EP0793212B1 - Control method for a display screen with gradation display and display device for carrying out the same - Google Patents

Description

The present invention relates to a control method image viewing screens displaying halftones. It applies to screens whose elementary image points are cells operating with two stable states and exhibiting a memory effect, and more particularly in the case where the halftones of the image are obtained by modulating the duration of light emission. The invention also relates to an image display device implementing the process.

By memory effect is meant the effect which allows cells to keep one or the other of two stable states, when a signal having ordered this state disappeared.

Such display screens are constituted for example by plasma panels (abbreviated "PAP"), of the current type continuous with memory, or of the alternative type, or even for example by screens whose elementary cells use a phenomenon of "effect of tip "to each produce an electron beam.

Taking for example the case of plasma panels (or abbreviated "PAP"), these are flat screen display devices, which operate on the principle of an electric discharge in gases. The PAP generally have two insulating tiles defining a space filled with gas. The tiles support two or more arrays of electrodes cross.

Each intersection of electrodes defines a cell at which corresponds to a small gas space. In each gas space, electric shocks can be caused in each of these spaces, applying appropriate voltages to both electrodes corresponding crossovers.

In continuous PAPs, the discharge current takes place always in the same direction, unlike alternative type PAPs whose operation is based on an excitation in alternating mode of electrodes.

In the case of PAP of the alternative type, the electrodes are covered with a dielectric material, so that, not being in contact with gas, electric charges accumulate on the dielectric at each gas discharge.

These charges remain after the end of the discharge and their presence, at the level of a cell, then makes it possible to cause a discharge into this cell by applying a lower voltage than that which would be necessary in the absence of these charges, which constitutes the memory effect "already mentioned. Cells which have such charges are said to be in the "on" state. The other cells, which require more tension high to produce a discharge, are said to be "off".

This memory effect is used with alternating signals called maintenance signals, applied to all cells to activate those which are in the "lit" state, that is to say cause in these cells so-called maintenance discharges which produce light, without changing their state "on" or modify the state of those which are in the "off" state.

All so-called "alternative" PAPs benefit from the above memory effect described.

Some alternative PAPs use only two electrodes to define and order a cell, as described for example in the French patent published with the number 2 417 848. In this case, the two crossed electrodes are used for both addressing (i.e. "on" or "off" state of the cell), and the maintenance discharges.

We can also cite alternative PAPs of the "maintenance" type coplanar ", as known in particular from the European patent document EP-A-0 135 382. In these PAP each cell is defined at the crossing between a so-called "addressing" electrode and a pair of parallel electrodes. Maintenance discharges are carried out using the two electrodes parallel, and addressing is done using one of these two electrodes and the addressing electrode.

In the different types of PAP which have a memory, all cells are powered in parallel. The great number possible cells can therefore lead to large currents, and the supply of the cells can then present defects which generate image defects.

The authors of the invention believed that the defects in cell feeding, due in particular to operating amplifiers at the limits of their characteristics, were aggravated by the principle of controls the halftone of the image.

Indeed, the elementary cell of a PAP only knows two states: the "on" state or the "off" state. Analog modulation of the quantity of light emitted by a pixel, i.e. by a cell not being not possible, the production of halftones is accomplished by modulation of the duration of light emission of the pixel in an image period, or in other words by modulating the time during which the cell is put in the "on" state within the image period.

In document EP-A-0 0306 011 is explained, in particular for a liquid crystal display, a method of obtaining halftones.

The control and the supply of the cells of a PAP are explained below.

Figure 1 shows schematically an alternative PAP. For simplify the description, the latter is of the type with two crossed electrodes to define a cell, as described in the French patent 2 417 848 cited upper.

The PAP includes a network of electrodes Y1 to Y4 called "line electrodes", crossed with a second network of electrodes called "column electrodes" X1 to X4. At each intersection of line electrodes and column corresponds to a cell C1 to C16. These cells are thus arranged in line L1 to L4 and in columns CL1 to CL4.

Each line electrode Y1 to Y4 is connected to an output circuit SY1 to SY4 of a line 1 control device, and each electrode column C1 to C4 is connected to an output circuit SX1 to SX4 of a column 2 command.

The operations of these two control devices 1, 2 are managed by an image management device 3.

Each output SY1 to SY4 of the line 1 control device delivers slots of tension which form the maintenance signals previously cited. These maintenance signals are thus applied simultaneously to all the line electrodes Y1 to Y4.

Figures 2a to 2d show maintenance signals applied respectively to the line electrodes Y1 to Y4. Figure 2a shows in particular that the maintenance signals are formed by a succession of niches of tension, established on either side of a potential of reference Vo which is often the potential of the mass. These slots vary between a negative potential V1 where they have a level, and a positive potential V2 where they present another step. The potential of reference Vo is applied to the column electrodes X1 to X4 in such a way that the application of maintenance signals develops across the cells C1 to C16 alternately positive and negative voltages, 150 V per example, which generate discharges in all PAP cells which are in the "on" state.

These discharges occur each time the polarity of the maintenance slots, i.e. at each positive Tp and negative transition Tn of these signals.

The cells are put into the "on" or "off" state by addressing operations which are managed by the management device image 3. They can consist, for example, of superimposing signals specific addressing on niche maintenance signals. In this Indeed, the line electrodes Y1 to Y4 are individualized, that is to say connected to an output circuit SY1 to SY4 specific to each of them, and each circuit output comprises for example a mixing circuit (not shown) by through which it receives maintenance signals and signals that come from different paths.

The maintenance signals have a period p which can be for example of 10 microseconds, during which the addressing of all cells belonging to a selected line L1 to L4, i.e. of all cells defined using a line electrode Y1 to Y4 selected.

Assuming that at an instant to begins addressing of the first line L1 corresponding to the first line electrode Y1, the addressing can for example be of a type such that, at this instant to, the signal applied to this electrode Y1 (and only to this one) is a transition negative of Tne erasure, of duration (shown in broken lines) more greater than the other transitions, and which causes the "off" state of all the cells connected to this line electrode Y1. Then at time t1 where the signal presents its positive plateau, a niche called Cl inscription (shown in broken lines) is superimposed (in positive) on this level. This registration window has the effect of putting all the cells connected to this line electrode, except those whose electrodes columns X1 to X4 deliver a so-called "masking" signal (not shown) which has the effect of inhibiting the effects of the registration slot Cl.

This operation can be repeated at each of the periods following maintenance signals at times t2 and t3, t4 and t5, t6 and t7 where the corresponding L2, L3, L4 lines are thus addressed respectively to the line electrodes Y2, Y3, Y4. At time t8, a new addressing of the first line L1.

These addresses carried out successively for each L1 line to L4 of the screen constitute a sub-scan, and several sub-scans are performed during an image cycle time or image period, in order to achieve the halftone of the image, by setting it to the "on" state or to the state "off" of cells C1 to C16 of each line L1 to L4 at each subscanning.

To this end, the image period PI is divided into n sub-periods S1, S2, ..., Sn of different durations, respectively equal to To, 2 To, ..., 2 ⁿ -1 To, with To = PI / 2 ⁿ -1.

FIG. 3 illustrates the division of the image period PI into n sub-periods S1, S2, ..., Sn with n equal to 4 in the example. The PI image period begins at time to with a first sub-period S1 which lasts a time To, and Ta ends at an instant. A second sub-period S2 begins at the instant ta and lasts a time equal to 2 TB to end at an instant tb, where begins a third sub-period S3. The third S3 subperiod lasts a time equal to 4 TB and ends at an instant tc. A fourth sub-period S4 starts at time tc and lasts 8 TB, until the end of the period PI which marks the instant to 'of a following image period.

With a PI image period of 20 ms for example, the sub-periods S1, S2, S3, S4 respectively have a duration of the order of 1.33 ms; 2.66 ms; 5.33 ms and 10.66 ms.

So in the example of figure 3, we can address each line L1 to L4 four times during the image period of this line, at instants to, ta, tb and tc. We can therefore for each line L1 to L4 put each cell C1 to C16, in the "off" state or in the "on" state at each of these instants, that is to say at the start of each sub-period S1 to Sn, and each cell keeps this state until the start of the next subperiod where it is again put in one or other of the two states "off", "on".

For the cells which are put in the "lit" state by the beginning of one or more sub-periods S1 to Sn, they are activated by the maintenance signals and they produce light during the duration of this or these subperiods. It is therefore possible by combination of the n sub-periods S1 to Sn to obtain 2 ⁿ -1 different durations of light emission by each cell, durations which each correspond to a desired luminance level for this cell during the image period PI, plus the zero luminance level which corresponds to the case of a cell which is put in the "off" state for all the sub-periods S1 to Sn of this image period.

Thus in the example of FIG. 3, for a cell brought to the state "on", ie activated during only the first sub-period S1, its luminance level is 1/5 of the level of the activated one during the first and third sub-periods S1, S3 and 1/15 of the level of the one activated during the entire PI image period.

This principle of controlling the luminance levels of the cells from a line L1 to L4, applies to all lines, of course with a time shift from one line to another; for example from an L1 line to the next line L2 with an offset which corresponds to a period p of maintenance signal as shown in figure 2, which can be for example on the order of 10 microseconds. In fact, the PI image period has the same duration for all lines L1 to L4, regardless of the number N of these lines, with a time offset for example of a period between two consecutive lines, offset which is found in the distribution of the sub-periods S1 to Sn.

It should be noted that the desired luminance levels for the different cells of each row L1 to L4, correspond to video input luminances which are coded and stored in the device image management 3, generally using n bits of different weights each corresponding to one of the sub-periods S1 to Sn.

The cells C1 to C16 in the "on" state being activated by the maintenance signals issued by the line 1 control device, they constitute a load applied to the latter.

Service signals can be produced in different ways ways, in themselves known. In all cases, the line command for this purpose comprises at least one amplifier A which delivers maintenance signals to the output circuits SY1 to SY4, either directly as shown in Figure 1, either through several stages of output (not shown) each assigned to supply several circuits of output, i.e. several line electrodes Y1 to Y4.

In the example of figure 1 only four line electrodes and four column electrodes are shown, but a PAP can have made over a thousand electrodes of each of these types, which define more of a million cells.

Consequently, the maintenance signals issued by amplifier A must be supplied by amplifier A under a current which can vary considerably depending on the content of the image, that is to say as a function of the number of cells which are in the "lit" state. Given the non-zero source impedances of amplifier A, so that the cell access impedances (linked in particular to inductances and resistances of the printed circuit tracks and connections, etc.) quantity of charges actually applied to a given cell C1 to C16 depends on the overall content of the image. In other words, the higher the charge applied to amplifier A is strong, the more the luminance of the cells in the "on" state which constitute this charge.

This variation in luminance as a function of the content of the image, is observed in particular in the case illustrated in FIG. 4 which represents a image formed mainly of a peripheral zone Z1 with low luminance, and a second zone Z2 with high luminance and having a coding constant video. There is a significant variation in luminance displayed as a function of the surface variation of the second zone Z2.

To this defect in the image, there is added another called defect of highlight, which consists in particular of an exaggeration even a inversion of luminance differences between zones. Referring again to the Figure 4, assuming that the second zone Z2 is made of two surfaces contiguous R1, R2 whose second R2 is located in the center of the first R1, and that we want to display luminances on these two surfaces different but similar: for example a luminance I2 corresponding to a video coding equal to 128 (in the case of 8-bit video coding, i.e. with 8 sub-periods as previously explained) for the second surface R2, and a luminance I1 coded 127 for the first surface R1.

It can be seen in the image exaggeration and a reversal of the difference in luminance shown by the two surfaces R1, R2: instead of a theoretical ratio I2 / I1 of 1.008 ^(128/127 = 1.008), obtained by makes an actual ratio which can be 0.54.

Furthermore, if we vary the first surface R1, all things also equal, there is a variation in the luminances I1 and I2 which shows that the luminance I2 of the second surface R2, is dependent on the content of the rest of the image outside this surface R2.

In order to remedy these defects, a known solution consists in decrease source impedances and connection impedances, and the impedances presented by the electrodes themselves. This is obtained by a choice and a selection of the components, by a drawing and a particularly careful realization of discharge current paths as well as by multiplying the paths offered to discharge currents (in particular by putting several power transistors in parallel, at the level of the signal amplifier (s) (such as amplifier A) as well as in the output circuits (such as the circuits SY1 to SY4).

But the resulting improvements are only partial, everything being however very expensive, because we multiply the number of components and / or their individual cost, and the size thus increases that the manufacturing complexity and therefore its cost.

The object of the present invention is to limit variations in the luminance according to the content of the image, and reduce or even remove highlighting faults.

To this end, the invention proposes to act on the load in a way relatively simple and inexpensive.

The invention relates to a method for controlling a screen of visualization whose elementary image points are cells arranged in rows and columns, the cells being placed, either in a state called "off", or in a state called "on" in which they are activated by signals delivered by a control device, the process consisting for each line, by function of a desired luminance level for each cell, to be set each cell in the "off" state or in the "on" state for n sub-periods successive of different durations distributed during a time of given cycle, the process being characterized in that it consists to distribute the n sub-periods in a different order at least two rows of cells close or consecutive so as to reduce the amplitude variations in an applied load in control device, this load being constituted by cells in the "lit" state of the two lines that produce light.

This allows on the one hand, to limit the load constituted by the activated cells, and on the other hand to limit the variations in time of this burden, and therefore to act particularly effectively on highlighting defects of which the authors of the present invention believe that they are due to a combination of the fall in luminance as a function of the load, and the variation over time of this load.

The invention also relates to an image display device. comprising cells arranged in rows and columns, the cells being put either in a state known as "extinct", or in a state known as "lit" in which they are activated by signals delivered by a command, the cells in the "on" state constituting a load applied to the control device, the cells being put in the "on" state or in the state "off" during sub-periods of different durations applied to each line, the sub-periods being distributed successively under the control of an image management device, characterized in that the image management device comprises means for distributing the sub-periods at least two consecutive lines or close to cells with orders different distribution for each line so as to reduce the amplitude of the variations a load applied to the control device, this charge being constituted by the cells to the "on" state of the two lines that produce light.

• la figure 1 déjà décrite représente un panneau à plasma ;
• les figures 2a à 2d déjà décrites illustrent le fonctionnement de cellules d'un panneau à plasma ;
• la figure 3 déjà décrite représente une division d'une période image en n sous-périodes ;
• la figure 4 déjà décrite représente une configuration d'image révélant des défauts que l'invention peut réduire ;
• les figures 5a, 5b représentent chacune dans le temps des variations de la charge constituée par une ligne de cellules ;
• la figure 5c représente dans le temps des variations de la charge constituée par une ligne de cellules activées suivant le procédé de l'invention ;
• la figure 6 montre de manière schématique un exemple de réalisation d'un dispositif de gestion d'image permettant la mise en oeuvre du procédé de l'invention.

The invention will be better understood on reading the description which follows, given by way of nonlimiting example with reference to the appended drawings, among which:

• Figure 1 already described shows a plasma panel;
• Figures 2a to 2d already described illustrate the operation of cells of a plasma panel;
• FIG. 3 already described represents a division of an image period into n sub-periods;
• FIG. 4, already described, represents an image configuration revealing defects that the invention can reduce;
• FIGS. 5a, 5b each represent over time variations in the charge constituted by a row of cells;
• FIG. 5c represents over time variations in the charge constituted by a line of cells activated according to the method of the invention;
• FIG. 6 schematically shows an exemplary embodiment of an image management device allowing the implementation of the method of the invention.

a) la zone périphérique Z1 a une luminance égale à 0 ;

b) la première surface R1 a une luminance de niveau 127 ;

c) la seconde surface R2 a une luminance de niveau 128.

Taking for example the case of an image configuration such as that of FIG. 4, and supposing that:

a) the peripheral zone Z1 has a luminance equal to 0;

b) the first surface R1 has a level luminance 127;

c) the second surface R2 has a luminance of level 128.

For two rows of close or consecutive cells as per example the second and the third line L2, L3 (already shown in FIG. 1), both of which would pass into the peripheral zone Z1 and into the surfaces R1 and R2 (fig. 4), a charge Q constituted by the cells in the state "lit", for each of these two lines L2, L3 is shown in Figures 5a, 5b.

Figure 5a illustrates the variations in charge Q for the second line L2 over time and for the image configuration of FIG. 4, and FIG. 5b illustrates these variations for the third line L3. Note that on these two lines L2, L3 the charge Q and its variations are the same, that the description given below therefore applies to both lines, and that the time marks are valid for these two lines. he however, there is a time difference between these two lines, which corresponds to the time interval required to address two lines consecutive, as already explained with reference to FIG. 2. This interval of time which is for example of the order of 10 microseconds is therefore very weak and insignificant compared to that of a PI image period of 20 ms which is used to show the evolution of the charge Q of the lines L2, L3.

At the instant to start the PI image period and with it the distribution from n sub-periods S to Sn, with n = 8 in the example, to achieve 256 luminance levels. The number of cells in the "lit" state is constant for the seven sub-periods S1 to S7 which follow one another at times t1, t2, t3, t4, t5, t6, and the charge Q therefore has a constant Q1 value until the end of the seventh sub-period S7 at an instant t7. The sum (in duration) of subperiods S1 to S7 define the luminance level 127.

For cells to display equal luminance levels or greater than 127, they must be set to "on" during the eighth sub-period S8, while the one having "off" must have luminance levels below 128. As a result and account given the example in which the number of cells coded 128 and much larger than that of cells coded 127, the charge Q shows a large increase from the first value Q1 to a second value Q2, at time t7 when the eighth sub-period S8 begins. This last having a duration equal to 1/2 image period, it ends at a instant tFP which marks the end of the image period Pl.

The increase in load Q of the lines L2, L3 results well heard as an increase in the load applied to the command line 1, and such an increase or variation of the load exists for all cell lines that define such images, and these variations are added in such a way that the overall variation of the load can be considerable and cause the various image defects already mentioned.

On the contrary, with the process of the invention, there is a tendency to oppose the load variations of two close or consecutive cell lines like lines L2, L3 and which therefore are interested in the same image configuration.

To this end, the method of the invention consists in distributing the n sub-periods S1 to Sn in a different order for the two lines of cells considered, and therefore to be distributed differently for these two lines, cell activation times within the image period Pl.

• pour la seconde ligne L2, conserver le séquencement des sous-périodes S1 à S8, c'est-à-dire l'ordre de leur distribution, déjà décrit en référence à la figure 5a qui montre que des sous-périodes sont distribuées dans l'ordre de leur accroissement de durée,
• et pour la troisième ligne L3 adopter un séquencement différent, tel que constituant par exemple par rapport à la seconde ligne L2, une inversion partielle du séquencement organisée autour d'un niveau de luminance donné. Par exemple un niveau correspondant à l'un des n-1 cas où le niveau de luminance correspond à l'activation par une unique sous-période.

One can for example in the case of the two consecutive lines L2, L3:

• for the second line L2, keep the sequencing of the sub-periods S1 to S8, that is to say the order of their distribution, already described with reference to FIG. 5a which shows that sub-periods are distributed in l 'order of their duration increase,
• and for the third line L3 adopt a different sequencing, such as constituting for example with respect to the second line L2, a partial inversion of the sequencing organized around a given luminance level. For example, a level corresponding to one of the n-1 cases where the luminance level corresponds to activation by a single subperiod.

In the example given, where the significant variation in charge Q is produced for luminance level 128, the sequencing of the sub-periods S1 to S8 can be such that for the third line L3 it begins with the eighth sub-period S8, as shown in Figure 5c.

FIG. 5c represents the variations of the charge Q of the third line L3, during an image period Pl.

At time to, the eighth S8 subperiod begins, which ends substantially at time t7. Between to and t7, the load has the second Q2 value. (We can note that in the same time interval between to and t7, the charge formed by the second line L2 has the first value Q1 weaker, caused by activation of cells by the seven subperiods S1 to S7).

At time t7 (in FIG. 5c): the eighth sub-period S8 takes end, and begins the succession of the seven sub-periods S1 to S7. At time t7 we also observe that the charge constituted by the third line L3, passes from the high value Q2 to the first value Q1 which is lower (at contrary to what is happening at this moment for the second line L2, whose load then passes from the first value Q1 to the second value Q2 which is higher).

The seventh S7 sub-period ends (for the third line L3) at the end of the image period PI, at the instant tFP where a new image period.

We see that for the example given, the difference in sequencing n sub-periods S1 to SN between two consecutive lines L2, L3 allows to perfectly compensate between two lines for variations in load corresponding to luminance levels 127 and 128, because the sum of charges formed by these two lines keeps a constant value throughout the duration of a PI image period. Such a difference from sequencing of the sub-periods S1 to SN between two lines L2, L3 therefore makes it possible to perfectly average the restitution of loads corresponding to levels 127 and 128, and it tends to standardize the charge for luminance levels greater than 128.

We see that by extending such a solution to all the lines of a display screen we can considerably reduce the overall variations in charge and therefore in luminance as a function of this last.

• on peut pour l'une des lignes L2, L3 de cellules conserver le séquencement traditionnel des sous-périodes S1 à S4 (illustré par exemple à la figure 5a),
• et pour l'autre de ces deux lignes on fait débuter la séquence de distribution des sous-périodes S1 à S8 par la sous-période dont la durée (à elle seule) représente le niveau de luminance autour duquel on désire effectuer les compensations de charge.

If we wish to standardize or reduce the overall charge presented by two consecutive lines such as L2, L3 for luminance levels other than above, corresponding for example to levels 63 and 64, the modification of the sequencing is the same type:

• it is possible for one of the lines L2, L3 of cells to keep the traditional sequencing of the sub-periods S1 to S4 (illustrated for example in FIG. 5a),
• and for the other of these two lines, the sequence of distribution of the sub-periods S1 to S8 is started with the sub-period whose duration (alone) represents the level of luminance around which it is desired to carry out the load compensations .

In the case of luminance level 64, this sequence of distribution according to the invention must begin with the seventh S7 subperiod, it can be followed by the eighth sub-period S8 then by the sub-periods S1 to S6.

The invention can also be applied to treat in the above manner described two consecutive lines (for example lines L1 and L2) to average the overall load compared to a first level luminance, for example 128, and operate on two consecutive lines following (for example lines L3 to L4) in order to act around a level of different luminance, for example 64.

It is possible to extend this principle of load compensation from several cell lines to the n-1 critical cases present in a system designed to process a video coded on n bits.

The advantageous compensation effect of load variations of two consecutive lines, can also be obtained with sequencing of the partially inverted type for the two consecutive lines, therefore that it is different, between these two lines, for example organized by a inversion from 64 (S7) for one and from 128 (S8) for the other.

The means useful for implementing the method of the invention for the most part already exist in most management systems picture 3 like the one shown in figure 1, which are used to control a display screen.

Figure 6 shows schematically by blocks functional, some of the functions in themselves known which are provided by an image management device 3 of the plasma panel.

It includes for example a video input circuit 10 which realizes an adaptation of the video signals, and classifies them for example for each row of cells, depending on the luminance to be displayed by each cell.

The video input circuit 10 delivers video data which is applied to a video coding circuit 11. The latter may include example a TC coding table, using which it defines the different sub-periods S1 to Sn during which each cell of a given row must be activated during a PI image period, to restore the level of desired luminance.

The video coding circuit 11 delivers coded data to a memory circuit 12, which can for example include as many planes memory PM1, PM2, ... PMn that there are sub-periods S1 to Sn. Every sub-period S1 to Sn can thus correspond to a memory plane in which, for each line L1, L4, the addresses of cells C1 to 16 which must be set to the "on" state.

Memory circuit 12, determined by exchange of information with row and column control devices 1, 2 (shown in the Figure 1), the execution of the "on" or "off" state settings for the different cells of each line, at the start of each of the sub-periods S1 to Sn which are distributed for each line within a PI image period.

The memory circuit 12 can for this purpose comprise a circuit sequencer 13, connected to each of the memory planes PM1 to PMn, and which for each line can then determine the order of distribution of the sub-periods S1 to Sn, so that for a given line, this distribution takes place following for example a simple sequencing such as the classic mode shown in figure 5a, and that for the next line it operates according to the mode called reverse sequencing, shown in Figure 5c. This can be obtained from simple way, for example by programming the sequencer 13, to so as to alternate a simple sequencing and a reverse sequencing.

But it is also possible to order the distribution order sub-periods S1 to Sn by detecting for at least one line one or several strong imbalances in the evolution of the number of cells in front be set to the "on" state during a PI image period. This can be detected for example by the video coding circuit 11 using a test 14, using the data present in the TC coding table, to forward a decision to the sequencer 13.

The above explanations relating to the operation of a image management device, are given by way of nonlimiting example, different other ways all within the reach of a specialist, allow to modify from one line to another, the sequencing of the n sub-periods S1 to Sn, systematically or according to tests.

Claims (12)

1. Method of controlling an image display screen displaying half-tones having cells (C1 to C16) arranged in rows (L1 to L4) and in columns (C1 to C4), the cells being placed either in a so-called "off" state or in a so-called "on" state in which they are activated by signals delivered by a control device 1, the method consisting, for each row (L1 to L4) as a function of a desired luminance level for each cell (C1 to C16), in placing each cell in the "off" state or in the "on" state for n subperiods (S1 to Sn) of different durations distributed during a given cycle time (P1), characterized in that it consists in distributing the n subperiods in a different order to at least two rows (L2, L3) of closely spaced or consecutive cells in such a way as to reduce the amplitude of the variations of a load applied to the control device, this load consisting of the cells in the "on" state of the two rows which produce light.
2. Control method according to Claim 1, characterized in that the n subperiods (S1 to S8) have different durations, increasing from one to another in a ratio of two, so as to allow by combination 2ⁿ-1 different durations each corresponding to a possible luminance level of the cells (C1 to C16).
3. Control method according to one of Claims 1 or 2, characterized in that for one of the two rows (L1, L2), it consists in beginning the distributing of the subperiods (S1 to Sn) with the subperiod whose duration corresponds to a luminance level for which a variation in load (Q1, Q2) to be compensated occurs.
4. Control method according to one of Claims 1 to 3, characterized in that it consists, for one of the two rows (L1, L2) in distributing the subperiods (S1 to Sn) in ascending order of their duration.
5. Control method according to one of Claims 1 to 3, characterized in that it consists in distributing the subperiods (S1 to Sn) to the two rows (L1, L2) in different orders from that of the increase of their duration.
6. Control method according to Claim 5, characterized in that it consists, for a set of rows (L1 to L4) comprising at least two groups (L1, L2 - L3, L4) of two consecutive rows, in distributing the subperiods (S1 to Sn) in a different order for the rows of one and the same group, and in such a way that each group effects a compensation of load centred on a different luminance level.
7. Control method according to one of the preceding claims, characterized in that the display screen is an AC plasma panel.
8. Image display device to which the method according to one of Claims 1 to 7 is applied, comprising cells (C1 to C16) arranged in rows (L1 to L4) and in columns (CL1 to CL4), the cells being placed either in a so-called "off" state or in a so-called "on" state in which they are activated by signals delivered by a control device (1), the cells (C1 to C16) being placed in the "on" state or in the "off" state during n subperiods (S1 to Sn) of different durations applied to each row (L1 to L4), the subperiods (S1 to Sn) being distributed successively under the control of an image management device (3), characterized in that the image management device (3) comprises means (13, 14, Tc) for distributing the subperiods (S1 to Sn) to at least two consecutive or closely spaced rows (L2, L3) of cells with different orders of distribution for each row in such a way as to reduce the amplitude of the variations of a load applied to the control device, this load consisting of the cells in the "on" state of the two rows which produce light.
9. Display device according to Claim 8, characterized in that the image management device (3) comprises a sequencer circuit (13) making it possible to control the distributing of the subperiods (S1 to Sn) according to at least two different orders of distribution.
10. Display device according to Claim 9, characterized in that the image management device (3) furthermore comprises a test circuit (14) cooperating with the sequencer circuit (13) so as to define the order of distribution of the subperiods (S1 to Sn).
11. Display device according to one of Claims 9 to 10, characterized in that it consists of a plasma panel.
12. Display device according to Claim 11, characterized in that the plasma panel is of the AC type.

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