DE69321804T2 - LIQUID CRYSTAL DISPLAY WITH AN ADDRESSING SCHEME FOR REACHING HIGH CONTRAST AND HIGH BRIGHTNESS WITH MAINTENANCE OF FAST SWITCHING - Google Patents

Description

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for supplying data signals to the column electrodes from a first state to an optically different second state, the control circuit applying data voltages of magnitude ±Vd to the column electrodes over the period of time during which a row electrode is scanned in such a way that the optical state brought about in a display element is determined by the so-called root mean square (RMS) voltage value across the element in question.

The RMS voltage value V₂ for the non-selected display elements, i.e. the display elements in the ON state, is given by:

V2² = (Vs + Vd)²/N + (N - 1) · V²d/N. (1)

The RMS voltage value V₁ for the non-selected display elements, i.e. the display elements in the OFF state, is given by:

V1² = (Vs - Vd)²/N + (N - 1) · V²d /N. (2)

Fig. 2 shows in diagrammatic form a transmission-voltage characteristic of an image cell belonging to that display device.

Alt & Pleshko have derived relationships which, for a given value of the ratio S = V₂/V₁ (sometimes called the threshold slope of the transmission-voltage characteristic), give the maximum number of lines Nmax that can be addressed by this method while maintaining a given contrast value, and the manner in which the line select pulse voltage Vs and the data voltages ±Vd must be selected to achieve this. These relationships are as follows:

Nmax = {(S² + 1)/(S² - 1)} (3)

(V₂/Vd)² = Nmax (4)

V²d = V1² *{0.5/1 - Q)},

where Q² = Nmax⊃min;¹.

By then choosing the row selection voltage Vs and the data voltage Vd according to expressions (2) and (3), the resulting RMS voltage when Nmax rows are used across a selected pixel is equal to V2, and the resulting RMS voltage across a non-selected pixel is equal to Vj. A larger multiplexing rate, in other words a higher value for Nmax, requires a larger slope of the transmission-voltage characteristic, i.e. a value for the quantity S = V2/V1 closer to 1.0.

By means of the currently known (and already used) so-called "SUPER-TWISTED" liquid crystal effects it is possible to obtain very high values for Nmax, since the threshold slope S of the transmission-voltage characteristic of these effects has a value that is very close to the limit value 1.0. Fig. 1 shows in diagrammatic form a part of a matrix-oriented display device 1 with Nmax selection lines (row electrodes) 2 and describes in principle the functioning of the RMS multiplex addressing method mentioned earlier.

The information to be displayed is fed to the data lines (column electrodes) 3. The display elements 4 are located at the intersection points of the selection lines 2 and the data lines 3. Depending on the information fed to the data lines 3, the display elements 4 are in an EIT state or an OFF state.

In synchronism with the selection of the rows or row electrodes by means of the row selection voltage Vs (selected according to expressions (4) and (5)), the image information (data voltage ±Vd) is applied across the column electrodes. Thus, from the time t1 and over a period of time t1 (sometimes referred to as the row time), the row 2a is selected, which together with the information then present on the data lines 3a, 3b, 3c (i.e. +Vd) determines the optical state of the pixels 4aa, 4bb, 4cc.

During this time period t1, when the row 2a is selected, all other pixels corresponding to the row electrodes 2b, 2c, etc. are at a voltage ±Vd.

From a time t₂ (where t₂ - t₁ = t₁), row 2b is selected for a time period t₁. The information then present on data lines 3 (i.e., ±Vd) determines the state of pixels 4ba, 4bb, 4bc.

After the line time t₁, the next line is selected. Thus, the entire image is written line by line. After selecting the last line of the matrix, the entire cycle is repeated (so-called "repeated scanning procedure"). The duration of a single write cycle is called the field time tf: tf = N · t1, where N represents the number of lines that are scanned one after the other.

An important point in this RMS addressing method is that both the rise time and the fall time (in other words the switching time for the transition to the "ON" or "OFF" state, respectively) of the optical effect are much longer than the field time.

Under these circumstances, the display element responds to the cumulative effect of a number of addressing pulses (or selection pulses). In particular, under these conditions, a liquid crystal display element responds in the same way as if it had been addressed by a sinusoidal signal or a step-shaped signal or the like having the same RMS voltage value as that of the "ON" and "OFF" voltages V₂ and V₁ given by expressions (1) and (2).

As already discussed, the maximum number of selection lines Nmax is related to the value of the ratio V₂/V₁ (threshold slope).

As the multiplex rate increases, increasingly higher voltages are required when the RMS multiplex addressing method described above is used.

In particular, the row selection voltage Vs becomes large (the data or column voltage Vd in this addressing scheme should always be chosen lower than the threshold voltage of the optical effect).

The high selection voltages result in the liquid crystal effect no longer responding to the RMS voltage value (average RMS voltage over a field time), but instead the pixel shows an optical response determined by the "instantaneous" voltage value sampled by the element in question during the line selection time. Fig. 3 shows in diagrammatic form the transmission behavior over a field time of such a display element in the "ON" state.

The "ON" state is already reached during the line time t1, during which the element in question samples a voltage of the magnitude Vs + Vd, since the "ON" switching time becomes less than or equal to the line time at sufficiently high voltages. After selection, i.e. after the line time t1, the pixel in question only samples a voltage ±Vd, which is less than the threshold voltage. This means that the pixel in question returns to its "OFF" state during the remaining field time. In Fig. 3, for the sake of simplicity, it is assumed that the fall time (in other words the "OFF" switching time) is the same as the field time.

This characteristic, the so-called "non-RMS" transmission behavior, has been observed, among other things, with liquid crystal display devices having "super-twisted" configurations, and in particular in the case of displays with very thin liquid crystal layer thicknesses. It is known that the switching times of a liquid crystal effect depend strongly on the layer thickness. As the layer thickness becomes smaller, the switching time decreases. See, among others, Okada et al. in SID Digest of Technical Papers XXII, 1991, pages 430 to 433.

The indicated transmission behavior (often referred to as "PARTIAL IMAGE RESPONSE") in Fig. 3 results in a loss of brightness and contrast compared to a liquid crystal effect that responds in a true RMS manner.

The IEEE conference paper of the 1988 International Display Research Conference, 1988, New York; T.N. Ruckmongathan: 'A generalized addressing technique for RMS responding matrix LDCs', pages 80 to 85 discloses an addressing method in which more than one address line is selected simultaneously. This requires a lower supply voltage and results in better brightness uniformity of pixels in comparison to conventional techniques known up to that point.

DE-A-27 52 602 discloses an electro-optical display system with a matrix of display elements and a drive circuit, the elements comprising oppositely positioned electrodes which are mutually isolated by an electro-optically activated medium, electrically connected row by row and column by column and periodically supplied with control signals. A signal independent of the display image is applied to each of the n row electrodes and a further signal is applied to column electrodes depending on the image and selected from 2n-1 signals.

According to the present invention, a display device is provided comprising a liquid crystal material between two carrier plates which are held at a defined distance and whose surfaces are opposite each other, a pattern of N row electrodes being arranged on one surface and a pattern of column electrodes being arranged on the other surface, the row electrodes crossing the column electrodes and thus forming display elements at the locations the intersections are formed, and wherein the device includes a control circuit for supplying data signals to the column electrodes and further includes a row scanning circuit for periodically scanning the row electrodes and supplying row selection voltage signals, wherein during the field time of the periodic scanning of the row electrodes a plurality of rows n are selected simultaneously during a selection time (ta), wherein the row selection voltage signals to be applied during the selection time (ta) are different for each of the rows to be selected simultaneously, wherein the amplitudes of the row selection voltage signals are the same for each of the rows to be selected simultaneously and the row selection voltage signals of the rows to be selected simultaneously are mutually orthogonal,

characterized in that the selection time (ta) for a group of n lines is divided into a number of selection time intervals distributed over the field time in such a way that the sum of the time intervals is equal to the selection time (ta) and in such a way that during the scanning of the N lines no more than n lines are selected simultaneously, the line selection voltages to be applied to each of the lines to be selected simultaneously during these time intervals being identical to the parts of the original line selection voltage signals of the lines concerned which correspond to the time intervals concerned.

An even number of rows can be selected simultaneously during a selection time, wherein one of the row selection voltage signals to be applied is a unipolar voltage existing for a period of time equal to the selection time, and the other row selection voltages are alternating polarity signals.

The invention is based on the insight that the use of a plurality of smaller pulses during the field time instead of the single large selection pulse per line is possible if a plurality of lines are selected simultaneously during the field time.

A simultaneous selection of a plurality of lines during scanning of the matrix (multiplexing) does not lead to a reduction in the maximum number of lines that can be addressed if the voltage waveform of the selection voltage and the data signals are selected in a suitable manner. For a given transmission-voltage characteristic with a slope V₂/V₁, Nmax is again according to expression (3), which was derived for the case of RMS behavior.

The simultaneous selection of a plurality of lines during the field time not only results in a selection signal with a smaller amplitude compared to a conventional RMS addressing, but at the same time provides the possibility of dividing the selection signal with its associated selection period into a plurality of separate selection pulses with a corresponding shortened pulse duration.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram illustrating a portion of a matrix-oriented display device;

Fig. 2 shows a transmission-voltage characteristic of an image cell of the display device;

Fig. 3 shows a transmission behavior of the display device over a field time;

Figs. 4a and 4b show an embodiment of the present invention;

Fig. 5a to 5c show diagrams illustrating signal waveforms for the second selection period ta/2 which is selected from a point in time following half of the field time;

Figs. 6a and 6b show another embodiment of the present invention;

Fig. 7 shows the resulting voltages of the row selection signals and the data signals for elements 11 to 22 as shown in Figs. 6a and 6b during the selection time tb;

Fig. 8a and 8b show the relationship between the selection period tb/4 and the corresponding voltage over the field time; and

Fig. 9 shows the row selection signals used to define selection periods smaller than the total selection period tc.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will now be explained in more detail with reference to a number of illustrative embodiments.

In the first illustrative embodiment (Fig. 4a and 4b) it is assumed that During field scanning, two lines are selected simultaneously.

For simplicity, it is assumed that the number of rows N of the matrix is a multiple of "two".

The selection time is then defined as the addressing time ta. Fig. 4a shows voltage waveforms for the row signals and the data signals that can be used to write the information as represented in this figure by the designations "ON" and "OFF".

After the addressing time ta, as shown in Fig. 4a, the next two rows are selected and, with the help of a column control circuit, the appropriate data voltages are applied to the columns according to the information then to be written. The field time tf in this example is given by: tf = (N/2) · ta.

The row selection voltage signals of the two simultaneously selected rows are mutually orthogonal.

In Fig. 4a, it is assumed for simplicity that two adjacent rows are selected. However, this is not necessary since any two rows of the N-row matrix can be selected simultaneously using this addressing method. Obviously, each row is always selected only during one addressing time ta for each field time.

Just like in conventional multiplex addressing as described earlier herein, "repeated sampling" takes place here.

The amplitude of the row selection voltage signals for the two rows to be selected simultaneously is equal to ±A.

Instead of the voltage waveforms shown for the two lines to be selected simultaneously, it is also possible to select voltage waveforms with a higher frequency. In this case, the data voltage signals must be adjusted and they differ from those shown in Fig. 4a.

The amplitude of the data voltages in Fig. 4a, which are applied to the columns during the addressing time ta to write the desired information, is equal to ±D during one half of the addressing time and equal to 0 during the other half of the addressing time. The actual shape of a data voltage signal is determined by the information to be displayed (i.e. the pixels 'ON' or 'OFF').

Fig. 4a indicates the four data voltage signals by which the possible information contents of the respective pixels appearing on the selected lines can be influenced using the line selection voltage waveforms shown: i.e. the combination 'OFF'/'OFF' (column 1), 'OFF'/'ON' (column 2), 'ON'/'OFF' (column 3), 'ON'/'ON' (column 4).

It is assumed that pixels 1, 2, 5, 7 of the matrix shown in Fig. 4a are in the 'OFF' state and pixels 3, 4, 6, 8 are in the 'ON' state.

To prevent DC components, it is possible, for example, to switch the polarity of the line selection voltages and the data voltages after each field time (analogous to the usual Alt & Pleshko multiplex addressing).

For simplicity, no such polarity change is shown in Fig. 4a.

Fig. 4b shows the resulting voltages across pixels 1 to 8 contained in Fig. 4a over the addressing time ta. The resulting voltage was derived as the difference voltage Vrow - Vcolumn.

Fig. 4b shows that the RMS voltage values during the addressing time ta of the pixels 1, 2, 5, 7, which are in the 'OFF' state, are equal to each other.

Similarly, the RMS voltage values of the ‘ON’ elements during ta are equal to each other.

It is possible to verify, using Fig. 4a, that any 'ON' element and 'OFF' element in the matrix has the same RMS voltage value during the remainder of the field time (i.e. during tf - ta).

The following expression can be derived for the RMS voltage value Von:

V²on = {ta · (A²/2 + (A + D)²/2) + ta · (N/2 - 1) · D²/2}/{ta · N/2}. (6)

The RMS voltage value Voff of an ‘OFF’ element is given by the following expression:

V²off = {ta · (A²/2 + (A - D)²/2) + ta · (N/2 - 1) · D²/2}/{ta · N/2). (7)

Using the "Lagrange multiplier" technique, it is possible to determine the maximum Von as a function of the voltages A and D under the condition that Voff is equal to the Threshold voltage V₁.

Then you will find the following expressions:

(A/D)² = N/4 (8)

D2 = V1² · 2 · {0.5/(1 - Q)}, (9)

where Q² = N⊃min;¹.

If equations (8) and (9) are inserted into expressions (6) and (7), the ratio Von/Voff is found to be:

(Von/Voff)² = {Q⊃min;¹ + 1}/{Q⊃min;¹ - 1} (10)

With Voff = V₁ and the definition for the slope S = V₂/V₁, it follows that the expression (10) is actually identical to the expression (3) which shows the relationship between the number of lines to be multiplexed (in this illustrative embodiment: T) and the slope of the transmission-voltage characteristic.

Comparing expressions (8) and (9) with expressions (4) and (5), it follows that the amplitude D of the data signals in case of simultaneous selection of two rows is by a factor of 21/2 larger than the data voltage Vd required according to the usual Alt & Pleshko RMS multiplex addressing (for an identical number of rows N of the matrix).

The amplitude A of the row selection voltages for simultaneous selection of two rows is smaller by a factor of 21/2 than the selection voltage Vs required according to the usual Alt & Pleshko RMS multiplex addressing (for an identical number of rows N of the matrix). All this means that the maximum voltage amplitude (A + D) of an 'ON' element in the case of the above-described addressing scheme with simultaneous selection of two rows is significantly smaller than the maximum voltage amplitude (Vs + Vd) for the same element in the case of addressing according to the usual Alt & Pleshko principle.

Consider again the voltage waveforms of the row selection signals and the data signals of Fig. 4a.

The complete addressing time ta is actually composed of two equal time periods ta/2 with associated characteristic voltage values for the row selection signals and the data signals.

In order to achieve the desired RMS voltage values for Von and Voff (according to expressions (8) and (9)), it is necessary that the selection periods ta/2 with the corresponding voltages occur directly one after the other, as assumed in the voltage waveforms in Fig. 4a.

The second selection period ta/2 can, for example, be selected from a point in time following half of the field time. This is illustrated in Fig. 5 using a matrix of 10 lines. For simplicity, only the line selection voltage signals of the 10 lines are shown during the scanning of the matrix over a field time. The situation shown is that in which two adjacent lines are selected. As mentioned earlier, this is not necessary. Fig. 5a shows the scanning cycle in which the addressing time ta has not been divided. In Fig. 5b, the line selection period ta has been divided into two time periods ta/2, with the second half of the total addressing time ta running from a point in time which is half the length of the field time. Fig. 5c shows, for comparison, the addressing scheme according to the usual RMS multiplexing method with the selection of a single line.

It is clear that it is necessary that the scanning scheme as shown in Fig. 5b adapts the control circuit for the data signals so that the appropriate data voltages are applied to the columns.

The principle of the scanning scheme according to Fig. 5b is obviously applicable not only to the 10-row matrix shown, but to any matrix with any number of rows N. In the case that N is not a multiple of 2, matrix addressing according to this principle can be carried out by inserting a so-called 'dummy' or virtual row.

The addressing scheme described in Fig. 5b results in an addressing scheme in which the single selection pulse of the usual Alt & Pleshko multiplex addressing has been replaced by two separate selection pulses with smaller amplitudes, which preferably occur at times evenly distributed over the field time. As already discussed, the maximum amplitude of the voltage across a pixel (during selection) is smaller for this addressing scheme than in the case of the usual Alt & Pleshko addressing.

Both the occurrence of a plurality of selection pulses with a smaller amplitude than that of the single selection pulse in the Alt & Pleshko addressing as well as the fact that the maximum voltage across a pixel during selection is smaller than the corresponding voltage in the Alt & Pleshko addressing scheme, have a positive influence on the reduction or elimination of "FILE RESPONSE".

In the event that the addressing scheme described above, which involves selecting two lines simultaneously and dividing the total addressing time ta into two separate selection periods ta/2 as shown in Fig. 5b, is not sufficient to reduce or eliminate "FILE RESPONSE", an addressing scheme may be chosen in which more than two lines are selected simultaneously, for example 4 or 6 or 8 etc.

However, it is not necessary that an even number of rows be selected at the same time.

In the following illustrative embodiment of the invention (Figs. 6a and 6b) it is described how addressing of an N-row matrix can take place when 4 rows are selected simultaneously during field scanning.

For simplicity, it is assumed that the number of rows N of the matrix is a multiple of 'four'. However, this is not necessary (analogous to addressing where two rows are selected at a time, as discussed earlier).

The selection time is now defined as the addressing time tb. Fig. 6a shows voltage waveforms for the row signals that can be used to write the information as shown in this figure. For simplicity, Fig. 6a shows only three columns i, j, k, which contain the associated information contents of pixels 11 to 22 corresponding to the crossing points of columns i, j, k, and the illustrated first group of four rows to be selected simultaneously. To represent the required data signals, the pixels 11, 12, 13, 14 belonging to column i are assumed to be 'OFF', 'OFF', 'ON' and 'OFF' respectively. The elements 15, 16, 17, 18 belonging to column j are 'ON', 'OFF', 'OFF' and 'ON' respectively. Pixels 19, 20, 21, 22 belonging to column k are ‘ON’, ‘ON’, ‘OFF’ and ‘ON’ respectively.

After the addressing time tb, the next 4 lines are selected. The field time tf in this example is given by: tf = (N/4) · tb.

The row selection voltage signals of the four simultaneously selected rows are mutually orthogonal such that the selection signal from one of the four simultaneously selected rows rows to be selected has a half period corresponding to the addressing time tb. In Fig. 6a, this is the first (top) row of the group of rows to be selected simultaneously. However, it is not necessary that this particular row has this row selection voltage waveform.

In Fig. 6a, for simplicity, it is assumed that a selection of four adjacent rows takes place. However, this is not necessary since any set of four rows of the N-row matrix can be selected simultaneously using this addressing method. Obviously, each row is always selected only during one addressing time tb for each field time. "Repeated scanning" takes place.

The amplitude of the line selection voltage signals for the four lines to be selected simultaneously is equal to ±B. To prevent DC components, it is possible, for example, to change the sign of the polarity of both the line selection voltages and the data signals after each field time. For simplicity, this polarity change is not shown in Fig. 6a.

Instead of the voltage waveforms shown for the four lines to be selected simultaneously, voltage waveforms with a higher frequency can alternatively be selected. In that case, the data signals must be adapted and they differ from those shown in Fig. 6b.

Fig. 6b illustrates how the data signals can be determined to write the information as set out in Fig. 6a.

In this context, consider first pixels 11, 12, 13, 14 belonging to column i. Pixel 11 should be in the 'OFF' state. To effect this, the data signal applied to pixel 11 during selection should be a data voltage signal in phase with the row select signal of the corresponding selected row. This signal is drawn in Fig. 6b in the column headed COLUMN i. The amplitude X of this signal (and the amplitude of the row select signals) is determined by the requirement that the RMS voltage value of the 'OFF' elements must have a predetermined value while the RMS voltage value of the 'ON' elements should be as large as possible. Pixel 12 should be in the 'OFF' state and, analogous to the situation described above, the data voltage signal applied to pixel 12 during selection should be in phase with the row selection signal of the corresponding row. This signal is plotted in Fig. 6b in the column headed COLUMN i. The amplitude of this signal is equal to the amplitude of the above data signal for pixel 11.

Pixel 13 should be in the 'ON' state. Therefore, during selection, pixel 13 should be applied with a data signal that is in antiphase with the row selection signal of the corresponding row. This data signal for pixel 13 again has the same amplitude as the two previously mentioned data signals of pixels 11 and 12 and is shown in the column whose heading is COLUMN i. The data signal for pixel 14 during selection follows, based on reasoning, analogous to that given for the other pixels in the relevant column i. In summary, the four data signals produce the data signal as drawn in Fig. 6b in the column whose heading is COLUMN i. During selection of the four rows to which elements 11, 12, 13, 14 belong, this signal is applied to column i. In a completely analogous manner, the data signal to be applied to column j can be determined in order to generate the required information contents of pixels 15, 16, 17, 18. Fig. 6b illustrates all these aspects by means of the voltage waveforms during selection for the respective elements and the total voltage (signal j) applied to column j during selection (see column headed COLUMN j). The column headed COLUMN k shows, for illustrative purposes, the data voltage signals and the total voltage of signal k for the respective column k (see signals drawn in the column headed COLUMN k).

When four lines are selected simultaneously, data signals should be applied in which 5 levels can be distinguished, namely ±E/2.0. The combination of these levels in a data signal applied to a column 1 during selection is determined by the image contents of the elements in the column 1 concerned.

Fig. 7 shows the resulting voltages (defined as Vline - Vcolumn) for elements 11 to 22 included in Fig. 6a during the selection time tb using the row selection signals drawn in Fig. 6a and the data signals (signal i, signal j, signal k) drawn in Fig. 6b. The RMS voltage values during the selection time tb of the 'OFF' elements 11, 12, 16, 17, 21, 14 are equal to each other. The RMS voltage values during the selection time tb of the 'ON' elements 15, 19, 20, 13, 18, 22 are equal to each other. After the selection and during the rest of the field time tf, i.e. over a period tf - tb, the RMS voltage value of each ‘OFF’ element is equal to the RMS voltage value of each ‘ON’ element.

The following expression can be derived for the RMS voltage value of an ‘ON’ element:

From² = [tb · (B² + B · E/2 + E²/4) + (N/4 - 1) · tb · E² * tb/4). (11)

The RMS voltage value Voff of an ‘OFF’ element is given by the following expression:

Voff² = [tb · (B² - B · E/2 + E²/4) + (N/4 - 1) · tb · E²/4]/(N · tb/4). (12)

Under the condition that Voff = V₁, Von can be maximized for a given N as a function of B and E.

The maximum is found:

(B/E)² = N/16 (13)

E² = 4 · V₁² · {0.5/(1 - Q)}, (14)

where Q² = N⊃min;¹.

Then the following is found for the ratio Von/Voff:

(Von/Voff)² = {Q⊃min;¹ + 1}/{Q⊃min;¹ - 1}. (15)

With Voff = V₁ and the definition for the slope S = V₂/V₁, it follows that the expression (15) is actually identical to the expression (3) which shows the relationship between the number of lines to be multiplexed (in this illustrative embodiment: N) and the slope of the transmission-voltage characteristic.

Comparing expressions (13) and (14) with expressions (4) and (5), it follows that the amplitude E in the case of a simultaneous selection of four rows is larger by a factor of 2 than the data voltage Vd which, according to the usual Alt & Pleshko multiplex addressing (for an identical number of rows N of the matrix), is required. The amplitude B of the row selection voltages for a simultaneous selection of four rows is smaller by a factor of 2 than that of the selection voltage Vs required according to the usual Alt & Pleshko RMS multiplex addressing (for an identical number of rows N of the matrix).

Consider again the voltage waveforms of the row selection signals of Fig. 6a and the voltage waveforms of the data signals of Fig. 6b.

The complete addressing time tb is composed of four equal time periods tb/4 with associated characteristic voltage values for the row selection signals and the data signals. In order to generate the desired RMS voltage values for Von and Voff (according to expressions (11) to (14) inclusive), it is not necessary that the selection time periods tb/4 with the associated voltages occur in direct succession, as was assumed in the voltage waveforms in Fig. 6a and Fig. 6b. The selection time periods tb/4 and associated voltages can be distributed over the field time.

This is illustrated in Fig. 8 using a matrix of 12 rows. For simplicity, only the row selection voltage signals of the 12 rows are shown during the scanning of the matrix over one field time. The situation illustrated is that in which four adjacent rows are selected simultaneously. As shown earlier, this is not necessary. Fig. 8a shows the scanning cycle in which the row selection period tb has not been divided. In Fig. 8b, the row selection period tb has been divided into four periods tb/4 which are evenly distributed over the field time. Other distributions are obviously also possible, for example an even distribution over the field time of two selection periods both equal to tb/2.

It is necessary for the scanning scheme as shown in Fig. 8b (but also for other distributions of the total line scanning period tb over the field time as shown above) to adapt the control circuit for the data signals so as to apply the appropriate data voltages to the columns. The principle of the scanning scheme according to Fig. 8b in the case of simultaneous selection of four lines and a distribution of the total selection period into smaller selection periods (with associated voltages) distributed over the field time is obviously applicable not only to the 12-line matrix shown, but to any matrix with any number of lines N. In the case that N is not a multiple of ches of 4, matrix addressing with simultaneous selection of four lines can take place by inserting so-called 'dummy' or virtual lines. The addressing scheme shown in Fig. 8b results in an addressing method in which the single selection pulse of the usual Alt & Pleshko multiplex addressing has been replaced by four separate selection pulses which have smaller amplitudes and preferably occur at times evenly distributed over the field time. The maximum amplitude of the voltage across the field time. The maximum amplitude of the voltage across a pixel (during selection) is smaller for this addressing method than in the case of usual Alt & Pleshko addressing. In the case of simultaneous addressing of four lines it is also possible, as already discussed, to use, for example, two separate selection periods of equal time duration tb/2, which may or may not be evenly distributed over the field time.

If the addressing scheme described above, which involves selecting four lines simultaneously and dividing the total selection period tb into four separate, equal selection period periods as indicated in Fig. 8b, is not sufficient to reduce or eliminate "FIELD RESPONSE", an addressing scheme may be chosen in which more than four lines are selected and the total selection period is again divided into a number of selection periods which may or may not be equal and which may or may not be evenly distributed over the field time, analogous to the manner illustrated by the addressing schemes in which two or four lines are selected simultaneously. Based on the descriptions and explanations given of the procedure to be followed to construct the correct addressing scheme in the case of simultaneous selection of four and two rows, anyone with reasonable technical knowledge will be able to derive the row selection signals and the data signals that can be used for simultaneous addressing of other numbers of rows (other than two and four).

Although not really necessary, in the illustrative embodiment of the invention now to be described, row selection voltage signals that can be used in the case of simultaneous selection of eight rows are indicated in Fig. 9. The selection signals in Fig. 9 are again orthogonal, and one of the voltage signals used has a half period corresponding to the addressing time (selection time). The data signals that in combination with the row selection signals can be determined according to the principle described for the case of addressing a matrix with simultaneous selection of four rows (see, inter alia, Fig. 6b and the relevant text).

The row selection signals of Fig. 9 can in turn be used to define smaller selection periods than the total selection period tc shown in this figure, for example eight selection periods of size tc/8. The eight selection periods can for example be evenly distributed over the field time, analogous to the descriptions of the distributions in Fig. 8b and Fig. 5b.

Obviously, the amplitudes of the row selection voltages and the maximum amplitudes of the data signals are to be chosen in such a way that the ratio of the resulting RMS voltage values of an 'ON' element and an 'OFF' element: Von/Voff is maximum when Voff = V₁.

If n lines are selected simultaneously, the amplitude Yn of the line selection signals and the maximum amplitude Xn of the data signals are to be selected according to:

Yn = N1/2 · Xn/n (16)

Yn = n1/2 · V1 · {0.5/(1 - Q)1/2, (17)

where Q² = N⊃min;¹.

The table below shows, for a number of values of n, how many and which voltage levels can occur in the data signals, which in combination with line selection signals, the waveform of which is shown in the illustrative embodiments given earlier, leads to a desired image content.

n = 2: ±X₂,0

n = 3: ±X₃, ±X₃/3

n = 4: ±X 4 , ±X 4 /2.0

n = 5: ±X5 , ±3 x X5 /5, ±X5 /5

n = 6: ±X 6 , ±4 x X 6 /6, ±2 x X 6 /6.0

Etc.

Claims (9)

1. Display device with a liquid crystal material between two support plates which are kept at a defined distance and whose surfaces lie opposite one another, wherein a pattern of N row electrodes is arranged on one surface and a pattern of column electrodes is arranged on the other surface, wherein the row electrodes cross the column electrodes and thus display elements are formed at the points of intersection, and where the device includes a control circuit for supplying data signals to the column electrodes and further includes row scanning circuits for periodically scanning the row electrodes and supplying row selection voltage signals, wherein during the field time of the periodic scanning of the. Row electrodes, a plurality of rows n are selected simultaneously during a selection time (ta), wherein the row selection voltage signals to be applied during the selection time (ta) are different for each of the rows to be selected simultaneously, the amplitudes of the row selection voltage signals for each of the rows to be selected simultaneously being the same and the row selection voltage signals of the rows to be selected simultaneously being mutually orthogonal, characterized in that the selection time (ta) for a group of n rows is divided into a number of selection time intervals which are distributed over the field time in such a way that the sum of the time intervals is equal to the selection time (ta), and in such a way that during the scanning of the N rows no more than n rows are selected simultaneously, wherein the row selection voltages to be applied during these time intervals to each of the rows to be selected simultaneously are identical to the parts of the original row selection voltage signals of the relevant rows that correspond to the relevant time intervals. 2. A display device according to claim 1, wherein an odd number of lines are selected during a selection time. 3. Display device according to claim 1, wherein an even number of lines simultaneously selected during a selection time, wherein one of the row selection voltage signals to be applied is a unipolar voltage existing for a period of time equal to the selection time, and wherein the other row selection voltages are signals of alternating polarity. 4. A display device according to claim 3, wherein the other line selection voltages with alternating polarity have a frequency as low as possible. 5. A display device according to claim 3 or claim 4, wherein the selection time is divided into a number of time intervals of equal time such that the sum of the time intervals is equal to the selection time, the time intervals being distributed over the field time and during the scanning of the N-line matrix no more than n lines are selected simultaneously, the line selection voltages to be applied to each of the lines to be selected simultaneously during the time intervals being identical to the corresponding parts of the original line selection voltage signals of the lines concerned. 6. Display device according to one of claims 1, 4, 5 or 6, wherein 2, 4 or 8 lines are selected simultaneously. 7. A display device according to any one of claims 1, 2, 5 or 6, wherein the selection period is divided into equal time intervals which are uniformly distributed over the field time. 8. A display device according to any preceding claim, wherein the amplitude Yn of the row selection voltages in case of simultaneous selection of n rows is given by Yn = N1/2 Xn/n, where Xn is the maximum data voltage appearing in the data signals, and where Xn is given by Xn = n1/2 V₁ · {0.5/(1 - N1/2)}1/2, where V₁ is equal to the threshold voltage or the effective RMS voltage value of a display element in the "OFF" state. 9. A display device according to any one of claims 1 to 7, wherein the number of rows N of the matrix is not a multiple of the number of rows n to be simultaneously selected, an additional number of virtual or "dummy" rows nv is added to the matrix such that the sum of N and nv is a multiple of n, and wherein the amplitude Yn of the row selection voltages for simultaneously selecting n rows is given by Yn = N1/2 · Xn/n, where Xn is the maximum data voltage occurring in the data signals, and where Xn is given by Xn = n1/2 · V₁ · {0.5/(1 - N1/2)}1/2 where V₁ is equal to the threshold voltage or the effective RMS voltage value of a display element in the "OFF" state.