Matrix type liquid crystal display

In a method for driving row electrodes (or column electrodes) of a matrix type liquid crystal display panel having a predetermined number of the row electrodes and a predetermined number of the column electrodes, different drive voltages are supplied to the row electrodes or the column electrodes so as to assure, the highest contrast despite different viewing or displaying positions.

BACKGROUND OF THE INVENTION 
The present invention relates to a drive circuit for a matrix type liquid 
crystal display providing a visual display of characters, patterns or the 
like. 
A scheme of circuit construction for driving a matrix type liquid crystal 
display by a line sequential scanning method is illustrated in FIG. 1. 
Data contained within a master memory 1 is converted into display pattern 
signals via a character signal converter 2, stored temporarily within a 
buffer memory in a column driver 3 and then supplied to column electrodes 
Y.sub.1, Y.sub.2, . . . Y.sub.n of a matrix type liquid crystal display 6. 
There is further provided a row driver 4 which scans row lines X.sub.1, 
X.sub.2, . . . X.sub.m of the matrix type liquid crystal display 6 one by 
one. A control circuit 5 provides all controls for the master memory 1, 
the column driver 3 and the row driver 4. 
With such a matrix type liquid crystal display driven by the row sequential 
scanning method, the greater the number of the row electrodes the shorter 
a period of time of voltage applied per row with respect to a full 
scanning period, in other words, the so-called duty factor is reduced, 
presenting a crosstalk problem. This is critical particularly for a liquid 
crystal display having the inherent properties of a threshold level that 
is not well defined and a response that is too slow. This results in an 
insufficient contrast in the liquid crystal display. 
Some approaches to resolve this problem have been suggested, for example, 
to modify a matrix electrode structure so as to increase the duty factor. 
As seen from FIG. 2 showing an equivalent circuit of the electrode 
structre, one way is to cause one row electrode X.sub.i to confront two 
independent column electrodes Y.sub.1j, Y.sub.2j, thereby doubling, of 
quadrupling, . . . the number of energizable rows without deteriorating a 
display contrast. It seems possible to develop a matrix type liquid 
crystal display having as many as 100 rows. 
Nevertheless, while a twisted nematic field effect mode liquid crystal 
display (referred to as "TN-FEM-LCD" hereinafter) can be used with a 
matrix drive and will assume a comparatively good contrast when viewed 
from a specific direction, it is disadvantageous in that variations in the 
viewing direction cause a reduction in contrast and difficulties in 
recognizing a visual display thereof. For the planar type display, the 
viewing angle differs to a great extent from the top of the bottom, viz., 
.theta., and .theta..sub.2, with the resulting difference in contrast from 
the top to the bottom. 
It is therefore an object of the present invention to provide a drive 
circuit which overcomes the shortcomings with the prior art device, with 
the feature that a drive voltage is varied in accordance with scanning 
electrodes in practising the row sequential scanning method.

DETAILED DESCRIPTION OF THE INVENTION 
A practical drive method embodying the present invention will be now 
described together with some performance characteristics and a driver 
circuit arrangement. 
There are two possible methods of using the matrix type liquid crystal 
display having the electrode equivalent circuit of FIG. 2. As shown in 
FIG. 3, one of the two methods is an upper and lower dividing type wherein 
each of column electrode is divided into the upper half and lower half 
within the cell structure. This produces upper and lower half cells or 
panel portions. As shown in FIg. 4, the other method is a two layer matrix 
type wherein two cells are provided in a stack. 
While the present invention will be described in terms of the upper and 
lower dividing type, it is obvious that the present invention is really 
applicable to the two layer matrix type or the combined type thereof. 
In matrix driving the liquid crystal display, a 1/n bias drive method which 
is developed from a well known 1/3 bias method is employed. As best seen 
from FIG. 6, a row electrode or an X electrode is supplied with 
##EQU1## 
when selected and O when non-selected while a column electrode or a Y 
electrode is always supplied with .+-.1/a V.sub.0, so that the voltage 
across the X and Y electrodes is V.sub.o when selected and .+-.1/a V.sub.0 
when non-selected. Therefore, an operating margin .alpha.=V.sub.ON 
/V.sub.OFF is maximized wherein a=.sqroot.N+1 and N is the number of 
scanning lines. 
This implies that the operating margin is maximized as far as the 
relashinship V.sub.1 =.sqroot.N.V.sub.2 is satisfied wherein V.sub.1 is a 
peak to peak voltage of a V.sub.X signal and V.sub.2 is the counterpart of 
a V.sub.Y signal. As the absolute values of V.sub.1 and V.sub.2 are varied 
while satisfying the above relationship, a constant ratio between 
respective viewing angles will be varied. FIG. 7 shows viewing 
characteristics wherein a contrast ratio is plotted as a function of a 
viewing angle .theta.. The curve a of FIG. 7 shows a contrast ratio where 
drive voltages V.sub.11 and V.sub.12 are established in the following 
relationship: 
##EQU2## 
The curve b shows the case where drive voltages V.sub.12 and V.sub.22 are 
correlated: 
##EQU3## 
The curve c shows the case where drive voltages V.sub.13 and V.sub.23 are 
correlated: 
##EQU4## 
In other words, 
V.sub.11 &gt;V.sub.12 &gt;V.sub.13 
V.sub.21 &gt;V.sub.22 &gt;V.sub.23 
Analysis of these experimental results revealed that, while variations in 
drive voltage cause less or no variations in the range of viewing angles 
having a suitable contrast ratio, the viewing angle .theta. where the 
highest contrast is available is in positive correlation with the drive 
voltage V. 
To this end, according to one preferred embodiment of the present 
invention, a single display unit is divided into the upper half and lower 
half as shown in FIG. 8 and a driver circuit arrangement is set up as 
shown in FIG. 9. The drive voltages V.sub.12 and V.sub.22 to be applied to 
the X electrode group 9 and the Y electrode group 11 of the upper half 
cell 13 are selected so as to develop the highest contrast with the 
viewing angle .theta..sub.11 toward a middle point P in the upper half 
cell. Similarly, the drive voltages V.sub.11 and V.sub.12 to the X 
electrode group 10 and the Y electrode group 12 of the lower half cell 14 
are selected so as to develop the highest contrast ratio with the viewing 
angle .theta..sub.22 toward another middle point Q in the lower half cell. 
This eliminates non-uniformity in contrast ratio throughout the display 
panel as is of the case where the dimension of the display panel is 
halved. 
This upper and lower separate drive method, of course, is applicable to the 
upper and lower dividing matrix of FIG. 3. In the case where the two layer 
matrix is combined with the upper and lower dividing type, it is also 
possible to realize the highest contrast ratio by quartering the display 
panel and changing the drive voltages to the respective quarters thereof. 
This leads to the display panel free of nonuniformity in contrast due to 
different displaying positions. It is obvious that the present invention 
is useful within a wide range covering from a hand held type display to a 
floor type. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such modifications are not to be regarded as a 
departure from the spirit and scope of the invention, and all such 
modifications are intended to be included within the scope of the 
following claims.