GB2320357A

GB2320357A - Liquid crystal display

Info

Publication number: GB2320357A
Application number: GB9626104A
Authority: GB
Inventors: Jonathan Rennie Hughes; John Clifford Jones
Original assignee: UK Secretary of State for Defence; Sharp Corp
Current assignee: UK Secretary of State for Defence; Sharp Corp
Priority date: 1996-12-16
Filing date: 1996-12-16
Publication date: 1998-06-17
Also published as: GB9626104D0

Abstract

A ferroelectric liquid crystal display comprises an addressable matrix of pixels and addressing circuitry for selectively addressing each pixel in order to vary the transmission level of the pixel relative to the transmission levels of the other pixels. The addressing circuitry includes spatial and/or temporal dither circuits for addressing separately addressable subpixels with different spatial dither signals and/or for addressing each pixel or subpixel with different temporal dither signals in separate subframes. In addition to such spatial and/or temporal dither, the addressing circuitry switches each pixel or subpixel between different grey states corresponding to different transmission levels, with at least two of the bits of spatial or temporal dither being switchable between more than two different grey states and at least one bit being switchable between a lesser number of different grey states than the or each other bit, so that a plurality of different overall transmission levels are achievable by different combinations of spatial and/or temporal dither and such grey states. This allows a larger number of substantially linearly spaced or suitably weighted grey levels to be produced than has previously been possible without giving rise to unacceptable complications.

Description

"DisplaylShutter Devices" This invention relates to display/shutter devices, and is concerned more particularly, but not exclusively, with liquid crystal display and optical shutter devices including spatial light modulators.

Liquid crystal devices are commonly used for displaying alphanumeric information and/or graphic images. Furthermore liquid crystal devices are also used as optical shutters, for example in printers and as spatial light modulators. Such liquid crystal devices comprise a matrix of individually addressable display/shutter elements which can be designed to produce not only black and white, but also intermediate tones, or colour variations in devices in which colour filters are used. The so-called grey-scale response of such a device may be produced in a number of ways.

For example, the grey-scale response may be produced by modulating the transmission of each element between "on" and "off' states in dependence on the applied drive signal so as to provide different levels of analogue grey scale. In a twisted nematic device, for example, the transmission of each element may be determined by an applied RMS voltage and different shades of grey may be produced by suitable control of the voltage. In active matrix devices the voltage stored at the picture element similarly controls the grey shade. On the other hand, it is more difficult to control the transmission in an analogue fashion in a ferroelectric liquid crystal device, although various methods have been reported by which the transmission may be controlled by modulating the voltage signal in such a device. In devices having no analogue grey-scale, a grey-scale response may be produced by so-called spatial or temporal dither techniques, or such techniques may be used to augment the analogue grey-scale.

In a spatial dither (SD) technique each element is divided into two or more separately addressable sub-elements which are addressable by different combinations of switching signals in order to produce different overall levels of grey. For example, in the simple case of an element comprising two equal sized sub-elements each of which is switchable between a white state and a black state, three grey levels (including white and black) will be obtainable corresponding to both sub-elements being switched to the white state, both sub-elements being switched to the black state, and one sub-element being in the white state while the other sub-element is in the black state. If both subelements are of the same size, the same grey level will be obtained regardless of which of the sub-elements is in the white state and which is in the black state, so that the switching circuit must be designed to take account of this level of redundancy. It is also possible for the sub-elements to be of different sizes which will have the effect that different grey levels will be produced depending on which of the two sub-elements is in the white state and which is in the black state. However a limit to the number of subelements which can be provided in practice is imposed by the fact that separate conductive tracks are required for supplying the switching signals to the sub-elements and the number of such tracks which can be accommodated is limited by space constraints.

In a temporal dither (TD) technique at least part of each element is addressable by different time modulated signals in order to produce different overall levels of grey.

For example, in a simple case in which an element is addressable by two sub-frames of equal duration, the element may be arranged to be in the white state when it is addressed so as to be "on" in both sub-frames, and the element maybe arranged to be in the dark state when it is addressed so as to be "off' in both sub-frames. Furthermore the element may be in an intermediate grey state when it is addressed so as to be "on" in one subframe and "of?' in the other sub-frame The dither frequency should be much greater than the frequency at which the device is refreshed to provide normal switching of the element so that the resulting dither will not be observable as flickering. Furthermore it is possible to combine such a temporal dither technique with spatial dither by addressing one or more of the sub-elements in a spatial dither arrangement by different time modulated signals. This allows an increased range of grey levels to be produced at the cost of increased circuit complexity.

In many applications, and particularly in display devices for displaying moving graphic images, there is a requirement for a large number of suitably spaced grey levels to be generated, with minimum (preferably no) redundancy of grey levels. Usually the grey levels are linearly spaced as far as possible. To this end the elements may be binary weighted, for example by dividing each element into three sub-elements having surface areas in the ratio 4 : 2 : 1 in a spatial dither arrangement. In this case, assuming that each sub-element is separately switchable between a black state B corresponding to a unit grey level of 0 and a white state W corresponding to a unit grey level of 1, and that the total grey level is given by adding together the grey levels of the three sub-elements with the appropriate binary weighting, 8 different grey levels without redundancy are obtainable by addressing of the three sub-elements concurrently as follows: 4 2 1 Total B B B 0 B B W 1 B W B 2 B W W 3 W B B 4 W B W 5 W W B 6 W W W 7 For the case where one form of digital dither (either spatial or temporal) is used exclusively, the number of grey levels achieved for b bits of dither is 2b, where the optimum distribution dither weightings are 20. 2l: 22...

EP 0453033A1 discloses a display device of this type, as well as a means of attempting to minimise the number of conductive tracks required for producing a maximum number of grey scales by providing an optimum relationship between the ratios of the surface areas of the column electrode tracks and the ratios of the surface areas of the row electrode tracks.

It is an object of the invention to provide a display/shutter device with means for enabling a large number of grey levels to be produced, with the grey levels preferably being spaced apart substantially linearly or in a sequence of desired weightings.

According to the present invention there is provided a display/shutter device comprising an addressable matrix of display/shutter elements, and addressing means for selectively addressing each element in order to vary the tone or colour level of the element relative to the tone or colour levels of the other elements, wherein the addressing means includes spatial and/or temporal dither means for addressing separately addressable spatial parts of each element with different combinations of spatial dither signals and/or for addressing at least part of each element with different time modulated temporal dither signals in order to be capable of producing a plurality of different tone or colour levels, and wherein the addressing means further includes analogue level selection means for switching at least a part of each element between n different tone or colour analogue levels in response to different analogue switching signals, where n is an integer greater than 2, whereby a plurality of different overall tone or colour levels are obtainable by selection of different combinations of spatial and/or temporal dither signals and analogue switching signals.

It should be understood that the black and white states are to be considered as constituting tone or colour levels in the context of this specification.

The combination of spatial and/or temporal dither with analogue grey levels (which term includes black and white) such that at least a part of each element has three or more different analogue grey (tone or colour) levels means that a larger number of substantially linearly spaced or suitably weighted grey levels can be produced than has previously been possible without giving rise to unacceptable complications. Furthermore the weighting can be chosen so as to minimize, or preferably remove altogether, any redundancy in the overall grey levels.

Where both spatial and temporal dither are combined to give multiple grey levels in an arrangement in which each sub-element has only 2 analogue levels B/W as described above, it is also preferable to use binary weighted levels to give no redundancy and linearly spaced levels. Consider the case of a bits of spatial dither (aSD) and b bits of temporal dither (bTD) where a and b are both positive integers. The maximum number of grey levels is given by 2"b. For 2SD and 3TD, 64 grey levels are possible using weightings such as 1:2 SD and 1:4:16 TD. This is because the least significant spatial bit gives the temporal levels 1, 4 and 16, whereas the most significant spatial bit gives the levels 2, 8 and 32. The optimum digital weightings follow the pattern 20:2L .. 2 '-' and 2 . 2 :2 2.2 3'...2 *iAwhere a and b may be either spatial or temporal dither interchangeably. Following these principles, the optimum weightings required to achieve, say 212=4096 levels using a = 3 bits SD (or TD) and b = 4 bits TD (or SD) dither are the weightings 1:2:4 SD (or TD) and 1:8:64:512 TD (or SD).

EXAMPLES In order that the invention may be more fully understood, a number of different arrangements for producing different grey levels in display devices in accordance with the invention will now be described by way of example only.

Each of the examples to be described uses a ferroelectric display device (FLCD) comprising a layer of ferroelectric material in the smectic phase contained between two glass substrates arranged parallel to one another, and oppositely facing electrode structures applied to the inwardly directed faces of the substrates in the form of electrode tracks arranged in rows and columns and crossing one another to form an addressable matrix of display elements. The electrode tracks may alternatively be arranged to form a polar coordinate (r, 6) matrix, a seven bar numeric matrix or some other x-y matrix. As is well known, the display elements or pixels at the intersections of the row and column electrode tracks are addressable by the application of suitable strobe and data pulses to the row and column electrodes. One such addressing scheme is disclosed in "The Joers/Alvey Ferroelectric Multiplexing Scheme", Ferroelectrics 1991, Vol. 122, pages 63 to 79. Furthermore each pixel, or each sub-element of each pixel where the pixel is sub-divided into two or more sub-elements, has n different analogue grey levels dependent on the voltage waveform applied to switch the pixel or sub-element, so that, in addition to the black state B and the white state W referred to above, the pixel or sub-element has one or more intermediate grey states G.

In a first example, each pixel is divided into three sub-elements arranged side-by side and having surface areas in the ratio 9 : :1 in a spatial dither (SD) arrangement, each sub-element being separately switchable between three different linearly spaced analogue grey levels, that is between a black state B corresponding to a unit grey level of 0, a white state W corresponding to a unit grey level of 1, and an intermediate grey state G corresponding to a unit grey level of 0.5. The following table shows that 27 different linearly spaced total grey levels are obtainable without redundancy in this example by combining the grey levels of the three sub-elements with the appropriate digital weightings.

9 3 1 Total B B B/G/W 0/0.5/1 B G B/G/W 1.5/2/2.5 B W B/G/W 3/3.5/4 G B B/G/W 4.5/5/5.5 G G B/G/W 6/6.5/7 G W B/G/W 7.5/8/8.5 W B B/GIW 9/9;5/10 W G B/G/W 10.5/11/11.5 W W B/G/W 12/12.5/13 More generally, if each pixel is divided into b sub-elements, each of which is separately switchable between n different linearly spaced analogue grey levels, the subelements have surface areas which are preferably weighted in the ratio n :n' .. . nb-' so that a maximum of nb different linearly spaced total grey levels are obtainable without redundancy. For example, where three different analogue grey levels are provided, the surface areas ofthe sub-elements are preferably weighted in the ratio 1 3 3I, and, where four different analogue grey levels are provided, the surface areas of the sub elements are preferably weighted in the ratio 1: 4 4 4C'. Similar digital weightings may be applied to an arrangement utilizing temporal dither rather than spatial dither.

Greater complexity is introduced if the greyscale progression is required to be nonlinear, for example logarithmic.

In a further example, each pixel comprises three sub-elements arranged side-byside in a spatial dither (SD) arrangement and having surface areas in the ratio 16 : 2 1.

However, only the largest of the three sub-elements has more than two analogue grey levels, in this example five linearly spaced analogue grey levels 0, 0.25, 0.5, 0.75 and 1 (giving levels of 0, 4, 8, 12 and 16 when weighted by the factor 16) where 0 corresponds to the black state and 1 corresponds to the white state, the other two subelements having only unit grey levels of 0 corresponding to the black state and 1 corresponding to the white state. This enables a maximum of 22 X 5 = 20 different linearly spaced total grey levels to be obtained, as shown in the table below, whilst restricting the drive circuitry required for the device by virtue of the fact that only one of the sub-elements of each pixel requires the application of multiple voltage levels thereto for switching between the different analogue grey levels. Since only the largest sub-element has multiple grey levels, the number of such grey levels may be maximized whilst keeping the cost of the associated drive circuitry within reasonable bounds.

Similar cost advantages are obtainable by such restriction of the drive circuitry in the alternative arrangements where either only temporal dither is used (and not spatial dither) or temporal and spatial dither are combined.

16 2 1 Total 0 0 0 0 0 0 1 1 0 1 0 2 0 1 1 3 4 0 0 4 4 0 1 5 4 1 0 6 4 1 1 7 8 0 0 8 8 0 1 9 8 1 0 10 8 1 1 11 12 0 0 12 12 0 1 13 12 1 0 14 12 1 1 15 16 0 0 16 16 0 1 17 16 1 0 18 16 1 1 19 Many more examples can be given where the digital bits have different numbers of analogue grey levels. However, it is reasonable to assume that, in cases where mixed analogue grey levels are provided, it is preferable for the more significant bits to have the largest number of analogue grey levels. This may help reduce the cost of driving circuitry as stated above, or may have other advantages. For example, it is reasonable to assume that a large number of analogue grey levels may be difficult to obtain reproducibly in the smallest spacial bit where the number of domains used to obtain the analogue grey levels is relatively small. Thus it may be preferable to restrict the analogue greyscale to the most significant bit, or the more significant bits. Alternatively, the number of analogue grey levels may change systematically from one digital bit to the next. Thus, if three digital bits are provided, the 256 grey levels may be obtained from 256 = 2 x 8 x 16 where the least significant bit has two analogue grey levels (0, 1), the second bit has eight analogue grey levels (0, 2, 4, 6, 8, 10, 12, 14) and the most significant bit has sixteen analogue grey levels (0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 198, 208, 224, 240) Where n analogue grey levels are used with both TD and SD, the maximum number of grey levels is n"b, with the optimum weightings being given by n :nl:....ntl and n0:n2:n2':n3' n (bl) where a and b may be either spatial or temporal dither interchangeably. For example three analogue levels (n = 3) with 2SD (a = 2) and 3TD (b = 3) give 729 grey levels if the weightings are SD = 1:3 and TD = 1:9:81. Five analogue levels give 244, 140, 625 (i.e. 53X4) grey levels with 3SD (or TD) and 4TD (or SD) ifthe weightings are in the ratio SD (or TD) = 1:5:25 and TD (or SD) = 1: 125: 15,625: 1,953,125.

Generally it is useful to calculate the number of digital bits b (for example the number of sub-elements required in a spatial dither arrangement) required to generate close to 2' = 256 total grey levels. For the case where each digital bit has the same number n of analogue grey levels, the number b of digital bits is calculated from the expression: b log10n = log,0256 hence b = log10 256 / log10n and b = 2.4082 / log10n The following solutions to this expression may be calculated, the number of grey levels which may be acceptable in practice being given by nINT(b): n b INT(b) nINT(b) 2 8 8 256 3 5.047 5 243 4 4 4 256 5 3.445 3 125 6 3.095 3 216 7 2.850 3 343 8 2.667 3 512 9 2.524 3 729 10 2.408 2 100 11 2.312 2 121 12 2.231 2 144 13 2.162 2 169 14 2.101 2 199 15 2.048 2 225 16 2 2 256 Where both temporal and spatial dither are combined, each having n analogue grey levels where n is a positive integer over 2, then 256 grey levels may be achieved from log n 2 log 256 /(axb). Examples of numbers n of grey levels required to obtain 256 overall greys when combining spatial and temporal dither are shown below: a b minimum n required for 256 or greater overall grey levels 1 2 16 1 3 7 2 2 4 2 3 3 2 4 2

Claims

CLAIMS 1. A display/shutter device comprising an addressable matrix of display/shutter elements, and addressing means for selectively addressing each element in order to vary the tone or colour level of the element relative to the tone or colour levels of the other elements, wherein the addressing means includes spatial and/or temporal dither means for addressing separately addressable spatial parts of each element with different combinations of spatial dither signals and/or for addressing at least part of each element with different time modulated temporal dither signals in order to be capable of producing a plurality of different tone or colour levels, and wherein the addressing means further includes analogue level selection means for switching at least a part of each element between n different tone or colour analogue levels in response to different analogue switching signals, where n is an integer greater than 2, whereby a plurality of different overall tone or colour levels are obtainable by selection of different combinations of spatial and/or temporal dither signals and analogue switching signals.
2. A display/shutter device according to claim 1, wherein the addressing means includes either spatial or temporal dither means for addressing each element with b bits of dither, where b is an integer equal to or greater than 2, and the maximum number of overall tone or colour levels obtainable by selection of different combinations of spatial or temporal dither signals and analogue switching signals is nb.
3. A display/shutter device according to claim 2, wherein the relative weightings of the b bits of dither are in the ratio n : .... nSl
4. A display/shutter device according to claim 2 or 3, wherein only one of the b bits of dither is switchable between n different tone or colour analogue levels, each other bit being switchable between a lesser integral number of different tone or colour analogue levels.
5. A display/shutter device according to any preceding claim, wherein the addressing means includes spatial dither means and each element is divided into b subelements individually addressable by said spatial dither means, where b is an integer equal to or greater than 2, and wherein at least one of the b sub-elements of each element is switchable between n different tone or colour analogue levels in response to different analogue switching signals, whereby a plurality of different overall tone or colour levels are obtainable by addressing of the b sub-elements of the element with different combinations of spatial dither signals and analogue switching signals.
6. A display/shutter device according to claim 5, wherein the b sub-elements of each element are arranged side-by-side such that the tone or colour levels of the subelements are added together to obtain the overall tone or colour level of the element.
7. A display/shutter device according to claim 5 or 6, wherein the b sub-elements of each element have surface areas which are in the ratio nO . . nsl and which together constitute the overall surface area of the element within the display/shutter field of the matrix, whereby a maximum number nb of different overall tone or colour levels are obtainable by addressing of the b sub-elements with different combinations of spatial dither signals and analogue switching signals.
8. A display/shutter device according to claim 1, wherein the addressing means includes spatial dither means and temporal dither means in combination for addressing each element with a bits of temporal dither and b bits of spatial dither, where a and b are integers equal to or greater than 2, and the maximum number of overall tone or colour levels obtainable by different combinations of spatial or temporal dither signals and analogue switching signals is naxb
9. A display/shutter device according to claim 8, wherein the relative weightings of the bits of temporal dither are in the ratio nO- ....nut' and the relative weightings of the bits of spatial dither are in the ratio n :....n ', or vice versa, where a and b denote the bits of spatial and temporal dither interchangeably.
10. A display/shutter device according to any preceding claim, wherein each element is addressable with b bits of dither having relative weightings chosen so as to minimise the redundancy of overall tone or colour levels obtained by the different combinations of spatial and/or temporal dither signals and analogue switching signals.
11. A display/shutter device comprising an addressable matrix of display/shutter elements, and addressing means for selectively addressing each element in order to vary the tone or colour level of the element relative to the tone or colour levels of the other elements, wherein the addressing means includes spatial dither means for addressing separately addressable spatial parts of each element with different combinations of spatial dither signals and temporal dither means for addressing at least part of each element with different time modulated temporal dither signals in order to be capable of producing a plurality of different tone or colour levels, whereby a plurality of different overall tone or colour levels are obtainable by selection of different combinations of spatial and temporal dither signals.
12. A display/shutter device according to claim 11, wherein the addressing means further includes analogue level selection means for switching at least a part of each element between n different tone or colour analogue levels in response to different analogue switching signals, where n is an integer greater than 1.
13. A display/shutter device substantially as hereinbefore described with reference to any of the foregoing examples.