Abstract:
A method may be provided for setting an average picture level. This may include setting an average picture level with a fixed number of steps based on a pre-determined gray level range, deriving a low brightness intensifying coefficient, deriving a high brightness intensifying coefficient, deriving a first constant representing an average increase rate of brightness, and calculating a number of sustain pulses corresponding to a gray level of a video signal currently inputted based on the derived low brightness intensifying coefficient, the derived high brightness intensifying coefficient and the derived first constant.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a plasma display panel, and more particularly to a method of setting an average picture level that is adaptive for preventing a brightness inversion phenomenon and a method of driving a plasma display panel using the same. 
   2. Description of the Related Art 
   A plasma display panel PDP is a display device using a phenomenon that visible ray is generated from a fluorescent substance when ultraviolet ray generated by gas discharge excites the fluorescent substance. The PDP is thinner and lighter than a cathode ray tube CRT, which has been used as main display means so far, and can be embodied of high definition and wide screen. 
   Referring  FIG. 1 , a discharge cell of a three-electrodes AC surface discharge PDP includes a first electrode Y and a second electrode Z formed on an upper substrate  1 , and an address electrode X formed on a lower substrate  4 . 
   The address electrode perpendicularly intersects a pair of sustain electrodes each including either the first electrode Y or the second electrode Z. 
   There are a dielectric layer  2  and a protective film  3  deposited on the upper substrate to cover the first electrode Y and the second electrode Z. 
   There is a dielectric layer  5  deposited on the entire surface of the lower substrate  4  to cover the address electrode X and there are barrier ribs  6  formed parallel to the address electrode X on top of it. 
   There is inactive mixture gas as discharge gas interposed into a discharge space of a discharge cell provided between the upper/lower substrates  1  and  4  and the barrier ribs  6 . 
   In order to realize gray level of a picture, there is a frame (field) driven by being divided into several sub fields that have different light-emission frequency. Each sub field is divided into a reset interval (or initialization interval) for initializing cells of a whole screen, an address interval for selecting the cell and a sustain interval for realizing the gray level in accordance with a discharge frequency, then to be driven. 
     FIG. 2  illustrates a driving apparatus of a conventional plasma display panel. 
   Referring to  FIG. 2 , the conventional driving apparatus of the PDP includes a first reverse gamma corrector  10 A connected between an input line  9  and the PDP  26 , a gain controller  12 , an error diffuser  14 , a sub-field mapping unit  16  and a data aligner  18 ; a frame memory  20  connected between the input line  9  and the PDP  26 , a second reverse gamma corrector  10 B, an average picture level APL controller  22  and a waveform generator  24 . 
   The first and the second reverse gamma corrector  10 A and  10 B applies reverse gamma correction to a gamma corrected video signal to linearly convert the brightness value depending on the gray scale value of a video signal. The frame memory  20  stores the data R,G,B of one frame portion and supplies the stored data to the second reverse gamma corrector  10 B. 
   The APL controller  22  receives the video data corrected by the second reverse gamma corrector  10 B to generate N (N is an integer) step signals for controlling the number of sustaining pulses. The gain controller  12  amplifies the corrected video data from the first reverse gamma corrector  10 A as much as effective gain. 
   The error diffuser  14  diffuses an error component of a cell to the adjacent cells to finely control the brightness value. The sub-field mapping unit  16  re-allots the video data corrected from the error diffuser  14  by sub-fields. 
   The data aligner  18  converts the video data inputted from the sub-field mapping unit  16  to be suitable for the resolution format of the PDP  26 , and then supplies to an address driving integrated circuit IC of the PDP  26 . 
   The waveform generator  24  generates a timing control signal by the inputted N step signal from the APL controller  22  and supplies the generated timing control signal to the address driving IC, a scan driving IC and a sustain driving IC of the PDP  26 . 
   In this way, the APL controller  22  of the driving apparatus of the conventional plasma display panel is used to emphasize the area that is relatively bright when the luminosity of the whole image is dark. 
   There is an operation process of the APL controller  22  explained in detail in reference to  FIG. 3 . 
   Referring to  FIG. 3 , the number of sustain pulses decreases as the step of APL gets higher. In other words, when the APL step is below a threshold APLth (to be set approximately between 17 to 24), the number of sustain pulses is set to be the maximum sustain number (approximately between 800 to 1200). When the APL step is over the threshold APLth, the number of sustain pulses gradually diminishes. Here, when the APL step is maximal, the number of sustain pulses is set to be the minimum sustain number (approximately near 200). 
   On the other hand, power consumption increases in proportion to APL when the APL is below the threshold APLth, and is uniformly maintained on the whole when the APL is over the threshold APLth. That is, the APL controller  22  of the conventional PDP is used in order to emphasize the relatively bright area when the luminosity of the whole image is dark and to have the power consumption maintained uniformly. 
   On the other hand, the number of sustain pulses in accordance with APL steps is determined by the following Equation 1.
 
 N sus=1/( a+b×x )  [Equation 1]
 
   Herein, x represents a current APL step, Nsus denotes the number of sustain pulses. And, a and b can be determined from a maximum sustain number, an APL threshold Ath, a minimum sustain number and a maximum APL step. 
   For instance, a linear equation in relation to a and b can be calculated by setting the maximum sustain number to be 1200 (Nsus=1200) and the threshold APLth of APL to be 17 (x=17). Further, quadratic equation can be calculated by setting the minimum sustain number to be 182 (Nsus=182) and the maximum APL step to be 255 (x=255). If the value of a and b is calculated in use of the linear equation and the quadratic equation, a has the value of 4.9×10 −4  and b has the value of 1.96×10 −5 . 
   At this moment, by the Equation 1, the number of sustain pulses Nsus is set to be 680.2721 . . . if the APL is 50, and the number of sustain pulses Nsus is set to be 408.1632 . . . if the APL is 100. Further, the number of sustain pulses Nsus is set to be 226.7573 if the APL is 200. In other words, the conventional APL controller  22  determines the number of sustain pulses by rounding off a fraction below a decimal point in use of Equation 1. 
   There is explained how to derive Equation 1 in reference to  FIG. 4 . 
   Referring to  FIG. 4 , a conventional equivalent circuit of a PDP includes a first capacitors Cd formed between the first Y and second Z electrodes and the address electrode X respectively, a second capacitor Cg formed by a gap between the first electrode Y and the second electrode Z, a third capacitor Cdi formed by the dielectric layers  2  and  5 , a fourth capacitor formed by a plasma and a inactive gas, a first resistor formed by the resistance value of the plasma, a second resistor Rd formed by a data driver and a third resistor Re formed by a scan driver. 
   In such an equivalent circuit of a conventional PDP, a first electric power P 1  consumed in a panel is proportional to the multiplication of the number of sustain and a current APL step (P 1 ∝Nsus×x). Further, a reactive power P 2  that charges the capacitors Cd, Cg, Cdi and Cv formed in the panel is proportional to the number of sustain (P 2 ∝Nsus). 
   A second electric power P 3  consumed in an energy recovering device for recovering the electric power supplied to the first and second electrodes is proportional to the reactive power P 2  (P 3 ∝P 2 ∝Nsus). A third electric power P 4  consumed by internal resistance in a power supplier that supplies electric power to the panel and the energy recovering device is proportional to the sum of the first electric power P 1  and the second electric power P 3  (P 4 ∝P 1 +P 3 ∝Nsus(K+Kx)). 
   Accordingly, the consumed power in the whole PDP is defined as Equation 2.
 
 P tot∝P1 +P 3 +P 4 ∝N sus( a+b×x )  [Equation 2]
 
   At this moment, an average brightness of a screen is determined by Equation 3.
 
 B∝N sus×x  [Equation 3]
 
   Even if the APL step changes in Equation 2, a maximum sustain number needed for sustaining power consumption uniformly is as the followings.
 
 N sus( x )= k /( a+bx )  [Equation 4]
 
   Herein, Equation 1 is derived since k is set to be 1. On the other hand, when the maximum number of sustain is 1200 (Nsus=1200) and the threshold APLth of the APL is 17 (x=17), if the brightness of B(x)−B(x−1)-herein, x is a natural number of 1 or more-is calculated in use of Equation 3, it can be shown as in  FIG. 5 . 
   In other words,  FIG. 5  is a graph represented by subtracting the brightness value of a previous APL step from the brightness value of a current APL step. 
   Referring to  FIG. 5 , it can be seen that the brightness is inverted when setting the number of sustain pulses when using a conventional Equation 1. In other words, there is shown an area where the brightness is inverted (where values are below ‘0’ in a Y axis) when the APL step is over 120. Such a brightness inversion phenomenon takes place by rounding off a fraction below a decimal point when calculating the number of sustaining pulses with Equation 1. On the other hand, if the brightness is inverted in the PDP, flicker phenomenon may occur, thereby causing picture quality deterioration. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a method of setting an average picture level that is adaptive for preventing a brightness inversion phenomenon and a method of driving a plasma display panel using the same. 
   In order to achieve these and other objects of the invention, a method of setting an average picture level according to an aspect of the present invention includes setting an average picture level with a fixed number of steps on the basis of a pre-determined gray level range; deriving a low brightness intensifying coefficient from a low brightness range of steps that are not greater than a middle step of the average picture level in order to correct a linearity of brightness; deriving a high brightness intensifying coefficient from a high brightness range of steps that are equal to or not less than the middle step in order to correct a brightness inversion phenomenon; deriving a first constant representing an average increase rate of brightness in relation to an increase of the average picture level when a current average picture level increases as compared to a previous average picture level; and calculating the number of sustain pulses corresponding to a gray level of a video signal currently inputted in use of the derived low brightness intensifying coefficient, the high brightness intensifying coefficient and the first constant. 
   The method further includes a step of setting a maximum sustain pulse number and a minimum sustain pulse number; 
   The method further includes a step of setting a second constant and a third constant in use of the following Formula 1 after receiving a threshold of average picture level that makes the maximum sustain pulse number sustained uniformly, the minimum sustain number and a maximum step value of the average picture level;
 
 N= 1/( a+b×x 1)  Formula 1
 
   Herein, N is the maximum sustain pulse number of the minimum sustain pulse number, a is the second constant, b is the third constant and x1 is the threshold or the maximum step value. 
   The sustain pulse number is determined by the following Formulas 2 and 3;
 
 N sus( x )= n ( x )/( a+bx )  Formula 2
 
 N ( x )=(( a+bx )/ x )×[ d×ln ( x )+ e ( x−i )+ f ( x−i ) 2 ]  Formula 3
 
   Herein, x is the current average picture level, Nsus(x) is the number of sustain pulses determined by the current average picture level, d is the low brightness intensifying coefficient, e is the first constant, f is the high brightness intensifying coefficient and i is the middle step of the average picture level. 
   The method further includes a step of setting a maximum power value and a minimum power value of the plasma display panel on the basis of an average power value of a power supply for supplying electric power. 
   The maximum power value is obtained by adding the average power value and 5%˜25% of the average power value. 
   The minimum power value is obtained by subtracting 5%˜25% of the average power value from the average power value. 
   The method includes steps of inputting the lower intensifying coefficient d, the first constant e and the higher intensifying coefficient f, which are pre-determined, into the Formulas 2 and 3; and increasing the low brightness intensifying coefficient by “1” until it satisfies the maximum sustain pulse number after inserting the threshold of the average picture level into the Formulas 2 and 3. 
   In the method, the first constant is made to increase by “1” until a high brightness value is obtained in a high average picture level if the maximum sustain pulse number is satisfied when the threshold of the average picture level is inputted into the Formulas 2 and 3. 
   In the method, the value of the first constant is made to decrease by “1” and the low brightness intensifying coefficient is made to increase by “1” in order to allow a power value currently consumed in the plasma display panel to be positioned between the maximum power value and the minimum power value. 
   In the method, the value of the high brightness intensifying coefficient is made to increase by “0.01” until the high brightness value is obtained in the high average picture level after the first constant and the low brightness intensifying coefficient are adjusted. 
   The low brightness intensifying coefficient, the first constant and the high brightness intensifying coefficient are substituted into the Formulas 2 and 3, and determined if they satisfy the following Conditions 1, 2 and 3;
 
 N ( x 2)= N (max)  Condition 1
 
   Herein, N(x2) is the threshold of the average picture level and N(max) is the maximum sustain pulse number.
 
 B ( x )−( Bx− 1)&gt;0  Condition 2
 
   Herein, B(x) is a brightness value of the current average picture level, B(x−1) is a brightness value of the previous average picture level.
 
 P 1 &lt;P ( x )&lt; P 2  Condition 3
 
   Herein, P 1  is a minimum power value, P 2  is a maximum power value and P (x) is a power value of the current average picture level. 
   The first constant and the high brightness intensifying coefficient are set to be minimum values that satisfy the Condition 1 to 3. 
   A method of setting an average picture level according to another aspect of the present invention includes steps of detecting a value of a current average picture level; subtracting a middle value of an average picture level from the value of the current average picture level; and calculating the number of sustain pulses corresponding to the value of the current average picture level in use of the subtracted value. 
   A method of driving a plasma display panel according to still another aspect of the present invention includes steps of detecting a value of a current average picture level; subtracting a middle value of an average picture level from the value of the current average picture level; and calculating the number of sustain pulses corresponding to the value of the current average picture level in use of the subtracted value, and wherein the sustain pulse are applied to the plasma display panel as many as the calculated number of the sustain pulses. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects of the invention will be apparent from the following detailed description of the embodiments of the present invention with reference to the accompanying drawings, in which: 
       FIG. 1  illustrates a perspective view of a discharge cell structure of a conventional three-electrodes AC surface discharge plasma display panel; 
       FIG. 2  depicts a block diagram of a driving apparatus of a conventional plasma display panel; 
       FIG. 3  is a graph conventionally representing the number of sustaining pulses in accordance with APL steps; 
       FIG. 4  is a circuit diagram equivalently representing a discharge cell shown in  FIG. 1 ; 
       FIG. 5  is a graph conventionally representing a brightness inversion phenomenon in according with APL steps; 
       FIG. 6  depicts a flow chart of the process of calculating a high brightness intensifying coefficient, a low brightness intensifying coefficient and an average increase constant; 
       FIG. 7  is a graph representing the number of sustaining pulses in accordance with APL steps determined by Equation 5 and 6; 
       FIG. 8  is a graph representing brightness values defined by Equation 5 and 6; 
       FIG. 9  is a graph representing values obtained by subtracting the brightness value of a previous APL step from the brightness value of a current APL step; and 
       FIG. 10  is a graph representing power consumption in accordance with APL steps. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to  FIGS. 6 to 10 , there are explained preferred embodiments of the present invention as follows. 
   According to an embodiment of the present invention, the number of sustain pulses in accordance with APL steps is determined by the following Equation 5 and 6.
 
 N sus( x )= n ( x )/( a+bx )  [Equation 5]
 
   Herein,
 
 n ( x )=(( a+bx )/ x )×[ d×ln ( x )+ e ×( x− 128)+ f ×( x− 128) 2 ]  [Equation 6]
 
   Herein, a constant d is used as a low brightness intensifying coefficient for improving linearity of brightness when an APL step (x) is not greater than half of the maximum step. A constant f is used as a high brightness intensifying coefficient for preventing a brightness inversion phenomenon when an APL step (x) is half of the maximum step or greater. A constant e is a value representing an average increase rate of brightness in accordance with APL increase. 
     FIG. 6  is a flow chart representing a method of determining constants d, e and f shown in Equation 6. 
   Referring to  FIG. 6 , firstly, a maximum sustain number N (max), an APL threshold (x1), a minimum sustain number N (min) and APL&#39;s maximum step value X(max) are inputted. Further, a minimum power value P 1  and a maximum power value P 2  of a sustain power supply, a constant j with a value of 0, and i having half of a maximum gray scale value (herein  128 ) are inputted. 
   The minimum power value P 1  and the maximum power value P 2  inputted in a step S 2  is determined as ±15% of an average power value of the sustain power supply. For example, if the average power value of the sustain power supply is 100, the minimum power value P 1  is set to be 85, the maximum power value P 2  is set to be 115. Practically, the minimum power value P 1  and the maximum power value P 2  may be set within ±5˜±25% of the average power value in the present invention. 
   The maximum sustain number N(max), the APL threshold (x1), the minimum sustain number N(min) and APL&#39;s maximum step value X(max) inputted in the step S 2  are substituted for the variables in Equation 1 to obtain the values of the constant a and b. (S 4 ) 
   After obtaining the values of the constant a and b in the step S 4 , the constants d, e and f are set to be 0, 1 and 0 respectively, i.e., d=0, e=1 and f=0. (S 6 ) After certain values are substituted for the constants d, e and f in the step S 6 , it is checked in use of Equation 5 and 6 whether the maximum sustain number N (max) satisfies the value inputted in the step S 2  when the APL threshold (x1) is substituted for the variable; herein, assuming that the maximum sustain number N(max) is 1200. (S 8 ) 
   When the APL threshold (x1) is substituted for the variable in the step S 8 , if the maximum sustain number N(max) is not satisfied, the value of the constant d is made to increase by one. (S 10 ) After then, the steps S 8  and S 10  are repeated to have the maximum sustain number N(max) when the APL threshold (x1) is substituted for the variable. 
   It is checked whether the brightness inversion phenomenon is generated if the maximum sustain number N(max) is obtained when the APL threshold (x1) is substituted for the variable in the step S 8 . (S 12 ) Equation 3 is used to check the brightness inversion phenomenon. 
   The value of the constant e is made to increase by one if there occurs the brightness inversion phenomenon in the step S 12  (at this moment, j=0). (S 14 ) The steps S 8  through S 14  are repeated until no brightness inversion phenomenon occurs. 
   If no brightness inversion phenomenon occurs in the step S 12 , the value of the constant j inputted in the step S 2  is made to increase by one. (S 16 ) After then, it is checked whether the consumed power value of the PDP is positioned between the minimum power value P 1  and the maximum power value P 2  inputted in the step S 2  while sequentially increasing the APL step. (S 18 ) 
   If the value of the power consumed in the PDP in the step S 18  is not positioned between the minimum power value P 1  and the maximum power value P 2 , the value of the constant e is made to decrease. (S 20 ) After decreasing the value of the constant e in the step S 20 , the constant d is made to increase in the step S 10 . (S 10 ) After then, the steps S 8  and S 12  are repeated. (S 8 , S 12 ) 
   It is checked whether the brightness inversion occurs in the step S 12  and if the brightness is inverted, it is inputted to the step S 14 . (S 12 ) In the step S 14 , it is checked whether the constant j is equal to the value of 0, if the constant j has the value of 0 or more, the constant f is made to increase by the value below decimal point; increment by 0.01. (S 14 ) After then, the steps S 8  through S 20  are repeated until the value of the power consumed in the PDP is positioned between the minimum power value P 1  and the maximum power value P 2 . 
   On the other hand, in the step S 18 , if the power value consumed in the PDP is positioned between the minimum power value P 1  and the maximum power value P 2 , the constant values calculated up to this point of time are substituted into Equation 5 and 6 to calculate the number of sustain pulses. 
   For instance, when the maximum sustain number is set to be 1200, the minimum sustain number 200, the threshold of APL  26  and the maximum step of APL  255 , the constants are set like {a=4.9×10 −4 , b=1.96×10 −5 , d=10400, e=60 and f=0.35}. 
   On the other hand, if the values of the foregoing a, b, d, e and f are substituted into Equation 5 and 6, it can be seen that the number of sustain pulses slowly decrease after the threshold of APL as in  FIG. 7 . Further, the brightness value checked by Equation 3 increases in proportion to the APL steps as in  FIG. 8 . 
   In addition, when the brightness of B(x)−B(x−1); herein, x is a natural number of 1 or more, is calculated, it is shown as in  FIG. 9 . 
     FIG. 9  is a graph representing values obtained by subtracting the brightness value of a previous APL step from the brightness value of a current APL step. 
   Referring to  FIG. 9 , there is no area where brightness is inverted according to a method of setting an average picture level of the embodiment of the present invention. In other words, there is no area where the value of B(x)+B(x−1) is 0 or less. Accordingly, it is possible according to the method of setting the average picture level of the present invention that flickers are eliminated and a picture quality is improved. 
   On the other hand, it can be seen that the power consumed in the PDP is sustained uniformly within ±15% as in  FIG. 10 . 
   Equation 5 and 6 according to the embodiment of the present invention is derived through the process as follows. 
   The following expression is to be satisfied by Equation 2 and 3 in order to satisfy P 1 &lt;P (x)&lt;P 2  (herein, P 1  is minimum power value, P 2  maximum power value) and B(x)&gt;(Bx−1) in an arbitrary APL level x.
 
 P 1/( a+bx )&lt; N sus( x )&lt; P 2/( a+bx )
 
   The number of sustain pulses satisfying the above expression is determined like Equation 5. A calculation of brightness can be done using Equations 5 and 3, as follows.
 
 B ( x )= x×N sus( x )=( x /( a+bx ))× n ( x )  [Equation 7]
 
   A differential function B′ (x) in relation to x in Equation 7 is as follows.
 
 B ′( x )= an ( x )/( a+bx ) 2   +xn ( x )/( a+bx )=ε( x )&gt;0  [Equation 8]
 
   If Equation 8 is converted as follows
 
 n ( x )+{ a/x ( a+bx )}× n ( x )≡ n ( x )+ p ( x ) n ( x )=ε( x )×{( a+bx )/ x}≡q ( x )
 
   At this moment, the solution of the differential equation is as follows.
 
 n ( x )=exp (−∫ P ( x ) dx └∫q ( x )exP(∫ p ( x ) dx+C ┘ 
 
   Herein,
 
∫ p ( x ) dx=∫{a/x ( a+bx )} dx=−ln {( a+bx )/ x} 
 
   Since the above equation is given, N(x) is expressed as follows.
 
 n ( x )={( a+bx )/ x }[∫ε( x ) dx+C] 
 
   Herein, assuming that it is given in a series form of ε(x)=d/x+e+2fx (d, e and f are constants), N(x) is expressed as follows.
 
 n ( x )={( a+bx )/ x }×( C+dln ( x )+ ex+fx   2 )
 
   Herein, a constant C does not much affect a brightness increase rate. Accordingly, if the constant C is included in e and f, Equation 6 is derived. 
   As described above, according to the method of setting the average picture level of the present invention, it is possible to prevent the brightness inversion phenomenon in setting the number of sustain in accordance with APL. By preventing such brightness inversion phenomenon, it is possible to prevent flickers generated in the plasma display panel and to provide a good picture quality. 
   Although the present invention has been explained by the embodiments shown in the drawings described above, it should be understood to the ordinary skilled person in the art that the invention is not limited to the embodiments, but rather that various changes or modifications thereof are possible without departing from the spirit of the invention. Accordingly, the scope of the invention shall be determined only by the appended claims and their equivalents.