Patent Publication Number: US-7710354-B2

Title: Plasma display apparatus and driving method thereof

Description:
This Nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 10-2005-0045417 filed in Korea on May 30, 2005, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a plasma display apparatus and a driving method thereof. 
   2. Description of the Background Art 
   In a plasma display panel generally, barrier rib provided between front panel and rear panel forms one unit cell. Main discharge gas, such as neon (Ne), helium (He), or a combination (Ne+He) of neon and helium, and inertia gas containing a small amount of xenon are filled within each cell. Discharge being executed by high frequency voltage, the inertia gas generates vacuum ultraviolet rays and excites a phosphor provided between the barrier ribs, thereby embodying image. 
     FIG. 1  illustrates a driving waveform for driving a plasma display panel in a related-art plasma display apparatus. 
   As shown in  FIG. 1 , the plasma display panel is driven with a subfield divided into a reset period for initializing all cells, an address period for selecting the cell to be discharged, a sustain period for sustaining discharge of the selected cell, and an erasure period for erasing wall charges within the discharged cell. 
   In a setup period of a reset period, a ramp-up waveform (Ramp-up) is concurrently applied to all scan electrodes. By the ramp-up waveform, a weak dark discharge is generated within the discharge cells of a whole screen. By a setup discharge, positive wall charges are accumulated on an address electrode and a sustain electrode, and negative wall charges are accumulated on a scan electrode. 
   In a setdown period, after the supplying of the ramp-up waveform, a ramp-down waveform (Ramp-down), which falls starting from a positive voltage lower than a peak voltage of the ramp-up waveform to a specific voltage level of lower than a ground (GND) level voltage, generates a weak erasure discharge, thereby sufficiently erasing the wall charges excessively formed in the scan electrode. By the setdown discharge, the wall charges of an extent generating a stable address discharge uniformly remain within the cells. 
   In the address period, a negative scan pulse is sequentially applied to the scan electrodes and at the same time, a positive data pulse is synchronized to the scan pulse and applied to the address electrode. A voltage difference between the scan pulse and the data pulse and a wall voltage generated in the reset period being added, the address discharge is generated within the discharge cell to which the data pulse is applied. 
   The wall charges of the extent generating the discharge at the time of applying the sustain voltage (Vs) are formed within the cell selected by the address discharge. A positive voltage (Vz) is supplied to the sustain electrode so that a voltage difference from the scan electrode is reduced during the address period and erroneous discharge with the scan electrode is prevented. 
   In the sustain period, the sustain pulse (Sus) is alternately applied to the scan electrodes and the sustain electrodes. In the cell selected by the address discharge, the wall voltage within the cell and the sustain pulse being added, whenever each sustain pulse is applied, the sustain discharge, that is, a display discharge between the scan electrode and the sustain electrode is generated. 
   After the sustain discharge is completed, in the erasure period, a voltage of an erasure ramp waveform (Ramp-ers) whose pulse width and voltage level are low is supplied to the sustain electrode, thereby erasing the wall charges remaining within the cells of the whole screen. 
   In the plasma display panel, the driving waveform is supplied every subfield of the frame. 
   Meantime, a rising ramp (Ramp-up) supplied to the scan electrode in the reset period is generally equal to a high voltage pulse of about 400 V and thus, an amount of light generated depending on discharge caused by the rising ramp relatively gets larger. Accordingly, luminance in an off state of all the discharge cells of the plasma display panel, that is, a black luminance relatively gets larger, thereby deteriorating a characteristic of contrast. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is to solve at least the problems and disadvantages of the background art. 
   The present invention is to provide a plasma display apparatus and a driving method thereof, for controlling a level of a reset pulse supplied to a scan electrode of a reset period, thereby improving a characteristic of contrast. 
   To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, there is provided a plasma display apparatus. The apparatus comprises a plasma display panel comprising a plurality of scan electrodes, and a scan driver. The scan driver drives the plurality of scan electrodes, divides the plurality of scan electrodes into a plurality of scan electrode groups, and distinguishes a level of reset pulse supplied to at least one of the plurality of scan electrode groups from a level of reset pulse supplied to the others of the plurality of scan electrode groups. 
   A method of driving a plasma display apparatus comprising a plurality of scan electrodes comprises dividing the plurality of scan electrodes into a plurality of scan electrode groups, and supplying a different level of reset pulse to at least one of the plurality of scan electrode groups and the others, resectively. 
   In another aspect of the present invention, there is provided a method of driving a plasma display apparatus comprising a plurality of scan electrodes. The method comprises dividing the plurality of scan electrodes into odd scan electrodes and even scan electrodes, supplying a first reset pulse with a rising ramp voltage to the odd scan electrodes and supplying a second reset pulse with a predetermined positive voltage to the even number scan electrodes in setup period of reset period of one subfield of an odd frame, and supplying the second reset pulse to the odd scan electrodes and supplying the first reset pulse to the even number scan electrodes in setup period of reset period of a subfield corresponding to one subfield among subfields of an even frame that is next frame of the odd frame. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail with reference to the following drawings in which like numerals refer to like elements. 
       FIG. 1  illustrates a driving waveform for driving a plasma display panel in a related art plasma display apparatus; 
       FIG. 2  illustrates a plasma display apparatus according to an exemplary embodiment of the present invention; 
       FIG. 3  illustrates a method for dividing a plurality of scan electrodes into a scan electrode group in a plasma display panel according to an exemplary embodiment of the present invention; 
       FIG. 4  illustrates a method for dividing scan electrodes formed in a plasma display panel, into scan electrode groups comprising the scan electrodes having different number according to an exemplary embodiment of the present invention; 
       FIG. 5  illustrates a method for dividing scan electrodes formed in a plasma display panel, into scan electrode groups each comprising one scan electrode according to an exemplary embodiment of the present invention; 
       FIG. 6  illustrates a driving method of a plasma display apparatus according to an exemplary embodiment of the present invention; 
       FIG. 7  illustrates a difference between frames of a reset pulse supplied to one scan electrode group in a driving method of a plasma display apparatus according to an exemplary embodiment of the present invention; 
       FIG. 8  illustrates a plasma display apparatus according to an exemplary embodiment of the present invention; 
       FIG. 9  illustrates a driving method for driving a plasma display panel in the plasma display apparatus of  FIG. 8  according to an exemplary embodiment of the present invention; and 
       FIG. 10  illustrates a difference between frames of a reset pulse supplied to one scan electrode group in a driving method of a plasma display apparatus according to an exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will be described in a more detailed manner with reference to the drawings. 
     FIG. 2  illustrates a plasma display apparatus according to an exemplary embodiment of the present invention. 
   As shown in  FIG. 2 , the plasma display apparatus comprises a plasma display panel  500  comprising a plurality of scan electrodes (Y 1  to Yn), a sustain electrode (Z), and a plurality of address electrodes (X 1  to Xm); and a scan driver  503  for driving the plurality of scan electrodes (Y 1  to Yn), dividing the plurality of scan electrodes (Y 1  to Yn) into a plurality of scan electrode groups, and distinguishing a level of a reset pulse supplied to at least one of the plurality of scan electrode groups, from levels of reset pulses supplied to the others of the plurality of scan electrode groups. 
   The plasma display apparatus comprises the plasma display panel  500  comprising the scan electrodes (Y 1  to Yn), the sustain electrode (Z), and the plurality of address electrodes (X 1  to Xm); a data driver  502  for supplying data to the address electrodes (X 1  to Xm); the scan driver  503  for driving the scan electrodes (Y 1  to Yn); a sustain driver  504  for driving the sustain electrode (Z) that is a common electrode; and a driving voltage generator  505  for supplying a necessary driving voltage to each of the drivers  502 ,  503 , and  504 . 
   In the plasma display panel  500 , a front panel (not shown) and a rear panel (not shown) are sealed at regular intervals. A plurality of electrodes, for example, a plurality of maintenance electrodes comprising the scan electrodes (Y 1  to Yn) and the sustain electrode (Z) are formed. The address electrodes (X 1  to Xm) are formed intersecting with the maintenance electrode comprising the scan electrodes (Y 1  to Yn) and the sustain electrode (Z). 
   The data driver  502  receives data that is inverse gamma corrected and error diffused by an inverse gamma correction circuit and an error diffusing circuit not shown) and then is mapped to each sub field by a sub field mapping circuit. 
   The scan driver  503  supplies a ramp up waveform (ramp-up) and a ramp down waveform (ramp-down) to the scan electrodes (Y 1  to Yn) during the reset period. The scan driver  503  sequentially supplies a scan pulse of a scan voltage (−Vy) to the scan electrodes (Y 1  to Yn) during an address period, and supplies a sustain pulse to the scan electrodes (Y 1  to Yn) during a sustain period. 
   The scan driver  503  divides the plurality scan electrodes into the plurality of scan electrode groups, and distinguishes the level of the reset pulse supplied to at least one of the scan electrode groups from those of the others of the scan electrode groups in the reset period. 
   The sustain driver  504  supplies a bias voltage of a sustain voltage (Vs) to the sustain electrodes (Z) during the address period, and alternately operates with the scan driver  503  and supplies the sustain pulse to the sustain electrodes (Z) during the sustain period. 
   The driving voltage generator  505  generates a setup voltage (Vsetup), a scan common voltage (Vscan-com), the scan voltage (−Vy), the sustain voltage (Vs), and a data voltage (Vd). The driving voltages can be varied depending on a composition of a discharge gas and a discharge cell structure. 
   The scan driver  503  comprises a first reset driver  506  and a second reset driver  507 . The scan driver  503  controls the first and second reset drivers  506  and  507  so that the first reset driver  506  supplies the reset pulses to odd number scan electrode groups of the plurality of scan electrode groups during the reset period, and the second reset driver  507  supplies reset pulses having different levels from the reset pulses supplied to the odd number scan electrode groups, to even number scan electrode groups during the reset period. 
   In a driving method of the plasma display apparatus according to an exemplary embodiment of the present invention, the plurality of scan electrodes are divided into the plurality of scan electrode groups, and the reset pulses each having a different level are supplied to the scan electrode groups different from at least one of the plurality of scan electrodes groups. 
     FIG. 3  illustrates a method for dividing the plurality of scan electrodes into the scan electrode groups in a plasma display panel according to an exemplary embodiment of the present invention. 
   As shown in  FIG. 3 , in the plasma display panel  600 , the scan electrodes (Y) are divided into an A scan electrode group  601 , a B scan electrode group  602 , a C scan electrode group  603 , a D scan electrode group  604 , an E scan electrode group  605 , an F scan electrode group  606 , a G scan electrode group  607 , an H scan electrode group  608 , an I scan electrode group  609 , and a J scan electrode group  610 . 
   For example, one hundred scan electrodes being totally formed in the one plasma display panel  600 , the scan electrodes ranging from the scan electrode (Y 1 ) to the scan electrode (Y 10 ) are divided into the A scan electrode group  601 , and the scan electrodes ranging from the scan electrode (Y 11 ) to the scan electrode (Y 20 ) are divided into the B scan electrode group  602 . Like this method, the C scan electrode group  603 , the D scan electrode group  604 , the E scan electrode group  605 , the F scan electrode group  606 , the G scan electrode group  607 , the H scan electrode group  608 , the I scan electrode group  609 , and the J scan electrode group  610  are distinguished. 
   The scan driver  503  of  FIG. 2  drives the plurality of scan electrode groups divided as above. For example, the first reset driver  506  of the scan driver  503  supplies the reset pulses to the odd number scan electrode groups, that is, the A, C, E, G, and I scan electrode groups  601 ,  603 ,  605 ,  607 , and  609  of the plurality of scan electrode groups during the reset period. The second reset driver  507  supplies the reset pulses to the even number scan electrode groups, that is, the B, D, F, H, and J scan electrode groups  602 ,  604 ,  606 ,  608 , and  610  of the plurality of scan electrode groups during the reset period. 
   The scan electrode group all comprises the scan electrodes of the same number, respectively. The number of the scan electrode groups is at least two and less than the total maximal number of the scan electrodes. 
   All the scan electrodes comprised in the one scan electrode group are sequential in their scan sequence. In other words, depending on the scan sequence, the scan electrodes of the predetermined number are collected and set as the scan electrode group. 
   In  FIG. 3 , the scan electrode groups  601 ,  602 ,  603 ,  604 ,  605 ,  606 ,  607 ,  608 ,  609 , and  610  comprise ten scan electrodes, respectively, to have same number. But, it is also possible to set the number of the scan electrodes comprised in at least one scan electrode group, different from those of the others of the scan electrode groups. The scan electrode groups are also controllable in number. 
     FIG. 4  illustrates a method for dividing scan electrodes formed in a plasma display panel  700 , into scan electrode groups comprising the scan electrodes of different number according to an exemplary embodiment of the present invention. 
   As shown in  FIG. 4 , the scan electrodes (Y) are divided into an A scan electrode group  701 , a B scan electrode group  702 , a C scan electrode group  703 , a D scan electrode group  704 , an E scan electrode group  705 , an F scan electrode group  706 , a G scan electrode group  707 , an H scan electrode group  708 , and an I scan electrode group  709 . At least one of the scan electrode groups  701 ,  702 ,  703 ,  704 ,  705 ,  706 ,  707 ,  708 , and  709  comprises the scan electrodes of the number different from those of the others of the scan electrode groups. 
   All the scan electrodes comprised in the one scan electrode group are sequential in their scan sequence. In other words, depending on the scan sequence, the scan electrodes of the predetermined number are collected and set as the scan electrode group. 
   The scan driver  503  of  FIG. 2  drives the plurality of scan electrode groups divided above. For example, the first reset driver  506  of the scan driver  503  supplies the reset pulses to the odd number scan electrode groups, that is, the A, C, E, G, and I scan electrode groups  701 ,  703 ,  705 ,  707 , and  709  of the plurality of scan electrode groups during the reset period. The second reset driver  507  supplies the reset pulses to the even number scan electrode groups, that is, the B, D, F, and H scan electrode groups  702 ,  704 ,  706 , and  708  of the plurality of scan electrode groups during the reset period. 
     FIG. 5  illustrates a method for dividing scan electrodes formed in a plasma display panel  800 , into scan electrode groups each comprising one scan electrode according to an exemplary embodiment of the present invention. 
   As shown in  FIG. 5 , each scan electrode group comprises one scan electrode. The scan driver  503  of  FIG. 2  drives a plurality of scan electrode groups. For example, the first reset driver  506  of the scan driver  503  supplies the reset pulses to the odd number scan electrode groups of the plurality of scan electrode groups, and the second reset driver  507  supplies the reset pulses to the even number scan electrode groups of the plurality of scan electrode groups. 
     FIG. 6  illustrates a driving method of a plasma display apparatus according to an exemplary embodiment of the present invention. 
   As shown in  FIG. 6 , in the plasma display apparatus of  FIG. 2 , the plurality of scan electrode groups comprise the first scan electrode group (Ya) and the second scan electrode group (Yb). The scan driver  503  comprises the first reset driver  506  and the second reset driver  507 . The first reset driver  506  supplies a first reset pulse equal to a rising ramp voltage to the first scan electrode group (Ya) during the setup period of the reset period of one subfield. The second reset driver  507  supplies a second reset pulse equal to a predetermined positive voltage to the second scan electrode group (Yb) during the setup period of the reset period of the one subfield. 
   It is possible that the first reset pulse rises from the predetermined positive voltage to the setup voltage, and the second reset pulse is equal to the sustain voltage. 
   It is possible that the one subfield is equal to a subfield whose weight is the lowest or a subfield whose order in time is the first among subfields of a frame. 
   The predetermined positive voltage is supplied to the first scan electrode group (Ya) and the second scan electrode group (Yb) in a setup period of a reset period of another subfield that is at least one of subfields with exception of the one subfield. 
   A maintenance period of a predetermined positive voltage supplied in the setup period of the reset period of another subfield is shorter than a maintenance period of the predetermined positive voltage supplied in the setup period of the reset period of the one subfield. 
   The reason of being set above is that low weight causing relatively great unstable discharge in a first subfield for embodying low graylevel, the maintenance period of the sustain voltage (Vs) of the reset pulse get longer for stable discharge, thereby getting a distribution of wall charges more uniform within a discharge cell. 
   As a result, in a subfield with exception of the first subfield, the stable discharge can be guaranteed even though the maintenance period of the sustain voltage (Vs) of the reset pulse is short in length. 
   As such, the reset pulse comprising a rising ramp is supplied in the setup period of the reset period only in one subfield among the subfields of the frame and thus, a total of the number of the rising ramps within one frame is decreased, thereby improving a characteristic of contrast. 
   It is desirable that the first reset driver  506  and the second reset driver  507  supply the same reset pulse to all the scan electrodes comprised in the same scan electrode group, in the reset period. 
   In the driving method of the plasma display apparatus according to an exemplary embodiment of the present invention, the first reset pulse equal to the rising ramp voltage is supplied to the first scan electrode group in the setup period of the reset period of the one subfield, and the second reset pulse equal to the predetermined positive voltage is supplied to the second scan electrode group in the setup period of the reset period of the one subfield. 
   The first reset pulse rises from the predetermined positive voltage to the setup voltage, and the second reset pulse is equal to the sustain voltage. 
   The predetermined positive voltage is supplied to the first scan electrode group and the second scan electrode group in the setup period of the reset period of another subfield that is at least one of subfields with exception of the one subfield. 
     FIG. 7  illustrates a difference between the frames of the reset pulse supplied to one scan electrode group in the driving method of the plasma display apparatus according to an exemplary embodiment of the present invention. 
   As shown in  FIG. 7 , after the first reset pulse is supplied to the first scan electrode group (Ya) in the setup period of the reset period of the one subfield, the second reset pulse is supplied in a setup period of a reset period of a subfield corresponding to the one subfield among subfields of a next frame. After the second reset pulse is supplied to the second scan electrode group (Yb) in the setup period of the reset period of the one subfield, the first reset pulse is supplied in a setup period of a reset period of a subfield corresponding to the one subfield among subfields of a frame after the next frame. 
   The first reset driver  506  of  FIG. 2  supplying the first reset pulse to the first scan electrode group (Ya) in the first subfield whose weight is the lowest among subfields of one frame, it is possible to supply the second reset pulse in a setup period of a reset period of a first subfield whose weight is the lowest in a next frame. 
   The reset pulse supplied to the first scan electrode group (Ya) in the setup period of the reset period, and the reset pulse supplied to the second scan electrode group (Yb) in the setup period of the reset period, are alternately supplied to the first scan electrode group (Ya) and the second scan electrode group (Yb) every one frame. 
   The first reset pulse being sequentially supplied to the first scan electrode group (Ya) and the second reset pulse being sequentially supplied to the second scan electrode group (Yb), the discharge relatively gets unstable in the second scan electrode group (Yb) to which the rising ramp is not supplied, compared to the first scan electrode group (Ya) to which the rising ramp is sequentially supplied. Thus, luminance gets different in the first scan electrode group (Ya) and the second scan electrode group (Yb), thereby deteriorating a picture quality. 
   In the driving method of the plasma display apparatus according to an exemplary embodiment of the present invention, the plurality of scan electrodes are divided into odd number and even number scan electrodes. In a setup period of a reset period of one subfield of an odd frame, the first reset pulse equal to the rising ramp voltage is supplied to the odd number scan electrodes, and the second reset pulse equal to a predetermined positive voltage is supplied to the even number scan electrodes. In a setup period of a reset period of a subfield corresponding to the one subfield among subfields of an even frame equal to a next frame of the odd frame, the second reset pulse is supplied to the odd number scan electrodes, and the first reset pulse is supplied to the even number scan electrodes. 
     FIG. 8  illustrates a plasma display apparatus according to an exemplary embodiment of the present invention. 
   As shown in  FIG. 8 , in the plasma display apparatus, a plurality of scan electrode groups comprise a first scan electrode group, a second scan electrode group, and a third scan electrode group. A scan driver  1103  comprises a first reset driver  1106 , a second reset driver  1107 , and a third reset driver  1108 . 
   The first reset driver  1106  supplies a first reset pulse rising from a predetermined positive voltage to a setup voltage to the first scan electrode group in a setup period of a reset period of one subfield. The second reset driver  1107  supplies a second reset pulse, which rises from a predetermined positive voltage to a voltage lower than the setup voltage and maintains a voltage lower than the setup voltage for a predetermined time, to the second scan electrode group in the setup period of the reset period of the one subfield. The third reset driver  1108  supplies a third reset pulse equal to a predetermined positive voltage to the third scan electrode group in the setup period of the reset period of the one subfield. 
   It is possible that the one subfield is equal to a subfield whose weight is the lowest or a subfield whose order in time is the first among subfields of a frame. 
   The predetermined positive voltage is supplied to the first scan electrode group, the second scan electrode group, and the third scan electrode group in a setup period of a reset period of another subfield that is at least one of subfields with exception of the one subfield. A maintenance period of the predetermined positive voltage supplied in the setup period of the reset period of another subfield is shorter than a maintenance period of the predetermined positive voltage supplied in the setup period of the reset period of the one subfield. 
   The number of the reset drivers  1106 ,  1107 , and  1108  are shown only three. But, unlike this, it is possible to embody all cases with more than three drivers such as four, five, and six. 
     FIG. 9  illustrates a driving method for driving a plasma display panel in the plasma display apparatus of  FIG. 8  according to an exemplary embodiment of the present invention. 
   As shown in  FIG. 9 , in the driving method of the plasma display apparatus, the first reset pulse is supplied to the first scan electrode group in the setup period of the reset period of the one subfield, and the second reset pulse is supplied to the second scan electrode group in the setup period of the reset period of the one subfield, and the third reset pulse is supplied to the third scan electrode group in the setup period of the reset period of the one subfield. 
   As such, only in one subfield among the subfields of the frame, the reset pulse comprising the rising ramp is supplied in the setup period of the reset period to the selected scan electrode groups of a predetermined number and thus, a total of the number of the rising ramps within one frame is decreased, thereby improving a characteristic of contrast. 
   In a driving waveform of  FIG. 9 , the first reset pulse is supplied to the first scan electrode group (Ya). The third reset pulse is supplied to the third scan electrode group (Yc). A reset pulse of voltage that is lower than the rising ramp supplied the first scan electrode group (Ya) and is higher than the predetermined positive voltage supplied to the third scan electrode group (Yc) is supplied to the second scan electrode group (Yb) positioned between the first scan electrode group (Ya) and the third scan electrode group (Yc). Thus, a luminance difference between the first scan electrode group (Ya) and the second scan electrode group (Yb) is lower than a luminance difference between the first scan electrode group (Ya) and the second scan electrode group (Yb) shown in the driving waveform of  FIG. 6 , thereby more improving a picture quality. 
   It is desirable that the first reset driver  1106 , the second reset driver  1107 , and the third reset driver  1108  supply the same reset pulse to all scan electrodes comprised in the same scan electrode group, in the reset period. 
     FIG. 10  illustrates a difference between the frames of the reset pulse supplied to one scan electrode group in the driving method of the plasma display apparatus according to an exemplary embodiment of the present invention. 
   As shown in  FIG. 10 , after the first reset pulse is supplied to the first scan electrode group in the setup period of the reset period of the one subfield, the second reset pulse is supplied in a setup period of a reset period of a subfield corresponding to the one subfield among subfields of a next frame. After the supplying of the second reset pulse, the third reset pulse is supplied in a setup period of a reset period of a subfield corresponding to the one subfield among subfields of a frame after the next frame. 
   After the second reset pulse is supplied to the second scan electrode group in the setup period of the reset period of the one subfield, the third reset pulse is supplied in a setup period of a reset period of a subfield corresponding to the one subfield among subfields of a next frame. After the supplying of the third reset pulse, the first reset pulse is supplied in a setup period of a reset period of a subfield corresponding to the one subfield among subfields of a frame after the next frame. 
   After the third reset pulse is supplied to the third scan electrode group in the setup period of the reset period of the one subfield, the first reset pulse is supplied in a setup period of a reset period of a subfield corresponding to the one subfield among subfields of a next frame. After the supplying of the first reset pulse, the second reset pulse is supplied in a setup period of a reset period of a subfield corresponding to the one subfield among subfields of a frame after the next frame. 
   As shown in  FIG. 10 , the reset pulse supplied to the first scan electrode group (Ya) in the setup period of the reset period, the reset pulse supplied to the second scan electrode group (Yb) in the setup period of the reset period, and the reset pulse supplied to the third scan electrode group (Yc) in the setup period of the reset period are alternately supplied to the first scan electrode group (Ya), the second scan electrode group (Yb), and the third scan electrode group (Yc) every frame. 
   The present invention has an effect of distinguishing the level of the voltage of the reset pulse supplied to the scan electrode group comprising one or more scan electrodes in the setup period of the reset period of one or more subfields of one frame, from those of the others of the scan electrode groups, thereby improving the contrast characteristic. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.