Patent Publication Number: US-6700562-B1

Title: Active matrix liquid crystal display devices

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an active matrix liquid crystal display device comprising a row and column array of liquid crystal display elements, each display element having an associated switching device, sets of row and column address conductors connected to the display elements via which selection signals and data signals, respectively, are applied to the display elements, a row drive circuit for applying selection signals to the set of row address conductors and a column drive circuit for applying data signals to the set of column address conductors, which column drive circuit is operable to transfer the data signals for the display elements of a row to groups of column address conductors in sequence, in respective group address periods, each group comprising a plurality of column address conductors. 
     Active matrix liquid crystal display devices, are well known, typical examples of such, and the general manner in which they operate, are described in U.S. Pat. No. 5,130,829. In these devices, display element electrodes are provided on a first substrate together with the switching devices, in the form of TFTs (thin film transistors), and sets of row and column address conductors. A second substrate carrying a transparent common electrode is arranged spaced from the first substrate and LC (liquid crystal) material is disposed between the two substrates. Each He display element electrode is connected to the drain electrode of its associated TFT. The gates of all of the TFTs in a row of display elements are connected to a respective row address conductor and the source electrodes of all the TFTs in a column of display elements are connected to a respective column address conductor. A row drive circuit connected to the set of row address conductors scans the row conductors by applying a selection (gating) signal to each row conductor in sequence to turn on the TFTs of a row of display elements, and a column drive circuit connected to the set of column conductors applies data signals to the column conductors in synchronism with the scanning of the row conductors by the row drive circuit, whereby the display elements of a selected row are charged via their respective TFTs to a level dependent on the value of the data signal on their associated column conductors to produce a required display output. The rows 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, and the array of display elements is repeatedly addressed in a similar manner in successive field periods. 
     For convenience of manufacture and compactness, the row and/or column drive circuits in some display devices, and especially those using polysilicon TFTs, have been integrated on the substrate carrying the TFTs peripherally of the display element array using the same large area electronics technology as that employed for the active matrix circuitry of the array with the circuitry of the drive circuits being fabricated simultaneously with this circuitry and similarly comprising TFTs, conductor lines, etc. Due to certain limitations in operational performance of the TFTs and the kinds of circuit possible when using TFTs, the column drive circuit is customarily provided in the form of a simple multiplexing circuit, examples of which are described in U.S. Pat. No. 4,890,101, the paper entitled “Fully Integrated Poly-Si TFT CMOS Drivers for Self-Scanned Light Valve” by Y. Nishihara et al in SID 92 Digest, pages 609-612, and in the paper entitled “A 1.8-in Poly-Si TFT—LCD for HDTV Projectors with a 5-V Fully Integrated Driver” by S. Higashi et al in SID 95 Digest, pages 81 to 84. This type of column drive circuit operates in the manner described in the opening paragraph, the operation being based on a multiplexing technique in which analogue video information (data) is sequentially transferred via multiplexing switches from a plurality of video input lines, to which video information is applied simultaneously, to corresponding groups or blocks of column address conductors in the display with each column conductor in a group being connected via a multiplexer switch to a different video input line. Each column address conductor is connected to a respective output of the circuit and typically in these circuits, the operation is such that an output associated with one column conductor becomes high impedance prior to, or while, the data signal for an adjacent column conductor is applied. During a row address (video line) period the multiplexer circuit operates to charge each group of column conductors in turn until all the column conductors in the display device have been charged to a level corresponding to the level of the video information on the input lines. Once a group of column conductors has been charged the associated multiplexing switches open and the column conductors become high impedance nodes with the voltage applied being maintained on the column conductor capacitance, and then the next group is charged. The circuit operates in this manner so as to charge all the groups in sequence and to drive each row of display elements in turn in this way during respective row address periods. 
     Whilst the provision of an integrated, multiplexing type, column drive circuit has benefits with regard to the simplication of fabrication of the display device, it has been found that problems can occur in the display output from the display element array during operation of the device. Particularly, certain columns in the array may show errors by virtue of the display elements in these columns having incorrect drive levels which results for example in a lack of display uniformity when displaying uniform grey fields that manifests itself as highly visible vertical lines in the displayed image. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an active matrix display device of the kind using a column drive circuit which operates in the manner of a multiplexing circuit in which the problem of the aforementioned undesirable display output artefacts is overcome or reduced at least to some extent. 
     According to the present invention there is provided an active matrix liquid crystal display device of the kind described in the opening paragraph which is characterised in that the column drive circuit is arranged to apply during an address period for one group a pre-charging signal to the adjacent column address conductor of the next-addressed group according to the value of a data signal for a column address conductor in that one group. Such pre-charging of a column conductor in a group at the time the preceding group is being charged is intended to reduce the size of the voltage transition on that column conductor occurring when that column conductor is charged in the subsequent group address period. As a result of this pre-charging, it has been found that the extent of the unwanted display artefacts is considerably reduced. 
     The invention stems from an appreciation of the reason for these display artefacts when using a multiplexing type of column drive circuit. Capacitive couplings can occur between adjacent column address conductors, either directly or indirectly. Where each column conductor extends between facing edges or sides of an adjacent pair of display element electrodes in a row, capacitance coupling between an adjacent pair of column address conductors indirectly via the electrode can be significant. Direct capacitive coupling between two column conductors can occur in the case of an alternative lay-out in which pairs of column conductors are provided adjacent one another and columns of display element electrodes are provided to either side of the pair, one column of electrodes being addressed via one of column conductors and the other addressed via the second column conductor. The presence of such indirect or direct capacitance means that as the voltage on the first column conductor of one group is changed in operation of the column drive circuit, the change in voltage can be coupled onto the last column conductor of the previously addressed group through this capacitance, thereby disturbing the voltage set on that last column conductor. This results in errors occurring in the voltage on the last column conductor of each group (apart from the last) which errors cause the aforementioned visible vertical lines in the displayed image. The problem is particularly apparent in high aperture type display devices, in which the display element electrodes are carried on an insulating layer that extends over the active matrix circuitry, comprising the TFTs and sets of row and column address conductors, on the substrate and in which portions of the display element electrodes are arranged to overlap partially the two adjacent column address conductor (and row address conductors) so as to increase their effective apertures. Such overlap can result in significant capacitance existing between a column address conductor and the adjacent portions of the display element electrodes. 
     By pre-charging, in accordance with the invention, the column conductor causing this interference, the magnitude of the voltage transition occurring on that column conductor when subsequently addressed by the column drive circuit is reduced, thereby reducing the error caused on the adjacent column conductor in the prior—addressed group. 
     For the greatest benefit the column conductor is preferably pre-charged to a value close to its expected final voltage, as dependent on the value of the intended data signal for that column conductor, and the pre-charging is effected in a way such that the voltage transition and the induced error tends to zero for plain fields where the visible effects of the induced error are most significant. 
     In a preferred embodiment, the data signals are transferred from a plurality of video input lines via multiplexing switches whose outputs are each coupled to a respective column conductor, as in known multiplexing column drive circuits, and the said adjacent column conductor of the next-addressed group is connected to a video input line via a supplementary switch, in addition to its associated multiplexer switch. The supplementary switch is arranged to be operated at the same time as the multiplexer switches of the previously addressed group. The supplementary switch and the multiplexer switch associated with the column conductor preferably are connected to the same video input line. Thus, when that preceding group is selected, the first column conductor in the next group is charged through the supplementary switch to the potential level existing on its associated video line at that time. Thereafter, when the next group is selected the first column conductor of this group is again addressed, via its respective multiplexer switch, and charged to the potential which then exists on the video line, i.e. the intended data signal level. The voltage transition on this first column conductor in the subsequently addressed group then corresponds just to the difference in the potential on its associated video line between successive time periods in which the two groups are selected. In the case of a plain field display, the video line voltage remains constant so the difference will be zero. In some cases, for example when a column inversion drive scheme is employed, it may be desirable to connect the supplementary switch to a video input line different from that to which the multiplexing switch is connected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of active matrix display devices in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a simplified schematic circuit diagram of an active matrix LC display device; 
     FIG. 2 illustrates schematically the lay-out of the display element electrodes and row and column address conductors in a typical part of a known active matrix LC display device of the high aperture kind; 
     FIG. 3 illustrates schematically a part of a known multiplexing type column drive circuit, together with some column conductors and their associated capacitances; 
     FIG. 4 shows the equivalent circuit of a part of the display element array of the display device of FIG. 1; 
     FIG. 5 illustrates typical drive waveforms present in operation of the display device; 
     FIG. 6 shows schematically a part of the column drive circuit of an embodiment of a display device according to the present invention; and 
     FIG. 7 illustrates an example of column voltage and control signal waveforms in operation. 
     It will be appreciated that the Figures are not drawn to scale and that certain dimensions may have been exaggerated whilst other dimensions may have been reduced. The same reference numerals are used throughout the Figures to denote the same or similar parts. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a simplified schematic circuit diagram of a generally conventional active matrix liquid crystal display device comprising a row and column array of liquid crystal display elements  10  is shown. The display elements each have an associated TFT  12  acting as a switching device, and are addressed via sets of row and column address conductors  14  and  16 . Only a few display elements are shown here for simplicity. In practice there can be several hundred rows and columns of display elements. The drain of a TFT  12  is connected to a respective display element electrode  18  situated adjacent the intersection of respective row and column address conductors, while the gates of all the TFTs associated with a respective row of display elements  10  are connected to the same row address conductor  14  and the sources of all the TFTs associated with a respective column of display elements are connected to the same column address conductor  16 . The sets of row and column address conductors  14 ,  16 , the TFTs  12 , and the picture element electrodes  18  are all carried on the same insulating substrate, for example of glass, and fabricated using known thin film technology involving the deposition and photolithographic patterning of various conductive, insulating and semiconductive layers. A second glass substrate, (not shown) carrying a continuous transparent electrode common to all display elements in the array is arranged spaced from the substrate  25  and the two substrates are sealed together around the periphery of the display element array and separated by spacers to define an enclosed space in which liquid crystal material is contained. Each display element electrode  18  together with an overlying portion of the common electrode and the liquid crystal material therebetween defines a light-modulating display element. 
     Both the general structure and operation of this device follow conventional practice, for example as described in U.S. Pat. No. 5,130,829 whose contents are incorporated herein. Scanning (gating) signals are applied to each row address conductor  14  in turn by a row drive circuit  30 , comprising for example a digital shift register, and data signals are applied to the column conductors  16 , in synchronisation with the gating signals, by a column drive circuit  35 . Upon each row conductor being supplied with a gating signal, the TFTs  12  connected to that row conductor are turned on causing the respective display elements to be charged according to the level of the data signal then existing on their associated column conductors. After a row of display elements has been addressed in a respective row address period, corresponding for example to the line period of an applied video signal, their associated TFTs are turned off, upon termination of the gating signal, for the remainder of the field period in order to isolate electrically the display elements and ensure the applied charge is stored on the LC capacitance to maintain their display outputs until they are addressed again in a subsequent field period 
     For a transmissive mode of operation, the display element electrodes  18  are formed of a light transparent conductive material such as ITO and the individual display elements serve to modulate light, which may be directed onto one side of the device, e.g. the substrate  25 , from a backlight, according to their applied data signal voltage so that a display image, built up by addressing all the rows of display elements in the array, can be viewed from the other side. For a reflective mode of operation, the display element electrodes  18  are formed of light reflecting conductive material such as a metal, and light entering the front of the device through the substrate carrying the common electrode is modulated by the LC material at each display element and, depending on their display state, reflected by the reflective display element electrodes back through that substrate to generate a display image visible to a viewer at the front of the device. 
     An example of a typical physical arrangement of the display element electrodes and row and column address conductors in a portion of the array is depicted schematically in FIG.  2 . The TFTs  12  are omitted here for the sake of clarity, but are located adjacent the intersection of the row and column conductors associated with the display element concerned. The individual display element electrodes  18  are labelled Pn,m where n and m denote their respective row and column numbers. Thus, the electrode Pn,m is addressed via associated row and column conductors Rn and Cm, the electrode Pn,m+1 is addressed via the row and column conductors Rn and Cm+1, the electrode Pn+1, m is addressed via the row and column conductors Rn+1 and Cm, etc. In this particular example, the display device structure is of the kind providing a high aperture. To this end, the display element electrodes  18  are carried on layer of insulating material, for example of silicon nitride or an organic material such as polyimide or resist, that is disposed over the active matrix circuitry, comprising the sets of address conductors and the TFTs carried on the substrate, and are extended so as to partly overlap at their opposing vertical side edges the adjacent column conductors  16  and at their top and bottom edges the adjacent row conductors  14 , as shown in FIG.  2 . As will be apparent, therefore, each column conductor is overlapped by portions of the display element electrodes in two adjacent columns of display elements. Each display element electrode  18  is connected to the drain of its associated TFT underlying the insulating layer through a contact opening (not shown) formed in the insulating layer. The individual display element electrodes  18  are separated from their neighbours by a small gap lying over the row and column conductors. Examples of this type of structure are described in U.S. Pat. No. 5,641,974 and EP-A-0617310 to which reference is invited for a more detailed description. 
     The row and column drive circuits  30  and  35  are for convenience and simplicity integrated on the substrate  25  and fabricated simultaneously with the active matrix array, comprising the TFTs and the sets of row and column address conductors, using the same thin film processing technology. Integrated drive circuits are well known, examples of such being described in the aforementioned papers. Normally polysilicon technology is used, although amorphous silicon technology can be employed instead in certain cases. The integrated column drive circuit  35 , is provided in the form of a multiplexing type of circuit. The general operation of such a circuit is based on a multiplexing technique in which analog video information is sequentially transferred from a plurality of video input lines to groups of a corresponding number of the column address conductors in the display device. The video information is transferred via multiplexing switches which may consist of NMOS TFTs, PMOS TFTs or CMOS transmission gates. The switches, which each constitute an output of the circuit associated with a respective column conductor, are operated in groups and when a group of switches is turned on the corresponding columns are charged to the data signal voltage levels then existing on the respective video lines. When the switches turn off, the voltages on the column conductors are stored on the capacitance of the column conductors and any additional storage capacitors which may be connected in parallel with them. During a video line row address period each group of multiplexing switches is turned on in sequence until all of the columns of display elements have been charged with the appropriate video information. 
     FIG. 3 illustrates in simplified, schematic, form a part of a known multiplexing column drive circuit  35 . In this relatively simple example, there are three video input lines, V 1 , V 2  and V 3 , to which parallel input video signals are applied, and the multiplexing switches,  36 , are arranged in groups of three with their outputs connected to respective consecutive column address conductors  16 . A control circuit  37 , comprising a shift register, which may or may not be integrated on the substrate  25  with the multiplexing circuit, sequentially selects each of the groups of multiplexing switches using the control signals G 1 , G 2 , G 3 , etc so that at the end of the video line period all of the columns in the array have been charged. When G 1  goes high the first three multiplexer switches  36  close and the first three columns S 1 , S 2  and S 3  are charged to the voltage level on the video lines V 1 , V 2  and V 3  respectively. G 1  then goes low, the three multiplexer switches open, and the columns S 1 , S 2  and S 3  become isolated from the video lines. The voltage on the columns is then stored on the column capacitance. Next, the control signal G 2  goes high, closing the next group of switches  36 , and the second group of three columns, S 4 , S 5  and S 6 , is charged to the voltage then existing on the video lines. The operation of the multiplexing circuit continues in this way with each group of columns being charged appropriately in succession until all the column conductors in the array have been charged. 
     In the operation of display devices using a multiplexing column drive circuit, problems have been experienced with display artifacts occurring in the form of visible vertical lines at regular intervals. It has been determined that these artifacts are caused by voltages being unintentionally capacitively coupled onto particular column conductors in the operation of the column drive circuit which result in an error in the voltages of the display elements associated with those column conductors and hence their output brightness. Such capacitive couplings arise due to the fact that the column address conductors  16  extend adjacent to columns of display element electrodes  18 . As a consequence, significant capacitance exists between a column address conductor and the adjacent display element electrodes. This is particularly the case in a high aperture type of display element lay-out in which the display element electrodes partially overlie the column conductors. 
     This effect will be described with reference also to FIG. 4 which shows an approximate equivalent circuit for a typical small number of display elements in the array, and to FIG. 5 which illustrates examples of certain voltage waveforms in operation of the circuit of FIG.  3 . The display elements of the display device each contain a number of capacitances, some of which are shown in FIG. 4. C 1  and C 2  represent the capacitance between display element electrode  18  and the two adjacent column conductors  16 . C 3  represents the display element capacitance, which may be a combination of the liquid crystal capacitance and a display element storage capacitor. C 4  represents the capacitance of the column conductor and will include the capacitance between the column conductor and the row conductor and the capacitance between the column conductor and the common electrode of the display array and the gate—source capacitances of the TFTs. Other capacitances may also be present and may contribute to the effects described here but have been omitted for clarity. 
     The source of the column voltage errors will become apparent by considering the effect of a change in the video information on, for example, column conductor S 2 . This change in voltage is coupled onto the display element capacitor C 3  via capacitor C 2  and therefore causes a change in the display element voltage. If the voltage on column conductor Si is not being maintained by the column drive circuit, i.e. the relevant column drive circuit output becomes high impedance and column conductor Si is floating, then this change in display element voltage can be further coupled onto the column conductor S 1  via the capacitor C 1 . This coupling of a change in voltage on one column conductor onto an adjacent column conductor can be denoted by a coupling factor Kc. If the voltage on the first column conductor changes by an amount ΔV, then the change in voltage that this produces on the second column conductor is given by KcΔV. This effect will be further explained with reference also to FIGS. 3 and 5. It is assumed here for simplicity that the display array is being addressed with a uniform grey field, and that row inversion, as well as field inversion of the polarity of the video information drive voltage is used, and also that the column drive circuit  35  has just three video input lines. Similar effects will occur for other inversion methods, for circuits with different numbers of video lines and when the displayed video information is more complex. In FIG. 5, G 1 , G 2  and G 3  are the control waveforms applied to the first three groups of multiplexer switches  36  which include voltage signals for turning on these switches, and S 1  to S 9  are the voltage waveforms appearing on the first nine column conductors. As the display is showing a uniform grey field, the voltage waveforms applied to the three video lines, V 1  to V 3 , are the same, as signified in FIG.  5 . The polarity of the video signals inverts after each video line period (TI). The control circuit  37  within the column drive circuit sequentially selects each of the groups using the control signals G 1 , G 2 , G 3 , etc so that at the end of the video line period TI all of the columns in the display have been charged. When G 1  goes high the first group of three column conductors S 1 , S 2  and S 3  is charged to the voltage level on the respective video lines. When the second group is selected immediately thereafter by G 2  going high then as the voltage on column conductor S 4  changes this change is coupled, with reduced amplitude, onto the display element electrodes of the display elements in the third column, represented by the node p 3  in FIG.  3 . The change in display element voltage is further coupled onto the last column conductor, S 3 , in the previously addressed group since this conductor is now isolated from the video lines. This results in an error in the column voltage as indicated in the voltage waveform for S 3  in FIG.  5 . The voltage change is also coupled further to column conductor S 2  via the display element node p 2  and then to column conductor S 1  via node p 1 . However, at each stage of coupling the magnitude of the coupled signal is reduced by the factor Kc. It is therefore the error in the voltage on S 3  which is of the most importance to the uniformity of the displayed image. 
     Once the second group of three columns has been charged the signal G 2  goes low and the second set of multiplexer switches turns off. When the third group of columns is charged by G 3  going high, coupling of the change in voltage on column conductor S 7  causes an error in the voltage on column conductor S 6 . This effect occurs in a similar way for each group of columns in the array so that in general the last, or end, column in each multiplexer group will be subject to a significant voltage error due to the voltage change on the adjacent column conductor in the next group, i.e. the first column conductor in the subsequently selected group. This error, which is a kind of cross-talk, manifests itself in the form of vertical lines being visible in the displayed image, the pitch of the lines corresponding to the width of the multiplexer groups. 
     In order to avoid, or significantly reduce, the visibility of such errors, the first column in one group is pre-charged before the group concerned is actually selected by the control signal associated with that group going high and during the period in which the immediately preceding group, in terms of selection order, is selected. The pre-charging is such as to minimise the amplitude of the voltage transition on the first column of that group, and hence the amplitude of the error voltage capacitively coupled onto the last, end, column of the preceding group, i.e. the group selected immediately before. In particular, the error voltage is reduced at least close to zero in plain areas of the display image where the brightness is not varying, and which are the parts of the image where the error is most highly visible. Desirably, the column is pre-charged to a value close to its expected final voltage, as determined by the value of the data signal intended for that column. 
     FIG. 6 shows schematically part of the column drive circuit in a preferred embodiment of display device according to the present invention for achieving this objective. The part of the circuit shown comprises two adjacent multiplexer groups, each again coupled to three successive column conductors. The six multiplexer switches  36 , comprising TFTs, are labelled T 1  to T 6 . TFTs T 1  to T 3  are the multiplexer switches of a first multiplexer group operable by a control signal G 1  and TFTs T 4  to T 6  are the multiplexer switches of the succeeding, i.e. next-addressed, multiplexer group operable by the control signal G 2 . In this circuit, an additional switch, again in the form of a TFT, is connected between the first column conductor in each group, (apart from that of the first group in the sequence), and the appropriate video line in parallel with the multiplexer switch associated with that column conductor. Thus, referring to FIG. 6, a supplementary TFT, Tx, is connected in parallel with the TFT T 4  between the column S 4  and the video line V 1 . Unlike, the multiplexer switch T 4  (and also T 5  and T 6 ) which is operated by the control signal G 2  applied to its gate, the gate of the supplementary TFT Tx is connected to the control line carrying the control signal G 1  and hence is operable simultaneously with the TFTs T 1  to T 3  of the preceding group. Each multiplexer group, apart from the first, has a supplementary TFT connected in this way so that, in general, the first column conductor in the Nth group has a supplementary TFT connected thereto which is operable by means of a control signal for the (N−1) th  group. 
     The operation of the column drive circuit  35  in respect of these two groups is illustrated in FIG. 7 in which Vf and VI represent respectively the voltage appearing on the first column of one group, e.g. the column S 4  in the (N+1) th  group, and the voltage on the last column of the preceding group, e.g. the column S 3  in the N th  group. These two column voltage waveforms are shown offset vertically in FIG. 7 for the sake of clarity. When the selection signal for the first group, i.e. the N th  group, is active (i.e. the control signal G 1  for that group goes high), all the columns of the N th  group, S 1  to S 3 , are charged to their appropriate data signal levels on the video lines V 1  to V 3  via the TFTs T 1  to T 3  respectively. Thus, V 1 , (S 3 ), increases to a certain value according to the level on V 3 . Simultaneously, the first column, S 4 , of the next, (N+1) th  group is also charged to the potential of the video line V 1  via the TFT Tx so that Vf, (S 4 ), also rises during this period. When this next, (N+1) th , group is immediately thereafter selected by G 2  going high, the voltage transition on the first column S 4  now only corresponds to the difference in the potential on the video line V 1  between the adjacent time periods corresponding to the selection periods of the control signals G 1  and G 2 . The change in Vf in this period, Vstep, and the voltage error, Verr, consequently induced on the column S 3  are minimised. 
     In a plain field, where the effects of any errors in the column voltages would be most noticeable, the video line voltages, determined by the video signal applied to the column drive circuit  35 , will be constant and so the first column of block N+1, i.e. S 4 , is charged to the correct voltage via Tx. Thus, the voltage step, Vstep, will be zero and likewise the error voltage, Verr, will be zero. In other cases, where there is a change in voltage on the relevant video line, then Vstep will not actually be zero. However, it will be smaller than the step corresponding to the full transition from a positive signal to a negative signal and, because it is in an area of the picture where there is a horizontal gradation in image brightness, the remaining small error will not be visible. The overall effect is that the size of Vstep, and hence the size of Verr, is proportional to the difference in voltage between the last column in the N th  group and the first column in the (N+1) th  group. As this same voltage determines the difference in brightness in display elements in a horizontal direction, the effect of the error will be invisible as it is small where the brightness changes little and only large where there is a large brightness change which will hide the error. 
     The arrangement shown in FIG. 6 is suited to the case where the display is driven in any mode where the polarity of the video signal on the input video lines does not change during the line period. This is always the case if the display is driven in a field or row (line) inversion mode and in some implementations of pixel and column inversion. However, if the display is driven in column or pixel inversion with the inversion applied by periodically inverting the incoming video signals on each input video line, then the TFT, Tx, needs to be connected to a different video line to the arrangement shown in FIG.  6 . The reason can be seen by considering the arrangement of FIG.  6 . If along a given row, column S 1  is, for example, to be charged positive, column  32  negative, column S 3  positive etc, then the polarity of the video line V 1  will be positive during the G 1  group select period but negative during the G 2  group select period. This means that column S 4  would be pre-charged to the wrong polarity (positive) during the G 1  group select. This will produce a large voltage transition on column S 4  when group G 2  is selected (and charged negative) giving the same undesirable high level of coupling onto column S 3  as would occur if Tx were not present. The solution in such a cases is to connect the extra TFT, Tx, to one of the video lines which has the same polarity during the G 1  group select period as V 1  has during the G 2  group select period. In this case, this can be achieved by connecting it to video line V 2 . 
     In a colour display device, colour filter elements are carried on the other substrate in conventional manner and in this case the video input lines V 1 , V 2  and V 3  may each carry a respective colour, red, green and blue, video information component with adjacent columns in the array being arranged to display red, green and blue information. 
     While the invention has been described in relation to a kind of display device structure in which the display element electrodes are carried above the active matrix circuitry on an insulating layer in particular, it is applicable to other types of display structures in which the electrodes  18  are situated at a similar level to, and laterally of, the TFTs and sets of addressed conductors, for example of the kind described in U.S. Pat. No. 5,130,829. 
     The part of the column drive circuit  35  which supplies the video signal to the video input lines (e.g. V 1 , V 2  and V 3 ) and the control circuit  37  which applies control signals, G 1 , G 2 , G 3 , etc to the multiplexer switches need not be integrated on the substrate  25  but instead may be formed separately and connected to the multiplexing circuit on the substrate. 
     Moreover, while it is particularly convenient for at least the multiplexing circuit of the column drive circuit to be fully integrated on the same substrate as the active matrix circuitry, this part of the drive circuit, and likewise the row drive circuit, could be fabricated as a separate component and electrically interconnected with the active matrix circuitry, for example, using chip-on-glass technology. 
     It is envisaged that the invention can be used beneficially in display devices using column drive circuits other than of the multiplexing type but which likewise operate in such a way that an output associated with one column conductor becomes high impedance before or while an adjacent column conductor is being supplied with a data signal as similar problems would be experienced. 
     From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the field of active matrix liquid crystal display devices and component parts thereof and which may be used instead of or in addition to features already described herein.