A PDP for Plasma Display Panel utilizes a matrix array of discharge cells, which can only be “ON”, or “OFF”. Therefore, it can be defined as a pure digital display. 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 amplitude video 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.
This PWM is responsible for one of the PDP image quality problems: the overall noise level, especially in the darker regions of the picture. This is due to the fact that displayed luminance is linear to the number of pulses, but the eye response and sensitivity to noise is not linear. In darker areas the eye is more sensitive than in brighter areas. This means that even though modern PDPs can display ca. 255 discrete video levels, quantization error will be quite noticeable in the darker areas. Moreover, all video pictures are pre-corrected to compensate the traditional gamma curves from standard display (e.g. CRT). Since, the plasma display is a pure linear display and does not provide such a non-linear gamma behavior, an artificial gamma function should be performed at the display level and in a digital form. This gamma function increases the quantization steps in the dark areas whereas the quantization steps will be reduced in luminous areas. In addition, an increasing of the quantization step will drastically increase the level of the noise present in the picture.
In the following, the quantization noise after gammatization of a video signal is described.
The method used to render video levels on a plasma (PWM) is responsible for one of the PDP image quality problems: the big quantization steps, especially in the darker regions of the picture increase strongly the noise level in those areas. This is due to the fact, that displayed luminance is linear to the number of impulses for driving the luminous elements, but the eye response and sensitivity to noise is not linear. In darker areas the eye is more sensitive than in brighter areas. This means than even though modern PDPs can display ca 255 discrete video levels, quantization error will be quite noticeable in the darker areas.
Moreover, all video pictures are pre-corrected by a γ−1 function to compensate the traditional gamma curves (γ) from standard display (e.g. CRT). Since, the plasma display is a pure linear display and does not provide such a non-linear gamma behavior, an artificial gamma function should be applied to the display level and in a digital form. This degamma function increases the quantization noise in the dark areas whereas the quantization noise will be reduced in luminous areas.
A standard gamma function applied on 8-bit level using the following formula:
      Out    ⁢                  ⁢          (              x        ,        y            )        =                    225        ·                              (                                          In                ⁢                                                                  ⁢                                  (                                      x                    ,                    y                                    )                                            255                        )                    γ                    ⁢                          ⁢      with      ⁢                          ⁢      γ        ≈    2  shall be taken as example. FIG. 1 illustrates such a function. Itshows that a gamma function applied to 8-bit level generates a strong quantization effect in the dark area. For instance, all input levels below 12 are set together to 0 after the gammatization, i.e. the application of the γ function. The following table presents the detail of the computation for some video levels:
Input (8-bit)(Output (float)Output (8-bit)00010,003921569020,015686275030,035294118040,062745098050,098039216060,141176471070,192156863080,250980392090,3176470590100,3921568630110,4745098040120,5647058821130,6627450981140,7686274511150,8823529411161,0039215691171,1333333331181,2705882351191,4156862751201,5686274512211,7294117652221,8980392162232,0745098042. . . . . . . . . 250245,0980392245251247,0627451247252249,0352941249253251,0156863251254253,0039216253255255255
This table shows that, in the dark areas, there are less output values than input values which means that the quantization steps have increased. On the opposite, in high levels, there are less input than output values (e.g. no input to generate the value 246) which means that the quantization noise has been reduced.
Standard digital pictures suffer from quantization noise which depends on the number of bits used for the digitalization. In addition to that, all natural sequences contain some natural noise (mainly gaussian noise). The overall visibility of these noise effects also depends on its temporal variation which generates a kind of bustling effect.
FIG. 2 presents the video values of a standard digital video picture before gammatization. It shows an example of quantization noise and natural noise for the three color-components R,G,B of a part of the picture. This noise is enhanced by its temporal variation.
Now, there shall be given an estimation of the effect obtained on a CRT disposing of an analog gammatization function (tube behavior). For that estimation, the assumption is taken that the luminance of the white will be 100 cd/m2 and that the CRT behavior can be represented by:
      CRT    ⁢                  ⁢          (              x        ,        y            )        =                    100        ·                              (                                          In                ⁢                                                                  ⁢                                  (                                      x                    ,                    y                                    )                                            255                        )                    γ                    ⁢                          ⁢      with      ⁢                          ⁢      γ        =    2.  In that case, the noise pattern on the CRT will be transformed as shown in FIG. 3. From the luminance values of the three patterns R,G,B, is calculated for each component R,G,B a mean noise value and a mean error value on a CRT screen.
This shall be compared with the noise generated in the case of a plasma display. First, the gammatization will be performed at digital level (8-bit) as shown in FIG. 4. The degammatization is performed on the input values as those given in FIG. 2 for the three components R,G,B. At the output a digital value is obtained.
Then, for each digital value, a luminance value can be computed taking the assumption that the plasma is a pure linear system, the value 255 is matched with 100 cd/m2 The visibility of the noise structure can be estimated as shown in FIG. 5 which corresponds to FIG. 3 but in the case of a PDP.
The estimation of the noise structure on a plasma showed that the increased quantization step in the dark areas lead to a strong noise pattern. Therefore the bustling effect of the noise will increase strongly on a plasma screen in comparison to standard displays (the mean error may be up to 80%). This is also enhanced by the fact that the human visual system behavior follows a logarithm law, more sensitive for low-levels than for high levels.
As explained in the previous paragraph, the noise is more visible on a plasma than on other display in the dark areas (e.g. CRTs). Therefore, it is judicious to implement a kind of noise reduction algorithm on PDPs. Actually, various displays already dispose of such algorithms. Nevertheless, standard noise reduction algorithms also have drawbacks like a loss of sharpness, moving artifacts (trail behind strong edges).