Voltage driving waveforms for plasma addressed liquid crystal displays

A plasma-addressed electro-optic display device having a layer of electro-optic material, data electrodes coupled to the electro-optic layer and adapted to receive data voltages for activating portions of the electro-optic layer, and a plurality of plasma channels extending generally transverse to the data electrodes for selectively switching on the electro-optic portions. The plasma channels each contain spaced elongated cathode and anode plasma electrodes and an ionizable gas filling. The cathode and anode electrodes are arranged in groups to reduce the connections required. Reference voltages are applied to the grouped cathode and anode electrodes such that the data voltages required to operate the panel are reduced decreasing vertical crosstalk. Various driving voltage waveforms can be used for further crosstalk reduction, to reduce the number of voltage levels required, and cause the cathode and anode electrodes to exchange roles during operation to extend lifetime.

This invention relates to plasma-addressed electro-optic display panels, 
and in particular to the electrode connections within and voltage driving 
waveforms for operating such displays. 
BACKGROUND OF INVENTION 
Plasma-addressed liquid crystal display panels, commonly referred to as 
"C" display devices, comprise, typically, a sandwich of: a first 
substrate having deposited on it parallel transparent column electrodes, 
commonly referred to as "ITO" columns or electrodes since indium-tin 
oxides are typically used, on which is deposited a color filter layer for 
a color display, or ITO columns fabricated on a color filter that is 
provided on the first substrate; a second substrate comprising parallel 
sealed plasma channels, corresponding to rows of the display crossing all 
of the ITO columns, each of which is filled with a low pressure ionizable 
gas, such as helium, and containing spaced cathode and anode electrodes 
along the channel for ionizing the gas to create a plasma, which channels 
are closed off by a thin transparent dielectric sheet; and an 
electro-optic material such as a liquid crystal (LC) material located 
between the substrates. The structure behaves like an active matrix liquid 
crystal display in which the thin film transistor switches at each pixel 
are replaced by a plasma channel acting as a row switch and capable of 
selectively addressing a row of LC pixel elements. In operation, 
successive lines of data signals representing an image to be displayed are 
sampled at column positions and the sampled data voltages are respectively 
applied to the ITO columns. All but one of the row plasma channels are in 
the de-ionized or non-conducting state. The plasma of the one ionized 
selected channel is conducting and, in effect, establishes a reference 
potential on the adjacent side of a row of pixels of the LC layer, causing 
each LC pixel in the row to charge up to the applied column potential of 
the data signal. The ionized channel is turned off, isolating the LC pixel 
charge and storing the data voltage for a frame period. When the next row 
of data appears on the ITO columns, only the succeeding plasma channel row 
is ionized to store the data voltages in the succeeding row of LC pixels, 
and so on. As is well known, the attenuation of each LC pixel to backlight 
or incident light is a function of the stored voltage across the pixel. A 
more detailed description is unnecessary because the construction, 
fabrication, and operation of such C devices have been described in 
detail in the following U.S. patents, and publication, the contents of 
which are hereby incorporated by reference: U.S. Pat. Nos. 4,896,149; 
5,077,553; 5,272,472; 5,276,384; 5,349,454; and Buzak et al., "A 16-Inch 
Full Color Plasma Addressed Liquid Crystal Display", Digest of Tech. 
Papers, 1993 SID Int. Symp., Soc. for Info. Displ. pp. 883-886 
It is known from U.S. Pat. No. 5,077,553 that the number of connections of 
a C display panel can be decreased using a method as depicted in FIG. 
1. Assuming a C display panel 12 with N rows of pixels, both the 
cathodes 30 and anodes 31 electrodes are taken together in groups of 
N.sup.1/2 lines, with one connection 8, 9 per group. This leads to 
2N.sup.1/2 connections. For instance, with N=1024, there are 2.times.32 
groups of 32 lines, resulting in 64 connections. FIG. 1 illustrates one 
form of this system in which all the cathodes are arranged in groups 30-1 
. . . 30-.sqroot.N, and in which the anode electrodes are also arranged in 
groups 31-1 . . . 31-.sqroot.N but with each anode associated with one of 
the cathode groups. In other words, each of the cathode groups include no 
more than one electrode of each anode group. In the same way, each of the 
anode groups include no more than one electrode of each cathode group. The 
adjacent cathode-anode electrode pairs are each located in a channel, and 
the channels whose electrodes form any one of a first group thus include 
no more than one electrode of any one of the second group. 
This display may be driven in the following way. The maximum voltage needed 
is equal to or larger than the ignition voltage V.sub.ig for igniting the 
plasma in a channel, and the minimum voltage needed is equal to or less 
than the sustain voltage V.sub.sus for sustaining the discharge in a 
channel once ignited. One of the cathode groups which includes the cathode 
electrode in the channel to be selected is driven with a voltage 
-1/2(V.sub.ig +V.sub.sus) during an ignition time T.sub.ig, while all 
other cathode groups have zero voltage as a reference during that ignition 
time T.sub.ig. After the time T.sub.ig, all cathode voltages are set to 
the reference zero voltage during the rest of the row time (T.sub.ROW 
-T.sub.ig). One anode group which includes the anode electrode in the 
selected channel is driven with a voltage 1/2(V.sub.ig -V.sub.sus) during 
T.sub.ig, while all other anode groups are driven with a voltage of 
-1/2(V.sub.ig -V.sub.sus) during that same time. During the rest of the 
row time, the anode voltages are maintained at the zero reference. With 
these applied voltages, only for one pair of cathode and anode electrodes 
in the selected channel is the voltage difference equal to V.sub.ig, so 
this pair only will ignite their channel during T.sub.ig. For all other 
anode-cathode pairs, the voltage difference during T.sub.ig is either 
-V.sub.sus or .+-.1/2(V.sub.ig -V.sub.sus), so these channels will not 
ignite. In this way the panel can be scanned from top to bottom. 
The disadvantage of this method is that a large column voltage range is 
needed, equal to 2V.sub.sat *, where V.sub.sat * is the voltage required 
to be applied to the series combination of the liquid crystal cell and the 
thin dielectric microsheet separating the plasma channel and the LC to 
drive the LC to its saturation voltage V.sub.sat. For this background 
analysis, the color filter, which if present, also contributes to the 
required value of V.sub.sat * is ignored. One can define V.sub.sat 
*=.alpha..sub.sat V.sub.sat, where .alpha..sub.sat is the attenuation 
ratio of the column voltage to the LC voltage at saturation. Similarly, 
one can define V.sub.th *=.alpha..sub.th V.sub.th, where V.sub.th is the 
threshold voltage of the LC and .alpha..sub.th is the attenuation ratio of 
the column voltage to the LC voltage at threshold. These values are 
indicated in FIG. 2, which is a known graph 2 plotting relative 
transmission for a typical LC material as a function of the voltage across 
an LC pixel, and graph 3 is a similar characteristic but now taking into 
account the voltage drop across the thin microsheet. 
It will be clear from FIG. 2 that this value 2V.sub.sat * is a large value, 
difficult to generate in the column-driving integrated circuit components 
(ICs), and will lead to capacitive crosstalk on pixels in the same column 
that are in the non-addressed mode storing a charge from a previous 
addressing step, known in the art as vertical crosstalk. 
SUMMARY OF INVENTION 
An object of the invention is an improved display device. 
Another object of the invention is a display device requiring reduced 
column driving voltages. 
A further object of the invention is a display device exhibiting reduced 
vertical crosstalk. 
Still another object of the invention is a display device requiring fewer 
different types of column driving ICs. 
In accordance with a first aspect of the invention, a display device 
comprises an electrode connection arrangement of the reduced connection 
type as described which is driven with voltages such that during the 
remaining row time (T.sub.ROW -T.sub.ig) , when the data voltage is 
applied to the column electrode, reference voltages are applied to the 
anode and cathode electrodes in the relevant groups substantially equal to 
.+-.V.sub.c *, where V.sub.c *=1/2(V.sub.th *+V.sub.sat *). In this way, 
the required column voltage swing is reduced to (V.sub.sat *-V.sub.th *). 
This will lead to less vertical crosstalk. By relevant groups is meant the 
groups containing the cathode and anode electrodes in the selected channel 
just previously ignited. 
An even further reduction in vertical crosstalk can be obtained in 
acordance with another aspect of the invention by rearranging the 
connections to the cathode and anode electrodes such that the anode 
connections to all even-numbered cathode groups are shifted by one, and 
modifying the scanning of the even-numbered cathode group electrodes. 
In accordance with a further aspect of the invention, the number of 
different voltages needed to operate the display panel is reduced by 
applying a voltage to a selected cathode group substantially equal to 
-1/2V.sub.ig, and applying to a selected anode group a voltage 
substantially equal to +1/2V.sub.ig. With this feature, only five voltage 
levels are required to operate the display panel, and the voltages are the 
same for both cathode and anode groups, thus reducing the types of ICs 
needed to drive both groups. 
In a prefered embodiment, the display is written in the row-inversion mode. 
This means that in one field the pixels of the first row are written in a 
positive sense, the pixels of the second row in a negative sense, the 
pixels of the third row in a positive sense, and so on. On the next field 
these signs are inverted. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its use, reference 
should be had to the accompanying drawings and descriptive matter in which 
there are illustrated and described the preferred embodiments of the 
invention, like reference numerals or letters signifying the same or 
similar components.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 3 shows a flat panel display system 10, which represents a typical 
C display device and the operating electronic circuitry. With reference 
to FIG. 3, the flat panel display system comprises a display panel 12 
having a display surface 14 that contains a pattern formed by a 
rectangular planar array of nominally identical data storage or display 
elements 16 mutually spaced apart by predetermined distances in the 
vertical and horizontal directions. Each display element 16 in the array 
represents the overlapping portions of thin, narrow electrodes 18 arranged 
in vertical columns and elongate, narrow channels 20 arranged in 
horizontal rows. (The electrodes 18 are hereinafter referred to from time 
to time as "column electrodes"). The display elements 16 in each of the 
rows of channels 20 represent one line of data. 
The widths of column electrodes 18 and channels 20 determine the dimensions 
of display elements 16, which are typically of rectangular shape. Column 
electrodes 18 are deposited on a major surface of a first electrically 
nonconductive, optically transparent substrate 34, and the channel rows 
are usually built into a second transparent substrate 36. Skilled persons 
will appreciate that certain systems, such as reflective display of either 
the direct view or projection type, would require that only one substrate 
be optically transparent. 
Column electrodes 18 receive data drive signals of the analog voltage type 
developed on parallel output conductors 22' by different ones of output 
amplifiers 23 (FIG. 4) of a data driver or drive drive circuit 24, and 
channels 20 receive data strobe signals of the voltage pulse type 
developed on parallel output conductors 26' by different ones of output 
amplifiers 21 (FIG. 4) of a data strobe or strobe means or strobe circuit 
28. Each of the channels 20 includes a reference electrode 30 (FIG. 4) to 
which a reference potential common to each channel 20 and data strobe 
circuit 28 is typically applied. 
To synthesize an image on the entire area of display surface 14, display 
system 10 employs a scan control circuit 32 that coordinates the functions 
of data driver 24 and data strobe 28 so that all columns of display 
elements 16 of display panel 12 are addressed row by row in row scan 
fashion. Display panel 12 may employ electro-optic materials of different 
types. For example, if it uses such material that changes the polarization 
state of incident light rays, display panel 12 is positioned between a 
pair of light polarizing filters, which cooperate with display panel 12 to 
change the luminance of light propagating through them. The use of a 
scattering liquid crystal cell as the electro-optic material would not 
require the use of polarizing filters, however. All such materials or 
layers of materials which attenuate transmitted or reflected light in 
response to the voltage across it are referred to herein as electro-optic 
materials. As LC materials are presently the most common example, the 
detailed description will refer to LC materials but it will be understood 
that the invention is not limited to display panels with liquid crystal 
materials. A color filter (not shown) may be positioned within display 
panel 12 to develop multi-colored images of controllable color intensity. 
For a projection display, color can also be achieved by using three 
separate monochrome panels 12, each of which controls one primary color. 
FIG. 4 illustrates the C version of such a display panel using LC 
material. Only 3 of the column electrodes 18 are shown. The rows 20 are 
constituted by a plurality of parallel elongated sealed channels 
underlying (in FIG. 4) a layer 42 of the LC material. Each of the channels 
20 is filled with an ionizable gas 44, closed off with a thin dielectric 
sheet 45 typically of glass, and contains on an interior channel surface 
first and second spaced elongated electrodes 30, 31 which extend the full 
length of each channel. The first electrode 31 in the prior art 
arrangement is typically grounded and is commonly called the anode. The 
second electrode 30 is called the cathode, because to it will typically be 
supplied relative to the anode electrode a negative strobe pulse 
sufficient to cause electrons to be emitted from the cathode 30 to ionize 
the gas. As explained above, each channel 20, in turn, has its gas ionized 
with a strobe pulse to form a plasma and a reference potential connection 
to a row of pixels in the LC layer 42 above. When the strobe pulse 
terminates, and after deionization has occurred, the next channel is 
strobed and turned on. Since the column electrodes 18 each cross a whole 
column of pixels, only one plasma row connection at a time is allowed on 
to avoid crosstalk. 
All of the methods described in the referenced patents and publication will 
be suitable for making the channels and electrodes for the panel of the 
invention. 
In accordance with the invention, a C display having the reduced number 
of connections of the type illustrated in FIG. 1 is driven with the 
waveforms of FIGS. 5a-5d in a row-inversion mode and in such manner that 
the voltage applied during the remaining row time to the relevant cathode 
and anode electrodes are substantially equal to .+-.V.sub.C *. 
Row-inversion mode means that, in one field the pixels of the first row 
are written in a positive sense, the pixels of the second row in a 
negative sense, the pixels of the third row in a positive sense, and so 
on; thus, all odd-row pixels in a positive sense and all even-row pixels 
in a negative sense. On the next field, the polarities are inverted so 
that all odd-row pixels are written to in a negative sense and all 
even-row pixels in a positive sense. By using the extra voltage levels, 
.+-.V.sub.C *, as the reference voltages in the row drive waveforms for 
the selected channel, vertical crosstalk can be reduced. 
FIGS. 5a-5d are aligned vertically and show the corresponding row time 
T.sub.ROW and the ignition time T.sub.ig. During the ignition time 
T.sub.ig. the column voltage is maintained at zero (for clarity reasons) 
but this is not necessary. During this same time, the anode and cathode 
electrode voltages are as shown in FIG. 5, namely, during the first row 
time 60, the group 1 cathode of the selected channel is set to a voltage 
of -1/2(V.sub.ig +V.sub.sus), the anode group 1 of the selected channel is 
set at a voltage +1/2(V.sub.ig +V.sub.sus), and the remaining anode groups 
are set at -1/2(V.sub.ig -V.sub.sus). During the remaining row select 
time, T.sub.ROW -T.sub.ig (where the row time T.sub.ROW is equal to the 
field time T.sub.F divided by the number of rows N, T.sub.ROW =T.sub.F 
/N), the voltages of the anode and cathode groups included in the selected 
channel are maintained at the new reference voltage values .+-.V.sub.C 
*=.+-.1/2(V.sub.th *+V.sub.sat *). This means that during the ignition 
time, only a selected cathode of group 1 and a selected anode of group 1 
have applied between them a voltage equal to the plasma ignition voltage 
and the gas filling in that channel will ignite. In all the other 
non-selected channels, the maximum voltage present during this ignition 
time is V.sub.sus only, well below the ignition voltage. V.sub.C *, as 
shown in FIG. 2, is the average voltage value of V.sub.th * and V.sub.sat 
*, which equals 1/2 the sum of V.sub.th * and V.sub.sat *. During the 
remaining row time, -V.sub.C * as a reference voltage is applied to the 
cathode and anode electrodes of group 1 while the column voltages are 
applied. Thus, when a column voltage is zero (not shown), the voltage 
applied across the pixel is V.sub.C * whereas the maximum peak-to-peak 
column voltage required is (V.sub.sat *-V.sub.th *), which is considerably 
below the value of 2V.sub.sat * required for the prior art system. Since 
the column voltage range is decreased to (V.sub.sat *-V.sub.th *), less 
vertical crosstalk on pixels in the same column that are in the hold mode 
will occur. 
When possible, the anode and cathode electrodes in the other non-addressed 
groups of the non-addressed channels are maintained at zero voltage to 
assist in maintaining this decrease of crosstalk. 
It will be understood that the column voltage values shown in FIG. 5d are 
arbitrary to indicate different data values, though it will also be 
evident that FIG. 5d shows the row inversion mode. 
During the second row time 62, the voltages previously applied to anode 
group 1 are applied to anode group 2 and vice-versa, with the result that 
the next channel ignites, and so on during the remaining row times. Thus, 
the driving voltages required for this method of operation are as follows: 
for the cathode groups, 0, -1/2(V.sub.ig +V.sub.sus), V.sub.C * and 
-V.sub.C *; for the anode groups, 0, -1/2(V.sub.ig -V.sub.sus), 
1/2(V.sub.ig -V.sub.sus), V.sub.C * and -V.sub.C *. 
When there is an even number of connections in the cathode groups, the 
anode voltage waveform of a row will always have the same polarity in one 
field, as can be seen in FIGS. 5b and 5c. This may lead to a certain 
vertical crosstalk. To eliminate the latter, in accordance with another 
aspect of the invention, the anode voltage of a row can be made 
alternating by shifting the anode connections by one or an odd number for 
all even-numbered cathode groups, which can be defined as the ith 
electrode of the second group is paired with one of the (i+1)th, (i+3)th, 
(i+5)th . . . , or with one of the (i-1)th, (i-3)th, (i-5th)th . . . , 
electrodes of the first group, where, for the example given, the first 
group is the cathode group and the second group is the anode group. This 
is illustrated in FIG. 6. For instance, in cathode group 2, the anode of 
anode group 1 is located in the 2nd row and paired with the second group 2 
cathode, the anode of anode group 2 is located in the third row and paired 
with the third group 2 cathode, and so on. The location of the anodes in 
the odd-numbered cathode groups remains the same as in FIG. 1. The anode 
of the last anode group (also an even number) is located in the 1st row of 
cathode group 2. The same way of connecting is used in the 4th, 6th, etc. 
cathode groups. When scanning such an even-numbered cathode group, the 
scanning procedure of the anode group is modified so that the scanning 
starts with the last anode group, then the 1st, 2nd, and so on. The 
pairing of the cathode and anode electrodes for a channel is illustrated 
in FIG. 6 by a dashed S between the paired electrodes. A similar 
representation is used in FIG. 1. It will be understood that this feature 
of the invention is not limited to use with the driving voltage scheme 
described in connection with FIG. 5 or FIGS. 7 and 8 described below but 
can also be employed with the voltage driving schemes used for the prior 
art arrangements such as in FIG. 1. 
As indicated above, a number of different voltage levels totalling six in 
number are needed to drive the system illustrated in FIG. 5. This may 
require two different types of driving ICs, one type for driving the 
cathodes and one type for driving the anodes which can increase costs and 
possibly lead to assembly errors. 
In accordance with another aspect of the invention, the voltage waveforms 
illustrated in FIGS. 7a-7b can be used to drive the cathode and anode 
electrodes, the column voltages being the same as in FIG. 5 and thus not 
being shown. With an ignition voltage V.sub.ig required to ignite a 
plasma, a voltage of 1/2V.sub.ig will be insufficient for ignition, Thus, 
a selected cathode group 1 in FIG. 7a can be driven with a voltage of 
-1/2V.sub.ig during the ignition time while all the other cathode groups 
have 0 volts applied during that same time period. To a selected anode 
group 1 is applied a voltage of 1/2V.sub.ig while all the other anode 
groups have 0 volts applied during that same time period. The result is 
that only the channel with the selected group 1 cathode and group 1 anode 
has a voltage difference applied equal to V.sub.ig and will ignite, 
whereas all other anode-cathode pairs have a voltage difference of 
1/2V.sub.ig, -1/2V.sub.ig, or 0, so that no ignition can take place there. 
During the remaining row time, the voltages of the relevant anode and 
cathode electrodes are kept at V.sub.C * or -V.sub.C * as described in 
connection with FIG. 5. 
FIG. 7a illustrates this aspect for two groups for one scanning field. 
During the next scanning field shown in FIG. 7b, all the voltages are 
inverted, including the ignition voltages. This means that an electrode 
that previously functioned as a cathode now functions as an anode, and 
vice-versa, as illustrated in FIG. 7b. 
As a result of these modifications, both the anode and cathode electrode 
groups need the following voltages: 0, 1/2V.sub.ig, -1/2V.sub.ig, V.sub.C 
*, and -V.sub.C *. Thus, both groups can be driven by the same IC type, 
and the circuitry is simplified as only 5 voltage levels are required. 
Another advantage is that sputtering of the cathode that occurs during the 
existence of the plasma and limits the lifetime of the display is now 
distributed over two electrodes thereby effectively doubling the display 
lifetime. 
FIGS. 8a and 8b show another way of operating the display to exchange the 
roles of the cathode and anode and extend the lifetime of the display. In 
this embodiment, the cathode and anode functions are exchanged not only at 
alternate fields but also at each row time. Thus, comparing FIG. 7a and 
FIG. 8a, as an example in the FIG. 7a embodiment, the cathode group 1 
functions as a cathode during each odd-numbered field, and as an anode 
during each even-numbered field, whereas in the FIG. 8a embodiment, the 
cathode group 1 functions as a cathode during the odd-numbered rows and as 
an anode during the even-numbered rows of each odd field, and as an anode 
during the odd-numbered rows and as a cathode during the even-numbered 
rows of each even field. 
It will be understood that functioning as a cathode means that the voltage 
applied to it is negative relative to the anode so that it emits electrons 
which are attracted to the more positive anode. It will also be understood 
that the waveforms shown can be modified to the extent that they are 
inverted or displaced in time and the even and odd rows and fields 
reversed while retaining the benefits of the invention. 
As an example to illustrate operation in accordance with the embodiment of 
FIG. 5, which is not meant to be limiting, for a conventional TN LC of a 
thickness of about 4.3 .mu.m, for 99% transmission, V.sub.th equals about 
1.7V, and for 1% transmission V.sub.sat equals about 4.7V. For a typical 
display panel, .alpha.=4.47@V.sub.th and 7.94@V.sub.sat. With a 30 .mu.m 
dielectric sheet of glass (dielectric constant=6.7), V.sub.th *=7.6V and 
V.sub.sat *=37.3V. The reference voltage V.sub.C * will then equal for 
this combination of parameters 32.45V. 
It will also be appreciated that the invention is not limited to the 
specific values given above. It is possible to obtain many of the benefits 
of the invention with voltages that vary by as much as 10-20% from the 
values stated above, except that, in the case of the ignition voltage, the 
value must exceed the ignition value for the gas plasma to be created. 
The invention can be used in all kinds of C displays that typically have 
a small channel pitch for use in computer monitors, workstations or TV 
applications. 
While the invention has been described in connection with preferred 
embodiments, it will be understood that modifications thereof within the 
principles outlined above will be evident to those skilled in the art and 
thus the invention is not limited to the preferred embodiments but is 
intended to encompass such modifications.