Liquid crystal display in which data values are adjusted for cross-talk using other data values in the same column

An active matrix liquid crystal display device having a row and column array of picture elements which each include a switching device and which are driven by a drive circuit via sets of row and column address conductors to which selection and data signals are applied respectively, includes a data signal adjustment circuit which adjusts a data signal intended for a picture element prior to its application to the column conductor and according to a compensation value derived from the values of data signals intended for other picture elements using the same column conductor in the period until the picture element is next addressed so as to compensate for the subsequent effects of vertical cross-talk due to capacitive coupling effects at the picture element. In determining the compensating adjustment, blanking intervals in an applied video signal and current leakage effects can also be taken into account.

BACKGROUND OF THE INVENTION 
This invention relates to an active matrix display device having a display 
panel comprising an array of picture elements each comprising a liquid 
crystal display element and a switching device connected to the display 
element, and sets of row and column address conductors connected to the 
picture elements, and a drive circuit for applying data signals to the 
column address conductors and for repetitively scanning the row address 
conductors to select each row of picture elements in sequence to drive the 
display elements of a selected row in accordance with the data signals 
applied to the associated column address conductors. The invention relates 
also to a method of driving such a display device. 
Display devices of the above kind are well known. The switching devices 
used in such display devices can comprise two terminal non-linear devices, 
such as thin film diodes, (TFDs), MIMs and the like, or three terminal 
devices such as TFTs. Examples of diode type display devices and TFT type 
display devices are described respectively in U.S. Pat. No. 5,159,325 and 
U.S. Pat No. 4,845,482. In the case of display devices using two terminal 
switching devices the crossing sets of row and column address conductors 
are normally carried on respective transparent plates. The switching 
devices are carried on one of the substrates adjacent their respective 
display elements and connected in series between the display element and 
its associated address conductor, usually the row address conductor, on 
that substrate. In the case of display devices using TFTs, the sets of row 
and column addresses are carried on one of two spaced substrates together 
with a display element electrode and a TFT adjacent each intersection 
between the sets of address conductors, while the other substrate carries 
a common electrode. Each TFT is connected to its associated display 
element electrode and respective row and column address conductors. In 
both kinds of display device the driving circuit connected to the row and 
the column address conductors applies a selection signal to each row 
conductor in turn and data signals to the column conductors whereby the 
display elements of a selected row are charged via their respective 
switching device to a level dependent on the value of the data signal on 
their associated column conductor so as to produce a required display 
output effect. The rows of picture elements are driven individually in 
turn during respective row address periods in this manner so as to build 
up a display picture over one field period, the picture elements being 
repeatedly addressed in similar manner in successive field periods. Such 
display devices are suitable for datagraphic display purposes or video 
pictures, the data signals being derived in this case by sampling an input 
video, e.g. TV, signal. 
A problem with these display devices is that of column coupling phenomenon 
and the effect of vertical crosstalk associated with this phenomenon. This 
phenomenon is caused by parasitic or stray capacitive effects in each 
picture element circuit, for example between the column address conductors 
and display element electrodes or as a result of the self capacitance of 
the switching devices, for example the TFT whose source and drain 
terminals are connected to a column conductor and a display element 
electrode respectively. As a result of such capacitances, data voltage 
signals present on the column conductors and intended for use in driving 
picture elements associated with that column conductor as they are 
selected are coupled to the non-selected picture elements in the column 
thereby affecting the outputs of supposedly isolated display elements. 
This vertical crosstalk can be regarded as the dependence of the RMS 
voltage on a given display element upon the data signals intended for 
other display elements in the same column. 
Such cross-talk problems are discussed in U.S. Pat. No 4,845,482 which 
describes a proposal for reducing the effects that involves applying a 
gating (selection) signal to a row conductor for a time shorter than the 
standard row address period, applying the data signal to the column 
conductor during this time, and applying a compensation signal to the 
column conductor during the remainder of the period, the compensation 
signal being a function of the complement of the data signal, to reduce 
any cross-talk produced in other picture elements connected to the column 
conductor as a result of that data signal. However, because the row 
address period is shortened the display elements have to be charged in 
less time than normal and this requires using higher gating voltages which 
has a number of disadvantages including an increase in ageing effects on 
the TFTs as well as the need for comparatively high voltage row drive 
circuit. 
The extent of vertical cross-talk in display devices using two terminal 
switching devices between the display elements and the column or row 
conductors is largely dependent on the capacitance of the switching device 
in relation to the display element capacitance. 
In a display picture the vertical crosstalk manifests itself most obviously 
as bands of different luminance extending above and below particularly 
bright or dark areas of the picture. The magnitude of the effect is 
dependent on the method of driving the display device. If field inversion 
is used, the effect can be considerable. The effect can be reduced by 
using a line inversion drive scheme, intended to eliminate flicker. In 
this, the data signals applied to a column conductor are inverted every 
row as a result of which the coupled column voltages have alternating 
positive and negative values thereby making the overall coupled RMS 
voltage closer to zero and reducing the amount of vertical cross-talk. If 
single line inversion is used then a problem can occur in colour display 
devices that use the delta colour filter pattern where each column 
conductor is connected to picture elements having only two colours. In 
this case the data signal for large areas of a primary colour like red is 
the same as that for a plain black or white area with field inversion and 
large amounts of cross-talk can occur. Also, in computer datagraphic 
displays, the nature of some video patterns can cancel the inversion 
procedure, making vertical cross-talk more noticeable. 
In U.S. Pat No. 4,892,389 there is described a method of driving a liquid 
crystal display device using two terminal non-linear switching devices so 
as to reduce cross-talk effects which involves applying a data signal to a 
column conductor for a part only of the available row selection period and 
substantially simultaneously with the application of a row selection 
signal to a row conductor and by applying a non-selection signal to the 
row conductor and a reference signal to the column conductor during the 
remaining part of the available row selection period. A disadvantage of 
this approach, like that of U.S. Pat. No. 4,845,482,is that the necessary 
reduction in the time for driving a display element with its data signal 
to a fraction of the available selection period causes problems. Higher 
peak currents through the switching devices are necessary which can damage 
the switching devices or at lease cause undesirable ageing effects. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved active 
matrix display device, and a method of operating such, in which the 
visible effects of vertical cross-talk are reduced without the 
aforementioned drawbacks. 
According to one aspect of the present invention there is provided an 
active matrix display device of the kind described in the opening 
paragraph which is characterised in that the drive circuit includes a data 
signal adjustment circuit for compensating for the effects of vertical 
cross-talk in the display panel due to capacitive coupling between the 
display elements and their associated column address conductors, which 
data signal adjustment circuit has an input to which data signals are 
applied and adjusts an input data signal for a picture element according 
to a cross-talk compensation value which is derived from the data signals 
intended for at least some of the other picture elements connected to the 
same column address conductor as that picture element in the period until 
the picture element is next addressed, the data signal adjustment circuit 
having an output from which the adjusted data signals are supplied to the 
column address conductors for driving the picture elements. 
According to another aspect of the present invention, there is provided a 
method of driving an active matrix display device having a row and column 
array of picture elements which each comprise a liquid crystal display 
element connected to a switching device and which are connected to sets of 
row and column address conductors, in which selection signals are applied 
to the row address conductors to select each row of picture elements in 
turn and data signals are applied to the column address conductors whereby 
the display elements of a selected row of picture elements are driven 
according to the data signals on their associated column address 
conductors, the rows of picture elements being driven repetitively in 
successive field periods, which is characterised in that, before being 
applied to a column address conductor, a data signal for a picture element 
associated with the column address conductor is adjusted to compensate for 
the effects of vertical cross-talk in the display panel due to capacitive 
coupling between the display elements and their associated column address 
conductors according to a cross-talk compensating value determined by the 
values of the data signals intended to be used for driving at least some 
of the other picture elements associated with that column address 
conductor in the period until the picture element is next addressed. 
The invention stems from a recognition that rather than trying simply to 
reduce the amount of vertical cross-talk due to the data signals on the 
column address conductors, the effects of vertical cross-talk through 
column coupling phenomenon can be compensated by altering the data signals 
intended for a column of picture elements before they are applied to the 
picture elements to allow for the expected column coupling due to the data 
signals for those picture elements so that after their application to the 
appropriate picture elements the effect of vertical cross-talk on an 
individual picture element leads to the display element having 
substantially the intended, correct, voltage, and consequently to the 
display element producing an output which is closer to the intended output 
as determined by the value of the data signal before such adjustment. 
Unlike the approaches described in the aforementioned U.S. Pat. No. 
4,845,482 and U.S. Pat. No. 4,892,389, the invention does not need the 
picture element address periods to be reduced, and hence the problems 
caused thereby are avoided. The need to provide means for performing the 
necessary correction of data signals for individual picture elements ahead 
of their application to the picture elements is more than outweighed by 
the substantial benefits achieved. 
In addition to reducing the cross-talk effects, the invention offers a 
further significant advantage. Previously, the consequences of vertical 
cross-talk have imposed a limitation on the size of the picture elements. 
As picture element sizes are reduced, for example to provide higher 
density arrays, the column coupling factor increases and vertical 
cross-talk becomes worse. There is a limit where row inversion drive 
techniques cannot reduce cross-talk sufficiently. With the present 
invention, such picture element size limitations can be overcome. 
The cross-talk compensation value for an input data signal intended for a 
picture element, derived according to intended data signals for at least 
some of the other picture elements connected to the same column address 
conductor in the field period subsequent to the addressing of the picture 
element, is preferably determined in the data signal adjustment circuit 
according to a capacitive coupling factor for the picture element whose 
value is dependent at least on the capacitance of the switching device. 
The parasitic capacitance of the switching device is likely to be the most 
influential in determining the extent of vertical cross-talk, but other 
stray capacitances in the picture element circuit, for example between a 
column conductor and a display element electrode in a TFT display device 
or between a row conductor and a display element electrode in a TFD 
display device, could be taken into consideration when determining the 
capacitive coupling factor to improve the effectiveness of the correction 
afforded by the compensation. 
Although it may be that for certain kinds of display applications adequate 
improvement can be obtained by compensating data signals in accordance 
with the values of the data signals intended for some, but not all, of the 
picture elements associated with the column address conductor, preferably 
the modification of the data signals is accomplished taking into account 
the data signals intended for substantially all the other picture elements 
associated with that column address conductor for optimum results. It has 
been found that the reduction in cross-talk as a result of the invention 
varies approximately linearly with the number of data signal voltages 
which are applied to the column address conductor that are taken into 
account. 
To provide effective compensation in most display situations then the 
adjustment made to an input data signal is preferably made according to 
the values of the input data signals for other picture elements in the 
same column during the field period which follows the addressing of the 
particular picture element with that input data signal. In a preferred 
embodiment, therefore, the input data signal is held in a store in the 
adjusting circuit for a field period and then adjusted according to a 
compensation value which is determined from the values of input data 
signals for picture elements in the same column that are held in the store 
during that field period. The store is required because there is a need to 
know the actual data signals, as determined by the applied video signals, 
which are intended for those other picture elements ahead of the 
addressing of a picture element with the input data signal concerned. The 
intended data signals used in the derivation of the compensation value are 
then the actual data signals according to the applied video signal, to be 
used. In practice, a field store may be used to hold the data signals. 
A simpler approach may be used in certain circumstances and particularly 
where the display device is used to a large extent for displaying 
principally stationary images or images which contain stationary part. In 
another embodiment, therefore, the data signal adjustment circuit adjusts 
an input data signal according to a crosstalk compensation value that is 
derived from the values of data signals input during the immediately 
preceding field period. With this approach, therefore, the intended data 
signals used in the derivation of the compensation value are not the 
actual input data signals for other picture elements in the same column 
but instead are postulated data signals and are predicted on the basis the 
data signals for a following field period will, apart from, for example, a 
change of sign in the case of field inversion being used, remain the same 
for a stationary image. In other words, the actual, future, data signals 
voltages can be assumed to be simply the negative of the current data 
signal voltages. Thus, the current data signals values can be used to 
predict the future data signal values. The need to provide a field store 
is thus avoided. The data signal predictions will, of course, be incorrect 
in the event that the input data signals are changed to provide a 
different display image. However, the effects of such a change between two 
display images before the data signal adjustments are corrected can be 
limited to two fields which is unlikely to be noticeable. Preferably, 
however, in order to accommodate a situation where continuous motion is to 
be displayed, the data signal adjustment circuit is arranged so as to 
compare values dependent on the input data signals for a column in 
consecutive fields and to disable the adjustment to the input data signal 
for a column in the event that the values in consecutive fields differ by 
a predetermined amount. Thus, the input data signals are used to address 
the picture elements of the column concerned without adjustment for 
crosstalk compensation. Although the effects of crosstalk will then be 
present, they are likely to be less visible than the effects caused if 
adjustment, on the basis of incorrect, predicted, data signals, were to 
continue. 
The data signal is preferably adjusted substantially according to a 
compensating factor which is determined by the intended data signals for 
the other picture elements connected to the same column address conductor, 
the intended display element voltage, and a capacitive coupling factor for 
a picture element circuit. This coupling factor would be dependent on, for 
example, the display-element capacitance and stray capacitance, especially 
the parasitic capacitance of the switching device. 
In the case of the data signals being derived from an applied video, e.g. 
TV, signal, in which successive fields are separated by a field blanking 
interval, then because the blanking interval can be a significant part of 
the field period it may also be taken into account in the derivation of 
the adjusted data signals. 
The compensation value need not be dependent solely on capacitive coupling 
effects but can be derived also to take into account leakage current 
effects in the switching devices. These leakage current effects, as can 
occur in TFTs or two terminal devices such as MIMs and TFDs due for 
example to their inherent behaviour or photosensitive properties, are 
similarly dependent on the levels of voltages appearing on the column 
address conductor. A correction for such leakage current can be 
incorporated in the compensation value by appropriately modifying the 
formula given above used in the calculation of this value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the active matrix display device comprises a liquid 
crystal display device intended to display video, e.g. TV, pictures, or 
datagraphic information and includes a liquid crystal display panel 10. 
The panel 10 has a row and column array, comprising n rows and m columns, 
of picture elements 12 each of which is located adjacent a respective 
intersection between sets of row and column address conductors 14 and 16 
to which drive signals are applied by row and column drive circuits 20 and 
21. The panel 10 is of a known kind and can be of the type using TFTs or 
two terminal non-linear devices as switching devices for the picture 
elements. FIGS. 2A and 2B show respectively the circuit configurations of 
a typical picture element of a TFT active matrix panel and a two terminal 
non-linear device active matrix panel. In the former, FIG. 2A, the gate of 
the TFT, 25, is connected to a row address conductor 14 and its source and 
drain terminals are connected respectively to a column address conductor 
16 and an electrode of a display element 30. The sets of conductors 14 and 
16, the TFTs and the display element electrodes of the panel are all 
carried on a first transparent substrate of the panel, for example of 
glass, which is spaced from a second transparent substrate with liquid 
crystal material e.g. twisted nematic LC material, disposed between the 
substrates. Respective portions of a continuous transparent electrode 
carried on the second substrate constitute second electrodes of the 
display elements whereby each display element 30 consists of a pair of 
spaced electrodes with LC material sandwiched therebetween. In the latter 
case, FIG. 2B, the two terminal non-linear switching device, 26, for 
example, a MIM device, back to back diodes, MSM 
(metal-semiconductor-metal) device or other TFDs or the like is connected 
between a row address conductor and a display element electrode. The 
display element electrodes are carried together with the set of row 
address conductors and the devices 26 on a first transparent, glass, 
substrate which is spaced from a second substrate with liquid crystal 
material disposed between the two substrates. The second substrate carries 
a set of strip electrodes, constituting the set of column address 
conductors and portions of these column conductors overlying the display 
element electrodes form second display element electrodes such that, 
again, each display element 30 consists of two spaced electrodes with LC 
material sandwiched therebetween. Instead of being connected between the 
display element and a row conductor, the switching devices may be formed 
on the same substrate as the column conductors and connected between the 
column conductors and the display elements. In both types of panel, all 
picture elements in the same row are connected to a respective one of the 
set of row address conductors 14 and all picture elements in the same 
column are connected to a respective one of the column address conductors 
16. The substrates carry respectively on their outer and inner surfaces 
polarising and LC orientation and protection layers respectively in 
conventional manner. 
The row and column drive circuits 20 and 21 of the display device are each 
also of a conventional kind. The row drive circuit 20, for example a 
digital shift register circuit, applies a selection signal to each row 
conductor 14 sequentially in turn during a respective row address period. 
This operation is controlled by timing signals from a timing and control 
circuit 22 to which synchronisation signals, derived by a synchronisation 
separator circuit 27 from an incoming video, e.g. TV, signal applied to an 
input 28, are supplied. The column drive circuit 21 comprises one or more 
shift register/sample and hold circuits for which data, (video 
information) signals derived from the applied video signal are provided 
from a video signal processing circuit 24. The circuit 21 operates to 
sample these signals, under the control of the timing and control circuit 
22 in synchronism with row scanning to provide serial to parallel 
conversion appropriate to the row at a time addressing of the panel. As 
each row conductor 14 is scanned with a selection signal, the switching 
devices, 25 or 26, of the associated row of picture elements are turned on 
so as to charge the display elements 30 of the row to a desired display 
element voltage according to the level of the data signal then subsisting 
on their respective column conductors 16, the display element voltage 
being proportional to the data signal voltage. Upon termination of the 
selection signal, the switching devices of the picture elements are turned 
off, thereby isolating the display elements from the column conductors 
until they are next addressed in the subsequent field period. Each row of 
picture elements of the panel is addressed in this manner so as to build 
up a display picture in a field period and the operation is repeated in 
successive field periods to produce a succession of display image fields. 
In the case, for example, of a TV display, each row of display elements is 
provided with picture information, data, of a TV line with the duration of 
selection signal corresponding to TV line period or less so that for a 
half resolution standard TV display having a line period of 64 .mu.s, 
each row address conductor is supplied with a selection signal at 
intervals of 20 ms. 
In order to avoid electrochemical degradation of the LC material, the 
polarity of the drive signals is periodically inverted, for example after 
every field, (field inversion). Polarity inversion may also be carried out 
after every row or every two rows, commonly referred to as line (row) 
inversion and double line (row) inversion, in order to reduce flickering 
effects. 
From the foregoing description, it will be apparent that during operation 
of the display device each column address conductor 16 carries a voltage 
waveform which consists of a series of data signal voltage levels each of 
which is intended for a respective one of the picture elements in the 
column of picture elements connected to that column conductor. Ideally, 
every display element in a column will be accessed when its associated row 
conductor is selected and remain electrically isolated for the remainder 
of the display cycle. However, there exists a capacitance at least across 
each switching device which couples the column conductor voltage waveform 
to its associated display element and this coupling leads to vertical 
cross-talk. To reduce the effects of such cross-talk, the display device 
includes a data signal adjustment circuit 40 in its drive circuit that 
operates to adjust the supplied data signals, intended to produce desired 
outputs from the display elements, before they are applied to the column 
conductors in such a way as to compensate for the anticipated effects of 
this cross-talk so that, after the display elements have been driven using 
the adjusted data signals, the effect of the cross-talk resulting from the 
voltage waveform on the column conductors is to cause the display elements 
to produce display outputs approaching those intended had there been no 
cross-talk. To this end, the value of an input data signal from the input 
video signal and intended for application to a picture element via a 
column conductor is adjusted having regard to the values of the data 
signals from the video signal which it is intended are to be used for at 
least some of the other picture elements subsequently addressed via that 
column conductor up till the time the picture element is next addressed. 
The adjustment made to each data signal, in the form of a cross-talk 
compensation value which is derived from, and thus determined by, intended 
data signals for other picture elements connected to the same column 
conductor in the period until the picture element concerned is next 
addressed, compensates for the likely effects on the display element 
voltage due to cross talk caused by the capacitive coupling of later 
applied data signals on the column conductor so that when the display 
element voltage is affected by this coupling, the resulting voltage, and 
hence display output, of the element is close to that originally intended 
as determined by the value of the data signal prior to adjustment. 
To understand the nature of this adjustment, the column coupling effects 
leading to vertical cross-talk need to be considered. 
The amount of coupling depends on the coupling factor "c" which is defined 
as: 
##EQU1## 
where k is the ratio of the average display element capacitance over the 
total parasitic capacitance between the column conductor and the display 
element electrode in the case where the switching device is connected 
between the column conductor and the display element or between the row 
electrode and the display element electrode in the case where the 
switching device is connected between the row conductor and the display 
element. From this, it will be appreciated that as the capacitance between 
the column conductor and the display element electrode increases so does 
the coupling factor and that if the capacitance of the display element is 
reduced the coupling factor increases yet again. Therefore, display 
elements of small size will suffer more from vertical cross-talk than 
larger size display elements for a given size of switching device. The 
parasitic capacitance determining the coupling factor will largely be 
dependent on the capacitance of the switching device, which is represented 
in FIGS. 2A and 2B by the dotted capacitor Cs. There can be other stray 
capacitances in a picture element circuit which play a part, for example 
the stray capacitance between a column conductor and a display element 
electrode in a TFT display panel or between a row conductor and the 
display element electrode in MIM/TFD type display panel. In the general 
case, the coupling factor c may be taken to be equal to the sum of the 
switching device capacitance and the stray capacitance divided by the sum 
of the display element capacitance, the switching device capacitance and 
the stray capacitance. 
Considering a display element in the xth row, then the column voltages for 
display elements x+1 to n of the current field followed by the column 
voltages for display elements 1 to x-1 of the next display field will be 
coupled to the xth pixel. In other words, after the addressing of the 
display element in the xth rows, all the data voltage signals intended for 
the other n-1 display elements in the same column which appear on the 
associated column conductor 16 in the period, corresponding to a field 
period, before that display element is again addressed will be coupled. 
Thus, the coupled column voltage for any display element is the part of 
the column waveform which corresponds to the column voltages for the next 
n-1 display elements in time. Because in practice the display device is 
operated with some kind of inversion (field, line, double line), then the 
coupled voltages will change polarity accordingly. 
The voltage of a display element affected by capacitive column coupling can 
be expressed in its simplest form by the following formula: 
##EQU2## 
where V.sub.p is the intended display element voltage without accounting 
for column coupling, and is proportional to the applied column, data 
signal, voltage, and V'.sub.p is the display element voltage which 
includes the effects of column coupling. The number of display elements in 
a column is n and for every display element there is the corresponding 
column voltage, V.sub.c (i), where i signifies the individual column (data 
signal) voltage which is coupled to the affected display element via the 
coupling factor "c". The elements of the summation correspond to the next 
"n-1" column voltages as discussed above. It is important to remember that 
the value of V.sub.c (i) can be negative as well as positive. If the 
display device operates in field inversion, as well as field inversion, 
then the polarity of the column voltage changes with every new field while 
if the display operates in line inversion, as well as field inversion, the 
polarity alternates with every new line. 
For most kinds of display images, the total error due to column coupling in 
a display panel operating with line inversion is generally less than that 
for field inversion. One of the advantages of using line inversion is that 
for most images column coupling, and hence vertical crosstalk, the 
consequent visible artifact, is reduced. 
A further point to be considered is the difference between the number of 
display lines (rows) and the number of video lines in the applied video 
signal which may include blanking lines. Equation (1.2) ignores any 
blanking interval. The blanking time can be a significant part (more than 
5% for example) of the field period and if this is the case then it may be 
included in the calculations by modifying equation (1.2) to take into 
account the total number of lines in one field of the video signal 
including the blanking lines (for example 312 for standard TV display) 
as well as the number of displayed lines (i.e. the number of picture 
element rows), and the fixed column blanking voltage value which 
corresponds to the blanking interval lines. Because, however, blanking 
interval introduces a small, fixed, error this error can instead be 
corrected by adjustment of other drive voltages. 
The resulting shift in the display element voltage from the intended value 
to a new one affects the transmission of the display element. Considering, 
for example, a display device operating in field inversion and being used 
to display a central black square in a 30% transmission background, then 
the visible artifact of vertical crosstalk caused by column coupling will 
result in the display regions above and below that central black square 
having transmission levels different to that of the remainder of the 
background. Because the display device operates in field inversion the 
area directly above the black central square will appear darker since the 
coupled voltage will shift the display element of that region in the 
direction of black but the area directly below the square will appear 
lighter because the coupled voltages (from the next field now) will be of 
opposite polarity and therefore will shift the display element voltages of 
that region towards the other direction. 
Such vertical crosstalk is particularly noticeable on display devices 
operating in field inversion. Line inversion can reduce the problem up to 
a point but if the nature of display picture is such that it tends to 
cancel the inversion pattern (for example black lines alternating with 
white lines) then crosstalk can again be highly visible. Patterns of this 
kind are commonly found on computer generated images. The above 
description relates to simple monochrome displays. Colour display devices 
using the so-called delta-nabla colour display element configuration will 
also suffer from crosstalk since the effects of row inversion in these 
display devices can similarly be cancelled in display pictures which 
contain blocks of primary colours. 
An added complication of the column coupling phenomenon is the coupling of 
the first transition of the column voltage after the display element has 
been addressed, referred to as column kickback, which is another 
contributory factor to the overall RMS calculation (equation 1.2) because 
it will shift the overall voltage level of the display element. The column 
voltage waveform will be coupled to the display element at this new level. 
Column kickback changes the display element voltage from V.sub.p to 
V.sub.pc which is equal to V.sub.p minus cV.sub.c where the column voltage 
of the affected display element in the column is V.sub.c. The value of the 
column voltage must have the appropriate sign. 
If the column kickback phenomenon and the blanking interval are included in 
the calculations then the equation for column coupling becomes: 
##EQU3## 
where l is the total number of lines in one video signal field and V.sub.b 
is the fixed blanking voltage. 
In the display device of FIG. 1, the phenomenon of vertical crosstalk is 
compensated by appropriately modifying the data signal voltages applied to 
the display elements. Equation 1.2 gives the voltage, V'.sub.p of a 
display element when it is affected by column coupling. By applying 
instead an adjusted data voltage Vn to the display element according to 
the equation: 
EQU Vn=Vc-.DELTA.Vp (2.1) 
where .DELTA.Vp, the correction factor, is given by 
EQU .DELTA.Vp=Vp.sup.l -Vp (2.2) 
then, after column coupling occurs, the effects of such column coupling 
will be substantially compensated and the voltage on the display element 
will be close to the one required so that the display output obtained from 
the display element approaches that intended. For example, if for a given 
display element a voltage of 4V rms is required, and, after applying the 
equation 1.2, it is found that the actual voltage will be 4.3V rms where 
the additional 0.3V rms is the coupled voltage due to column coupling of 
the data voltages for other display elements connected to the same column 
conductor applied during a field period between addressing the display 
element concerned and next addressing that display element then by 
applying around 3.7V initially instead to the display element, the effects 
of the column coupling can be largely negated and the actual rms display 
element voltage would be very close to the intended value of 4V. Of 
course, this compensation is not exact, bearing in mind that the 
compensation is derived from the originally intended data signals for the 
other display elements in the same column before they too are adjusted. If 
those data signals are similarly compensated, the actual data signal 
levels applied to the column conductor will, of course, differ from those 
used in the computation of the adjusted data signal. Considering, for 
example, the display element in the xth row, then compensation for column 
coupling is derived from the anticipated column voltages for the display 
elements in rows x+1 to x+(n-1). When the next display element x+1, is 
compensated for column coupling, the compensated data signal will change 
the display element voltage very slightly from that envisaged. This 
implies that the calculations for display element x contain an error since 
the actual column voltage for display element x+1 is different from the 
one assumed. Exact compensation would only be feasible for stationary 
images and periodic moving images. However, it has been found that the 
above described approach is highly successful and can eliminate, or at 
least significantly reduce, the visible vertical cross-talk. In 
simulations involving worst possible envisaged display conditions for a 
display device operating with field inversion, a display element was found 
to have an additional 0.25v due to column coupling. After applying the 
algorithm, Equation 2.1 using the simple formula 1.2., to adjust the level 
of the data signal applied to the display element for column coupling 
compensation purposes, it was found that the additional voltage on the 
display element after actual column coupling was reduced to a mere 3 mV. 
The reduction obtained, of the order of almost 100 to 1, is entirely 
adequate for all practical applications. 
In experiments with a display device operating a field inversion, and 
without compensation, and displaying a central black or white square on a 
grey-scale background it was found that the transmission values of the 
regions directly above and below the central square showed considerable 
differences with the transmission levels of the remainder of the 
background but that after compensation these differences were effectively 
eliminated and vertical crosstalk was reduced to below visibility levels. 
Corrections using the formula 1.3 could provide still further 
improvements. Crosstalk under single line inversion shows an improvement 
of a similar order to field inversion crosstalk. Since vertical crosstalk 
manifests itself more as line flicker than as an overall transmission 
change on display devices operating under line inversion, the compensation 
reduces the flicker level accordingly. 
The data signal adjustment circuit may be arranged so as to provide 
additional adjustment to the data signals for effects other than those due 
solely to capacitive coupling and which similarly lead to vertical 
cross-talk, and in particular for leakage in the switching devices 
producing leakage current induced cross-talk. This leakage current may be 
caused by, for example, the inherent behaviour, and in some cases the 
photosensitive properties, of the switching devices (TFTs and 
two--terminal devices) and its extent is similarly dependent on the level 
of voltages applied to the column conductors. Adjustment for this effect 
can be accommodated in the data signal adjustment circuit by using an 
appropriately modified form of the formula 1.2 in the derivation of the 
data signal compensation value. 
Referring again to FIG. 1, the compensation for vertical cross-talk by 
pre-correcting the incoming video signal is performed by the compensation 
unit 40 connected between the video processing circuit 24 and the column 
drive circuit 21 which operates to modify the voltage levels of, analogue 
input video data signals, preferably supplied from the processing circuit 
24 to the unit 40 in digitised form, and to determine the data signals to 
be supplied by the column drive circuit 21 to the picture elements, 
according to the correction algorithm of equation 2.1. 
Embodiments of the display device, and in particular the unit 40, and their 
methods of operation will now be described. In the first embodiment, the 
adjustment of the input data signals is performed on the basis of the 
actual data signals, as determined by the input video signal, to be used 
for other picture elements connected to the same column in the next 
period. 
As equation 1.2 shows, implementation of the compensation algorithm 
requires extensive computation to generate a compensated data signal for 
each picture element. The summation term in equation 1-2 can be broken up 
and simplified to: 
##EQU4## 
The summation for the next display element, p+1, in the column will be: 
##EQU5## 
It should be noted that V.sub.c (n+1) is in fact V.sub.c (1) of the next 
field. The two summations for display element "p+1" differ by only a 
simple addition and subtraction from the summations for display element 
"p" since: 
##EQU6## 
If, therefore, two running sums are kept for every column, the calculations 
can be significantly simplified. 
Because it is necessary to know the actual data signals intended for the 
other picture elements in advance, the column coupling compensation unit 
40 requires a field store where the values of the data signals for the 
display elements which will affect the current display element will be 
stored. This effectively is a "rolling" store since a new row of display 
element values will enter when an old one is dropped out. The field store 
is necessary because the contents of the present display element(s) will 
be affected by future ones. In effect it is a field delay and this field 
delay should operate with the same number of luminance gray scales as the 
display. 
The operation of an embodiment of the circuit 40 when performing 
connections according to equations 1.2 and 2.1 will now be described with 
reference to FIG. 3 which shows schematically an example circuit 
implementation. 
The vertical crosstalk correction must be calculated separately for each 
picture element as the video signal passes through the compensation 
circuit. Therefore the hardware must solve equations 1.2 and 2.1 at 
picture element rate. A look-up table can be used to calculate the 
correction at the required speed. The look-up table requires the following 
three input variables in order to solve the equations: 
a) The picture element voltage V.sub.p for the picture element (y,x) where 
y and x designate the row and column respectively. 
b) The sum of the column voltages .SIGMA.V.sub.c that are intended to be 
applied to column x over the following field period. 
c) The sum of the squares of the column voltages that are intended to be 
applied to column x over the following field period. 
In practice these variables can be supplied to the look-up table in the 
form of digital video data rather than a direct binary representation of 
the actual voltages. A simple data-to-V.sub.p or V.sub.c conversion can be 
built into the look-up table without any extra complexity. 
Rather than calculating .SIGMA.V.sub.c and .SIGMA.V.sup.2 from scratch for 
each picture element, .SIGMA.V and .SIGMA.V.sup.2 are maintained as 
running totals and stored in a RAM. These running totals can be maintained 
as follows. Each time a picture element data signal for column x enters 
the field delay, the data and data squared for that picture element is 
added to the column sums. Each time a picture element data signal for 
column x emerges from the field delay, the data and data squared for that 
picture element is subtracted from the column x sums. Separate sums are 
maintained for all columns in the display. Thus, by the time the video 
data for a given picture element emerges from the field delay 
.SIGMA.V.sub.c and .SIGMA.V.sup.2 for all the picture elements in the same 
column have been summed and are ready to be used in the calculation of the 
vertical crosstalk correction for that picture element. In this way, the 
appropriate correction can be calculated and added to the video data for 
each picture element as it emerges from the field delay. 
Referring to FIG. 3, the field delay (RAM) to which digital 
representations, V data (x,y), of the analogue video are supplied serially 
from the processing circuit 24 via an analogue to digital converter (not 
shown) is designated at 44. The timing of the RAM field delay 44 is 
controlled by clock and reset signals CLK and RST from the timing and 
control circuit 22. The signals Vdata are supplied also to a look-up table 
45 from which the value Vhd c.sup.2 is derived (where Vc is the column 
voltage), having regard to the voltage transmission characteristics of the 
display elements. Similarly the signals Vdata outputted from the field 
store 44 from the preceding field period are supplied to a look-up table 
46 from which their V.sub.c.sup.2 values are obtained. These values of 
V.sub.c.sup.2 from look-up tables 45 and 46 are fed to an adder 47 whose 
output (V.sub.c.sup.2 the new field minus V.sub.c.sup.2 from the old 
field) is supplied to a further adder 48 where it is added to the previous 
summation (.SIGMA.V.sub.c.sup.2) stored in a line buffer (RAM) 49, 
controlled by clock and reset signals CLK and RST from the circuit 22, to 
derive a new summation (.SIGMA.V.sub.c.sup.2). This is then supplied as a 
first input to a correction look-up table 50 and written back to the line 
buffer RAM 49. A similar system, but with different look-up tables 45 and 
46, is used to derive the summation of V.sub.c (.SIGMA.V.sub.c) which is 
supplied as a second input to the look-up table 50. A third input of the 
circuit 50 is supplied with picture element data values, Vdata, outputted 
from the field delay 44. Adjusted picture element Vdata signals, duly 
corrected in accordance with equations 1.2 (or 1.3) and 2.1, obtained from 
the look-up table 50 are then fed in serial form to the column driver 
circuit 21 via a digital to analogue converter, giving the adjusted data 
voltage signals Vn (x,y) where they are then sampled to provide serial to 
parallel conversion and supplied to the appropriate column conductors to 
drive the picture elements. Instead of deriving .SIGMA.Vc and 
.SIGMA.Vc.sup.2 separately and supplying these values independently to the 
LUT50, it may be possible to derive an approximation for .SIGMA.Vc.sup.2 
from .SIGMA.Vc. However, while such a technique could allow a reduction in 
the necessary circuit componentry the data signal compensation achieved 
will inevitably be less accurate. The embodiment described above with 
reference to FIG. 3 is able to provide very effective compensation for 
most usual kinds of display pictures but for its implementation it 
requires a field store in order that the compensation is calculated from 
the actual data signals for display elements, as determined by the input 
video signal, for the next field. In another embodiment, an alternative 
approach is used which is capable of providing adequate compensation 
without the need for a field store in situations where it is expected that 
the display produced will entail to a large extent stationary pictures or 
pictures comprising stationary parts for periods of time. Whilst for this 
alternative approach compensation is obtained only so far as stationary 
parts of the displayed images are concerned, it offers the advantage that 
a field store is not required so that comparatively simple, and 
consequently less expensive, circuitry can be used. The vertical crosstalk 
compensation algorithm, which effectively is embodied in the LUT 50, uses 
the equation 1.2 to derive the display element voltage after the effect of 
column coupling. In the previous embodiment, the derivation of the display 
element voltage according to this formula was simplified, for 
implementation purposes, by the use of running sums for every column, by 
means of line buffers/stores, i.e. store 49 in FIG. 3. The field store 44 
is needed because the summations, and therefore the compensation to be 
applied, depend on the next actual n-1 column data signal voltages. The 
principles of the compensation algorithm (equation 1.2) in this 
alternative embodiment remain the same but the manner in which the column 
sums are calculated differ. Such calculations are based on the assumption 
that, because a static image is being displayed, the column voltage one 
field period later will simply be the negative (in the case of field 
inversion operation) of the current column voltage. Therefore, running 
column voltage sums: (corresponding to sums .SIGMA.Vc and .SIGMA.Vc.sup.2 
with reference to FIG. 3) can be maintained by using the current column 
voltage value to predict the future column voltage value (after one 
field). In the previous embodiment, therefore, an input data signal is 
adjusted according to a compensation value that is derived from data 
signals intended for other picture elements connected to the same column 
conductor over the subsequent field period which intended data signals are 
the actual data signals determined by an input video signal and it is this 
need to know the actual data signals according to the applied video signal 
for the forthcoming field period which necessitates a field store. In this 
alternative embodiment, on the other hand, the compensation value used for 
adjusting a data signal is similarly derived from the values of data 
signals intended for other picture elements connected to the same column 
conductor over the subsequent field period, but the intended data signals 
are postulated rather than actual and assumed to be merely the inverse of 
the corresponding picture element data signals in the current field. 
The operation of the data signal adjustment circuit to this end requires a 
pair of sums for each summation .SIGMA.Vc and .SIGMA.Vc.sup.2 to be 
maintained for each picture element column. These sums can be held in a 
linestore, each location in which contains the sums for a particular 
column. 
The operation of the data signal adjustment circuit is illustrated 
diagrammatically in FIG. 4 in the form of a kind of flowchart. Referring 
to FIG. 4, the operation as depicted covers two consecutive fields, field 
s and field s+1, and concerns only one column, column k, of the display 
panel is shown but the operation in so far as the generation of corrected 
signals for all other columns is concerned will be identical. 
In FIG. 4, the letter A represents a sum .SIGMA.Vc referred to previously 
and is a running prediction, (stored in a linestore) of the column sum for 
column k, (as indicated by the line in the boxes 60 representing the 
display array) over the next field period, so that As+1 (k,1) and As+1 
(k,n) denote the column sums for field s+1 for the picture element at 
column k, row 1 and row n respectively. The letter B signifies a fresh 
prediction of the value of the column sum, stored in the linestore, for 
column k at the end of the next field period. Thus, sum B is equivalent to 
.SIGMA.Vc but worked out from zero over each field and to this end sum B 
is reset to zero at the beginning of each field, as indicated at 61 in 
FIG. 4. Sum B is compared with Sum A and used to correct Sum A if they 
differ once every field period, as indicated at 62. 
The procedure for calculating each sum is as follows. 
Firstly, with regard to calculating Sum A, then Sum A.sub.p+1 (k,i) is a 
running prediction of the column sum for column k over the next field 
period from row i+1 in field s to row i-1 in field s+1. The sum is updated 
each time the data signal for a picture element in column k is received. 
The update is based on the assumption that V.sub.c(k,i) in the current 
field will be equal to -V.sub.c (k,i) in the next field. Therefore: 
EQU A.sub.s+1(k,i) =A.sub.s+1(k,i-1) -2V.sub.c(k,i) (4.1) 
With regard to calculating sum B, then Sum B.sub.s+1 (k,n) is a fresh 
prediction of what the column sum for column k will be at the end of field 
s+1. Sum B is set to zero at the start of field s. The predicted sum is 
then calculated during field s using the assumption that the V.sub.c(k,i) 
in field s will be equal to -V.sub.c(k,i) in the field p+1. Therefore, by 
the end of field p, sum B is given by: 
##EQU7## 
At the end of each field sum B is used to correct Sum A. If the picture is 
static then the two sums will be equal. However, if the picture has 
changed Sum A will be incorrect and will not be equal to Sum B. 
Sum A can be substituted into equation 1.2 and used in the LUT 50 in order 
to calculate the vertical crosstalk correction for picture element (k,i). 
This formula used in the compensation algorithm involves a squared sum as 
well. This squared sum, .SIGMA.Vc.sup.2, is derived in a similar way. 
This technique produces the same results on static images as the technique 
of the previous embodiment which uses the field store. This time, because 
the information does not change from field to field, the field store is 
not necessary. However, if a moving picture is displayed, the column 
voltage predictions, the running sums, and hence the crosstalk corrections 
will become incorrect. Sudden changes between two images will imply that 
the corrections on 2 fields would be wrong but it is highly unlikely that 
this will be noticeable. The wrong correction will only be present for two 
field periods, (about 33 ms for a 60 Hz display). 
Complications arise when continuous motion is portrayed which implies 
continuous changes. Under these circumstances the "wrong" correction may 
become visible in the displayed image since it would be continuously 
present. To avoid this possibility the correction for a particular column 
can be turned off depending on the values of sums A and B at the end of 
each field period. The columns which have not changed significantly can 
have the correction applied to them during the next field while the ones 
which have changed significantly can be excluded from the correction. 
Therefore, in order for compensation to be applied to the picture elements 
in column k during field s+1 the following condition is applied: 
EQU .vertline.A.sub.s+1(k,n) -B.sub.s+1(k,n) .vertline..ltoreq.d (4.3) 
This condition is determined at the "compare" stage in FIG. 4. The value of 
d can vary from 0 (most sensitive) to the theoretical maximum (effectively 
disabled). A correction on/off bit for each column must be stored for the 
duration of the field period. This can be held in the linestore, along 
with the sums of each column. 
FIG. 5 schematically illustrates for the purposes of comparison with that 
of FIG. 3 an example form of circuit implementation for the compensation 
unit 40 in this embodiment. This example shows how .SIGMA.Vc, i.e. sum A, 
is generated for supply to the LUT 50. From this Figure, it can be seen 
that a main difference is that rather than having a field store in the 
form of a field delay RAM, 44 in FIG. 3, the circuit in this embodiment 
has, in effect, a negative field delay in the form of an inverter 70 to 
which digital representations, V data (x,y), of the analogue video are 
supplied, and also to the LUT 50, and from which the `future` data, that 
is the predicted data for the subsequent field and comprising inversions 
of the present data are supplied to the LUT 45. The new .SIGMA.Vc, i.e. A, 
is corrected, at 71, once every field period by the sum B. This correction 
involves the comparison stage, as described with reference to FIG. 4, and 
formula 4.3 in which the value d is determined and as a result of which an 
`on` or `off` correction control signal (bit) cs, depending on the value 
of d, is supplied to the linestore RAM 49 to indicate whether or not 
corrected data signals are to be used, for the particular column 
concerned, in the next field period. 
The form of circuit implementation in FIG. 5 is intended for simple 
comparison purposes. A practical implementation of the circuit is shown in 
FIGS. 6 and 7, with FIG. 6 illustrating how the sum A would be generated 
and used to provide corrected data signals in practice. With regard to 
FIG. 6, video data, V data, from the processing circuit 24 and fed to an 
input is supplied both to the LUT 50 and an inverter 80 whose operation is 
controlled by an input signal L according to whether the line concerned is 
a positive or negative line. The output from the inverter 80 is fed to a 
look-up table 82, corresponding to the LUT 45 in FIG. 3, from which the 
values Vc are derived. The values of Vc are then supplied to a times minus 
two multiplier 83 to give the value of -2 Vc which is then used in the 
adder 48, together with the old Sum A obtained from the linestore 49, 
(having regard to the requirement of equation 4.1), to provide a new 
.SIGMA.Vc (Sum A). 
Referring to FIG. 7, the predicted Sum B, used to correct sum A at the end 
of each field period is calculated by taking the values of Vc from the 
output of LUT 82 and applying them to an adder 90 to which the old 
.SIGMA.Vc, (B), from row zero to the row concerned, obtained from the 
linestore 49 is also supplied to provide a new .SIGMA.Vc which is then 
written into the linestore 49 and also fed at the end of each field period 
to a two's complement inverter 91 which inverts all the bits of the sum 
and adds one to the result and whose output values, Sum B, are then used 
as a fresh prediction to correct the running sum A at the end of each 
field. The old .SIGMA.Vc values applied to the adder 90 are reset to zero 
at the start of each field, as signified in FIG. 7 by the reset signal R. 
The sum .SIGMA.Vc.sup.2 also input to the LUT 50 is generated using a 
similar circuit except that values of Vc.sup.2 rather than Vc are given by 
an LUT equivalent to LUT 82. As mentioned previously in relation to the 
FIG. 3 embodiment, it might be possible instead to derive an approximation 
for .SIGMA.Vc.sup.2 from .SIGMA.Vc. This would lead to a reduction in the 
amount of circuitry needed but less accurate compensation should be 
expected. 
In the above-described embodiments, the adjustment effected for each 
picture element is based on the data signal levels for all other picture 
elements in the same column. The nature of the circuits 40 in both 
embodiments and their manner of operation makes this reasonably 
straightforward to achieve. However, using for example alternative kinds 
of adjustment circuits, it is possible that the adjustment of the data 
signal voltage for a picture element may be accomplished using less than 
all the data signals which are intended to be applied to the same column 
conductor in the period following addressing the picture element and its 
next addressing. Using the data signals for a proportion of the other 
picture elements would provide less reduction in cross-talk but 
nevertheless could give results which are acceptable and adequate in 
certain situations. 
From reading the present disclosure, other modifications will be apparent 
to persons skilled in the art. Such modifications may involve other 
features which already known in the field of liquid crystal display 
devices and which may be used instead of or in addition to features 
already described herein.