DISPLAY DEVICE

Abstract: In a liquid crystal display, each pixel is formed of four separately energisable areas I, II, III, IV which have relative light outputs in the ratio 1:2:4:8 respectively, thereby allowing a range of 16 grey levels to be achieved by appropriate selection of areas. Each of the areas has a number of segments, which are spaced-apart but concentrically arranged, in order to have the same average position, so that different bit patterns cause the pixel to change brightness only, without any apparent positional change.

Description: The present invention relates to a display device and especially, but not solely, to a television receiver with a bistable display material e.g. ferroelectric liquid crystals. U.S. Patent Specification No. 3845243 discloses a gas discharge display panel in which each display element is divided into several areas which can be individually energised. To display a grey scale the number of energised areas in each element is selected according to the required brightness value. British Patent Specification No. 1390925 shows, in Figure 9, a display device comprising a lattice of pixels to provide an image representing a picture-signal, each pixel being divided into a plurality of discrete areas, each of which areas is individually settable to either of two states, only one said state producing a light output, the light outputs of the areas of a pixel not being the same. The present invention provides such a device which is characterised in that the areas of each pixel are arranged in substantially concentric manner, such that they have substantially the same average position. The human eye is very sensitive to small errors in apparent pixel position. However, the display device of the present invention overcomes any such positional error because the areas of a pixel may have essentially the same average position when viewed, so that different patterns of area driving in a pixel present a change of brightness, without any apparent positional change. Preferably, the light outputs of the areas in a pixel have a binary relationship therebetween. Advantageously, a pixel has four distinct areas with light outputs in the ratio 8:4:2:1. The present invention also provides a method of operating this display device, comprising effecting first and second setting scans of the lattice for each received picture-signal; applying, in the first scan, to each of the pixels an amount of the picture signal corresponding to the relevant least-significant-bit information to set the pixel for a first specified time period; and applying, in the second scan, to each of the pixels the remainder of the picture signal corresponding to the relevant most-significant-bits information to set the pixel for a second specified time period. A display device embodying the present invention can be driven in any appropriate manner. The binary relationship of the areas in a pixel of the present invention ensures that an amount of grey levels substantially greater than the number of areas can be achieved. Thus for four areas, sixteen grey levels are produced. Preferably, each pixel is formed of a plurality of selectively-settable liquid crystal cells. The present invention is applicable to colour displays and to black-and-white displays. The present invention also embodies a display panel for use in a display device as herein defined. Furthermore, the present invention also embodies equipment for the generation, and/or transmission, and/or processing, and/or reception, of signals suited and/or designed for a display device as herein defined. In order that the invention may more readily be understood, a description is now given, by way of example only, reference being made to the accompanying drawings, in which: Figure 1 is a simplified representation of a pixel of a display device according to the present invention; Figure 2 is a plan view of a pixel embodying the present invention; Figure 3 is a plan view of the metallisation row-patterning of a pixel in a display device according to the present invention; Figure 4 is a plan view of the metallisation column-patterning of a pixel in a display device according to the present invention; Figure 5 is a representation of the pixel arrangement when the patterning of Figures 3 and 4 are combined; Figures 6 and 7 are plan views of the metallisation patterning on the two plates for another form of pixel which embodies the present invention. The illustrated examples of the present invention involve the generation of a grey scale television image by the multiplexing of a bistable display material, namely ferroelectric liquid-crystal material, arranged to provide a lattice of pixels. Each pixel (see Figure 1) is formed of four areas I, II, III, IV which have light outputs in the ratio 1:2:4:8 respectively, thereby allowing a range of 16 grey levels which can be achieved by appropriate selection of areas. The concentric arrangement of the areas is significant. Since the human eye is very sensitive to small errors in apparent pixel position, it is important that each of the areas has substantially the same average position, so that different bit patterns cause the pixel to change brightness only, without any apparent positional change. In one implementation, the four areas are etched onto one piece of glass coated with ITO, and all four areas are addressed with one strobe line etched onto the opposite glass plate (Figure 2). However, the active area of each pixel is reduced because of the area taken up by metal tracks bringing in data, resulting in some dimming of the displayed image. Figure 3 to 5 illustrate a pixel metallisation pattern for a liquid crystal display device embodying the present invention. Figure 3 shows pixel boundary 1 and the row patterning formed by regions 2 of metal and regions 3 of ITO, with rows A₁ and A₂ relating to pixel areas II and IV and Row B relating to pixel areas I and III. Figure 4 shows the column patterning again formed by regions 2 of metal and regions 3 of ITO, with Columns C₁ and C₂ relating to pixel areas III and IV, and Column D relating to pixel areas I and II. When these row and column patterning are overlapped, the resulting formation of pixel is as shown in Figure 5. This arrangement is advantageous in that it avoids conductor crossovers on either display plate, thereby eliminating the possibility of electrical shorts otherwise caused by pinholes in crossover insulation. As can be seen in the Figures, there are three row metallisations and three column metallisations; however two of the row metallisations are connected together at the edge of the screen, as are two of the column metallisations, the net result being that two external row drivers and two column drivers address the four areas of the pixel. Omitted from Figure 5 for the reason of simplicity is an appropriate opaque insulating pattern to stop the viewer seeing right to the edge of the areas. Also, it may be necessary to alter the width of the metal conductors from the dimensions as shown so that all the row and column electrodes have approximately equal time constants. Figure 6 illustrates the arrangement of strobe and data lines on one plate 20 in respect of one pixel of a display while Figure 7 illustrates the arrangement of strobe and data lines on the other plate 21 in respect of the same pixel. In this arrangement, three electrodes address three areas on each glass plate, one strobe line and two data. The pixel is still split into selectively settable areas in the ratio of 8:4:2:1 but any combinations of strobe and data lines can be used. In this example, one strobe 22 covers the areas corresponding to outputs with the ratio values 1 and 2 and is on side one of the glass plate 20 while the other strobe 23 covers the areas with output ratio values 4 and 8 of plate 21 (see Figures 6 and 7). With this electrode scheme the display can be addressed in the same time as with other layouts but with minimisation of the active area lost to the electrode pattern. The metal tracking is arranged so that tracks on one glass plate cover tracks on the other plate. In this layout, the strobe pulses move in different directions on the screen, strobe 22 moving vertically while strobe 23 moves horizontally. This may make it necessary to process more data per frame but may reduce flicker in 25 Hz sequentially refreshed displays. Also, the splitting of the strobe lines between the two plates is that both electrode and tracking patterns become of similar complexity, which is less (hence easier to manufacture, and with a higher degree of reliability) than the more complex plate in the type of arrangement of Figure 2. Plate 20 also has a line 24 for data bit 3 and a line 25 for data bit 4. Plate 21 has a line 26 for data bit 1 and lines 27, 28 for data bit 2. In operating a display device having pixels of the illustrated type, the four least significant bits of the image are input to the pixel as the video signal arrives, and the four most significant bits are input to the same pixel 1/17 of the frame time later. Thus two display lines need to be addressed every video line. The most significant bits are displayed for 16/17 of the time, and the least significant for 1/17 of the time. Flicker will be minimal, and the data storage required is 1/34 of a frame store, being half the number of bits for 1/17 of the time. Such an operation enables the display device to achieve a range of 256 grey levels. If 256 levels are insufficent to avoid contours in the blacks or mid greys, then the operation may be extended to 12 bits by inputting the 4 least significant bits as the video signal arrives, the middle 4 bits 1/273 of a frame later, and the most significant bits 16/273 of a frame after that. The storage required for a 12 bit display is not significantly greater than that for an 8 bit display, but the time in which to address a pixel is less, as three display lines rather than two need to be addressed in one video line time. If an 8 bit display is sufficient, then during the 64 microsecond line time the row of pixels corresponding to the current video line should be addressed, together with the row corrsponding to 37 (i.e.625/17) lines previously. Thus 32 microseconds is allowed for the addressing of a row of pixels. Since the arrangement of Figure 2 allocates two independent row electrodes to each pixel row, there is 16 microseconds to address each 'row' of subpixels. The display device of the invention acts as an accurate D/A converter, converting the input digital data into visible light intensity. Therefore, to avoid contouring effects, the normal requirements of a D/A converter as regards bit accuracy and stability etc. must be met. For example, the time delay between displaying the sets of 4 bits determines the relative brightness, and should be accordingly accurate; this can be readily achieved. The bistable display material may not switch completely between states immediately, but may take some time to settle; such an effect could be compensated by adjusting the exposure time of the least significant bits, or by altering the stored bit pattern. To minimise errors caused by relative inaccuracy of the bit areas in the pixel patterning, and their sensitivity to misalignment of the front and back plates, the active transparent area of pixel area on one plate is arranged to be larger than that on the other plate, such that a small misalignment between the two plates does not alter the visible pixel area. The display device of the invention allows the maximum time for multiplexing bistable pixels, while still allowing the grey scale display to be updated at video rates. Also, it minimises display flicker, and minimises the amount of data storage required. Because of the maximum action of the time of addressing each row of the display, the pixels are updated in sequence and are continuously visible. If a colour display is required, separate red, green, and blue pixels must be used rather than field sequential colour with monochrome pixels.

Claims: A display device comprising a lattice of pixels to provide an image representing a picture-signal, each pixel being divided into a plurality of discrete areas (I, II, III, IV: A, B, C, D), each of which areas is individually settable to either of two states, only one said state producing a light output, the light outputs of the areas of a pixel not being the same, characterised in that the areas of each pixel are arranged in substantially concentric manner, such that they have substantially the same average position. A display device according to Claim 1, characterised in that the light outputs of the areas in a pixel have a binary relationship therebetween. A display device according to Claim 2, characterised in that a pixel has four distinct areas with light outputs in the ratio 8:4:2:1. A display device according to any one of the preceding Claims, wherein each area of each pixel is formed of a selectively-settable liquid crystal cell. A method of operating a display device as claimed in any one of the preceding claims, comprising effecting first and second setting scans of the lattice for each received picture-signal; applying, in the first scan, to each of the pixels an amount of the picture signal corresponding to the relevant least-significant-bit information to set the pixel for a first specified time period; and applying, in the second scan, to each of the pixels the remainder of the picture signal corresponding to the relevant most-significent-bits information to set the pixel for a second specified time period. A method according to Claim 5, characterised in that the first scan comprises applying four least-significent-bits, the first specified time period being substantially 1/17 of the total time period for the picture-signal, and the second scan comprises applying four most-significent-bits, the second specified time period being substantially 16/17 of the total time period for the picture signal.

Inventors: GREEN, IAN MCDONALD
Applicants: THORN EMI PLC
Application year: 1987
Main classification: G02F1
IPC codes: H04N5, G09G3, G09F9, G02F1