Display device

Abstract: In a bistable switching display device the occurrence of artefacts due to considerable changes of periodicity between successive grey scale stages is reduced by a suitable subdivision of the electrodes (112). To this end a drive unit (116) allocates fewer than 2 n grey scale stages to each pixel (113) which is subdivided into n sub-pixels (113 a , 113 b , 113 c ). The change of periodicity will decrease when a suitable division of the surface ratios and drive sequence are chosen.

Description: The invention relates to a display device according to the preamble of claim 1. The electro-optical medium used usually switches between two optical states with a steep transition characteristic (transmission/voltage characteristic curve) or with a hysteresis in this transition characteristic such as, for example, liquid crystal display devices (as supertwist display devices or ferro-electrical display devices). The two optical states define the two extreme transmission levels (possibly together with polarizers and/or reflectors) and hence the extremes on the grey scale. Grey scale stages are understood to mean intermediate transmission levels. The invention further relates to an electro-optical medium which is switchable between two optical states. A display device of the type described in the opening paragraph is described in EP-A-0 316 774. The display device shown in this Application is driven in the multiplex mode in which in a system of crossing selection or address lines (row electrodes) and data lines (column electrodes) the drive is effected by consecutively energizing the address lines while the information to be written is being presented on the data lines. Different transmission levels (grey scale stages) can be introduced in such a display device by subdividing the column electrodes into sub-electrodes having different surface areas (for example, in accordance with surface ratios of 8:4:2:1). With such an exponential subdivision (2 p :2 p-1 :...:2:1) a maximum number of grey scale stages (levels) can be selected, namely 2 n , including fully on and fully off, with a minimum number of connections of the sub-electrodes n per column. This number can be increased by subdividing also the selection electrodes or by means of weighted drive. The allocation of column sub-electrodes to be switched on is unambiguously coupled to a given grey scale stage by the exponential subdivision of the sub-electrodes. However, the number of variations, i.e . the number of sub-pixels switching on or switching off upon transition to a next higher or next lower grey scale stage is then also fixed. This may mean that large parts of the pixel change their optical state in the case of such transitions. For example, in an extreme case a transition may occur at a width ratio of 8:4:2:1 of the sub-columns, at which transition the widest sub-column switches from light to dark, whereas the other sub-columns switch from dark to light. The same objection applies to a device as shown in EP-A-0 361 981 in which a pixel is sub-divided into subpixels. If a display medium is used, which switches between two optical states at a certain transition, about half of the pixel area switches from light to dark, while the remainder of the pixel area switches dark to light. Notably in projection television such transitions are visible as artefacts in the image, also in less extreme cases at the recommended viewing distance (the viewing distance is approximately 6 times the image width and even further). To indicate a criterion for the extent of change in the case of such a transition, this Application refers to the change of periodicity. Periodicity is understood to mean the display, translated to amplitude and phase, of a fundamental wave related to the light/dark division across the pixel, as will be explained hereinafter. Viewed across the width of a pixel, the transmission (reflection) is to this end represented by means of a block function having, for example, the value of 1 for light parts and the value of 0 for dark parts. With the change described above this function acquires a complementary value throughout the width of the pixel and the change of periodicity is maximal. A possibility of reducing the visibility of transitions at the viewing distance is to subdivide the column into a large number of, for example 15 sub-electrodes of equal width and to introduce the stages (levels) by starting with one sub-electrode and by switching on an adjoining sub-electrode for each subsequent stage. However, this is at the expense of the number of connections; to realise 16 stages, including fully on and fully off, 15 connections instead of 4 are then required. The present invention has, inter alia, for its object to provide a display device of the type described in the opening paragraph in which a grey scale can be defined with transitions between adjoining grey scale stages which (at the viewing distance) are gradual to the observer, while the number of sub-electrodes in a column remains limited to an acceptable number. To meet this object a display device according to the invention is characterized according to the characterizing part of claim 1. As described above, an exponential subdivision is understood to mean such a division that the surfaces of the column sub-electrodes have a mutual ratio of 2 n-1 :2 n-2 :...:2:1. The invention is based on the recognition that the use of an additional sub-electrode yields such a redundance in the way of allocating grey scale stages that combinations of column sub-electrodes can always be allocated to subsequent stages in such a way that no transitions occur at which the light/dark-related block function acquires a completely complementary value. This can already be achieved by giving at least two column sub-electrodes a different width in a device according to the invention in which the grey scale has N stages, including the two extreme transmission levels and in which the widths of the column sub-electrodes are in a mutual ratio of an integer, while the widest of the column sub-electrodes has a width which is smaller than (N/(N-1)(L/2) if N is even and smaller than (L/2) if N is odd, L being the sum of the widths of the column sub-electrodes. Since at least two column sub-electrodes have a different width, a narrowest width can be chosen (which may be allocated to a plurality of column sub-electrodes). In the case of a suitably chosen drive the number of column sub-electrodes at consecutive stages can be switched on in such a way that the switched-on part increases by this width. By limiting the width of the widest column electrode, the switch-over (at consecutive stages) between two complementary situations is avoided. Said periodicity may be mutually compared in various manners. For example, the maximum change of periodicity, which is found when all grey scale stages are traversed, can be considered. For example, the distances in a Fourier diagram related to said block functions are considered for this purpose. Here again a maximum distance between two successive states can be considered. The influence of all other transitions, which may be considerably decisive for the total number, is, however, ignored. The total path length, i.e . the sum of all distances in the Fourier diagram between the grey scale stages may also be taken as a measure. A path norm is valid as a very good criterion for the change of periodicity: in which and in which f j (x) is the block pattern associated with a pixel having a width of L for the j 0 stage in the grey scale with values of 1 and 0 for the extreme values of the grey scale as a function of the position (x) within the pixel, and N is the number of grey scale stages, including the two extreme states. It is found that for a subdivision of a column electrode into 5 sub-electrodes a number of stages N of a grey scale with 12 ≤ N ≤ 16 can be allocated by means of the drive circuit in such a way that artefacts are much less visible. The improvement is even somewhat better when using 6 sub-electrodes. The maximum path norm as defined above is, for example, chosen to be 2.0. Dependent on the subdivision of the electrodes and the number of stages to be reduced in the grey scale, this path norm may be given a considerably lower value. Dependent on the number of stages and the number of sub-electrodes and their width distribution, this criterion is sometimes slightly more stringent, sometimes slightly less stringent than that based on the above-mentioned choice of width ratios and maximum width of the widest sub-electrode. The number of stages N of the grey scale is less than 2 n for a subdivision into n sub-electrodes, hence less than 32 in the case of 5 sub-electrodes, although better results are achieved at lower values of N, for example 12. To render the device according to the invention suitable for video applications, in which a much larger number of stages is required, this number can be increased by also subdividing the row electrodes. These are preferably subdivided into two sub-electrodes so that the double drive frequency is sufficient. In the case of a subdivision in accordance with the ratio N:1, N 2 stages of the grey scale of the pixel defined by n column electrodes and two row electrodes can be realised. To simplify the mode of connection and driving, the widest row sub-electrode sub-electrode, may be subdivided into two strips at both sides of the narrowest row said strips being interconnected in an electrically conducting manner at one end. On the other hand, the number of grey scale stages may be increased by weighted drive, in which a first pattern is displayed during a (N/(N+1)) th part of a frame period and a second pattern is displayed during the (1/(N+1)) th part of the frame period. A total number of N 2 stages of a grey scale can then be realised again. These and other aspects of the invention will now be described in greater detail with reference to some embodiments and the drawing in which Fig. 1 is a diagrammatic plan view of a part of a state-of-the-art display device, Fig. 2 is a diagrammatic cross-section taken on the line II-II in Fig. 1, Fig. 3 is a diagrammatic plan view of a part of a state-of-the-art display device at different transmission levels, Fig. 4 shows the associated light/dark distribution and a fundamental wave related thereto, Fig. 5 shows a Fourier diagram and the grey scale stages in such a display device and in a modification of such a display device, Fig. 6 shows a Fourier diagram and the grey scale stages in another display device, Fig. 7 is a diagrammatic plan view of a part of a display device according to the invention, Fig. 8 is a diagrammatic cross-section taken on the line VIII-VIII in Fig. 7, Fig. 9 shows a Fourier diagram and the grey scale stages for the device of Figs. 7 and 8, and Fig. 10 shows Fourier diagrams and the grey scale stages for a display device in which different drive modes (according to and not according to the invention) are shown, using the same subdivision of the columns. Fig. 1 shows diagrammatically a subdivision of electrodes 101, 102 between which an electro-optical material is present. In this embodiment the electrodes, for example a row electrode 101 and a column electrode 102 are subdivided into sub-electrodes. The column electrode 102 is subdivided into sub-electrodes 102 a , 102 b , 102 c , 102 d whose widths are in a mutual ratio of 8:4:2:1. The row electrode 101 is subdivided into sub-electrodes 101 a , 101 b whose widths are in a ratio of 16:1. At the area of the crossing of the electrodes 102 (sub-electrodes 102 a , 102 b , 102 c , 102 d ) and 101 (sub-electrodes 101 a , 101 b ) a display cell 103 is defined which can change its electro-optical properties entirely or partly by suitably driving the sub-electrodes. If a ferro-electric liquid crystal is chosen as an electro-optical material, or if the device is alternatively formed as a bistable switching device, as in a supertwistnematic liquid crystal display, it is possible to apply such a voltage to the (sub-)electrodes that a given voltage threshold is exceeded and the transmission state changes locally, for example, from light-absorbing to light-transmissive, or conversely. This behaviour may also be influenced by the position of polarizers, if any. Since the electrode 102 is subdivided into sub-electrodes, it is possible to drive only a portion of the display cell 103. For example, if the sub-electrode 101 a and the sub-electrode 102 a are energized correctly, the portion 103 aa (sub-pixel) of the display cell is driven so that this portion becomes, for example, light-absorbing, whereas the other portion of the display cell remains light-transmissive. This is shown in Fig. 3a, while Fig. 3b shows the drive which is complementary thereto. By energizing the sub-electrodes 101,102 in different manners, different surfaces of the display cell 103 can be driven so that different proportions of light-transmissive/light-absorbing (white/black) are obtained, in other words, different grey scales. Fig. 2 shows diagrammatically a cross-section of a part of the device, taken on the line II-II in Fig. 1. The electrodes 101 and 102 are provided as parallel strips of transparent conducting material (for example, indium-tin oxide) on transparent substrates 106, 107 of, for example glass or quartz. As described hereinbefore, said electrodes 101 and 102 are subdivided into column sub-electrodes 102 a , 102 b , 102 c , 102 d , while the row electrodes are also subdivided, if necessary. To give the liquid crystal molecules a given preferred direction at the location of the electrodes, the electrodes are coated with an orientation layer 108. A layer of liquid crystal material 109, in this case a ferro-electric liquid crystal material is present between the two substrates 106, 107. The device may be used as a display device and will conventionally be provided with polarizers, colour filters and/or mirrors as well as an illumination source. The sub-pixels 103 have a bistable switching behaviour, in other words, they switch between two extreme states, viz. substantially completely light-transmissive and substantially completely light-absorbing. In the device of Fig. 1 (and Fig. 3) the sub-pixel 103 db is the smallest switching unit. With the subdivision shown, 256 stages in a grey scale can be realised, including completely dark and completely light, with a minimum number of connections, viz. 6 (4 column sub-electrodes and 2 row sub-electrodes) per pixel. Fig. 3 shows how the change of periodicity at the transition of a grey scale stage (Fig. 3a, where 127/255 part is unshaded, i.e. light-transmissive) to a subsequent stage (Fig. 3b in which 128/255 part is light-transmissive) may be maximal when using such a minimum number of connections. Notably this type of transitions leads to the above-mentioned artefacts. To find a qualitative criterion, Fig. 4a shows the light variation of Fig. 3a once more, taken on the line IV-IV in Fig. 3a. This variation is shown as a block function f(x), in which f(x) = 1 for the light-transmissive part and f(x) = 0 for the light-absorbing part. This block function (periodically continued) is shown in Fig. 4b as a periodical function F(x), given by: F ( x ) = B 0 + B 1 cos (2π/ L ) + A 1 sin (2π x / L ), in which and It is true that F(x) is different from f(x), but this difference is found to comprise only components having wavelengths of L/2 or less, while said artefacts are found to be originating from components having the largest wavelength L. Also the fact that only the change of periodicity of a row sub-electrode is considered hardly influences the result of the considerations. Fig. 5a shows values of the Fourier components A 1 , B 1 associated with such an exponential subdivision with 4 column sub-electrodes, and, diagrammatically, the stages 0, 1, 2, ..., 14, 15 (N = 16) in the grey scale realised with this subdivision. At the transition from stage 7 to 8 there is a similar interchange of light-transmissive and light- absorbing as above, as has been described with reference to Fig. 3. This transition corresponds to a large jump in the Fourier diagram. More generally, to prevent such large jumps, it holds that the widest column sub-electrodes have a maximal width which is a multiple of the width of the narrowest column sub-electrode. As a total width of L and N stages in the grey scale, the width of the narrowest column sub-electrode is L/ (N-1) . If N is odd ((n-1) even), these widest column sub-electrodes should be narrower than (N-1)/2 units. i.e. narrower than (N-1)/2. L/(N-1) = L/2. If N is even ((N-1) odd), these widest column sub-electrodes should be narrower than N/2 units, i.e. narrower than N/2.L/N-1. The same applies to an electrode subdivision with the narrowest sub-electrode in the middle and the electrode having the double width at both sides thereof, etc. This is diagrammatically shown in Fig. 5b. Fig. 6 shows in a similar way the Fourier components and the stages in a grey scale of 16 stages, realised by means of 15 sub-electrodes of the same width. Although a transition between two successive stages yields the same jump in the Fourier diagram, this is at the expense of an unrealistically large number of connections in practice. Figs. 7 and 8 show a part of a display device according to the invention. Here the column electrodes 112 are subdivided into column sub-electrodes 112 a , 112 b , 112 c , 112 d , 112 e whose widths are in a mutual ratio of 2:2:2:1:4. Together with the row sub-electrodes 111, these electrodes define sub-pixels 113 (Fig. 7). The sub-electrodes 111, 112 are driven via connections 114, 115 (Fig. 8) by a drive unit 116 (shown diagrammatically) in which an energization of the sub-electrodes 111, 112 associated with a grey scale stage associated with an incoming signal 117 is generated. To this end, the drive unit 116 comprises, for example an A/D converter 118 which generates an address of a look-up table for each grey scale value (stage). The addresses associated with successive stages then supply signals at the output of the look-up table 119 in such a way that the change of periodicity is small for driving successive stages and that the path norm is minimal when all grey scale stages are being traversed. Sub-pixels 113 aa ... 113 ae (Fig. 7) can be selected by means of the row sub-electrode 111 a and the column sub-electrodes 112 a ... 112 e . Since the grey scale stages can now be defined in different manners, a stage can be represented (due to the redundance) in different manners in an associated Fourier diagram. Fig. 9 shows the Fourier components for different realisations of the stages for a display device with N = 12. Fig. 9 also shows the path with the smallest path norm in accordance with the above-mentioned definition and the associated stages 0, 1, 2 ... 11 in the grey scale. This path norm is 0.684. The same path norm is found when subdividing the column sub-electrodes in accordance with the ratio 4:2:2:2:1; 2:2:2:1:4; 2:2:1:4:2 or 2:1:4:2:2, in other words, in case of cyclic permutation. The same path norm is also found in case of mirroring, i.e. a width ratio of 4:1:2:2:2 and all its cyclic permutations. Fig. 10a shows a similar diagram and the associated grey scale stages for a subdivision of the column electrode in accordance with the ratio 3:2:1:2:3 in which N is also 12. A path having the smallest path norm (1.046) is shown by means of a solid line. The change of periodicity (and hence the path norm) is dependent on the allocation of the sub-electrodes 112 to successive grey scale stages. The broken line in Fig. 10 a illustrates another allocation having the same path norm. The solid line in Fig. 10 b indicates how the diagram is traversed in case of a completely different allocation, in this case the worst possible, and the related grey scale stages. The path norm is 6.23 in this case. As already noted, the number of grey scale stages may be increased, for example by subdividing the row electrode 111 into row sub-electrodes 111 a , 111 b as is shown in Fig. 7, with a mutual width ratio of N:1. This increases the number of stages to N 2 . The drive unit 116 then subdivides the signal 117 into sub-signals for the row sub-electrodes. The widest row sub-electrode may be subdivided into two strips at both sides of the narrowest row sub-electrode, which strips are interconnected in a conducting manner at one end. This provides a simpler possibility of connection at both sides. The display device may also be driven with a weighted drive. The drive unit 116 then subdivides, for example, the incoming signal 117. The sub-signals address the look-up table via the A/D converter in such a way that the most significant part of the information defining the stage drives the sub-electrodes 112 during an (N/(N+1)) th part of a frame period and the other information drives the sub-electrodes 112 during an (1/(N+1)) th part. Different subdivisions of the column sub-electrodes are alternatively possible. Some subdivisions are given in Table I for n = 4 and in Table II for n = 5, together with the path norm as defined above. N best subdivision path norm second subdivision path norm 12 1-4-2-4 1.795 1-2-3-5 1.953 13 1-2-3-6 2.352 1-2-4-5 2.758 14 1-2-3-7 2.264 1-2-6-4 2.333 15 1-2-7-4 2.408 1-2-4-7 2.653 16 1-2-4-8 2.514 this is the exponential subdivision N best subdivision path norm second subdivision path norm 12 1-2-2-2-4 0.684 1-2-2-4-2 0.770 13 1-2-3-4-2 0.948 1-2-2-5-2 1.042 14 1-2-3-3-4 0.874 1-2-2-3-5 1.020 15 1-2-5-2-4 1.173 1-5-1-5-2 1.205 16 1-2-3-4-5 1.257 1-2-5-2-5 1.264 It is apparent from the Tables that not only the width ratio but also the mutual subdivision of the sub-electrodes across the column electrode also influence the change of periodicity. For example, the combinations (n = 4, N = 15) and (n = 5, N = 12) result in different values of the path norm for different subdivisions of the sub-electrodes across the column electrodes. The width ratio of the sub-electrodes need not be maintained beyond the actual pixel. Notably for external connections, the narrower electrodes at the edge of the display device may be wider. The invention may not only be used for display devices comprising a bistable electro-optical medium, but also for devices having such a steep transmission/voltage characteristic curve that in practice they are only driven in the on and off-states. Even for display devices having a gradual transmission/voltage characteristic curve the use of only the on and off-states may be chosen. The invention may also be used for these devices.

Claims: A display device comprising an electro-optical medium (109) which is switchable between two extreme transmission levels and is arranged between a first supporting plate (106) provided with row electrodes (101) and a second supporting plate (107) provided with column electrodes (102), subdivided into n column sub-electrodes (102 a , 102 b , 102 c , 102 d ) where n ≥ 4, which define n sub-pixels at the area of a crossing with a row electrode, at least two column sub-electrodes having a different width at the area of the crossing, said device comprising a drive circuit (116) for energizing or non-energizing column sub-electrodes to produce grey-scale stages, characterized in that n=4 or 5 and in that the column sub-electrodes have width ratios in accordance with the Table below or in accordance with cyclic permutations of these ratios in the Table: n = 4 n = 5 N = 12 1:4:2:4 N = 12 1:2:2:2:4 1:2:3:5 1:2:2:4:2 N = 13 1:2:3:6 N = 13 1:2:3:4:2 1:2:2:5:2 N = 14 1:2:3:7 N = 14 1:2:3:3:4 1:2:6:4 1:2:2:3:5 N = 15 1:2:7:4 N = 15 1:2:5:2:4 1:5:1:5:2 N = 16 1:2:3:4:5 1:2:5:2:5 A display device as claimed in claim 1, characterized in that the column sub-electrodes have a width ratio and the device has a grey-scale of N-stages in accordance with the Table or in accordance with cyclic permutations of the ratios in the Table. A display device as claimed in Claim 1, characterized in that a row electrode (101) is subdivided into two row sub-electrodes (101 a , 101 b ) each defining at the area of the pixel together with the column sub-electrodes N stages of the grey scale, the row sub-electrodes having a mutual width ratio of 1:N. A display device as claimed in Claim 1, characterized in that the drive circuit comprises means for subdividing an incoming signal into two sub-signals and in that the most significant part of the information defining the grey-scale stage drives the column sub-electrodes during an (N/(N+1)) th part of a frame period and the other information drives the column sub-electrodes during an (1/(N+ 1)) th part of a frame period.

Inventors: BLOMMAERT FRANCISCUS JOHANNES, HAAREN JOHANNES ALBERTUS M VAN, VERHULST ANTONIUS GERARDUS H, BLOMMAERT, FRANCISCUS JOHANNES JOSEPHUS, VAN HAAREN, JOHANNES ALBERTUS MATTHIJS MARIA, VERHULST, ANTONIUS GERARDUS HENDRIKUS
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: G09G3
IPC codes: G09F9, G09G3, G02F1