DISPLAY ARRANGEMENT WITH GRAY SCALE CONTROL

Abstract: A display arrangement comprises flat panel display device (5) having an array of picture elements. A driving circuit arrangement is provided for driving each of the picture elements by means of a two level signal which switches at a multiple of a frame rate so as to produce the visible effect of a multi-intensity image. The driving circuit comprises a plurality of intensity level registers (2), one for each intensity level to be reproduced. A source signal for a pixel of interest is applied to the register corresponding to the source intensity level. If the register content is greater than or equal to a given threshold value then the pixel value is set to a first level which is passed from decision circuit (3) via a driver circuit (4). A value of (the first level - source level) is then added to the contents of the register (2). If the register content is less than the threshold value then the pixel value is set to a second level. A value of (the second level - source level) is then added to the contents of the register (2). Subsequently a random number generated by a blue noise generator (6) is added (7) to the contents of the register (2).

Description: DESCRIPTION DISPLAY ARRANGEMENT WITH GRAY SCALE CONTROL The invention relates to a display arrangement and to a method of driving such a display arrangement. Standard colour LCD (Liquid Crystal Display) panels are normally controlled digitally and are therefor unable to display more than eight basic colour combinations (two intensity levels per colour, that is red, green, and blue). In order to overcome this limitation it is known to switch a picture element (pixel) quickly, that is at a rate greater than the frame refresh rate, between its two intensity levels to artificially generate intermediate intensity levels. This, however, introduces further problems. The simplest way of implementing the faster switching of pixels is to use a frame wide pulse width modulation arrangement but this requires an increase in frame rate by a factor equal to the required number of intensity levels. In addition there are limitations in the performance of typical display panels, such as the maximum shift clock rate and increased power consumption. In addition a higher frame rate requires a proportionate increase in the frame buffer bandwidth. It is possible to use static dithering techniques to imitate a higher colour depth. While this option works well with high resolution static images such as in colour printing, the larger pixel size associated with an LCD panel combined with the high contrast between adjacent pixels produces a high visible noise in the displayed image. One of the properties of LCD panels is the relatively slow response of the crystals to changes in the applied signal. The switching times of each LCD pixel can be of the order of tens and even sometimes hundreds of milliseconds. This behaviour improves the performance of a pixel switching algorithm, but switching complete frames is still too noticeable at low frame rates. It is known that the visibility of the flicker is dependent on the area of the flickering surface. Consequently, using different switching patterns for adjacent pixels can significantly reduce the flicker. The human brain is, however, highly specialised in pattern and shape recognition and as a result regular patterns in space and in time are very noticeable, usually as moving or trembling structures. To overcome these problems the use of a pseudo-random noise source as the basis of a dynamic dithering scheme has been proposed and described in US-A-5,703,621. The arrangement described uses a two dimensional error propagation scheme in order to produce correct shading for narrow vertical structures. While this is satisfactory as far as the displayed image is concerned it has the disadvantage of requiring complex hardware and/or software for its implementation. Consequently it is a relatively expensive solution. A much less expensive solution would be possible if a one dimensional error propagation scheme was used. This, however, results in an inability to produce correct shading for narrow vertical structures. It is an object of the invention to enable the production of images using an LCD having a greater number of levels of resolution than that provided by the pixel switching levels using dynamic dithering which is less expensive to implement than known two dimensional error propagation schemes. The invention provides a display arrangement comprising flat panel display device having an array of picture elements, a driving circuit arrangement for driving each of the picture elements by means of a two level signal, the two level signal switching at a multiple of a frame rate so as to produce the visible effect of a multi-intensity image, wherein the driving circuit comprises a plurality of intensity level registers, one for each intensity level to be reproduced, means for applying a source signal for a pixel of interest to the register corresponding to the source intensity level, means for setting the pixel value to a first level if the register content is greater than or equal to a given threshold value and then adding a value of (the first level - source signal level) to the contents of the register, means for setting the pixel value to a second level if the register content is less than the threshold value and adding ( the second level - source level) to the contents of the register, and means for adding a random number to the contents of the register. The provision of a separate register for each intensity level it is desired to reproduce enables narrow vertical structures to be reproduced without requiring the use of two dimensional error propagation schemes which require the storage of the propagation errors of a complete line. In effect a one dimensional error propagation scheme is implemented for each of the intensity levels it is desired to reproduce. Thus if it is desired to reproduce eight grey scale levels then six registers will be needed. Clearly separate registers are not needed for black and white. The means for adding a random number to the register may comprise an adding arrangement for adding the output of a blue noise generator to the contents of the register. The use of a blue noise generator reduces the possibility of producing slow flickering of the intensity of individual pixels because of the absence of a low frequency component in the noise spectrum. The spectral distribution of noise is often described by a colour. The best known is white noise which is so named because its power spectrum is constant across all frequencies of interest in the same way as white light in the visible spectrum. Low frequency noise is often known as pink noise while blue noise is used to refer to noise that has very little low frequency content and may be considered as the complement of pink noise. Robert A. Ulichney disclosed the concept of blue noise in a paper entitled "Dithering with Blue Noise" published in Proceedings of the IEEE, Vol. 76 No.1 , January 1998. The invention further provides a method of driving a flat panel display comprising an two dimensional array of picture elements (pixels) using a driving circuit arrangement to obtain a multi-intensity picture display comprising the steps of; i) providing a driving circuit arrangement for driving each of the pixels by means of a two level signal, ii) causing the driving circuit to produce said two level signal at a multiple of the frame rate, iii) providing in the driving circuit arrangement a plurality of intensity level registers, one for each intensity level to be reproduced, iv) applying a source signal for a pixel of interest to a register corresponding to the source signal intensity, v) setting the pixel level to a first value if the register content is greater than or equal to a threshold value and, if so, adding a value of (the first level - source level) to the contents of the register, vi) setting the pixel level to a second value if the register content is less than the threshold value and, if so, adding (the second level - source level) to the contents of the register, and vii) adding a random number to the contents of the register. The above and other features and advantages of the invention will be apparent from the following description, by way of example, of an embodiment of the invention with reference to the accompanying drawings, in which:- Figure 1 shows in block schematic form a display arrangement according to the invention, and Figure 2 shows an embodiment of a blue noise generator suitable for use in the embodiment of Figure 1. Figure 1 shows in block schematic form a display arrangement comprising a signal source 1 which produces a signal representative of the intensity level it is desired that a given pixel should produce. This signal may define a colour intensity or a grey level depending on whether a colour or monochrome display is required. In the following description the term "grey level" will be used for simplicity but it will be understood that such usage is intended to cover both monochrome and colour display arrangements and should be interpreted accordingly. The signal from the signal source 1 , which is preferably in digital form, is fed to an appropriate one of a plurality of registers forming a register bank 2. The particular register to which the signal is applied is selected according to the amplitude or grey level of the signal. The output of the selected register is connected to the input of a decision circuit 3 whose output is fed to an LCD driver circuit 4 which drives the display 5. A blue noise generator 6 has its output connected to a first input of a summing arrangement 7, a second input of which is connected to the output of the selected register of the register bank 2. The output of the summing arrangement 7 is connected to the input of the selected register. The output of the signal source 1 is further connected to a first input of a subtractor arrangement 8,while the output of the decision circuit 3 is further connected to a second input of the subtractor arrangement 8. The output of the subtractor arrangement 8 is connected to a third input of the summing arrangement 7. The following description will explain the operation of the apparatus described with reference to Figure 1. The signal from the signal source 1 is used to select the register in the register bank 2 that is allocated to the particular grey level represented by the signal. The output of that register is fed to the decision circuit 3 where it is determined whether the contents of the register are greater than or equal to one. If it is then the output of the decision circuit connected to the LCD driver circuit 4 causes the driver circuit to set the target pixel to black. The input signal is then subtracted from the black level by the subtracting arrangement 8 and the result is added to the value in the selected register and then entered into the register. If the decision circuit 3 determines that the register value is less than one then it causes the driver circuit 4 to set the target pixel to white. The input signal is then subtracted from the white level and the result is added to the value in the selected register and then entered into the register. In both cases a random value generated by the blue noise register 6 is also added to the value in the register. This affects the local accuracy of the reproduced intensity but produces the required dynamics over time. Because the mean value of the noise source is zero there is no global bias in the intensity distribution. An intentionally generated offset in the noise signal generated may, however, be used to compensate for non-linearity in the display. It will be apparent that a separate error register is used for each intermediate intensity level it is desired to reproduce to keep track of the cumulative error produced in the dithered image so far. Assuming an original image with sixteen distinct grey levels and a target display capable of black and white only a local error of +/-15 is possible at every pixel location. As an example, for a grey level of five in the source image, making the targeted pixel white produces a local error of +10, i.e. 15-5. If this is then added to the error register for that grey level and this value is used in the decision for the next pixel having the intensity level five the overall error in the image is kept between +/-15. A common problem of all error propagation algorithms is the global response to local events. In this context that is a small area at one level within a large one at a different level will influence the dither pattern for the large area. In some instances this produces highly visible artifacts in the image produced. In addition border effects in the direction of error propagation tend to occur with steps in the grey level. By using a separate register for each grey level the global response is damped and the border effects substantially reduced. In the present embodiment a plurality of registers are used and the input signal level selects which one of them is used. The grey level of the input signal can be regarded as the register address selecting which one is to be used to drive the pixel Since each grey level selects a different register within the register bank error propagation does not take place over successive pixels but instead takes place over successive pixels having the same desired intensity. Consequently these errors will not accumulate at boundaries, where intensity levels vary in a stepwise manner, and therefor narrow vertical structures can be more faithfully defined without requiring classical two dimensional error propagation. The present arrangement uses a one dimensional error propagation scheme but this error propagation is separately carried out for each intensity level. Figure 2 shows an embodiment of a blue noise source suitable for use as the blue noise generator 6 in the display arrangement of Figure 1. As shown in Figure 2, the blue noise generator comprises a pseudo-random sequence generator 21 followed by a high pass filter 22. The implementation of such functions is well known to those skilled in the art, but typically the generator 21 will comprise one or more shift registers with selected one(s) of the stages fed back to the input and the high pass filter 22 will conveniently be a digital filter so that its output will be a random bit stream with lower frequencies suppressed. In order to produce colour displays it is usual to provide for each pixel three sub pixels each comprising a liquid crystal element and having a filter overlaying it. These filters are normally red, green, and blue and as a result the combined display by the pixel will take an appropriate colour. In this case, the display would be driven by an RGB signal and each of the three components would be provided with a separate bank of registers, decision circuits, adders, and drivers. From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design and use of display methods and apparatus and component parts thereof and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation of one or more of those features which would be obvious to persons skilled in the art, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims: CLAIMS 1. A display arrangement comprising flat panel display device (5) having an array of picture elements, a driving circuit arrangement for driving each of the picture elements by means of a two level signal, the two level signal switching at a multiple of a frame rate so as to produce the visible effect of a multi-intensity image, wherein the driving circuit comprises a plurality of intensity level registers (2), one for each intensity level to be reproduced, means (1) for applying a source signal for a pixel of interest to the register corresponding to the source intensity level, means (3) for setting the pixel value to a first level if the register content is greater than or equal to a given threshold value and then adding (7, 8) a value of (the first level - source level) to the contents of the register, means (3) for setting the pixel value to a second level if the register content is less than the threshold value and adding (7, 8) (the second level - source level) to the contents of the register, and means (6, 7) for adding a random number to the contents of the register. 2. A display arrangement as claimed in Claim 1 in which the means for adding a random number to the register comprises an adding arrangement (7) for adding the output of a blue noise generator (6) to the contents of the register (2). 3. A display arrangement as claimed in Claim 1 or Claim 2 in which each pixel comprises three sub-pixels of different colours, each sub pixel being associated with its own plurality of registers. 4. A method of driving a flat panel display comprising an two dimensional array of picture elements (pixels) using a driving circuit arrangement to obtain a multi-intensity picture display comprising the steps of; i) providing a driving circuit arrangement for driving each of the pixels by means of a two level signal, ii) causing the driving circuit to produce said two level signal at a multiple of the frame rate, iii) providing in the driving circuit arrangement a plurality of intensity level registers, one for each intensity level to be reproduced, iv) applying a source signal for a pixel of interest to a register corresponding to the source signal intensity, v) setting the pixel level to a first value if the register content is greater than or equal to a threshold value and, if so, adding a value of (the first level - source level) to the contents of the register, vi) setting the pixel level to a second value if the register content is less than the threshold value and, if so, adding (the second level - source level) to the contents of the register, and vii) adding a random number to the contents of the register. 5. A method as claimed in Claim 4 wherein step vii) comprises the steps of; viii) generating a blue noise signal, and ix) adding the blue noise signal to the contents of the register. 6. A method as claimed in Claim 5 of driving a flat panel display comprising an two dimensional array of picture elements (pixels), each pixel comprising three sub pixels each arranged to display a different primary colour, the method comprising the further step of; x) providing a plurality of intensity level registers for each of the sub pixels. 7. A method as claimed in Claim 6 wherein for each of the sub pixels step vii) comprises the steps of viii) generating a blue noise signal, and ix) adding the blue noise signal to the contents of the register.

Inventors: JOHNSON NEIL M, MERGLER IWO, JOHNSON, NEIL, M., MERGLER, IWO
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: G09G3
IPC codes: G06T5, G09G3, H04N1