Method for driving plasma display panel

Focusing attention on a new fact that a difference in the operating margin between sub-fields becomes remarkable when preliminary discharge is not provided for all sub-fields, but is thinned out, the operating margin for a plasma display panel will be improved by restricting the difference in the operating margin. In the case of the thinned preliminary discharge system, particularly the dependence of maintenance blanking characteristics on the maintenance pulse number becomes remarkable and as a result, the operating margin difference among the sub-fields becomes remarkable, and therefore, parameters for blanking pulse of the sub-field during the maintenance blanking period are set in conformity with the maintenance pulse number (number of times of emission) for each sub-field in order to restrain this operating margin difference.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a method for driving a plasma display
 panel, and more particularly, to a method for driving a color plasma
 display panel capable of tonal display by dividing one field into a
 plurality of sub-fields to set the number of times of emission for each
 sub-field to different values.
 2. Description of the Related Art
 Conventionally, the tonal display on a plasma display panel has been
 implemented by controlling a number of times of discharge (emission
 luminance) during a maintenance period as shown in FIG. 11. More
 specifically, one field (F), which displays one screen, is repeated 50 to
 about 70 times a second, whereby screens of the respective fields are
 stacked by means of afterimages of a human eye and a flicker-free natural
 image can be obtained. This one-field period is divided into a plurality
 of sub-fields (SF), and these sub-fields are combined by varying a
 maintenance pulse number (a number of times of discharge) during the
 maintenance period of each sub-field to thereby implement tonal display.
 In, for example, display of 64 shades of gray, as shown in FIG. 11, one
 field is constituted by six sub-fields: SF1 to SF6, and a preliminary
 discharge period (preliminary lighting period+blanking period) is provided
 at the head of each sub-field, and subsequent to this period, there are
 provided a write period and a maintenance period respectively. The
 weighting is effected by reducing the number of times of discharge during
 these maintenance periods by about 1/2 for each successive sub-field, from
 the sub-field at the head (in SF1, the number of times of discharge is
 assumed to be 32n where n is a positive integer).
 When the foregoing sub-field is selected within one frame for maintenance
 discharge in accordance with this method, the emission luminance can be
 controlled by the number of times of maintenance discharge in the
 sub-field selected, and therefore, the display of 64 shades of gray can be
 implemented.
 In this respect, FIG. 12 is a sectional view showing a general plasma
 display panel. In FIG. 12, reference numeral 1 designates a front
 substrate; 2a, a scanning electrode; 2b, a maintenance electrode; 3, a bus
 electrode; 4, a dielectric layer; 5, a rear substrate; 6, a data
 electrode; 7, a white dielectric body; 8, fluorescent material; and 9, a
 discharge cell respectively.
 If the preliminary discharge periods are provided at the heads of all the
 sub-fields as described above, preliminary discharge occurs at least six
 times even in a non-display portion to cause light emission over the
 entire screen. This emission causes black float particularly in a dark
 place, thereby deteriorating the contrast. Also, if the sub-fields are
 arranged simply in decreasing order of the weighting of emission luminance
 (number of times of discharge) as shown in FIG. 11, a pseudo contour may
 appear on displaying a moving image.
 In order to suppress these defects, a driving sequence, as shown in FIG. 1,
 is used (this driving sequence diagram in FIG. 1 is the same as that for
 the present invention), in which this preliminary discharge is applied
 once per field, and the sub-fields are not arranged simply in decreasing
 order of the weighting of emission luminance (number of times of
 discharge), but their sequence has been determined by contriving. In such
 driving sequence, the preliminary discharge period is provided only for
 the sub-field SF6 at the head, and the sub-field SF6 is constituted by the
 preliminary discharge period, a write period, a maintenance period and a
 maintenance blanking period. Each of the sub-fields SF1 to 5 other than
 the sub-field SF6 is constituted by a write period, a maintenance period
 and a maintenance blanking period.
 In such driving sequence in which preliminary discharge is provided for all
 sub-fields as shown in FIG. 11, the sequence, in which light is certainly
 emitted over the entire screen at the beginning of each sub-field for
 blanking, is adopted, and therefore, the presence or absence of wall
 charge, which is caused by the presence or absence of maintenance
 discharge of the sub-field in question, is bound to be erased, and does
 not affect the next sub-field. In contrast, however, in such driving
 sequence, in which the preliminary discharge is thinned out, as shown in
 FIG. 1, the presence or absence of the maintenance discharge during a
 maintenance period of the sub-field in question remains as a difference in
 wall charge on the scanning electrode and maintenance electrode, and
 therefore, the blanking characteristics of a maintenance blanking period
 provided at the last of the sub-field becomes important as one of the
 elements for determining the operating margin.
 However, wall charge has conventionally been blanked by the use of
 microdischarge using wall charge during the maintenance blanking period,
 and therefore, the maintenance blanking period is susceptible to the
 amount of wall charge, and the blanking characteristics easily becomes
 unstable. Therefore, when it is adopted, such sub-field driving sequence
 as shown in FIG. 1 is defective, in that the operating margin is lowered
 and the yield is reduced as compared with the conventional method in which
 all sub-fields are provided with preliminary discharge.
 FIG. 13 shows the dependence of the operating margin in driving sequence in
 sub-fields of FIG. 1 on the sub-field. The "minimum operating voltage" in
 this figure is the minimum value of the drivable voltage, and the "maximum
 operating voltage" is the maximum value of the drivable voltage. This
 operable voltage range is the operating margin. When voltage exceeding
 this operating margin is applied, an erroneous display occurs, and when
 voltage below the operating margin is applied, a non-display portion
 occurs. From this figure, it can be seen that the operating margin of the
 sub-field next to a sub-field having low weighting of emission luminance
 is lowered.
 In other words, SF4, which is next to SF6 having the minimum emission
 luminance, has the highest minimum operating voltage, and the lowest
 maximum operating voltage. From this figure, therefore, it can also be
 seen that the operating margin for the entire plasma display panel is
 regulated by SF4 to be narrowed. The sub-field SF4, which is next to SF6
 having the minimum emission luminance, has the minimum operating margin.
 This is because the intensity of the maintenance discharge during a
 maintenance period prior to the maintenance blanking period is affected by
 the maintenance pulse number constituting the maintenance period.
 As shown in FIG. 14, the maintenance discharge during the maintenance
 period becomes stronger with the number of maintenance pulses PSUS to be
 applied, and will be saturated. Therefore, when the number of maintenance
 pulses is as small as 1 piece (case of n=1) like SF6, the maintenance
 discharge does not become strong during the maintenance period. On the
 other hand, at SF3, which follows SF1, the maintenance discharge becomes
 strong because the number of maintenance pulses at SF1 is as sufficiently
 great large as 32 pieces (case of n=1).
 Since the number of the maintenance pulses differs depending on the
 sub-field as described above, the intensity of the maintenance discharge
 differs, and the amounts of wall charge which are produced by the
 respective sub-fields during the maintenance period are different from one
 another. Since these different wall charge have been blanked (neutralized)
 during the maintenance blanking period having the same maintenance
 blanking pulse, the blanking (neutralization) of the wall charge becomes
 insufficient in a sub-field having a small number of maintenance pulses,
 leading to decrease in the foregoing operating margin.
 In this respect, a driving method in which the preliminary discharge is not
 provided for all the sub-fields, but the number of times of preliminary
 discharge per field is reduced in an attempt to enhance the display
 contrast, is discussed in Japanese Patent Application Laid-Open Nos.
 4-280289 and 7-49663. Also, a conventional example in which the waveform
 of the blanking pulse has been contrived in order to obtain sufficient
 blanking characteristics even if there are variations in the
 characteristics of the discharge cell, is discussed in Japanese Patent
 Application Laid-Open Nos. 8-30228 and 9-160522. They are aimed to
 eliminate variations in the blanking characteristics within one field and
 discharge cell.
 SUMMARY OF THE INVENTION
 The present invention is directed to on a new fact that in a case where the
 preliminary discharge is not provided for all the sub-fields (case of
 thinned preliminary discharge system in which preliminary discharge has
 been thinned out), particularly the dependence of the maintenance blanking
 characteristics on the maintenance pulse number becomes significant and as
 a result, an operating margin difference among the sub-fields becomes
 significant. One object of the present invention is to improve the
 operating margin of the plasma display panel by restraining this operating
 margin difference.
 According to the present invention, there is provided a method for driving
 a plasma display panel for dividing one field period displaying one screen
 of a plasma display panel into a plurality of sub-fields, and setting a
 number of times of light emission in each sub-field thus divided (setting
 a maintenance pulse member in each sub-field) to different values for
 tonal display, each of the foregoing sub-fields having at least a write
 period, a maintenance period and a maintenance blanking period, wherein
 parameters for blanking pulses (blanking parameters of blanking pulses)
 during the foregoing maintenance blanking period are set in conformity
 with the foregoing number of times of emission (maintenance pulse number)
 during the foregoing maintenance period.
 The foregoing maintenance blanking period is characterized in that a
 plurality of blanking parameters constituting the maintenance blanking
 period are at least one of the foregoing number of blanking pulses, crest
 value, pulse width and rise time, and that the preliminary discharge
 periods are thinned out and provided for a subset of sub-fields instead of
 being provided for all sub-fields.
 Further, the present invention is characterized in that the sequence of the
 foregoing sub-fields within one field is arranged so as to be different
 from the decreasing order of the number of times of emission, that the
 foregoing blanking pulse is a bipolar pulse having positive and negative
 polarities, and further that the foregoing blanking pulse is supplied to
 the scanning electrode and a common maintenance electrode.
 The operation of the present invention will be described. In the case of
 the so-called thinned preliminary discharge system, in which preliminary
 discharge is not provided for all sub-fields, particularly, the dependence
 of the maintenance blanking characteristics on the maintenance pulse
 number becomes significant and as a result, the operating margin
 difference among the sub-fields becomes significant. Therefore, the
 parameters for blanking pulses of the sub-fields during the maintenance
 blanking period are set in conformity with the maintenance pulse number
 (number of times of emission) for each sub-field in order to suppress the
 operating margin difference.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Hereinafter, embodiments of the present invention will be described in
 detail in conjunction with the drawings.
 FIG. 1 shows the sub-frame structure of one field according to an
 embodiment of the present invention and an example of driving sequence,
 upon first glance this driving sequence causes this example to appear to
 be the same as the foregoing conventional thinned preliminary discharge
 system. However, parameters (pulse number, crest value, pulse width, rise
 time, etc.) of blanking pulses during the maintenance blanking period in
 each sub-field have been set so as to be different respectively in
 response to the number of times of emission during the maintenance period
 of each sub-field.
 In order to perform tonal display, one field is structured so as to be
 divided into six sub-fields: SF1 to SF6, and with the aim of improving the
 contrast and preventing any pseudo contour from occurring during display
 of a moving image, the number of times of preliminary discharge is set to
 once a field, and the sequence of the sub-fields is changed from the
 simple order of the weighting of emission luminance during the maintenance
 period.
 More specifically, as shown in FIG. 1, the sequence is: SF6 (weighting of
 emission luminance: 1n), SF4 (weighting: 4n), SF2 (weighting 16n), SF1
 (weighting: 32n), SF3 (weighting: 8n) and SF5 (weighting: 2n). The "n" is
 a positive integer. However, this sequence is exemplary, and the driving
 sequence is not limited thereto. In this respect, the weighting of this
 emission luminance is effected on the basis of the number of maintenance
 pulses constituting the maintenance period in the same manner as before.
 The structure of each of these sub-fields will be described below. The
 preliminary discharge period is provided only for the sub-field SF6 at the
 head, and the sub-field SF6 comprises a write period, a maintenance period
 and a maintenance blanking period which follow the preliminary discharge
 period. Each of sub-fields SF1 to 5 other than the sub-field SF6 comprises
 a write period, a maintenance period and a maintenance blanking period. In
 this respect, the preliminary discharge period comprises a preliminary
 lighting period and a preliminary blanking period which display the entire
 display screen in the same manner as in the example of FIG. 11.
 FIG. 2 shows a part of driving waveforms for sub-fields SF6 and SF4. As the
 driving waveform, there are shown three types: a pulse train D which is
 applied to the data electrode; pulse trains S0 and Sm which, of a
 plurality of scanning electrodes, are applied to the 0-th one and m-th
 one; and a pulse train C which is applied to the common maintenance
 electrode.
 In the present invention, the blanking pulse trains during this maintenance
 blanking period are structured as below. FIG. 3 is a partially enlarged
 view showing these blanking pulse trains. In FIG. 3, a first blanking
 pulse P EC1 is applied to the maintenance electrode, subsequently a second
 blanking pulse P ES2 is applied to the scanning electrode, a third
 blanking pulse P EC3 and a fourth blanking pulse P ES4 are likewise
 applied to each electrode respectively. Of these blanking pulses, the
 first to third blanking pulses are called fine-width blanking pulses and
 the fourth blanking pulse is called a thick-width blanking pulse.
 The crest value, pulse width and rise time (negative polarity, negative
 rise), which are parameters by which these blanking pulses are
 characterized, are indicated by V1 to V4, .tau.1 to .tau.4 and t in FIG. 3
 respectively. Since the optimum values for these values vary as a function
 of sub-field, as shown in FIGS. 4 to 7, the parameters for blanking pulses
 for each sub-field SF are determined in accordance with the tendency.
 Next, the operation of the tonal display will be described with reference
 to FIG. 9.
 (1) The entire screen is caused to discharge and emit light once through a
 preliminary lighting pulse PP during the preliminary discharge period,
 positive charge, electrons, excitation atoms or molecules are generated
 within discharge cells to activate the discharge cells, and wall charges
 on the data electrode, scanning electrode and maintenance electrode are
 neutralized (blanked) through preliminary blanking pulses P E1, P E2 and P
 E3 to make preparations for causing the next write discharge with
 stability (S1).
 (2) Scanning pulses Pw are successively applied to a plurality of scanning
 electrodes during the write period, and in synchronization therewith, a
 data pulse PD corresponding to the displayed data is applied to generate
 write discharge, for writing displayed data (S2).
 (3) During the maintenance period, maintenance discharge is caused to occur
 through a maintenance pulse P SUS in accordance with the data written for
 displaying (S3).
 (4) The maintenance discharge is stopped through P EC1, P ES2, P EC3 and P
 ES4 during the maintenance blanking period, and wall charges on the data
 electrode, scanning electrode and maintenance electrode are neutralized
 (blanked) through maintenance blanking discharge to make preparations for
 stabilizing write discharge for the next sub-field (S4).
 (5) If this operation is not terminated (case of NO in S5), the procedure
 will return to S2 again to repeat the steps to S5, and if the operation is
 terminated (case of YES in S5), the process is terminated.
 By the foregoing process, the tonal display can be performed by causing any
 sub-field to emit light.
 Next, the operation of maintenance blanking discharge (S4) will be
 described with reference to FIG. 10.
 For the neutralization (blanking) of wall charges during the foregoing
 maintenance blanking period, optimum values as shown in FIGS. 4 to 7 for
 the foregoing parameters differ for the respective sub-fields because the
 sub-fields have different maintenance pulse numbers. Since parameters
 (shown in FIG. 3) for blanking pulses constituting the maintenance
 blanking period for each sub-field have been set to the optimum values
 shown in FIG. 4, the wall charges could be neutralized (blanked) under the
 optimum conditions in all the sub-fields (S11, S12). As a result, the
 write characteristics of all the sub-fields were stabilized.
 In this respect, as regards the maintenance blanking pulse number, it is
 qualitatively known that when the pulse number is increased, the
 maintenance blanking ability is improved. Since, however, the sub-fields
 exhibit complicated behavior depending on their order of selection and
 combination, the optimum pulse number was selected by cut-and-try methods.
 The general view is that the blanking pulse number in the maintenance
 blanking becomes large when the number of times of emission during the
 maintenance period is small, and conversely that the blanking pulse number
 becomes small when the number of times of emission is large.
 FIG. 8 shows another embodiment according to the present invention, and in
 this embodiment, the blanking pulse in the previous embodiment is
 allocated to pulses of positive polarity and negative polarity and applied
 to the scanning electrode and the maintenance electrode. Since the
 amplitude of the blanking pulse can be reduced according to this driving
 method, it becomes possible to lower the dielectric strength of the
 driving circuit, and to reduce the circuit cost. According to this
 embodiment, the blanking pulse is applied with plural and different crest
 values, and therefore, the circuit becomes complicated. Therefore, this is
 an important technique to provide low-priced products.
 As described above, according to the present invention, when a blanking
 period comprising a plurality of blanking pulses of the optimized
 parameters is applied for each sub-field, the dependence of the operating
 margin on the sub-field is eliminated, and the operating margin expands
 even if the operation is caused to be performed only by one preliminary
 discharge in one field.
 Therefore, it is possible to manufacture a plasma display panel with
 high-level display contrast in an excellent yield, and to reduce the cost.
 Also, since the operating margin is large, it is possible to extend the
 service life, and therefore, it is also possible to provide the products
 with high reliability at low cost.