DE69224292T2 - METHOD FOR CONTROLLING A LIQUID CRYSTAL DISPLAY MATRIX - Google Patents

Description

The present invention relates to a method of addressing a matrix-type liquid crystal cell, and the method is particularly, though not exclusively, relevant to such cells used in visible displays and the like.

A known method of addressing a matrix array type liquid crystal cell is disclosed in GB Patent 2,173,336B. In this method, the cell is treated as a matrix of orthogonal row and column electrodes. Waveforms are applied to both the rows and columns to selectively switch the individual elements to either the dark (no light transmitting) or the light transmitting state.

Two types of data waveforms can be applied to the column electrodes. "On" or "Unchanged". Synchronized to these waveforms are those applied to the row electrodes labeled "Select", "Non-Select" and "Blank". At any one time, only one row electrode is applied with the "Select" waveform and at least one other row electrode is applied with the "Blank" waveform. All other row electrodes are applied with the "Non-Select" waveform.

The above scheme operates in the following way: The column electrodes are supplied with data waveforms to produce the desired scheme on the row electrode to which the selection waveform is supplied. The arrangement is such that this electrode has previously been blanked by supplying the blanking waveform. This has the effect of placing the row in the dark (or "off") state, regardless of whether it is combined with an "on" or "unchanged" data waveform. all elements of the display common to that electrode are switched to the dark state before the selection waveform is applied to that row.

Simultaneously with the application of the data waveform, the blanking waveform is applied to a row electrode. It will be apparent to those skilled in the art that in such a system it is a requirement to provide DC compensation or charge equalization, i.e. neither the "on" nor the "unchanged" data waveform must have a DC component.

However, the system described in GB 2,173,336B suffers from the disadvantage that the response time of the liquid crystal material is slowed if the blanking waveform is applied to a row electrode shortly before the selection waveform is applied to the same row electrode. This is because the magnitude of the blanking waveform is larger than that required to bring the liquid crystal material into its dark state and therefore a degree of recovery must occur before the electrode addressing activity can occur. The disclosure in GB 2,173,336B generally indicates that the duration and amplitude of blanking pulses can be varied simultaneously to meet particular requirements, but there is no indication that these expedients could have any significance in addressing the above disadvantages. Indeed, when the deleterious effects on the switching function resulting from the occurrence of counter-voltage stimuli either immediately after or immediately before the switching stimulus are considered in GB 2,173,336B, the proposed solution results in the introduction of one or more gaps in the data pulse waveform. The introduction of gaps in the data pulse waveforms is undesirable because (being of extremely short duration) they require that the switching circuits and the associated components have an extremely high reaction rate.

DE 35 01 982 describes another addressing method, in which blanking pulses are applied simultaneously to both groups of electrodes; each individual blanking pulse has a smaller amplitude than the sampling (or selection) pulse, but the combined effect of the two synchronized blanking pulses is such that a voltage is generated at the pixels which exceeds that of the sampling (selection) pulse.

It is therefore an object of the present invention to provide an addressing scheme in which a reduction in the liquid crystal response time is at least mitigated.

Thus, according to the present invention there is provided a method of addressing a matrix array type liquid crystal cell containing ferroelectric liquid crystal material, the cell comprising a plurality of pixels defined by overlapping regions between elements of a first group of electrodes on one side of the material and elements of a second group of electrodes oriented orthogonal to the first group on the other side of the material; and wherein the pixels of the group are addressed on a line-by-line basis after blanking, wherein unipolar blanking pulses are applied exclusively to the elements of the first group of electrodes to effect blanking, and wherein for selectively addressing the pixels, unipolar selection pulses are applied serially and exclusively to the elements of the first group of electrodes, while charge balanced bipolar data pulses are applied exclusively and in parallel to the elements of the second group, the positive going portions of the data pulses being synchronized with the selection pulses for data significance and the negative going portions the data pulses are synchronized with the select pulses for the other data significance, the polarities of the select and blanking pulses being opposite and fixed, characterized in that reductions in the response time of the liquid crystal material to the selective addressing of the pixels when blanking pulses and select pulses are applied adjacent to one another in time are mitigated by the use of blanking pulses of magnitude VS/n and duration mT, where VS is the voltage of the select pulse, T is the duration of the select pulse, n and m are arbitrary values, and where m is greater than or equal to n, so that the amplitude of a given blanking pulse is less than that of a given select pulse.

Thus, the present invention provides a method of addressing a liquid crystal cell of the matrix array type, wherein the potential for the response time of the liquid crystal material to be reduced is reduced by providing a blanking pulse, the amplitude and duration of which can be modified as desired within operating limits.

An embodiment of the present invention is explained in more detail below by way of example only with reference to the accompanying drawings.

Fig. 1 illustrates schematically a typical matrix-group type cell;

Figure 2 graphically illustrates the various waveforms that can be applied to the row and column electrodes in both the prior art and the present invention;

Fig. 3 schematically illustrates a comparison between the general shape of a blanking waveform and a selection waveform;

Fig. 4 is a schematic representation of the modified blanking waveform occurring in the cell according to the present invention, and

Fig. 5 shows a graphical representation of the applied voltage versus time for a selected pixel.

From Fig. 1 it can be seen that a liquid crystal cell of the matrix group type, generally designated 2, comprises a group of overlapping orthogonal row electrodes 4 and column electrodes 6, between which liquid crystal material (not shown) is arranged.

Referring now to Fig. 2, the operation in terms of addressing cell 2 will be described. It should be noted that the general principles of operation used in known liquid crystal displays are also applicable to the display described in the present embodiment. Therefore, the data signals that can be applied to the column electrodes 6 are the "unchanged" waveform 8 and the "on" waveform 10. The waveforms that are applied to the row electrodes 4 in synchronism with the waveforms 8, 10 for the column electrodes are the "select" 12, "non-select" 14 and "blank" 16 waveforms. Fig. 2(a), (b), (c) and (d) illustrate the resulting waveforms at the intersections of the row and column electrodes 4, 6 as a result of the combination of the selected waveforms applied thereto.

The method of addressing a specific pixel 3 within the cell 2, which is defined by an overlap area between a row electrode 4 and a column electrode 6 is as follows: Before any addressing of the cell 2 with display data takes place, the row 4 of which the one particular pixel 3 forms a part must first be placed in the dark state. This is achieved by applying a blanking waveform 16. This is necessary to ensure that the entire row 4 is placed in a predetermined state from which display data addressing can take place.

The desired display data waveform 8 or 10, e.g. 8, is applied to the desired column electrode 6 which overlies or intersects the pixel 3. At the same time, the selection waveform 12 is applied to the corresponding electrode 4 which overlies the pixel 3. Referring particularly to Fig. 2(a), it can be seen that the combination of the unmodified waveform 8 and the selection waveform 12 at the pixel 3 produces a particular combined waveform. Similarly, if the on waveform 10 had been selected instead of the unmodified waveform 8, then the resulting waveform at the pixel 3 would be that shown in Fig. 2(b).

In any event, at the same time as the pixel 3 lying on the row electrode 4 is addressed as described above, the electrode for the next row to be addressed (though not necessarily the next row in the group) receives the blanking waveform 16 as above ready for addressing.

Note that the amplitude of the voltages shown in Fig. 2 is defined as follows: Vd is the voltage of the data waveform 8, 10, and T is the time period of a waveform having that voltage amplitude. Similarly, VS is the amplitude of the select waveform 12; Vb is the amplitude of the blanking waveform 16.

While in the prior art displays there is a disadvantage if the blanking waveform is applied to the electrodes for the next row only a few row addressing periods before the selection waveform because the response time of the liquid crystal is slowed and the temporal width T of the waveform must necessarily be increased for the display to function, it will be seen with reference to Figure 3 that the present invention, as embodied in the present example, alleviates this problem by applying the blanking waveform 16 temporally adjacent to the selection waveform 12 as desired without slowing the response time of the liquid crystal.

This is achieved by modifying the shape of the blanking pulse 16. This modification is achieved by reducing the amplitude of the blanking pulse 16 so that it is smaller than the amplitude VS of the following selection pulse 12. However, the duration of the blanking pulse 16 is then preferably extended in comparison with the duration T of this selection pulse 12.

Preferably, but not necessarily, the areas defined by the select pulse 12 and the blanking pulse 16 are equal, although the area defined by the select pulse is never smaller than that of the preceding blanking pulse.

The above is illustrated in Fig. 3 where it can be seen that the blanking pulse 16 has an amplitude VS/n and a duration mT in comparison to the selection pulse 12 which has an amplitude VS and a duration T. In this example m=n=2.

The reason for reducing the amplitude of the blanking waveform 16, which provides the above-mentioned advantage, can be explained with reference to Fig. 5. Fig. 5 illustrates the switching characteristic of the cell 2 by reducing the voltage supplied to it versus the time during which this voltage is applied. It is well known to those skilled in the art that the magnitude of the blanking pulse 16 is greater than is actually required to place each pixel 3 in the "dark" state. Therefore, some recovery time is then required immediately after the blanking pulse is applied before addressing takes place. Thus, the reduction in the amplitude of the blanking waveform 16 according to the present invention can overcome the need for a recovery period while remaining within the switching limitation of Fig. 5.

It should be noted that the limit of the reduction in the amplitude of the blanking waveform 16 and the temporal extension is governed by the requirement that VS/n > Vd.

In principle, any values for n and m can be used, although integer values have been found to be convenient. There is no requirement that m and n be equal, but m must be greater than or equal to n.

However, if n and m are chosen to be equal, it is preferable that values greater than or equal to 2 be chosen; this is because a value less than 2 would result in the amplitude of the blanking waveform 16 and a duration approaching that of Fig. 1(a) causing no switching. In this case, the blanking waveform 16 would be ineffective.

Finally, Figure 4 shows a representation of the general waveform that appears at pixel 3 when addressed with a blanking pulse 16 according to the present invention. This figure illustrates the requirement VS/n > Vd because it can be seen that if the amplitude of the blanking pulse 16 VS/n falls below Vd, then some time during the There is a blanking period when the voltage changes polarity, and this polarity change can promote switching in the opposite direction away from the desired blanking (or dark) state. Therefore, the limit is VS/n > Vd.

It should be noted that the amplitude of the blanking pulse must be greater than or equal to that of the data pulses 8,10. This is because during the application of the blanking pulse 16 there is no deviation of the difference between the blanking pulse 16 and either the data pulse 8 or 10 to a polarity opposite to that of the blanking pulse 16.

Thus, the present invention provides a method for addressing a matrix array type liquid crystal cell with a modified blanking waveform while maintaining the response time of the liquid crystal.

In the present invention, the number of rows of pixels between the application of the blanking pulse 16 and the selection pulse 12 can be reduced to 8 or less.

It will be apparent to those skilled in the art that modifications and changes can be made to the embodiment described here without departing from the scope of the invention, e.g. the possibility of blanking being carried out in such a way that elements are set to the light state instead of the dark state.

Claims (7)

1.) A method of addressing a matrix array type liquid crystal cell (2) containing ferroelectric liquid crystal material, the cell comprising a plurality of pixels (3) defined by overlapping regions between elements of a first group (4) of electrodes on one side of the material and elements of a second group (6) of electrodes oriented orthogonal to the first group on the other side of the material; the pixels of the group being addressed on a line-by-line basis after blanking, unipolar blanking pulses (16) being applied exclusively to the elements of the first group (4) of electrodes to effect blanking, and unipolar selection pulses (12) being applied serially and exclusively to the elements of the first group (4) of electrodes for selectively addressing the pixels, while charge balanced bipolar data pulses (8, 10) are applied exclusively and in parallel to the elements of the second group (6), the positive going parts of the data pulses being synchronized with the selection pulses for one data significance and the negative going parts of the data pulses (8, 10) being synchronized with the selection pulses (12) for the other data significance, the polarities of the selection pulses (12) and the blanking pulses (16) being opposite and fixed, characterized in that reductions in the response time of the Liquid crystal material to the selective addressing of the pixels (3), when blanking pulses (16) and selection pulses (12) are applied adjacent to one another in time, are attenuated by the use of blanking pulses (16) of magnitude VS/n and duration mT, where VS is the voltage of the selection pulse (12), T is the duration of the selection pulse (12) and m are arbitrary values, and where m is greater than or equal to n, so that the amplitude of a given blanking pulse (16) is smaller than that of a given selection pulse (12). 2.) Method according to claim 1, in which n and m have equal values so that the charge balance for the individual elements of the first group (4) of electrodes is maintained by the areas defined by the selection pulses (12) and the blanking pulses (16) which are equal. 3.) Method according to claim 1 or 2, wherein n and m are integer values. 4.) Method according to claim 3, wherein n and m have the value 2. 5.) Method according to claim 1, wherein m is greater than n. 6.) Method according to claim 5, wherein n and m are integer values. 7.) Method according to one of the preceding claims, in which for a given row of pixels (3) within the group, the blanking pulse (16) is supplied before the supply of the selection pulse (18), the number of rows between the blanking pulse (16) and the selection pulse (12) being less than or equal to 8.