The plasma display technology now makes it possible to achieve flat colour panels of large size and with limited depth without any viewing angle constraints. The size of the displays may be much larger than the classical CRT picture tubes would have ever been allowed. Referring to the latest generation of European TV sets, a lot of work has been made to improve its picture quality. Consequently, there is a strong demand, that a TV set built in a new technology like the plasma display technology has to provide a picture so good or better than the old standard TV technology. This picture quality can be decomposed in different parameters such as:                Good response fidelity of the panel: This means that only one pixel could be “ON” in the middle of a black screen and in addition, this panel has to perform a good homogeneity.        Good brightness of the screen: This is limited by the idle time of the panel, i,e time in which no light is produced.        Good contrast ratio even in dark room: This is limited by the brightness of the panel combined with the black level.All these parameters are completely linked together. So an optimised compromise has to be chosen to provide the best quality picture at the end.        
A plasma display panel utilizes a matrix array of discharge cells, which could only be “on” or “off”. Also unlike a CRT or LCD in which gray levels are expressed by analog control of the light emission, a PDP controls the gray levels by modulating the number of light pulses per frame. The eye will integrate this time-modulation over a period corresponding to the eye time response. Since the video amplitude determines the number of light pulses, occurring at a given frequency, more amplitude means more eye pulses and thus more “on” time. For this reason, this kind of modulation is known as PWM, (for pulse width modulation). To establish a concept for this PWM, each frame will be decomposed in sub-periods called “sub-fields”. For producing the small light pulses, an electrical discharge will appear in a gas filled cell, called plasma and the produced UV radiation will excite a coloured phosphor, which emits the light.
In order to select which cell should be lighted, a first selective operation called “addressing” will create a charge in the cell to be lighted. Each plasma cell can be considered as a capacitor, which keeps the charge for a long time. Afterwards, a general operation called “sustain” applied during the lighting period will add charges in the cell. Only in the cells addressed during the first selective operation, the two charges build up and that brings a firing voltage between two electrodes of the cell. UV radiation is generated and the UV radiation excites the phosphorous for light emission. During the whole sustain period of each specific sub-field, the cell will be lighted in small pulses at a given sustain frequency. At the end, an erase operation will remove all the charges to prepare a new cycle. In the standard addressing method known as ADS (Address Display Separated), all the basic cycles are made one after the other. This is represented on FIG. 1 which is an example of ADS based on a 8-bit encoding with only one priming pulse at the beginning of the frame. In that case, the gray level is represented by a combination of the 8 following bits:1-2-4-8-16-32-64-128
So, the frame period is divided in 8 sub fields, each one corresponding to a bit. The number of light pulses for the bit 2 is the double as for the bit 1 and so forth. So it is possible through sub fields combination to build the 256 gray levels. This is only an example, as the number of sub fields or of priming could be modified in view of the quality factor to improve.
In fact for this type of display, more brightness equals more sustain pulses. This also means more peak luminance. More sustain pulses correspond 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.
The picture introducing the higher power consumption is a full-white picture. Therefore, for a required power consumption and for a given electronic efficacy, the luminance of the full-white is fixed. Then, for all other picture content, the peak-luminance will be adapted to have stable power consumption as shown on FIG. 2. This figure shows the decrease of the luminance when the picture load increases from a peak white picture to a full white picture. More precisely, when a PDP screen displays a full white picture (right screen in FIG. 2), less luminance is needed by the eye to catch a nice impression of luminance since this luminance is displayed on a very large part of the visual field. On the other hand, when a PDP screen displays a picture having low energy (left screen in FIG. 2) the contrast ratio is very important for the eye. In that case, the highest available white luminance should be output on such a picture to enhance the contrast ratio.
Such a concept suits very well to the human visual system, which is dazzled in case of full-white picture whereas it is really sensitive to dynamic in case of dark picture (e.g. dark night with a moon). Therefore in order to increase the impression of high contrast on dark picture, the peak-luminance is set to very high values whereas it is reduced in case of energetic pictures (full-white). This basic principle will lead to a stable power consumption, as represented by the horizontal line in FIG. 2.
In the case of a plasma display, the luminance as well as the power consumption is directly linked to the number of sustain pulses per frame. This has the disadvantage of allowing only a reduced number of discrete power levels compared to an analog system.
In other words, the concept of power management adapted to a PDP is based on the change of the total amount of sustain pulses depending on picture content in order to keep the overall power consumption constant. Such a concept is illustrated on FIG. 3 that shows the number of sustain pulses in relation with the picture load.
In the case of fully digital displays like plasma, only discrete modes can be defined on the curve of FIG. 3 based on a measurement of the picture content or picture load. This measurement, mainly called APL for Average Power Level can be computed as following:
      APL    ⁡          (              I        ⁡                  (                      x            ,            y                    )                    )        =            1              (                  C          ×          L                )              ×                  ∑                  x          ,          y                    ⁢                          ⁢              I        ⁡                  (                      x            ,            y                    )                    where I(x,y) represents the displayed picture having C columns and L lines. The main objective leads in the determination of a discrete number of modes in an optimal manner.
Once the optimal power modes have been defined based on a given number of sustain pulses for various APL values, the distribution of sustain pulses among the sub-field sequence should be performed. On one hand, a high number of sub-fields is mandatory to ensure high quality display with reduced moving artifacts. On the other hand, every addressing operation required for each sub-field corresponds to idle time where no light pulse can be produced. Furthermore, the available sustain frequency is fixed and normally corresponds to an optimal panel functioning to avoid luminance variation depending on picture content.
In other words, in the past, the optimal sustain frequency was fixed for all APL values and set to the optimal value (e.g. 200 kHz in the present example). Obviously, this will reduce the capability of the panel to display high peak luminance for a high number of sub-fields. Therefore, new approaches have been defined in the past in order to reach higher peak-luminance at good panel homogeneity. Some of the solutions are described, for example, in WO00/46782 or WO02/11111 in the name of the applicant. Since high peak luminance is only mandatory for picture having low charge, which also means picture being less sensitive to the homogeneity problems, the optimal sustain frequency is not required there. Therefore the actual state of the art for optimized power management is based on a variation of the sustain frequency for low-charged pictures as shown on FIG. 4 for a 12 sub-fields distribution.
In this example, when the picture load is below 20%, an increase of the sustain frequency will be performed whereas this frequency is fixed for more loaded pictures. Obviously, all the values presented here are only example and should vary for one supplier to another (e.g. the value 20%). Indeed some suppliers keep the same frequency whereas other suppliers have, for every APL value and picture charge, an other sustain frequency.
However, the concept described above presents some limitations such as:                For a given APL value, the sustain frequency is fixed to a given value, for example 200 KHz at 100% charge and 320 KHz for low charge. There is only a shift of the sustain frequency value. In this case, the EMI (Electro-Magnetic Interference) peak observed at the sustain frequency will also evolve in its position as the sustain frequency. It will stay strong, always requiring a strong filter that decreases the brightness.        The panel efficacy as well as the voltage margin of the panel depend strongly of the sustain frequency. In other words, if the sustain frequency is too far away from the optimal value, a loss of margin as well as efficiency could happen. Moreover, the impact on the margin and efficacy will be stronger on low sub-fields (LSB) having less energy. In that case, if the APL changes between two pictures having a lot of similarities, changes in the dark areas can be perceptible (the eye is much more sensitive in those regions).        
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