Patent Grant
7796138
This application claims the benefit, under 35 U.S.C. §365 of International Application PCT/EP2004/053603, filed Dec. 20,2004, which was published in accordance with PCT Article 21(2) on Jul. 28, 2005 in English and which claims the benefit of Europeanpatent application No. 04100030.8, filed Jan. 7, 2004.
The present invention relates to a method for processing video data to be displayed on a display screen by providing said video data having video levels selected from a predetermined number of video levels, encoding said predetermined number of video levels with a corresponding number of codewords and illuminating pixels in a central area of said display screen in accordance with said codewords.
Furthermore, the present invention relates to a corresponding device for processing video data.
Referring to the last generation of CRT displays, a lot of work has been done to improve its picture quality. Consequently, a new technology like Plasma has to provide a picture quality at least as good or even better than standard CRT technology. For a TV consumer, high contrast is one main factor for a high subjective picture quality of a given display. The dark room contrast is defined as the ratio between the maximal luminance of the screen (peak-white) and the black level. Today, on plasma display panels (PDP), contrast values are inferior to those achieved for CRTs.
This limitation depends on two factors:
In the following, aspects of response fidelity and priming are presented in more detail.
A panel having good response fidelity ensures that only one pixel could be ON in the middle of a black screen and in addition, this panel has a good homogeneity.
A first solution to achieve good response fidelity, by standard PDPs and for a given addressing speed, leads to the priming operation mentioned above. In that case, each cell will be repeatingly excited. Nevertheless, since an excitation of a cell is characterized by an emission of light, this has to be done parsimoniously to avoid a strong reduction of the dark room contrast (i.e. to avoid more background luminance). Therefore a simple way to improve the dark room contrast leads to an optimization of the priming use.
Actually, two kinds of priming can be found on the market:
Obviously, the better solution should be based on the use of a “soft-priming” with the assumption that the total amount of “soft-priming” required to obtain an acceptable response fidelity will produce less light than a single “hard-priming”. This is not the case when the coding has not been optimized since one priming per sub-field should be required.
In fact, the best contrast ratio will be obtained by using a single soft-priming operation per frame. Such a concept is achieved by optimization of the coding concept as seen in the next paragraph.
The document EP-A-1 250 696 introduces a concept of one single “soft-priming”, where only one priming at the beginning of a frame is performed. In that case, only the first sub-fields will be near enough from the priming signal in the time domain to benefit from it. Now, the main idea was to use these first sub-fields as a kind of “artificial priming” for the next sub-fields taking the assumption that one lighted sub-field will help the writing of the next ones (cascade effect).
In the first case, the codeword used (P-101111111101) enables a good cascade effect from the priming P up to the last sub-field (MSB). Then, the distribution of the writing discharge is well concentrated and fully occur inside 1.1 μs which represents the new borderline for the address speed. This means, that the writing process can be performed within the addressing period.
In the second case, the codeword used (P-000000000001) does not permit any cascade effect and therefore the writing of the last sub-field is less efficient. Then, the distribution of the writing discharge is no more concentrated and is spread on a longer time period as shown by the envelope. Thus some writing process would be performed after the addressing period. In that case, more time should be given to the addressing for acceptable response fidelity.
The results presented in
Moreover, the optimal sub-fields encoding should enable to have not more than one sub-field OFF between two sub-fields ON. This property will be called Single-O-Level (SOL). An optimized sub-field weighting based on the mathematical Fibonacci sequence enables to fully respect the SOL criterion.
Nevertheless, some experiments have shown that, under some circumstances, even a SOL criterion combined with a single “soft-priming” is not enough to provide perfect response fidelity.
In the following the specific problem of the present invention is demonstrated. Experiments have shown that, when the number of sustains grows, the biggest sub-fields will suffer from response fidelity problems. These problems appear only under certain circumstances, for instance in the case of a horizontal greyscale at a high sustains number as shown in
In view of that it is the object of the present invention to provide a method and device for processing video data, which remove the PDP border problem.
According to the present invention this object is solved by a method for processing video data to be displayed on a display screen by providing said video data having video levels selected from a predetermined number of video levels, encoding said predetermined number of video levels with a corresponding number of codewords and illuminating pixels in a central area of said display screen in accordance with said codewords, as well as illuminating pixels in a border area surrounding said central area of said display screen by using only those codewords of said number of codewords, which have a constant bit value in a selectable part of the codewords.
Furthermore, according to the present invention there is provided a device for processing video data to be displayed on a display screen including data providing means for providing said video data having video levels selected from a predetermined number of video levels, encoding means for encoding said predetermined number of video levels with a corresponding number of codewords and illuminating means for illuminating pixels in a central area of said display screen in accordance with said codewords, wherein said illuminating means is adapted for illuminating pixels in a border area surrounding said central area of said display screen by using only those codewords of said number of codewords, which have a constant bit value in a selectable part of the codewords.
Preferably, codewords, which have a binary 0 between two binary 1, are not used for illuminating the border area. Thus, cells of the display screen being ON cannot pollute surrounding cells being OFF.
Video levels corresponding to codewords being not used may be recreated by dithering. With such dithering every video level can be created by temporarily switching on an off a higher video level.
In a preferred embodiment a part of the codewords having constant bit value may be determined by a power level of a picture to be displayed. Since the pollution of neighbour cells depends on the power level of a picture, it is advantageous to adapt the coding of the video levels to the power level.
Moreover, the part of the codewords being determined to have constant bit value should include the most significant bits of the codewords. Thus, especially those codewords are not used for coding video levels, the high level sub-fields of which are on and off alternatingly. Consequently, cells of the display screen being energized by a lot of sustain impulses according to high level sub-fields will not pollute neighbouring cells being OFF.
The border problem is reduced towards the centre of the display screen. Therefore, the border area is preferably divided into several sub-areas, wherein the non-usage of codewords is stepwise reduced. A first one of said several sub-areas may be illuminated by codewords with a first selectable part of constant bit value and a second one of the several sub-areas may be illuminated by codewords with a second selectable part of constant bit value, wherein the second selectable part includes the first selectable part of codewords or at least a portion of it or is different from the first selectable part. In a preferred embodiment the length of the part within a codeword in which the bit value is constant, is variable starting from the most significant bit of a codeword.
Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. The drawings showing in:
The present invention is based on the knowledge that the structure of a PDP in its centre is different from that in the border area. In detail plasma panels are built with two glass plates (front and back) sealed together and having electrodes on top of them (horizontal transparent electrodes on the front plate, vertical metallic electrodes on the back plate). The various plasma cells (Red, Green and Blue dots) are delimited through so-called barrier-ribs having a certain height. This height also normally defines the distance between the two plates. This basic concept is illustrated in
This geometrical situation will have a strong impact on the panel response fidelity, above all for very energetic pictures (pictures with a lot of sustains).
In the introductory part the concept enabling the use of only one single priming operation in the case of an optimized encoding has been presented. This concept of single priming works very well in case of full-white pictures having a limited maximal white value (e.g. 100 cd./m2 with around 150 sustains). In that case, since the soft-priming light emission is below 0.1 cd/m2 the contrast ratio is beyond 1000:1 in dark room.
However, as illustrated in
1-2-3-5-8-12-18-24-31-40-50-61.
In order to learn the reasons of the problems the sub-field codewords for these values should be compared. The comparison is shown in
Now, in order to learn more about the reason of the problems another zoomed part of the screen is shown in
The examples given above show that the problem of response fidelity appearing at a PDP border for high video level pictures are linked to the switching ON/OFF of MSB. Indeed, in the case presented
This problem is directly linked to the situation described above: the open cells at the PDP border. Indeed, when an open cell has a certain sub-field switched ON, it will pollute the neighbouring cells that are OFF (compare
The examples above show that, when a cell is open, there could be a migration of charges to the neighbouring cells. When those neighbours are ON, the migration will disappear during a discharging operation. However, when the neighbouring cells are OFF, the charges will remain. The amount of charges will depend on the number of sustains used for the sub-field ON. Then, if the amount of polluting charges is strong enough, this could disturb the writing of the next sub-field for the polluted cells.
Up to a certain degree this pollution problem can be solved by applying priming operation, since the priming operation acts as reset and is able to suppress the polluting charges. In order to do that, this concept described in EP-A-1 335 341 is based on a limit Δ representing a maximal number of sustain without priming. In other words, when a sub-field contains more than Δ sustains, its priming is activated. This leads to an evolving number of priming. However, this also reduces the maximal available darkroom contrast.
In order to go further and to reduce the total amount of priming, according to the present invention it is suggested to modify the codeword at the panel border so that critical situations like that depicted in
The codewords may be modified in dependence of the average power level of a picture to be displayed. A prerequisite of this is that an adequate power management is provided.
For every kind of active display, more peak luminance corresponds also to a higher power that flows in the electronic. Therefore, if no specific management is done, the enhancement of the peak luminance for a given electronic efficacy will introduce an increase of the power consumption. The main idea behind every kind of power management concept associated with peak white enhancement is based on the variation of the peak-luminance depending on the picture content in order to stabilize the power consumption to a specified value. This is illustrated in
In order to avoid overloading the power supply of the plasma, the number of sustains can be adjusted depending on the picture content. When the picture is full (e.g. full white page—100%) it is not possible to use the total amount of sustains (e.g. only 100 sustains are used) which leads to a reduced white luminance (around 100 cd/m2). This determines the power consumption (e.g. 300 W). Then when the charge of the picture decreases (e.g. night with only a small moon up to 0%), the number of sustains can be increased without increasing the power consumption. This only enhances the contrast for the human eye.
In other words, for every charge of the input picture computed through the APL (Average Power Level), a certain amount of sustain impulses will be used for the peak white as shown in
where I(x,y) represents the picture to be displayed, C the number of columns and L the number of lines of this picture. Then, for every possible APL values, the maximal number of sustains to be used is fixed.
Since, only an integer number of sustains can be used, there is only a limited number of available APL levels. This is illustrated in
According to
An other important aspect of the present new concept of codeword modification is its compatibility with the previous concept of dynamic priming. Indeed, both concepts can be utilized separately but a combination of both brings further improvements. On one hand, dynamic priming increases the dark level (reducing the darkroom contrast) without modifying the greyscale quality, on the other hand the concept of codeword modification limits the greyscale portrayal capability of the plasma panel in border areas while requiring no additional priming.
As already said, the inventive concept is based on a specific encoding for border areas.
It is important to notice here that the border areas are really small and do not represent a main part of the screen (e.g. only 4% of the screen).
In the following the basic concept of codeword limitation shall be explained in detail. For this, the example defined in
In fact, the values are obtained through measurements at the panel level.
The main idea behind this concept is to forbid the insertion of 0 between two 1 for critical sub-fields. In other words, in the total amount of existing codewords, the critical ones will be suppressed. In the following table one can find the standard encoding table for the sub-field sequences used above: 1-2-3-5-8-13-19-25-32-40-49-58 as well as the suppressed codewords for all areas.
In the example shown in the table, the first column corresponds to the video value to be rendered, the second column to the standard codeword (used in the standard area of the panel as described on
For instance, in the area Δ1, the video values 33 up to 38 are not rendered whereas they are rendered in the two other areas.
Indeed, the video level 33 is rendered with the codeword 111010100000 in the standard area. In case of APL=0%, the 6th sub-field has an energy of 71 sustains which is more than Δ1 but lower than Δ2 and Δ3. In this codeword, the 6th sub-field is set to zero whereas the 7th is set to one, which represents a critical situation as described in
Later on, the missing levels will be recreated by the means of dithering. Even if this concept will increase a bit the dithering noise in the border areas, it has to be remembered that those areas are very small (e.g. 4% of screen size) and do not represent the main area for the human eye. In that case the limitations introduced by the specific border coding will not be really noticeable for the viewer but the gain in terms of contrast (less priming used) will be quite strong. Indeed, in the example at APL=0%, one signal priming instead of 8 is enough, so that the contrast has been improved by a factor 8.
Following number of levels are suppressed in the example:
Moreover, fewer levels will be suppressed in the case of a combination with dynamic priming. In that case, a trade-off should be chosen between the number of sub-fields used for dropping and the number of additional priming. The ideal position for the primed sub-fields will be on the lowest sub-fields from the critical group (all sub-fields having more than Δn sustains) since the number of codewords to be dropped will be more reduced in that case.
Furthermore, the suppression is done only for law APL values as seen on
A hardware implementation of the border-coding concept for a PDP panel is shown in
Depending on the Average Power Level (APL), the control system 4 determines the sustain table and the encoding table with its sub-fields number. Furthermore, this basic information APL is sent to a border select block 5 so that a correct decision regarding the critical areas can be taken. To do that, the border select block also disposes of position information (H-line and Clock-pixel) so that the right Δ area can be determined. Additionally, the border select block 5 receives a control signal BORD from the system control block 4. This control signal BORD is used for activating the specific border coding. The Δ information output from the border select block 5 as well as a mapping information (related to the encoding and sustain table) is sent to the video mapping block 3 which modifies the video data so that the dropped video parts can be recreated correctly with the dithering function.
After the mapping stage in video mapping block 3, data are forwarded to a dithering block 6 replacing non-encodable video levels. Then, the encoding to codewords of a 10 bit RGB signal from the dithering block 6 is performed by the sub-field coding block 7 receiving coding information from the system control block 4 concerning the decision which LUT should be used for sub-field coding.
The system control block 4 also controls the writing of 16 bit RGB pixel data from the sub-field coding block 7 in a 2-frame memory 8 (WR), the reading (RD) of RGB sub-field data from a second frame memory integrated in the 2-frame memory 8, and the serial to parallel conversion circuit (SP) in a serial-parallel conversion block 9 receiving the output signals SF-R, SF-G,SF-B from the 2-frame memory 8.
The 2-frame memory 8 is required, since data is written pixel-wise, but read sub-field-wise. In order to read the complete first sub-field a whole frame must already be present in the memory 8. In a practical implementation two whole frame memories are present, and while one frame memory is being written, the other is being read, avoiding in this way reading the wrong data. In a cost optimized architecture, the two frame memories are located on the same SDRAM memory IC, and the access to the two frames is time multiplexed.
The serial-parallel conversion block 9 outputs top and bottom data for the plasma display panel 10. Finally the system control block 4 including an addressing and sustain control unit 42 generates the SCAN and SUSTAIN pulses required to drive the PDP driver circuits of the PDP 10.
In summary in this document, it was shown how the use of a new coding concept can optimize the picture quality regarding the contrast as well as the response fidelity. Subjective tests performed in dark room environment have shown good picture quality assessment regarding classical PDPs.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04100030 | Jan 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2004/053603 | 12/20/2004 | WO | 00 | 6/26/2006 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2005/069262 | 7/28/2005 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 6268890 | Kawahara | Jul 2001 | B1 |
| 6388677 | Doyen et al. | May 2002 | B1 |
| 6417824 | Tokunaga et al. | Jul 2002 | B1 |
| 6727913 | Hoppenbrouwers et al. | Apr 2004 | B2 |
| 6882351 | Morita | Apr 2005 | B2 |
| 6922181 | Tanaka et al. | Jul 2005 | B2 |
| 7158155 | Ueda et al. | Jan 2007 | B2 |
| 20030076338 | Hashimoto | Apr 2003 | A1 |
| 20030193451 | Kimura | Oct 2003 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20070154101 A1 | Jul 2007 | US |
