specifically, the selection pulse voltage for the scanning electrode S1 has 
a small amount of waveform distortion relative to the distortion of 
selection pulses for the scanning electrodes S3 to S8, whereas the 
selection pulse voltage for the scanning electrode S2 has a large amount 
of distortion relative to the waveform distortion of the selection pulses 
for the scanning electrodes S3 to S8. However, the scanning selection 
pulses for scanning electrodes other than the scanning electrodes S1 and 
S2 are not influenced by the induction from the segment side. For similar 
reasons, the selection pulse voltage level for the scanning electrodes S1 
and S2 largely decreases at the beginning of the period t1 due to waveform 
distortion. 
As a result, the waveform distortion occurring at the rising edge of the 
scanning selection pulses for the scanning electrodes S1 and S2 in the 
periods t1 and t5 is different (i.e., more or less drastic) from the 
waveform distortion occurring at the rising edge of the other scanning 
selection pulses. Therefore, the effective values of the applied voltages 
to the pixels (LC) corresponding to the scanning electrodes S1 and S2 
become smaller than the effective values of the voltages applied to the 
pixels (LC) corresponding to other scanning electrodes. 
Because of the difference between the effective voltages corresponding to 
the scanning electrodes S1 and S2 and the effective voltages corresponding 
to the scanning electrodes S3 to S8, the illuminance of a portion 
corresponding to the scanning electrodes S3 to S8 is lower than the 
illuminance of portions corresponding to the other scanning electrodes, 
thereby resulting in a horizontal zone (corresponding to the two scanning 
electrodes) of uneven or reduced illuminance. In summary, any difference 
between the waveform distortion at the rising edge of a scanning selection 
pulse corresponding to a point of change in the orthogonal function and 
the waveform distortion at other portions of the orthogonal function 
results in a horizontal zone (corresponding to the number of selected 
scanning electrodes) of unevenness in illuminance. 
Although the effective values of the voltages applied to the pixels 
corresponding to the scanning electrodes S1 and S2 are different from each 
other in the above description, they become substantially equal in actual 
driving because of processes such as averaging the frequency of scanning 
selection pulses and rotation of the orthogonal function for cancelling 
the DC component. 
Because of the above-mentioned difference in the waveform distortion at the 
rising edge of each scanning selection pulse and because of the waveform 
distortion at the falling edge of the scanning selection pulse, a zone of 
display unevenness as shown in FIG. 9B is observed when the image data 
shown in FIG. 9A is displayed on a display panel of 8.times.8 display 
pixels by using the orthogonal function of FIG. 2. 
Thus, the dispersion MLS driving method has the above-mentioned problem of 
display unevenness due to the operation principle thereof, i.e., all the 
scanning lines are divided by the number of selected lines into a 
plurality of subgroups in such a manner that the scanning selection 
waveform is dispersed within each subgroup. 
SUMMARY OF THE INVENTION 
In one aspect, the present invention provides a method for driving a simple 
matrix type display device including a display panel having a plurality of 
scanning electrodes and a plurality of data electrodes intersecting each 
other, and a matrix of pixels located at the respective intersections of 
the plurality of scanning electrodes and the plurality of data electrodes, 
the method including the steps of: applying a data voltage to the 
plurality of data electrodes, the data voltage corresponding to values 
obtained by performing an orthogonal transform of input data; applying a 
scanning voltage to the scanning electrodes, the scanning voltage 
corresponding to an orthogonal function used for the orthogonal transform; 
and reproducing the input data by performing an orthogonal inverse 
transform of the data voltage on the display panel, wherein the step of 
applying the scanning voltage includes the steps of: applying a scanning 
selection pulse signal which has at least two levels to the plurality of 
scanning electrodes as a scanning voltage; and fixing the scanning 
selection pulse signal to an unselected level during a first period, a 
second period, or both of the first and second periods, the first period 
being defined as a period from the beginning of the output of the data 
until a predetermined time later in a data voltage output period during 
which the data voltage is output to the plurality of data electrodes, and 
the second period being defined as a period from a predetermined short 
time before the completion of the output of data until the completion of 
the output of data in the data voltage output period. 
In another aspect, the present invention provides a liquid crystal display 
device including: a display panel having a plurality of scanning 
electrodes and a plurality of data electrodes intersecting each other and 
a matrix of pixels located at the respective intersections of the 
plurality of scanning electrodes and the plurality of data electrodes; a 
data driver for applying a data voltage to the plurality of data 
electrodes, the data voltage corresponding to values obtained by 
performing an orthogonal transform of input data; a scanning driver for 
applying a scanning voltage to the plurality of scanning electrodes, the 
scanning voltage corresponding to an orthogonal function used for the 
orthogonal transform; and a timing control circuit for receiving a 
synchronization signal which defines timing of outputting the data voltage 
from the data driver and for outputting a control signal which fixes the 
potential level of the scanning electrode at an unselected level during a 
first period, a second period, or both of the first and second periods, 
the first period being defined as a period from the beginning of the 
output of data until a predetermined time later in a data voltage output 
period which is determined by the synchronization signal, and the second 
period being defined as a period from a predetermined short time before 
the completion of the output of data until the completion of the output of 
data in the data voltage output period, wherein the control signal output 
from the timing control circuit controls the scanning driver to output a 
scanning selection pulse during each data voltage output period so that a 
pulse width of the scanning selection pulse is shorter than the data 
voltage output period. 
In one embodiment of the invention, the scanning selection pulse has at 
least two selected levels and the unselected level, and the scanning 
driver outputs one of the levels of the scanning selection pulse based on 
the orthogonal function used for the orthogonal transform, in accordance 
with the output timing of the corresponding data voltage from the data 
driver, and the scanning driver fixes the currently output scanning 
selection pulse to the unselected level, based on the control signal from 
the timing control circuit and independently of the outputting of the 
scanning selection pulses. 
Thus, the invention described herein makes possible the advantages of (1) 
providing a method of driving a display device capable of preventing 
display quality problems inherent to the dispersion type MLS driving 
method, e.g., the double-image phenomenon and a horizontal zone 
(corresponding to the number of selected scanning electrodes) of 
unevenness in illuminance while conserving the advantages inherent to the 
dispersion type MLS driving method, e.g., high contrast obtained with 
relatively small-scale circuitry; and (2) providing an LC display device. 
These and other advantages of the present invention will become apparent to 
those skilled in the art upon reading and understanding the following 
detailed description with reference to the accompanying figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First, the principle of the present invention will be described. 
According to the present invention, there is provided a simple matrix type 
LC display device including a plurality of scanning electrodes and a 
plurality of data electrodes disposed so as to intersect each other and a 
matrix of pixels provided so as to correspond to the intersections of the 
scanning electrodes and the data electrodes, in which a scanning voltage 
pulse is applied to a selected one of the scanning electrodes at a point 
in time shifted with respect to the point in time at which a data voltage 
is applied to the data electrodes, thereby preventing the display quality 
problems inherent to the dispersion type MLS driving method. 
FIG. 10A shows a conventional scanning signal waveform. FIG. 10B shows a 
scanning signal waveform according to the present invention. In FIGS. 10A 
and 10B, .tau.1 represents a time shift at the falling edge of the 
scanning selection pulse, and .tau.2 represents a time shift at the rising 
edge of the scanning selection pulse. 
The LC display device of the present invention includes a scanning driver 
for sequentially outputting scanning voltage pulses to the scanning 
electrodes, and a timing control circuit for controlling the timing at 
which the scanning driver outputs scanning voltage pulses to the 
respective scanning electrodes. 
The scanning driver is capable of outputting "selected" potentials (having 
two or more values) and one "unselected" potential in accordance with the 
timing signal for a data driver, which is supplied from an external 
controller, for example. Furthermore, the scanning driver is capable of 
fixing the current output potential at the "unselected" potential, in 
accordance with an output timing signal for controlling the output of the 
scanning voltage pulses provided from the timing control circuit, 
independently of the output operation of the scanning selection pulses. 
The timing control circuit receives a driver latch pulse signal (which is 
usually equivalent to a horizontal synchronization signal) from an 
external controller, the latch pulse signal indicating a period 
(hereinafter referred to as "data output period") during which data is to 
be output. The timing control circuit generates a timing control signal 
for fixing the potential of the scanning signal from the "selected" 
potential to the "unselected" potential during a first period (from the 
point at which the output of data begins until a predetermined time later) 
or a second period (from a predetermined short time before the completion 
of the output of data until the completion of the output of data), or 
during both first and second periods. The first period and the second 
period as recited herein correspond to the time shifts .tau.2 and .tau.1, 
respectively, in FIG. 10B. 
Specifically, the timing control circuit receives a latch pulse (i.e., a 
horizontal synchronization signal) from an external controller which 
determines a period during which the data driver outputs a predetermined 
voltage level (i.e., the "data output period"). The timing control circuit 
then generates a scanning pulse output enable signal which is activate (at 
a High level, for example) during a length of time excluding a short 
predetermined period of time immediately after the point at which the 
latch pulse is input, a short predetermined period of time immediately 
before the point at which the latch pulse is input, or both periods. The 
scanning pulse output enable signal is supplied to the scanning driver. 
The period from one latch pulse to a next latch pulse defines the data 
output period of the data driver. 
In every horizontal synchronization period, the scanning driver sets either 
the "selected" voltage or the "unselected" voltage for each scanning 
electrode in accordance with function data supplied from an orthogonal 
function generator. Then, in accordance with the scanning pulse output 
enable signal from the timing control circuit, the scanning driver 
actually outputs the "selected" voltage to the selected scanning 
electrodes while the scanning pulse output enable signal is active, i.e., 
during a length of time excluding the short predetermined period of time 
before and/or after a latch pulse. The scanning driver outputs the 
"unselected" voltage to the unselected electrodes, as well as to selected 
electrodes during inactive periods of the scanning pulse output enable 
signal. 
The output period of the "selected" voltage of scanning selection pulses 
according to the present invention is shorter than the conventional output 
period of the "selected" voltage, i.e., one horizontal synchronization 
period (or a period between one latch pulse and a next latch pulse) 
because the "selected" voltage is fixed to the "unselected" voltage during 
short periods of time within the originally "selected" time period, as 
described above. However, the temporary fixation of the "selected" voltage 
to the "unselected" voltage during the originally "selected" time period 
does not affect the reproduction of data because the orthogonality of the 
function is maintained. Although the effective value of the voltage 
applied to the LC may slightly be reduced during the time period(s) in 
which the voltage applied to the scanning electrodes is fixed at "0", this 
slight reduction is substantially harmless because the time period 
accounts for a very small portion of the horizontal synchronization 
period. Moreover, the data signal is subjected to an orthogonal transform 
using a predetermined orthogonal function, the data output voltage forms 
an alternating current regardless of the operation of the driving circuit 
of the LC display device of the invention. Therefore, no residual DC 
components remain to offset the data signal voltage in the LC panel. 
By supplying the scanning signal voltage after the above-described process 
to a high-speed (rapid response) simple matrix type LC display device 
along with a data signal voltage, the advantages of the dispersion type 
MLS driving method, e.g., high contrast obtained with relatively 
small-scale circuitry, can be attained while preventing display quality 
problems inherent in the dispersion type MLS driving method, thereby 
obtaining a uniform and beautiful image display. 
A technique for shifting the timing of the scanning selection pulse is 
disclosed in Japanese Laid-Open Patent Publication No. 5-150750, for 
example. According to this prior art technique, the falling edge of the 
scanning selection pulse is shifted in order to prevent induction waveform 
distortion in the scanning selection pulse due to a change in the voltage 
of the data signal. Moreover, the rising edge of the scanning selection 
pulse is shifted in order to prevent induction waveform distortion in the 
scanning selection pulse due to a change in the voltage of the data signal 
from the previous row. However, this prior art technique is directed to 
the duty driving. 
On the other hand, in accordance with an LC display device of the present 
invention including an LC display panel driven by an active driving method 
such as the MLS method, a time shift is adopted at the falling edge of the 
scanning selection pulse so as to ensure that the scanning selection pulse 
(which inevitably includes some waveform distortion due to the time 
constant of the LC panel and the like) fits within the appropriate 
selected period, thereby avoiding display quality problems inherent in the 
active driving method, e.g., the double-image phenomenon. Moreover, in 
accordance with the LC display device of the present invention, a time 
shift is adopted at the rising edge of the scanning selection pulse so as 
to prevent induction from the data signal voltage from a previous subgroup 
in the MLS driving, thereby reducing display unevenness particularly 
prominent in the dispersion MLS driving method. 
Hereinafter, the present invention will be described by way of examples, 
with reference to the accompanying figures. 
(Example) 
FIG. 11 is a block diagram illustrating the overall structure of an LC 
display device 100 according to an example of the present invention. 
As shown in FIG. 11, the LC display device 100 includes: a memory 2 for 
temporarily storing an input data signal scanned along the row direction, 
the input data signal being read out along the column direction in 
accordance with the predetermined number of simultaneously selected lines; 
an orthogonal transform circuit 3 for subjecting data read out from the 
memory 2 to an orthogonal transform; and a data driver 4 for outputting a 
voltage corresponding to the data signal after the orthogonal transform. 
The LC display device 100 further includes a scanning driver 11 for 
outputting scanning voltage pulses and a function generator 5 for 
supplying an orthogonal function to the orthogonal transform circuit 3 and 
the scanning driver 11. The LC display device 100 is completed by a timing 
control circuit 12 for controlling the scanning driver 11 by providing 
output timing of scanning selection voltage pulses; a data driver 4; a 
controller 6 for supplying a synchronization signal to the timing control 
circuit 12, the data driver 4, and the scanning driver 11; and an LC panel 
7 of a simple matrix type. The timing control circuit 12 and the scanning 
driver 11 constitute an LC driving circuit 1 on the scanning side of the 
LC display device 100. 
In the LC display device 100 having the above configuration, an externally 
input image signal (data signal) is written to the memory 2 along the row 
direction. The data is read out from the memory 2 from a plurality of rows 
along the column direction simultaneously. The number of rows is the same 
as the number of simultaneously selected lines, as in the prior art. The 
data read out from the memory 2 is subjected to an orthogonal transform by 
the orthogonal transform circuit 3 before being supplied to the data 
driver 4. 
The function generator 5 supplies an orthogonal function for the orthogonal 
transform and inverse transform to the orthogonal transform circuit 3 and 
the scanning driver 11, respectively. 
For comparison, a typical conventional structure supplies a driving voltage 
corresponding to the data signal (after an orthogonal transform) from the 
data driver 4, and a driving voltage corresponding to the orthogonal 
function (used for the orthogonal transform) from the scanning driver 11, 
to the LC panel 7 for every horizontal synchronization period, the two 
driving voltages being synchronized, thereby reproducing an image 
represented by the data signal on the LC panel 7. 
On the other hand, the LC driving circuit 1 on the scanning side (including 
the timing control circuit 12) of the LC display device 100 of the present 
example operates as follows. 
First, the controller 6 supplies a latch pulse as a synchronization signal 
to the timing control circuit 12, the data driver 4, and the scanning 
driver 11. The latch pulse is equivalent to a horizontal synchronization 
signal in principle. The drivers 4 and 11 output voltages as they receive 
the latch pulse. 
FIG. 12A shows an exemplary circuit configuration of the above-mentioned 
timing control circuit 12. The timing control circuit 12 includes an 
inverter 12c receiving the above-mentioned latch pulse LP and first and 
second one-shot multivibrators 12a and 12b each receiving the output of 
the inverter 12c at an input B thereof. An output Q of the first one-shot 
multivibrator 12a and an output Q of the second one-shot multivibrator 12b 
are coupled to the inputs of a two-input AND circuit 12d. A power level 
Vcc is supplied to an input CLR of the first and second one-shot 
multivibrator 12a and 12b. Inputs A and CEXT of the first and second 
one-shot multivibrator 12a and 12b are grounded. Furthermore, an input 
REXT/CEXT of the first one-shot multivibrator 12a is coupled to the power 
level VCC via a resistor R1, and an input REXT/CEXT of the second one-shot 
multivibrator 12b is coupled to the power level VCC via a resistor R2. A 
capacitor C1 is coupled between the input REXT/CEXT and the input CEXT of 
the first one-shot multivibrator 12a, and a capacitor C2 is coupled 
between the input REXT/CEXT and the input CEXT of the second one-shot 
multivibrator 12b. The resistor R2 is composed of a power-side resistor 
R2a and a vibrator-side resistor R2b. The resistors R2a and R1 are 
variable resistors. 
As shown in FIG. 12B, the first one-shot multivibrator 12a is adapted so as 
to output a signal which remains at the Low level for a predetermined 
period .tau.2 after receiving the latch pulse LP. The signal remains at 
the Low level for a predetermined period .tau.2 after receiving the latch 
pulse LP and then shifts to the High level. The second one-shot 
multivibrator 12b is adapted so as to output a signal which shifts from 
the Low level to the High level immediately after receiving the latch 
pulse LP, remains at the High level for a predetermined period .tau.3, and 
then shifts back to the Low level. As a result, the AND circuit 12d 
outputs a scanning pulse output enable signal DOFF as shown in FIG. 12B. 
In the circuit configuration shown in FIG. 12A, a switch 12e is provided 
after the AND circuit 12d for selecting between the scanning pulse output 
enable signal DOFF and the power level VCC. A resistor R4 is coupled 
between the switch 12e and the power level VCC. Therefore, in accordance 
with the LC display device 100 of the present example, it is possible to 
select between an operation in which the above-mentioned control of the 
width of the scanning selection pulse is made using the timing control 
circuit 12 and an operation which does not include such control. 
When the timing control circuit 12 having the above configuration receives 
the latch pulse LP in FIG. 12B at the input of the inverter 12c, the 
scanning pulse output enable signal DOFF in FIG. 12B is output from the 
two-input AND circuit 12d. 
The pulse width of the scanning pulse output enable signal during a period 
between one latch pulse and a next latch pulse (i.e., one horizontal 
synchronization period) can be controlled based on the time constant 
defined by the capacitor and the resistors connected to the respective 
one-shot multivibrators 12a and 12b. 
Specifically, the period .tau.2 (i.e., a period after the input of the 
latch pulse LP until the scanning pulse output enable signal DOFF is 
active) is determined based on the capacitor C1 and the resistor Rl. The 
period .tau.1 (i.e., a period after the scanning pulse output enable 
signal DOFF is inactive until a next latch pulse LP is input) is 
determined based on the capacitor C2 and the resistor R2. Although the 
timing control circuit 12 shown in FIG. 12A has a simple analog 
configuration, it will be appreciated that the timing control circuit 12 
having the same function can be implemented by a digital logic circuit. 
The scanning driver 11 receives the orthogonal function from the function 
generator 5. In accordance with the latch pulse LP from the controller 6 
and the scanning pulse output enable signal DOFF from the timing control 
circuit 12, the scanning driver 11 applies the "selected" voltage to the 
electrodes to be selected only while the scanning pulse output enable 
signal DOFF is active during one horizontal synchronization period, and 
applies the "unselected" voltage to the electrodes to be selected while 
the scanning pulse output enable signal DOFF is inactive. At the same 
time, the scanning driver 11 applies the "unselected" voltage to the 
unselected electrodes throughout the horizontal synchronization period as 
in conventional techniques. 
The data driver 4 outputs a data signal which has been subjected to the 
orthogonal transform during one horizontal synchronization period in 
accordance with the latch pulse LP. 
Although the effective value of the voltage applied to the LC is slightly 
reduced as compared with the applied voltage of the prior art (because of 
the time period .tau.1 and/or .tau.2 of the present invention during which 
the voltage applied to the scanning electrodes is fixed to the unselected 
potential), the effective value of the applied voltage can be compensated 
by increasing the "selected" voltage of the scanning driver and/or the 
voltage corresponding to the data signal, thereby preventing the 
illuminance of the LC panel from being lowered. 
The inventors conducted an experiment in which a VGAn LC panel (response 
speed: 130 ms) including 640.times.480 (.times.a number corresponding to 
the three primary colors of R, G, and B) pixels was driven while being 
split into upper and lower halves so as to display an image at a frame 
frequency of 120 Hz, using the block dispersion driving method under the 
conditions that the number of block scanning lines was 120 and that the 
number of simultaneously selected lines within each block was 7. The 
active period of the scanning pulse output enable signal was set to be the 
remainder of the horizontal synchronization period excluding a period of 
about 2 .mu.s immediately after the input of a latch pulse and a period of 
about 3 .mu.s immediately before the input of a next latch pulse. As a 
result, excellent display quality was obtained. 
In order to compensate the effective value of the applied voltage, the 
amplitude of the scanning voltage and the data voltage to be applied to 
the LC was increased by several percent. 
Usually the inactive period of the scanning pulse output enable signal is 
determined in view of the capacitance and resistance of the LC panel, the 
time constant due to the ON resistance of the driver, the length of one 
horizontal synchronization period, and the like. It is preferable to 
prescribe the inactive period of the scanning pulse output enable signal 
to be in the range of about 10% to about 20% of one horizontal 
synchronization period, in view of the slight decrease in the effective 
value of the applied voltage. The main reasons for this are described 
below. 
In the case of driving the above-mentioned VGA panel according to the 
present example, the effective voltage has the following ON/OFF ratio (or 
"driving margin"): 
##EQU2## 
In the above equation, it is assumed that the bias ratio is 1/a, and that 
the scanning selection pulse is fixed to the "unselected" potential during 
a period equal to b.times.100% of the conventional pulse width. The ON/OFF 
ratio takes the theoretical maximum value (about 6.5%) under the optimum 
bias when a=.sqroot.252 and b=0. 
Assuming that b is increased to 0.15 in the above state, the ON/OFF ratio 
is derived from eq.2 to be about 6.0%, indicating a decrease of about 10%. 
Specifically, the effective voltage for the ON pixels decreases by about 
4%, and the effective voltage for the OFF pixels decreases by about 3.5%. 
If the voltage applied to the LC is universally increased by 4% (equivalent 
to the decrease in the effective voltage for the ON pixels), for example, 
the effective voltage for the ON pixels takes the legitimate value but the 
effective voltage for the OFF pixels takes a value higher than the 
legitimate value. Therefore, the decrease in the ON/OFF ratio due to the 
inactive period(s) of the scanning pulse output enable signal cannot be 
corrected by adjusting the effective value of the voltage applied to the 
LC. 
Thus, it will be seen that there is no substantial decrease in contrast due 
to the slight decrease in ON/OFF ratio when the inactive period of the 
scanning pulse output enable signal is in the range of about 10% to about 
20% of one horizontal synchronization period, although an excessively 
large value of b would invite problems such as low contrast and crosstalk. 
Considering the breakdown voltage and the power consumption of actual 
driver ICs for driving the LC, the increase in voltage resulting from the 
operation should be contained within about 10% of the conventional level. 
Thus, in accordance with an LC display device of the present example, the 
points in time at which the level of the scanning selection pulse changes 
are slightly shifted with respect to the legitimate or conventional output 
timing. Specifically, a time shift is adopted at the falling edge of the 
scanning selection pulse so as to ensure that the scanning selection pulse 
(which inevitably includes some waveform distortion due to the time 
constant of the LC panel and the like) fits within the appropriate 
selected period, and furthermore the waveform of the scanning selection 
pulse is maintained by ensuring that any induction from the segment side 
(i.e., the data electrode side) appears during unselected periods of the 
scanning electrodes. As a result, the advantages of the dispersion type 
MLS driving method, e.g., high contrast obtained with relatively 
small-scale circuitry are attained while preventing display quality 
problems inherent in the dispersion type MLS driving method, thereby 
obtaining a uniform and beautiful image display. 
Although the above example illustrated a case where the scanning selection 
pulse applied to the scanning electrodes as a scanning voltage is fixed to 
the "unselected" voltage during both a first period (from the beginning of 
the output of data until a predetermined time later) and a second period 
(from a predetermined short time before the completion of the output of 
data until the completion of the output of data), it is also possible to 
fix the scanning selection pulse to the "unselected" voltage only during 
either the first or second period. 
For example, by fixing the scanning selection pulse applied to the scanning 
electrodes to the "unselected" voltage during the second period (from a 
predetermined short time before the completion of the output of data until 
the completion of the output of data) in the data voltage output period, 
any waveform distortion occurring at the falling edge of the scanning 
selection pulse is prevented from being applied to the scanning electrode 
longer than it should properly be applied, thereby preventing the 
double-image phenomenon inherent in the dispersion MLS driving method. On 
the other hand, by fixing the scanning selection pulse applied to the 
scanning electrodes to the "unselected" voltage during the first period 
(from the beginning of the output of data until a predetermined time 
later) in the data voltage output period, it becomes possible to prevent 
the change in potential of the data electrode from affecting the rise and 
fall of the scanning selection pulse, thereby preventing the generation of 
a horizontal zone (corresponding to the number of selected scanning 
electrodes) of unevenness in illuminance relative to the other electrodes. 
Although the dispersion MLS method was described in the above example, the 
present invention is also effective for any driving method that uses an 
orthogonal function for a simple matrix type display device, e.g., the AA 
method and the non-dispersion MLS method. 
As described above, in accordance with a method for driving a display 
device of the present invention, a data voltage, corresponding to values 
obtained by subjecting the input data to the orthogonal transform, is 
supplied to the data electrodes and a scanning voltage, corresponding to 
an orthogonal function used for the orthogonal transform, is supplied to 
the scanning electrodes so that the input data is reproduced by the 
display panel after being subjected to an orthogonal inverse transform. 
The scanning selection pulse applied to the scanning electrodes as a 
scanning voltage is fixed at the "unselected" voltage during a first 
period (from the beginning of the output of data until a predetermined 
time later) or a second period (from a predetermined short time before the 
completion of the output of data until the completion of the output of 
data) in the data voltage output period, or both the first and second 
periods. As a result, the advantages of the dispersion type MLS driving 
method, e.g., high contrast obtained with relatively small-scale 
circuitry, are attained while preventing display quality problems inherent 
in the dispersion type MLS driving method, e.g., the double-image 
phenomenon and a horizontal zone (corresponding to the number of selected 
scanning electrodes) of unevenness in illuminance. 
Thus, by fixing the scanning selection pulse applied to the scanning 
electrodes to the "unselected" voltage during a second period (from a 
predetermined short time before the completion of the output of data until 
the completion of the output of data) in the data voltage output period, 
any waveform distortion occurring at the foot of the falling edge of the 
scanning selection pulse is prevented from being applied to the scanning 
electrode longer than it should properly be applied, thereby preventing 
the double-image phenomenon inherent in the dispersion MLS driving method. 
Moreover, by fixing the scanning selection pulse applied to the scanning 
electrodes to the "unselected" voltage during a first period (from the 
beginning of the output of data until a predetermined time later) in the 
data voltage output period, it becomes possible to prevent the change in 
potential of the data electrode from affecting the rise and fall of the 
scanning selection pulse, thereby preventing the generation of a 
horizontal zone (corresponding to the number of selected scanning 
electrodes) of unevenness in illuminance relative to the other electrodes. 
An LC display device according to the present invention includes a timing 
control circuit which receives a synchronization signal defining the 
timing for outputting a data voltage from a data driver and outputs a 
control signal for fixing the potential of the scanning electrode at the 
"unselected" voltage during a first period (from the beginning of the 
output of data until a predetermined time later) or a second period (from 
a predetermined short time before the completion of the output of data 
until the completion of the output of data) in the data voltage output 
period as defined by the synchronization signal, or both the first and the 
second periods. The control signal from the timing control circuit 
controls a scanning driver to output a scanning selection pulse during 
each data voltage output period such that the scanning selection pulse is 
shorter than this period. As a result, display quality problems inherent 
in the dispersion type MLS driving method, e.g., the double-image 
phenomenon and a horizontal zone (corresponding to the number of selected 
scanning electrodes) of unevenness in illuminance are prevented. 
As a result, the advantages of the dispersion type MLS driving method, 
e.g., high contrast obtained with relatively small-scale circuitry, are 
attained while preventing display quality problems inherent in the 
dispersion type MLS driving method, thereby obtaining a uniform and 
beautiful image display having excellent contrast. 
Various other modifications will be apparent to and can be readily made by 
those skilled in the art without departing from the scope and spirit of 
this invention. Accordingly, it is not intended that the scope of the 
claims appended hereto be limited to the description as set forth herein, 
but rather that the claims be broadly construed.