Abstract:
A display device comprising a plurality of picture elements arranged in rows and columns, each picture element being switchable between at least two states having different optical output characteristics, a plurality of row and column drivers, connected to said rows and columns respectively, for switching said picture elements between said states, wherein at least one switch is provided for switching at least one of said drivers between a first configuration in which the driver drives at least one row or column, and a second configuration, different from said first configuration, in which the driver drives at least two rows or columns.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to switchable display devices, and particularly, although not exclusively, to switchable liquid crystal display devices using spatial dither (ie. pixel subdivision) to achieve different grey levels. Embodiments of the invention allow a display panel to be operated at different resolutions, while maintaining an optimum number of spatial dither grey levels in the lower resolution, without increasing the number of drivers required. 
     BACKGROUND OF THE INVENTION 
     Some prior art documents will first be discussed. 
     “New bistable liquid-crystal twist cell”—D. W. Berreman and W. R. Heffner; J. Appl. Phys. 1981, 52 (4), 3032—describes the Bistable Twisted Nematic (BTN) effect, where selection between two metastable states (strong twist, e.g. 360 degrees, and weak twist, e.g. 0 degrees) is achieved by reducing a voltage either abruptly or gradually after blanking to the homeotropic weak state. This effect has the disadvantage of not giving grey level control. 
     European Patent Publication No.0613116 describes addressing with very short address times by placing the short selection pulse at an optimised position in time after the blanking pulse (so there is a pause between the two). Two modes of addressing are described as very high and very low selection voltage pulses both generate the highly twisted state (black in thin cells between crossed polarisers) whereas intermediate pulse voltages generate the low twist (white) state. This patent is important for fast BTN addressing as required for high resolution passive video displays. It has the disadvantage of not giving grey level control. “A Bistable Twisted Nematic (BTN) LCD Driven by a passive Matrix Addressing”—-T. Tanaka, Y. Sato, A. Inoue, Y. Momose, H. Nomura, S. Iino; Proceedings of Asia Display &#39;95, 259—presents a first black and white BTN panel. The pixels of this panel are not capable of displaying intermediate grey levels. 
     The use of the BTN effect as described in the prior art does not allow the generation of intermediate grey levels within the pixel. 
     The following publications relate generally to bistable LC effects and provide background information: 
     G. D. Boyd, J. Cheng and P. D. T. Ngo, Appl. Phys. Lett. 36, 556 (1979) 
     N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 (1980) 
     R. Barberi, M. Boix and G. Durand, Appl. Phys. Lett. 55, 2506 (1989) 
     Barberi and G. Durand, Liquid Crystals 10, 289 (1991) R. Barberi, M. Giocondo and G. Durand, Appl. Phys. Lett. 60, 1085 (1992) 
     M. Pfeiffer et al., SID 95 Digest, 706 
     W. Hartmann, Ferroelectrics 122, 1 (1991) discusses grey scale techniques for FLCs. 
     R. A. Martin et al. Journal of the SID 4/2, 1996 p65 describes an active matrix display used as a binary mode. In this case greyscale is achieved by an error diffusion technique, the display does not have two separate resolution modes. 
     In matrix displays, greyscale can either be achieved within the pixel element, if the used effect offers an analogue or other range of configurations of different brightness, or by subdividing the pixel temporally or spatially if the effect offers only a black and a white state (or a limited number of states).m This invention is concerned principally with spatial subdivision of pixels into subpixels (spatial dither). The more subpixels you have, the more column (data) and row (strobe, select) drivers you normally need, and the cost of the display increases. It is often advantageous (and common practice in cathode ray tube (CRT) monitors) to have the choice between different screen resolutions. If such different resolutions are achieved in spatial dither panels, where a higher resolution with less greyscale and a lower resolution with more greys are offered, the optimal patterning of the subpixels is different for the two resolutions. To achieve both patterns the display has to be subdivided into more subpixels than will ever be addressed independently at one time. More subcolumns and/or subrows exist than are addressed independently, and therefore several drivers are used identically. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a display device comprising a plurality of picture elements arranged in rows and columns, each picture element being switchable between at least two states having different optical output characteristics, a plurality of row and column drivers, connected to said rows and columns respectively, for switching said picture elements between said states, wherein at least one switch is provided for switching at least one of said drivers between a first configuration in which the driver drives at least one row or column, and a second configuration, different from said first configuration, in which the driver drives at least two rows or columns. 
     The invention thus reduces the number, and therefore the manufacturing cost, of the drivers. 
     Each picture element may be switchable between only two states. 
     Said two states may correspond to the picture element being on and off. 
     In one embodiment of the invention, at least some picture elements emit or transmit light at a different wavelength to other picture elements. 
     In this case, some of said picture elements may emit or transmit red light, some emit or transmit green light, and some emit or transmit blue light. 
     In a further embodiment of the invention, said one switch is a row switch which switches at least one of said row drivers between said first and second configurations, and the display device also comprises at least one column switch for switching at least one of said column drivers between a third configuration in which the column driver drives at least one column, and a fourth configuration, different from said third configuration, in which the column driver drives at least two columns. 
     In a further embodiment of the invention, operation of said switch or switches allows the display device to operate in at least two resolutions, including a high resolution mode and a low resolution mode 
     In a further embodiment of the invention, the number of grey levels in the low resolution mode is higher than would be possible without said switch or switches, for a device also capable of operating in said high resolution mode. 
     In a further embodiment of the invention, said picture elements are arranged in an array of identical groups of picture elements, each group being formed by picture elements at the intersection of number of adjacent rows and columns of picture elements, and each group containing picture elements of different sizes. 
     Each such group of picture elements may be connected to an identical switch or set of switches. 
     The display device may be formed from a liquid crystal panel. 
     The liquid crystal panel may form a passively addressed array. 
     The display device may be a bistable liquid crystal display device for example a bistable twisted nematic liquid crystal display or ferroelectric liquid crystal diplay. 
     The invention allows operation of a panel in at least two different resolutions with optimised numbers of grey levels and a minimised number of line drivers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 shows four adjacent subpixels of equal size; 
     FIG. 2 shows four adjacent subpixels of differing sizes; 
     FIG. 3 shows an embodiment of the invention, incorporating nine adjacent subpixels of different sizes; and 
     FIG. 4 is a schematic showing an extension of the embodiment of FIG. 3 incorporating subpixels of three different colours. 
     FIG. 5 shows an alternative pixel element arrangement to be used with a similar switching configuration to that used in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments to be described relate to bistable twisted nematic (BTN) liquid crystal displays (LCDs). Consider, for example, such a display comprising 1200×1600 pixels. It can be seen from FIG. 1 that this display can be used to display an image of 600×800 pixels if each square of 2×2 pixels is treated as a single pixel  10  comprising four subpixels  12 . The four subpixels  12  are all of equal size and are driven by two column electrodes  14 , and two row electrodes  16 , each electrode having an associated driver (not shown). Each subpixel  12  can be either “ON” (light) or “OFF” (dark). The subpixels  12  therefore allow the single pixel  10  to display 5 different grey levels. These grey levels correspond to having 0, 1, 2, 3 or 4 subpixels  12  in the “ON” state. However, there is redundancy in such an arrangement because different combinations of subpixels  12  in the “ON” state can result in the same grey level. For example, there are four possible ways of having one subpixel  12  in the “ON” state. 
     However, the 1200×1600 display can still be used to display an image of 1200×1600 pixel if required by treating each subpixel  12  as a single pixel having no intermediate grey levels. The display is thus capable of providing two different screen resolutions. 
     FIG. 2 shows that the redundancy of FIG. 1 can be removed by creating four subpixels  18 ,  20 ,  22  and  24  of different sizes. If the original pixel  10  is considered as being divided vertically by a vertical line  30  in the ratio of 1:2, and horizontally by a horizontal line  32  in the ratio of 1:4, then the first subpixel  18  represents {fraction (1/15)} of the total area of the pixel  10 , the second subpixel  20  {fraction (2/15)} of the area, the third subpixel  22  {fraction (4/15)} of the area, and the fourth subpixel  24  {fraction (8/15)} of the area. In order to show this more clearly, imaginary dotted lines  34  have been drawn on the pixel  10 . When the pixel  10  is divided in this way it is possible to achieve  16  different, evenly spaced, grey levels by selecting different combinations of the subpixels  18 ,  20 ,  22  and  24 . 
     The display can then be used to display an image of 600×800 pixels, with  16  grey levels for each pixel. However, the display can no longer be used to display an image of 1200×1600 pixels. 
     FIG. 3 shows how this problem can be overcome, while at the same time continuing to use only two column drivers and two row drivers for each pixel  10 . In FIG. 3 the original pixel  10  is divided into 9 subpixels  41  to  49 , which form three columns  51 ,  52  and  53 , and three rows  61 ,  62  and  63 , as shown in FIG.  3 . 
     It is helpful to visualise the layout of FIG. 3 as being produced by modifying that of FIG. 2 by carrying out the following two steps. First of all, the row formed by subpixels  18  and  20  is moved downwards until the horizontal line  32  reaches the centre of the pixel  10 , and becomes the horizontal line  54  in FIG.  3 . The top of this row is indicated by horizontal line  55 . Then the pixel  10  is divided down its centre by vertical line  56 . Vertical line  30  remains in the same place. 
     It will thus be seen that the three columns  51 ,  52  and  53  split the pixel  10  in the ratios 2:1:3, and the three rows  61 ,  62  and  63  split the pixel  10  in the ratios 1.5:1:2.5 or equivalently 3:2:5. Thus by driving selected rows together, and selected columns together, it is possible to produce either the ratios of FIG. 1 or the ratios of FIG.  2 . In particular, the columns can be driven either in the ratio (2+1):3=1:1 (corresponding to FIG. 1) or in the ratio 2:(1+3)=1:2 (corresponding to FIG.  2 ). Similarly, the rows can be driven either in the ratio (3+2):5=1:1 (corresponding to FIG. 1) or in the ratio 2:(3+5)=1:4 (corresponding to FIG.  2 ). 
     Furthermore, this can be done without increasing the number of drivers. That is, it is still possible to use only two drivers for each pixel  10  provided that switches are used to connect each driver to different combinations of rows or columns. This will be explained in greater detail below. 
     The three columns  51 ,  52  and  53  are addressed by three column electrodes  57 ,  58  and  59  respectively, and the three rows  61 ,  62  and  63  are addressed by three row electrodes  67 ,  68  and  69  respectively. Two column drivers  70  and  72  are driven by column driving electronics  74  and two row drivers  80  and  82  are driven by row driving electronics  84 . Column drivers  70  and  72  remain permanently connected to column electrodes  57  and  59  respectively, and row drivers  80  and  82  remain permanently connected to row electrodes  68  and  69  respectively. 
     Two column MOSFET transmission gate switches  76  and  78  are provided to connect column electrode  58  either to column driver  70  or  72  under the control of a signal labelled RES in FIG.  3 . Similarly, two row MOSFET transmission gate switches  86  and  88  are provided to connect row electrode  68  to either row driver  80  or  82  under the control of the RES signal. When the RES signal is high transmission gates  76  and  88  conduct and transmission gates  78  and  86  behave as open circuits. The display then operates in high resolution mode (1200×1600 in our example) with each pixel  10  behaving as four separate pixels of equal size. When the RES signal is low, gates  78  and  86  conduct and gates  76  and  88  behave as open circuits The display then operates in low resolution mode (600×800 in our example) with each pixel providing 16 possible grey scales as in the case of FIG.  2 . For clarity the circuitry for only one pixel  10  is shown in FIG.  3 . The other pixels are provided with the same circuitry. 
     FIG. 4 is a schematic showing how the embodiment of FIG. 3 is extended to provide a colour display capable of operating in two different resolutions, without increasing the numder of drivers (over that needed for one resolution), while retaining the maximum number of grey scales, or colours, in the lower resolution. 
     As can be seen from FIG. 4, the pixel  10  is divided into 27 subpixels, which continue to form three rows  61 ,  62  and  63 . Colour filters are placed over the pixel  10  in six vertical stripes. These form red, green and blue filters  90 ,  91  and  92  respectively. The pixel  10  is also divided into subpixels along the vertical lines dividing these filters. Horizontal and vertical lines  54  and  56 , dividing the pixel  10  centrally, remain in the same positions, as does horizontal line  55  dividing the first and second rows  61  and  62 . However, vertical line  30 , which in FIG. 2 divided the pixel  10  in the ratio 1:2, is replaced by three separate vertical lines  30   a ,  30   b  and  30   c  which divide the first red, green and blue filters  90 ,  91  and  92  in the ratios 2:1 respectively. The effect of this can most easily be appreciated by considering each colour separately. The effect is that each colour is divided vertically in the ratios 2:1:3, which are the same ratios as used in FIG.  3 . 
     The embodiment of FIG. 4 can be driven with only two row drivers  94  and  96 , and six column drivers  101  to  106  provided suitable switching circuitry is used to connect each driver to either one or two rows or columns as indicated schematically in FIG.  4 . The pixel  10  of FIG. 4 can thus be used as a single pixel (in a low resolution mode) which is capable of displaying 4096 (ie 16 3 ) colours, or as four separate pixels (in a high resolution mode) each capable of displaying 8 (ie 2 3 ) colours. In the latter case, each of the four pixels contains three subpixels each of a different colour, thus allowing 16 possible colours. 
     FIG. 5 shows an alternative pixel element arrangement to be used with a similar switching configuration to that used in FIG.  3 . In this case the high resolution mode is again 1:1:1:1 ratio of pixel areas, but in the low resolution mode the selected areas are in the ratio 1:3:4:12. The column electrodes are divided in the ratio 1:1.5:2.5, whilst the rows are divided in the ratio 1.25:1.25:2.5. Although these are not ideal ratios, if each of the picture element were able to show 8 linearly spaced greyleveles 0, 1, 2, 3, 4, 5, 6, 7, then in the high resolution mode the display would be a 3 bit display whilst in the low resolution mode the panel would be greater than a 7 bit display. As an example a liquid crystal display used in this way might be a diode matrix addressed liquid crystal panel.