Patent Publication Number: US-8125500-B2

Title: Apparatus for driving a plasma display panel with APL pre-measurement and corresponding method

Description:
FIELD OF THE INVENTION 
     This application claims the benefit, under 35 U.S.C. §119 of European Patent Application 05292660.7, filed Dec. 12, 2005. 
     The present invention relates to an apparatus for driving a plasma display panel including brightness/contrast control means for receiving video input data, for modifying the video levels of the video input data in accordance with external adjustments data and for outputting modified video data and power measurement means for measuring a power level of the modified video data. Furthermore, the present invention relates to a corresponding method. 
     BACKGROUND OF THE INVENTION 
     A PDP (plasma display panel) uses a matrix array of discharge cells, which can only be “ON”, or “OFF”. Also unlike a CRT (cathode ray tube) or LCD (liquid crystal display) in which gray levels are expressed by analog control of the light emission, a PDP controls the gray level by modulating the number of light pulses per frame (sustain pulses). This time-modulation will be integrated by the eye over a period corresponding to the eye time response. Since the video amplitude is portrayed by the number of light pulses, occurring at a given frequency, more amplitude means more light pulses and thus more “ON” time. For this reason, this kind of modulation is also known as PWM, pulse width modulation. 
     For all displays using pulse width modulation, the number of real gray levels is limited. For PDP, in case of standard coding the number of gray levels is more or less equal to 256. 
     These various gray levels can only be used when the dynamic of the input picture is at its maximum (in case of 8 bit signal, video values between 0 and 255). In other cases, when the dynamic is reduced (in particular because of contrast or brightness parameters), the number of displayed levels will further decrease. 
     The problem is that the picture quality is affected when the number of displayed levels is reduced. 
     Unfortunately, when reducing the contrast (by dividing by a certain factor) and/or the brightness (subtracting a certain coefficient from the picture), the maximum value of the picture decreases and so the picture quality is reduced. 
     Contrast and brightness controls are usually part of the so called “front-end”, while PDP specific functions (gamma function, Sub-field encoding, etc) are part of the so called “back-end” of the display (see  FIG. 3 ). 
     In the back-end of a PDP an APL function is used to control the power. The computation of this Average Power Level (APL) is made through the following function: 
               APL   ⁡     (     I   ⁡     (     x   ,   y     )       )       =       1     C   ×   L       ·       ∑     x   ,   y       ⁢     I   ⁡     (     x   ,   y     )                 
where I(x,y) represents the picture to display, C the number of columns and L the number of lines of the PDP.
 
     The aim of power management is to keep the power consumption constant (see  FIG. 1 ) and to have a peak luminance as high as possible. So for every APL value, the maximal number of sustain pulses to be used is fixed. 
     This number of sustains decreases when the APL increases, and vice versa shown in  FIG. 2 . 
     In peak-white pictures (low APL at the left side of  FIG. 2 ), the number of sustain pulses is not limited by the power consumption, but by the available time for sustaining. For this reason, the power consumption of peak-white picture will be lower than for the other pictures. Consequently, also the power consumption decreases for low APL levels (compare  FIG. 1 ). 
     The following table shows an allocation of the values of the number of sustain pulses to the average power levels according to  FIG. 2 . The average power levels are coded on 10 bits. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                   
                 Total Number 
               
               
                   
                 APL 
                 of sustains 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                  0 
                 1000 
               
               
                   
                  1 
                 1000 
               
               
                   
                  2 
                 1000 
               
               
                   
                  3 
                 1000 
               
               
                   
                  4 
                 1000 
               
               
                   
                  5 
                 1000 
               
               
                   
                 . 
               
               
                   
                 . 
               
               
                   
                 . 
               
               
                   
                  50 
                 1000 
               
               
                   
                  51 
                 1000 
               
               
                   
                  52 
                 1000 
               
               
                   
                  53 
                 1000 
               
               
                   
                  54 
                 1000 
               
               
                   
                  55 
                 1000 
               
               
                   
                  56 
                 999 
               
               
                   
                  57 
                 998 
               
               
                   
                  58 
                 996 
               
               
                   
                  59 
                 994 
               
               
                   
                  60 
                 991 
               
               
                   
                  61 
                 988 
               
               
                   
                  62 
                 984 
               
               
                   
                  63 
                 979 
               
               
                   
                  64 
                 975 
               
               
                   
                  65 
                 971 
               
               
                   
                  66 
                 966 
               
               
                   
                  67 
                 962 
               
               
                   
                  68 
                 958 
               
               
                   
                  69 
                 954 
               
               
                   
                  70 
                 950 
               
               
                   
                  71 
                 946 
               
               
                   
                  72 
                 942 
               
               
                   
                  73 
                 938 
               
               
                   
                  74 
                 933 
               
               
                   
                  75 
                 929 
               
               
                   
                 . 
               
               
                   
                 . 
               
               
                   
                 . 
               
               
                   
                 295 
                 449 
               
               
                   
                 296 
                 448 
               
               
                   
                 297 
                 447 
               
               
                   
                 298 
                 446 
               
               
                   
                 299 
                 445 
               
               
                   
                 300 
                 444 
               
               
                   
                 301 
                 442 
               
               
                   
                 302 
                 441 
               
               
                   
                 303 
                 440 
               
               
                   
                 304 
                 439 
               
               
                   
                 305 
                 438 
               
               
                   
                 . 
               
               
                   
                 . 
               
               
                   
                 . 
               
               
                   
                 1005 
                 102 
               
               
                   
                 1006 
                 102 
               
               
                   
                 1007 
                 102 
               
               
                   
                 1008 
                 102 
               
               
                   
                 1009 
                 102 
               
               
                   
                 1010 
                 102 
               
               
                   
                 1011 
                 101 
               
               
                   
                 1012 
                 101 
               
               
                   
                 1013 
                 101 
               
               
                   
                 1014 
                 101 
               
               
                   
                 1015 
                 101 
               
               
                   
                 1016 
                 101 
               
               
                   
                 1017 
                 100 
               
               
                   
                 1018 
                 100 
               
               
                   
                 1019 
                 100 
               
               
                   
                 1020 
                 100 
               
               
                   
                 1021 
                 100 
               
               
                   
                 1022 
                 100 
               
               
                   
                 1023 
                 100 
               
               
                   
                   
               
            
           
         
       
     
     As indicated above, the problem of the standard implementation of power management is that when the energy of the input picture of the back-end decreases, the number of sustain pulses increases. 
       FIG. 3  shows a principle block diagram of the driving unit of a plasma panel  1 . The video input signal is first processed in the front end  2 . The front end includes a scaling unit  4  for adapting the size of the picture to that of the panel. The scaled input signal is supplied to a brightness/contrast control block  5 . This control block  5  receives external signals for tuning or modifying the brightness or the contrast of the picture. The video signal is processed respectively and supplied to the back end  3 . Within the back end  3  the signal is processed in a usual path including a gamma block  6 , a dithering block  7  and an encoding block  8 . The gamma block  6  performs a data transformation with a look up table in accordance to a nearly quadratic gamma function. The output signal of the gamma block  6  is transmitted to the dithering unit  7  which will add for example 4 bit dithering in order to have more discrete video levels at the output. Afterwards, the sub field encoding  8  generates sub field data for the video signal. The resulting sub field data are sent to the plasma panel  1 . 
     In a parallel path within the back end  3  the output signal of the front end  2  is input into an APL measurement block  10 . This block supplies an APL level of the brightness/contrast tuned video signal to the power management  9 . The power management  9  controls the gamma unit  6  and the encoding unit  8 . Furthermore, the power management  9  delivers sustain information to the plasma panel  1 . 
     With this arrangement, it is for example interesting to see what happens when the user is decreasing the contrast and/or the brightness. 
     When decreasing the contrast and/or the brightness, the APL (measured in the back-end  3 ) is decreasing; this means that the number of sustains is increasing. This increases partly the contrast. 
     For example, the user wants to reduce the contrast by 2 for a picture, which has an APL of 300 (10 bit value). So originally this picture has in average approximately 444*300/1024=130 sustains/cell, and can have a peak luminance of 444 sustains (compare table shown above). 
     To obtain in average 130/2=65 sustains/cell, the user in fact has to reduce the contrast of the picture by around 4. For an APL value of 70, according to the table, the average number of sustain is equal to 950*70/1024=65. The peak luminance in this case is also reduced since all brightness levels of the whole picture are divided by more than 4, the maximum value of the picture will not be higher than 255/4.3=60 (this represents 950/4.3=222 sustains). But since, the picture is divided by more than 4, the number of gray levels really used is also divided by around 4. The picture quality is rather low in this case. 
     SUMMARY OF THE INVENTION 
     In view of that, it is the object of the present invention to provide a driving apparatus for a plasma display panel which improves the picture quality, when the brightness and contrast of the picture are reduced. Furthermore, a respective method shall be provided. 
     According to the present invention this object is solved by an apparatus for driving a plasma display panel including brightness/contrast control means for receiving video input data, for modifying the video levels of the video input data in accordance with external adjustment data and for outputting modified video data, first power measurement means for measuring a power level of said modified video data and for supplying a first power level, second power measurement means for measuring a power level of said video input data and for supplying a second power level, generator means for generating a third power level comprised between said first power level and said second power level or equal to the larger one of said first and second power levels, and data processing means ( 9 ) for calculating the maximum number of sustain pulses per frame applicable to said modified video data on the basis of said third power level and for controlling the display of said modified video data on said plasma display panel ( 1 ) respectively. 
     Furthermore, there is provided a method for driving a plasma display panel by providing video input data, modifying the video levels of the video input data in accordance with external adjustment data in order to obtain modified video data, measuring a power level of said modified video data and providing a respective first power level, measuring a power level of said video input data and providing a respective second power level, generating a third power level comprised between said first power level and said second power level or equal to the larger one of said first and second power levels, and processing said modified video data for calculating the maximal number of sustain pulses per frame applicable to said modified video data on the basis of said third power level and controlling the display of said modified video data on said plasma display panel respectively. 
     The advantage of the present invention is that the APL level of the input video signal can be considered in the back end before the video signal is modified by the brightness/contrast control unit. Thus, the adjustment of brightness and contrast affects the picture quality on the plasma panel less negative. 
     According to a preferred embodiment of the present invention the third power level is the larger one of the first power level and the second power level. With this feature it is possible that the total power of the picture remains unchanged even if the brightness or contrast of the picture is varied. 
     Preferably, the power levels measured in the driving apparatus are average power levels related to one picture. 
     Furthermore, the data processing means may include power management means for keeping the power consumption of the plasma display panel constant irrespective of the power control information. 
    
    
     
       DRAWINGS 
       The present invention will now be explained in more detail along with the attached figures, showing in: 
         FIG. 1  a diagram of the power consumption over the average power level; 
         FIG. 2  a diagram of the number of sustain pulses over the average power level; 
         FIG. 3  a block diagram of a driving unit of a plasma panel according to the prior art; 
         FIG. 4  a block diagram of a driving unit of a plasma panel according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The purpose of the invention presented here is to improve the behavior of power management regarding contrast and brightness control. 
     The idea is that when contrast and/or brightness decrease, the power management should not increase the number of sustains. Otherwise the user needs to further decrease the contrast and/or the brightness. Then, the picture quality would further decrease, too. 
     This can be done by using for power management  9  the same APL value than the one before the contrast/brightness decrease. This value can be measured with the help of an additional APL measurement unit  11  placed in the front-end  2  before the contrast/brightness control unit  5  as shown in  FIG. 4 . 
     However, this value cannot be used directly. Otherwise when the energy is increased by the front-end  2  (by increasing the contrast and/or the brightness for example) the power on the display  1  could be higher than the maximum value allowed. Therefore in this case the power has to be reduced by the power management block  9 . 
     A comparison between  FIGS. 3 and 4  shows that except for the units  11  and  12  the other elements  1  to  10  of  FIG. 4  are also present in the apparatus of  FIG. 3 . Therefore, as to the description of these units it is referred to  FIG. 3 . 
     As already mentioned, there are two APL measurements: one in the front-end  2 , and the other one in the back-end  3 . The power management unit  9  will use the maximum of these two values to determine the number of sustains to be displayed. This maximum is provided by a comparator unit  12 . So the implementation is very simple. 
     The content of the front-end  2  and the back-end  3  are only given as examples. It is only mandatory in the front-end  2  to have the APL f  measurement unit  11  before the brightness/contrast control  5 . 
     Since this solution can only lead the power management unit  9  to use a higher value of APL, the number of sustains to be displayed can only be reduced. This means that the power consumption will be reduced in this case. This is a real advantage as to the tuning of contrast or brightness. 
     In a variant implementation, the comparator unit  12  can be replaced by a generator unit  12  that generates an APL level that is comprised between the two measured APL levels. This APL value should be greater than APL b  and, if APL f &gt;APL b , said APL value can be any value comprised between APL b  and APL f . 
     Now, the example of the introductory part of the description shall be regarded again. The APL measured in the front-end is equal to 300. The user wants to reduce the contrast by 2. Since the APL in the back-end  3  will decrease, the power management unit  9  will use the APL measured in the front-end  11 , this means 300, and so the same number of sustains is used. Therefore in order to reduce the contrast by 2, the video has to be divided by 2. The APL measured in the back-end  3  is equal to 150 in this case. 
     The power management  9  uses the value 300 as input. The average number of sustains is equal to 444*150/1024=65, but the maximum value of the picture will be 255/2=127. So the number of gray levels really used will be divided by around 2. This means that the number of gray levels really used is twice as big as in the standard implementation. So finally the picture quality is significantly improved. 
     In summary, the invention presented in this document aims at improving the picture quality when the contrast and/or the brightness are reduced. This is achieved by implementing an APL (Average Power Level) function in the front-end and using the measured value in the back-end.