Abstract:
The present invention describes a method for driving a PDP wherein motion artifacts like blur and double edges are minimized by using an upconversion to preferably 100 Hz as in CRTs with Natural Motion, combined with an LSC subfield distribution. Subfield groups from two images are combined to form a field for a display image.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method of driving a plasma display panel, the method comprising the steps of:  
           [0002]    receiving of a series of consecutive original images;  
           [0003]    distributing information from each original image over a first plurality of subfields.  
           [0004]    The invention also relates to a device for driving a plasma panel display and to a display apparatus.  
         PRIOR ART  
         [0005]    Plasma Display Panels (PDPs) are widely used for flat TV screens. These panels are simple to form a large unit, emit light by themselves, provide high display quality and achieve high response speed. In order to display an image on a display unit, the so-called “subfield driven method” is used. One image frame is shown in a number of successive periods called subfields. During a subfield, an amount of light is emitted which is dependent on the weight of the subfield. Each subfield has a different weight. A desired intensity level for a pixel in the image is realised by controlling the specific subfields. The human eye sees the sum of the intensity levels of the enabled subfields within a field (i.e. an image) due to the integrating character of the human eye. In this way a subfield driven method using 8 subfields can display a maximum of 2 8  halftone levels.  
           [0006]    Plasma Display Panels suffer from several motion artefacts. One type of these motion artefacts is Dynamic False Contouring (DFC). DFC may occur in relative large areas with little luminance differentiation. The DFC artefacts can sufficiently be reduced by using an adapted subfield distribution. The remaining motion artefacts, such as double/coloured edges and motion blur, are caused by the large time difference between the first and last subfields of a TV field period. To avoid these artefacts, a subfield distribution is usually selected showing one central peak. The subfields with the highest values are placed in the middle of the frame. Another variant is placing the subfields in increasing order so that the subfields form a kind of staircase. The staircase variant is used in the CLEAR method, see T. Tokunaga et al, “Development of new driving method for AC-PDPs: High-Contrast, Low Energy Address and Reduction of False Contour Sequence CLEAR”, Proceedings of the IDW &#39;99, page 787. An example of a distribution using the CLEAR method is shown in FIG. 1A. In FIG. 1A a histogram is shown which represents the distribution of the subfields within one frame period. The first (white) bar represents a set-up of all plasma cells. After the set-up phase, all cells are active. The other bars represent the luminance of a plasma cell during the consecutive subfields (SF 1 -SF 8 ). During the first subfield (SF 1 ) the activated cells will have the lowest luminance (L 1 ). After each subfield, plasma cells that should be inactive are turned off, and the other cells will remain active. Each cell can be turned off only once per frame period. This means that each extra luminance level is created by using one extra subfield. All cells are activated once during the set-up phase at the beginning of the frame period.  
           [0007]    If the CLEAR method subfield distribution is used at a 50 Hz frame rate, large area flicker will be visible. From B. Salters et al., “Reduction of large area flicker in plasma display panels”, Proceedings of the SID 2001, page 1098, 2001, San Jose, and H. Kuriyama et al., “A 50 Hz flicker reduction for PDP and evaluation system development”, Proceedings of the SID 2001, page 1102, 2001, San Jose, it is known that this large area flicker can be reduced by using a subfield distribution with two peaks. However, such a subfield distribution will cause more motion artefacts such as double/coloured edges or motion blur.  
           [0008]    Another subfield distribution with an inherent reduction of the dynamic false contour is called LSC (Limited Subfield Coding). This scheme is described in a prior patent application [PHNL010114EPP] of the present applicant, titled “Limiting number of subfield changes for next higher level”. The idea is that for each increasing grey level, only a limited number of subfields change value, which will result in a reduction of the DFC motion artefact.  
           [0009]    In Cathode Ray Tube (CRT) screens large area flicker and motion artefacts are a well known problem too. These artifacts are minimised by a concept called Motion Compensated Upconversion, also known as Natural Motion, see G. de Haan, “Video Processing for multimedia systems”, University Press Eindhoven, ISBN 90-9014015-8. In between two consecutive original images an extra image is inserted. In this way the display frequency is doubled from for example 50 Hz to 100 Hz. The inserted images are corrected for motion, if motion is present in the video sequence.  
           [0010]    Motion compensation is also used in plasma display panels. In R. van Dijk and T. Holtslag, “Motion Compensation in Plasma Displays”, Proceedings of the IDW 1998, page 543, motion vectors from a video sequence are used to reduce motion artifacts. Basically, all subfields are shifted on the motion vectors to make sure that the human eye will integrate all subfields in a correct way. This algorithm can give very good moving picture quality, but its implementation is very complex.  
           [0011]    It is an object of the present invention to provide a method for displaying moving pictures on a PDP display with a relative simple system that shows very little motion artifacts.  
         SUMMARY OF THE INVENTION  
         [0012]    The invention relates to a method of driving a plasma display panel, the method comprising the steps of:  
           [0013]    receiving of a series of consecutive original images;  
           [0014]    distributing information from each original image over a first plurality of subfields,  
           [0015]    characterized in that the method also comprises the steps of:  
           [0016]    producing a series of new images;  
           [0017]    inserting at least one of said series of new images between every two consecutive original images;  
           [0018]    distributing information from a field of each new image over a second plurality of subfields;  
           [0019]    generating a display image using a first group of subfields of an original image and a second group of subfields of at least one new image.  
           [0020]    Furthermore, the invention relates to a device for driving a plasma display panel, the device comprising:  
           [0021]    means for receiving of a series of consecutive original images; means for distributing information from a field of each original image over a first plurality of subfields;  
           [0022]    means for producing a series of new images;  
           [0023]    means for inserting at least one of said series of new images between every two consecutive original images;  
           [0024]    means for distributing information from a field of each new image over a second plurality of subfields;  
           [0025]    means for generating a display image using a first group of subfields of an original image and a second group of subfields of at least one new image.  
           [0026]    The present invention also relates to a display apparatus, comprising:  
           [0027]    a device as described above;  
           [0028]    a plasma display panel for displaying the image.  
           [0029]    A method according to the invention reduces large area flicker with reduction of motion artifacts, like blur or double edges. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    Below, the invention will be explained with reference to some drawings, which are intended for illustration purposes only and not to limit the scope of protection as defined in the accompanying claims.  
         [0031]    [0031]FIG. 1 shows two subfield distributions.  
         [0032]    [0032]FIG. 2 shows the principle of combining subfields out of two images.  
         [0033]    [0033]FIG. 3 is a block diagram of a first embodiment of the system of the present invention.  
         [0034]    [0034]FIG. 4 is a block diagram of a second embodiment of the system of the present invention.  
         [0035]    [0035]FIG. 5 shows subfield distributions to explain timing differences between theoretical and practical frame periods.  
         [0036]    [0036]FIG. 6 shows subfield distributions to explain the influence of automatic power control. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0037]    In the present invention a subfield distribution with two staircases is proposed, in contrast to only one single staircase in the CLEAR method, see FIG. 1A. Two staircases are created by introducing a second set-up phase. With this distribution, referred to as “double staircase distribution”, no large area flicker will be observed.  
         [0038]    In FIG. 1B a histogram is shown which represents the distribution of the subfields within one Frame period. The two highest (white) bars represent two set-ups of the plasma cells. The other bars represent luminances of a plasma cell during consecutive subfields (SF 1 -SF 8 ). During the first subfield (SF 1 ) the activated cells will have the lowest luminance (L 1 ). The fifth subfield (SF 5 ) represents the second luminance (L 2 ). The second subfield (SF 2 ) represents the third luminance (L 3 ), etc.  
         [0039]    In an embodiment of the present invention, the sequence described above, is changed at subfield SF 4  and SF 8 , i.e., the fourth subfield (SF 4 ) represents the highest luminance (L 8 ) and the last subfield (SF 8 ) represents the second highest luminance. In this way subfields SF 1 -SF 4  produce roughly the same amount of light as subfields SF 5 -SF 8 .  
         [0040]    Of course, the human eye will experience higher luminances than L 8  because it will integrate the luminances during a certain period of time. In fact, FIG. 1B shows a maximum grey level experienced by the eye during one field (period), i.e. all subfields are used.  
         [0041]    In the present invention an upconversion, to preferably 100 Hz, as in CRTs with Natural Motion is used, in addition with the double staircase distribution, as described above. It is not possible to display two complete images in a time period of 20 milliseconds on a PDP. This is because a present PDP will need this time period to address the cells and emit light. Therefore, the subfield values are extracted from one of two images dependent on the position of the subfield along the time axis.  
         [0042]    In FIG. 2, the extraction of subfields is explained. A video signal originally contains images  1 ,  2  every T milliseconds. In the invention, new image  3  is inserted between the original image  1 ,  2 . New image  4  is inserted between original image  2  and its original successor (not shown). This means that the frequency of the video signal increases with a factor 2. The new images  3 ,  4  are motion compensated, for example by a state of the art device already used in CRTs. During one field period one image will be shown. To show an image on a PDP, a subfield method is used with for example 8 subfields per field. All images are converted to subfield data using e.g. the LSC distribution or the CLEAR method as described in the prior art, but the idea can be used for any subfield distribution.  
         [0043]    In one embodiment, original image  1  will be converted to subfield coding  8 , and new image  3  will be converted to subfield coding  10 . (Remark: of course not all pixels in an image will have the same grey level, so the coding per pixel are different.)  
         [0044]    The next step is to combine subfields from the original image  1  with subfields from the new image  3 . As shown in FIG. 2, four subfields SF 1 -SF 4  are taken from the original image  1  and four subfields SF 5 -SF 8  are taken from the new image  3 . By doing this, motion artefacts resulting from the double staircase distribution will be reduced.  
         [0045]    In this method some subfield data is thrown away. The subfield group SF 5 -SF 8  of the original image  1  and the subfield group SF 1 -SF 4  of the new image  3  are not used anymore.  
         [0046]    If an image sequence is not moving, the original image  1  and original image  2  will be the same. This means that the image  3  will also be the same. By extracting the subfields as described above the resulting grey level (see distribution  16 ) will be exactly the same as the grey level of the original image (see distribution  8 ). So, no artefacts will occur with still images.  
         [0047]    [0047]FIG. 3 is a block diagram of a first embodiment of an apparatus of the present invention. The apparatus, e.g. a television, comprises a set and a panel: Both set and panel contain processing modules. The panel (i.e. the actual screen) receives Red Green Blue (RGB) signals from a tuner in the set.  
         [0048]    Interlaced YUV input data  20  is deinterlaced and upconverted by means of a frame rate converter  22 . Several commercial available modules exist, see G. de Haan, “Video Processing for multimedia systems”, University Press Eindhoven, ISBN 90-9014015-8.  
         [0049]    The output is a progressive 100 Hz video stream  24 . This video stream  24  is in standard YUV format. It is converted by YUV converter  26 , to a 100 Hz RGB signal  28 . This 100 Hz RGB signal is transferred from the set to the panel, which is indicated by a dashed line. The next step is a gamma/Floyd Steinberg (FS) dithering performed by a video processor  30 . The output of the video processor  30  is a processed 100 Hz progressive RGB signal  32 . Next, subfield generation is performed by subfield generating module  34 . In subfield generating module  34  also the subfield extraction from the two images  1 ,  3 , as shown in FIG. 2, is done. The resulting RGB subfield data  36  is used for driving a PDP  38  which will show an image  40  which will be experienced by a human eye  42 .  
         [0050]    An advantage of this embodiment is the fact that the motion estimation and the upconversion is performed in the set, and not in the panel. This is especially attractive because the used module is already incorporated in the set of present apparatus for deinterlacing purposes.  
         [0051]    In FIG. 4, a block diagram of a second embodiment of the apparatus of the present invention is shown. Interlaced YUV input data  50  is deinterlaced by a frame rate converter  52 . In frame rate converter  52 , motion vectors  53  are calculated out of two consecutive original images using a state of the art motion estimation process. Unlike in FIG. 3, no upconversion is performed in this module. The outgoing signal is a progressive 50 Hz video stream  54 . This video stream  54  is in YUV format. It is converted by YUV converter  56 , to a 50 Hz RGB signal  58 . This 50 Hz RGB signal  58  is transferred from the set to the panel, which is indicated by a dashed line. The next step is a gamma/FS dithering performed by a video processor  60 . The output of the video processor  60  is a processed 50 Hz progressive RGB signal  62 . Next, subfield generation is performed by a upconversion/subfield generation module  64 . At this point, an upconversion is performed by inserting new images between original images using the motion vectors  53  received from frame rate converter  52 . In upconversion/subfield generation module  64  also the subfield extraction from the two images  1 ,  3 , as shown in FIG. 2, is done. The resulting RGB subfield data  66  is used for driving a PDP  68  that will show an image  70 , which will be experienced by a human eye  72 .  
         [0052]    An advantage of this embodiment compared to the one described above, is that all processing is done on 50 Hz.  
         [0053]    The invention described above, may be used e.g. in plasma display panels (PDPs), plasma-addressed liquid crystal panels (PALCs), liquid crystal displays (LCDs), Polymer LED (PLEDs), Electroluminescent (EL), used for personal computers, television sets and so forth.  
         [0054]    In FIGS. 5 a  and  5   b , the timing difference of a theoretical (FIG. 5 a ) and practical (FIG. 5 b ) situation is shown. In the theoretical situation, the time difference between both subfield peaks (e.g., 10 ms) that are generated from the different images is equal to one half video field period (e.g. ½×20 ms). This time difference could be the difference between the subfields with the highest values, or between the first subfields of both peaks. However, the optimal timing difference depends on the subfield distribution. Normally the upconverted new image is calculated exactly in the middle of the frame time (so after 10 ms at 50 Hz). If the subfields use the entire field period, this closely equals the time of the first subfield of the second peak (which is generated with the upconverted image). The time difference between the original and upconverted images will in that case be correct. FIG. 5 b  shows the practical situation. In a configuration that can also display 60 Hz, 8 subfields only use 16.667 ms. The time difference between the two peaks is in that case smaller than 10 ms (i.e. 8.3 ms). The motion artefact reduction should be corrected for this difference.  
         [0055]    Another attention point concerning this timing is “automatic power control”, which means that the number of sustain pulses is reduced when the picture load exceeds a certain value. This is shown in FIG. 6. Due to the reduction of the sustain time, the starting points of the subfields will move. It is best to keep the starting time of the first subfield of the 2 nd  staircase fixed at a certain time instant (preferably equal to the half frame time).  
         [0056]    While the invention has been described in connection with preferred embodiments, it will be understood that modifications thereof within the principles outlined above will be evident to those skilled in the art, and thus the invention is not limited to the preferred embodiments but is intended to encompass such modifications. It is possible to interchange lines and columns. The invention is applicable to display devices in which the sub-field mode is applied. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer.