EP0349415A1 - Method and device for controlling a matrix screen displaying gray levels - Google Patents

Abstract

According to the invention, activation signals are sent to the columns of the screen, in the course of the line time T, for a time which depends on the grey level i of the image point under consideration and which equals (T/N). Nil with 0 </= i </= m </= N, the Nil forming a strictly ascending sequence in i, with zero first term and with last term less than or equal to N. The Nil are chosen so as to obtain a specified distribution for the luminous intensities of the various grey levels. …<??>Application to the control of liquid-crystal or microtip matrix screens. …<IMAGE>…

Description

The present invention relates to a method and a device for controlling a matrix display screen intended for displaying images having gray levels. It applies in particular to the control of fluorescent micro-tip screens or liquid crystal screens. The images can be in black and white or in color, the expression "gray level" meaning in the latter case "color halftone".

It is known that to control the display of images on a matrix screen, the following scanning mode is generally used: the lines are successively addressed, that is to say carried from an appropriate potential Vlp to another suitable potential Vla une times per image and during a time T called "line time" which is the same for all the lines and which is equal to the quotient of the duration of an image by the number of lines; simultaneously with the addressing of each line, the columns receive signals making it possible to control the respective states of the image elements, or pixels, of the line considered, according to the desired image: a column is brought to an appropriate potential Vca if the corresponding pixel must be on and at another suitable potential Vce if the corresponding pixel must, on the contrary, be off. At the end of time T, the addressing of the line considered ceases and the next line is addressed, the signals received by the columns depending on the respective states desired for the pixels of this next line and so on.

Two techniques are also known which make it possible to generate images comprising gray levels:

A first technique consists in subjecting a column to an intermediate potential between Vca and Vce so that the corresponding pixel has an intermediate brightness between that which corresponds to the lit pixel and that which corresponds to the extinct pixel.

However, in particular in the case of a fluorescent screen with microtips, it is very difficult to adjust an intermediate voltage between Vca and Vce for a given brightness, due to the stiffness of the voltage-brightness characteristic for such a screen.

A second technique consists in bringing a column to the Vca potential only for a fraction of the line time, proportional to the quantity of light desired for the corresponding pixel and then to bringing this column to the Vce potential during the rest of the line time (temporal modulation of the control potential of each column).

However, the relation between the time of application of Vac and the luminosity is never perfectly linear and, in particular in the case of a fluorescent screen with microtips, there exists a strongly non-linear relation between the time of application and brightness due to the time it takes to establish the voltage across a pixel.

In addition, in the case of one or the other of the two known techniques mentioned above, this time for establishing the voltage across a pixel further depends on the access resistance to this pixel linked to its position in the screen. It follows that the charge time of the pixel also depends on this position: for the same control potential, two pixels for example located at the two ends of the same column do not have the same brightness, the pixel closest to the column contact to which the control potential having the highest brightness is applied.

The present invention relates to a method and a device for controlling a matrix screen displaying gray levels, which uses a temporal modulation of the control potential of each column and therefore does not have the drawback of the first known technique mentioned above. , and which also does not cause a problem of non-linearity as posed by the second known technique mentioned above.

Specifically, the present invention firstly relates to a method for controlling a matrix display screen intended to display images having gray levels which are identified by whole numbers increasing from 0 to a number integer m at least equal to 1, this screen comprising a plurality of rows and a plurality of columns whose intersections are respectively associated with picture elements, method according to which, for each picture, these rows are successively activated for a given time T called line time which is the same for all the lines, and, when each line is activated, the columns are respectively controlled by signals intended to activate these columns, each signal being applied for a time which depends on the level of gray of the image element corresponding to the intersection of the activated line considered and of the column controlled by the signal considered,
process characterized in that the line time T is subdivided into N equal time intervals dt, N being an integer at least equal to m, in that each gray level i of each line is associated with a chosen integer Nil of intervals dt, l representing the number of the line considered, the numbers Nil forming, for all l fixed, a strictly increasing sequence of the variable i, of first term NOl zero and of last term Nml less than or equal to N, and in that said time during which said signal is applied is equal to the product of dt by that of the numbers in said sequence which corresponds to said line and to said gray level, said column being deactivated after said time during which said signal is applied, until the next line is activated, the Nil numbers being chosen so as to obtain a determined distribution for the light intensities of the different gray levels.

It can therefore be seen that the present invention makes it possible to correlate the time of application of the potential Vac during the line time with the voltage-brightness characteristic of the screen considered.

The use of the Nil quantities in the present invention and the possibility of choosing these quantities means that one can balance a posteriori - that is to say once the screen and the electronic circuits associated with it are ready to operate or even have already operated - the gray levels obtained with respect to each other, either to obtain a particular gray scale, regular or logarithmic for example, or to compensate for an aging of the screen-circuit assembly, or else to choose a better compromise between coupling and brightness.

In this connection, it is recalled that the coupling in question is a phenomenon which is linked to the resistance of access to different pixels and which visually results in "smudges" from one line of the screen to the other.

For any couple of lines l1 and l2, the sequences of numbers Nil1 and Nil2 can be identical (non-differentiation of the lines on the screen, the lines l1 and l2 not necessarily being successive lines.

Then the gray levels can be adjusted as follows:
at least two zones are formed on the screen corresponding respectively to the gray level 0 and to the gray level m,
the fraction of the line time during which the columns are activated for the image elements at the gray level m is varied, until a desired image quality is obtained on the screen,
- a uniform image is formed on the screen having the gray level m thus defined and the brightness of this uniform image is measured,
- the luminosity which must be obtained for each of the other gray levels 1 to m-1, on the basis of a chosen gray level scale, is calculated from this measured brightness value, and
- For each of these other gray levels, a uniform image is formed on the screen having this other gray level and the number of said sequence which corresponds to it is adjusted so as to obtain the brightness calculated for this other level of Grey.

On the contrary, for certain lines l1, l2 of the screen the sequences of numbers Nil1 and Nil2 may not be identical (differentiation of the lines on this screen).

It is then possible to correlate the time of application of the potential Vac during the line time with not only the voltage-brightness characteristic of the screen considered as already indicated, but also with the position of the pixel addressed in this screen.

When said sequences Nil1 and Nil2 are not identical for certain lines l1, l2 of the screen, the maximum gray levels can be adjusted as follows:
- the respective luminosities of all the lines of the screen are measured when these lines are at the maximum gray level and the lowest brightness line which is taken for reference is determined, and
- for each of the other lines l, the number Nml corresponding to the maximum gray level is adjusted so that the resulting brightness is equal to the reference brightness.

In this case, the other gray levels 1 to m-1 can then be adjusted as follows:
- the brightness which must be obtained for each of the other gray levels 1 to m-1 is calculated from the reference brightness value, according to a chosen gray level scale, and
- For each of these other gray levels and for each line, the image of this line having this other gray level is formed on the screen and the number of said series corresponding to it is adjusted so as to obtain the brightness calculated for this other gray level.

Preferably, Nml is less than N, which makes it possible to suppress the phenomenon of smearing from one line to the other, as will be seen more clearly below.

The present invention also relates to a device for controlling a matrix display screen intended to display images having gray levels which are identified by whole numbers increasing from 0 to an integer m at least equal to 1 , this screen comprising a plurality lines and a plurality of columns whose intersections are respectively associated with picture elements, this device comprising:
means provided for successively activating the lines for a given time T called line time which is the same for all the lines and this, for each image, and
- Column control means, provided to generate, when activating each line, signals intended to activate the columns respectively, each signal being applied for a time which depends on the gray level of the picture element corresponding to the intersection of the activated line considered and the column controlled by the signal considered,
device characterized in that the column control means comprise:
- means which are common to all the columns and which include:
. means provided for generating pulses of period dt equal to T / N, N being an integer at least equal to m,
. memorization means provided for memorizing, at least for each non-zero gray level i of each line, information linked to a chosen integer Nile, l representing the number of the line considered, the Nil numbers forming, for all l fixed , a strictly increasing sequence of the variable i of the last term Nml less than or equal to N, and
- means provided for applying said signal for a time equal to the product of dt by that of the numbers in said sequence which corresponds to said line and to said gray level, and for deactivating said column after said time during which said signal is applied, up to 'when the line is activated following, the application time of any signal corresponding to the display of a gray level image element 0 being zero, the Nil numbers being chosen so as to obtain a determined distribution for the light intensities of the different levels of Grey.

In a particular embodiment of the device that is the subject of the invention, the column control means further comprise a shift register whose number of positions is equal to the number of columns and which receives gray level information for the columns as input. , each position being associated with a given column, and occupied during the activation of a line, by the gray level information i relating to this column, the means provided for applying said signal comprise for each column:
a register which receives as input the information contained in the corresponding position of the shift register and which is controlled by line start signals, and
a comparator with two inputs, the first input of which is connected to the output of said register and the output of which controls the activation of the corresponding column by means of amplification,
and the means common to all the columns are provided for sending to the second input of each comparator information representing whole numbers k, this information varying from 0 to m in an increasing manner over the course of the line time so that the corresponding column to this comparator is activated as long as k is less than i then deactivated and maintained in the deactivated state as soon as k reaches i until the activation of the next line.

According to a first particular embodiment of the device which is the subject of the invention, the numbers Nil1 and Nil2 being equal, for any pair of lines l1 and l2 and for each gray level i, the means common to all the columns further comprise:
- a first counter provided for counting down, and
a second counter which is reset to zero at the start of a line, which is incremented by an end of counting signal sent by the first counter, and which sends the information representing the numbers k to the second input of each comparator,
this first counter is decremented by the means provided for generating the pulses, the storage means comprise at least m registers numbered from 0 to m-1 and an address bus on which are sent the information representing the numbers k, the signals of output of these storage means control the initialization of the first counter which takes these output signals into account when sending its end of count signal, and the information present at the address i of the storage means, i taking any of the values 0 to m-1, is equal to the difference between the numbers N (i + 1) l and Nil.

Finally, according to a second particular embodiment, the sequences of numbers Nil1 and Nil2 not being identical for certain lines l1, l2 of the screen, the means common to all the columns further comprise:
- a first counter designed to count down,
- a second counter which is reset to zero at the start of a line, which is incremented by a end of counting signal emitted by the first counter, and which sends the information representing the numbers k to the second input of each comparator, and
- a third counter which is reset to zero at the start of an image and incremented at the start of each line,
the first counter is decremented by the means provided for generating the pulses, the storage means comprise at least mxL registers, L being the number of lines, and an address bus on which the information representing the numbers k is sent in the form of binary words in two parts, the most significant part corresponding to the output signals of the third counter and the least significant part corresponding to the information representing the numbers k, the output signals from these storage means control the initialization of the first counter which takes these output signals into account when sending its end-of-count signal, and the information present at the address ixl of the storage means, i taking any of the values 0 to m-1 and l taking any one of the values 1 to L, is equal to the difference between the numbers N (i + 1) l and Nil.

• - la figure 1 illustre schématiquement le principe d'un affichage en tout ou rien pour un écran fluorescent à micro-pointes,
• - la figure 2 illustre schématiquement le principe de l'invention pour un tel écran fluorescent à micro-pointes,
• - la figure 3 montre les variations du courant électronique en fonction de la tension entre la cathode et la grille pour un écran donné du type précédent,
• - la figure 4 illustre schématiquement l'intérêt de subdiviser, dans la présente invention, le temps de ligne T en un nombre N d'intervalles dt, supérieur au niveau de gris maximum m,
• - la figure 5 est une vue schématique d'un premier mode de réalisation particulier du dispositif objet de l'invention, et
• - la figure 6 est une vue schématique d'un second mode de réalisation particulier de ce dispositif.

The present invention will be better understood on reading the description which follows, of exemplary embodiments given purely by way of indication and in no way limiting, with reference to the appended drawings in which:

• FIG. 1 schematically illustrates the principle of an all-or-nothing display for a fluorescent screen with microtips,
• FIG. 2 schematically illustrates the principle of the invention for such a fluorescent screen with microtips,
• FIG. 3 shows the variations of the electronic current as a function of the voltage between the cathode and the grid for a given screen of the previous type,
• FIG. 4 schematically illustrates the advantage of subdividing, in the present invention, the line time T into a number N of intervals dt, greater than the maximum gray level m,
• FIG. 5 is a schematic view of a first particular embodiment of the device which is the subject of the invention, and
• - Figure 6 is a schematic view of a second particular embodiment of this device.

FIG. 1 schematically illustrates the principle of the all-or-nothing display in the case of a particular fluorescent screen with microtips. By "all or nothing display" means a display for which each pixel can only be in the off state or in the on state, with no intermediate state. We see in Figure 1 the successive addresses of the first three lines of the screen L1, L2 and L3. Each line passes at a given time from a potential Vlp = 45V to a potential Vla = 90V which it keeps for the time of line T to then return to the potential Vlp = 45V at the moment when the next line passes from the potential 45V at 90V potential ... When all the lines have been addressed, the first one is addressed again and so on.

FIG. 1 also shows specific addressing signals for the first three columns of the screen C1, C2 and C3, these signals leading to the following image on the screen: the pixels corresponding to the intersections of the columns C1 , C2 and C3 with line L1 are respectively in the states off, on and off; the intersections of these same columns with the line L2 respectively lead to pixels in the on, off and off states and the same intersections with the line L3 respectively lead to pixels in the on, off and on states. Thus, for example, when the line L1 is activated, the potential applied to the contact of the column C1 changes from Vce = 0V to Vca = 45V to then return to 0V during the successive addresses of the lines L2 and L3.

The process which is the subject of the invention will now be explained: according to the invention, the line time T is divided into N equal intervals dt. We suppose that we want to be able to display m + 1 gray levels identified by the number 0 (pixel off), 1, ..., m (maximum gray level corresponding to a pixel lit). The number N is at least equal to m. In practice, N is much greater than m. We associate with each gray level i of each of the lines l of the screen a number Nil of intervals dt. The gray level 0 (pixel off) is associated with 0 intervals regardless of the number l of the line. In other words, N0l is zero whatever l.

In addition, the number of intervals dt associated with each of the gray levels is strictly increasing with the brightness of this gray level. In other words, for all l fixed, the sequence of numbers Nil is a strictly increasing sequence of the variable i.

In addition, the maximum gray level m (corresponding to a lit pixel) is associated with a number of intervals Nml less than or equal to N whatever l.

For a given addressed line, the column electrode whose pixel must have the non-zero gray level brightness i is carried, from the beginning of the line time T, at the activation potential Vca (0V for certain fluorescent micro-tip screens) and maintained at this potential for Nil time intervals dt, l being the number of the line considered, after which this electrode is brought back at the extinction potential Vce (45V for these fluorescent micro-tip screens) and this, until the start of the next line.

The process which is the subject of the invention is illustrated by an example in FIG. 2, in the case of a fluorescent screen with micro-tips in this example, the line time T is subdivided into 32 intervals dt (a) in view translate 8 levels of gray (0 to 7). The numbers N and m are therefore equal to 32 and 7 respectively.

We considered 4 gray levels 0,1, 4, and 7 and, for each of these levels, we represented the time diagram of the control signal applied to a column contact to display this level (in dotted lines) as well as the behavior of this column (in solid lines) during the line time T. It is noted in FIG. 2 that the gray level 7 ("white" points, ie lit) corresponds to N7l = 28 dt intervals (b), l representing the number of the line considered, that the gray level 4 is associated with N4l = 14 intervals dt (c), that the gray level 1 (pixel almost extinct) is associated with N1l = 5 intervals dt (d) and that the gray level 0 (black point, ie off) is associated with N0l = 0 interval dt (e).

An example showing the improvement in the performance of a fluorescent microtip screen thanks to the process which is the subject of the invention is given in table I which is at the end of this description and in which the lines are not differentiated: for any couple of lines l1, l2 and for each gray level i, the numbers Nil1 and Nil2 are equal.

In this table, the gray levels range from 0 to m = 15, the Nil numbers associated with them in accordance with the present invention range from N0l = 0 to N15l = 355. Furthermore, the gray levels obtained with a regular distribution over time were compared in accordance with the second known technique, mentioned above (application time of Vac proportional to the desired brightness) with the gray levels obtained with an adjusted distribution. in accordance with the present invention, for a fluorescent microtip screen whose emission characteristic is shown in FIG. 3. The load resistance of each column of this screen is 10 kilo-ohms, the capacity to charge per column is 1 nanofarad, the line time is 64 microseconds and this line time is subdivided into N = 640 equal dt intervals.

In FIG. 3, the variations of the intensity J of the electronic current, represented in milli-amperes per square millimeter, are represented as a function of the voltage v between a cathode (column) and a grid (line) of the screen. , expressed in volts.

On table I, for each gray level i, the value obtained for the ratio (in percentage) of the brightness Ii corresponding to this gray level to that corresponding to the maximum gray level (15) has been indicated, d on the one hand with the invention, by experimentally determining the Nile numbers so as to obtain a regular distribution of the brightness, and on the other hand with the prior art (second known technique, mentioned above).

It will be noted that the invention makes it possible to obtain luminosity ratios which increase substantially in arithmetic progression, which is not the case. in the prior art.

In addition, with the regular brightness distribution, chosen in accordance with the invention in this table I, the coupling is limited to 2.7% of the current emitted by a gray level point 15 and this coupling is zero for the other levels 0 to 14.

In FIG. 4, the advantage of not assigning N intervals dt to the maximum gray level m has been illustrated diagrammatically. We consider a line l of a fluorescent screen with microtips and the following line l + 1. It is assumed that a pixel PB of line l corresponds to a lit point (gray level m) and that the pixel PN belonging to the same column as PB and located on line l + 1 corresponds to an extinct point (level of gray 0). In the case (a) where N intervals dt are assigned to the most significant gray level, we see that there is a CPL coupling between the pixels PB and PN, the dotted lines corresponding to the control signal applied to the contact of the column considered and the solid line corresponding to the behavior of this column, during the line time T. As a result of this coupling, light is emitted parasitically on the line l + 1. On the contrary, in case (b) where the number of intervals dt allocated to the largest gray level is less than N, such interference does not exist.

We will now explain how to determine the number of intervals Nil to be associated with each gray level i. We first consider the case in which the lines are not differentiated. Nile numbers can be determined as follows:
- the image of a checkerboard is formed on the screen, or a succession of bands alternately lit (maximum gray level) and unlit (gray level 0). It suffices in fact to form an image comprising an extinct part and a lit part and more precisely, an image comprising at least on the same column a lit point immediately followed by a lit point.

The fraction of the line time during which the column electrodes are maintained at the activation potential is then varied, for the lit pixels, either by varying Nml to N constant, or by varying N to Nml constant. In this way, the best compromise between the coupling and the brightness is sought, knowing that the larger Nml / N, the better the brightness but the stronger the coupling.

A uniform gray level image m resulting from the previous compromise is then formed on the screen and the brightness of this image is measured for example by means of a photometer or by measuring the anode current (in the case a fluorescent screen with micro-tips).

From this luminosity value for the gray level m, the luminosity which must be obtained for each of the other gray levels is calculated according to a luminosity scale which has been set (regular or logarithmic scale by example).

Finally, for each of these other gray levels, a uniform image of this other level is formed on the screen and the number of intervals dt associated with this other level is adjusted so as to obtain the brightness previously calculated for this other level.

Note that the settings made are valid for all screens that have the same characteristics, the same number of rows and the same number of columns: in the case of identical screens produced in the chain, it is not necessary to redo these settings for each of these screens.

In the case where the lines are differentiated, we can start by adjusting the maximum gray level of each of the lines as follows:

The lowest brightness line is first determined by measuring the respective luminosities of all the lit lines, successively for example. The lowest brightness line is generally the last line, that is to say the one furthest from the contacts allowing the addressing of the screen columns.

The number of intervals dt to be assigned to the maximum gray level of this other line is then adjusted, for each other line, so that it has the same brightness as said lowest brightness, the latter being taken for reference. During this adjustment, only said other line considered is lit on the screen.

Then, one can calculate, from the value taken for reference, the luminosity which one must obtain for each of the other gray levels according to a scale which one fixed. After which, for each of these other gray levels, the lines of the screen are successively activated on the screen and the number of intervals dt associated with this other level and with the line considered is adjusted so as to obtain the brightness previously calculated for said other level.

FIG. 5 schematically represents a first particular embodiment of the device which is the subject of the invention, making it possible to control a matrix screen 2, for example a fluorescent screen with microtips, for which the lines of the point of view of their brightness. This screen includes a set of lines 4 parallel to each other and a set of columns 6 which are parallel to each other and perpendicular to the lines. On the same side of the screen, the end of each line is provided with a line contact. Similarly, on one side of the screen, adjacent to the previous one, the end of each column is provided with a column contact.

The device shown in FIG. 5 comprises means 8 for controlling the lines and means 10 for controlling the columns. The intersection of a given line and a given column defines a picture element 12 which appears on the screen when this line and this column are addressed in an appropriate manner.

We suppose for example m = 15, from where 16 gray levels identified by the numbers 0, 1, ..., 15 that we can code on 4 bits in binary system. (For m + 1 gray levels, these are coded on p bits such that 2 ^p ≧ m + 1).

The device represented in FIG. 5 further comprises means 13 provided for supplying the information concerning the gray levels of the pixels, this information being coded in binary system, on 4 bits and denoted GP, and the synchronization pulses, in particular those of start of line.

Furthermore, the means 10 include:
a shift register 14 having as many positions as there are columns in the screen, each position comprising 4 bits (if m = 15),
- for each column, a 4-bit register 16 which, in the example shown in FIG. 5, is a D-type flip-flop of 4 bits, as well as a comparator 18 and means 20 for amplifying the control signal of the column considered, and
- means 22 which are common to all columns and which will be described later.

The GP information is presented successively to the input of the shift register 14 and moved in this register 14 so that at the start of the addressing of a line, each information item, which is associated with a pixel, occupies the position of the shift register which is associated with the column corresponding to this pixel. At the start of the addressing of a line, each item of information GP is transferred from its position in the register 14 to the inputs D of the flip-flop 16 of 4 bits associated with this position. The non-inverting outputs Q of this flip-flop are sent to one P of the two inputs (4 bits) of the comparator 18 of 2x4 bits, the other Q input (4 bits) of this comparator receiving GC information which is common to all column commands and coded on 4 bits. This GC information, which comes from the means 22 common to all the columns, changes in an increasing manner over the course of the line time T. The output of the comparator 18 is connected to the input of the corresponding amplification means 20, the output of which controls the corresponding column.

As long as the value GP is greater than the value GC, the output of the comparator 18 remains at logic level 0 and the contact of the column corresponding to the comparator 18 considered is maintained at the potential 0 volt (activation). As soon as the value GC becomes equal to GP then greater than this value GP, the output of the comparator 18 passes and remains at logic level 1 and the contact in question is brought and maintained at the potential 45 volts (extinction).

The means 22 which are common to all the columns include a first 8-bit counter 24, intended for counting down, a second 4-bit counter 26, a clock 28 and a memory 30.

The counters 24 and 26 are for example of the type 74193.

The means 22 also comprise a first door 32 of the AND type and a second door 34 also of the AND type. The output of door 32 is connected to the clock input CK of the counter 26. The output of door 34 is connected to the (reversing) loading input LD ("load") of the counter 24. An input of the door 32 is connected to the (reverse) retaining output RE ("carry") of the counter 26 and the (reverse) output of end of countdown BO ("borrow") of the counter 24 is connected to the other input of the door 32 as well as at an entrance to door 34.

The means 13 are provided for sending line start information to the line control means 8 and to the reset input RESET of the counter 26. This line start information is also sent to the clock input CK ("latch") of each flip-flop 16 and at the other input of door 34 via an inverter 36.

We see in Figure 5 that the clock input of flip-flop 16 is inverting: the line start pulse (logic state 1) is reversed a first time (logic state 0) by the inverter 36 then a second times (logic state 1) at this input CK of the flip-flop 16 which therefore takes care of the information contained in the corresponding position of the register 14 when the line start pulse is sent.

The clock 28 is a regular clock of frequency 1 / dt, that is to say N / T. The pulses supplied by the clock are sent to the DC down counting input of counter 24.

The GC information coded on 4 bits comes from the counter 26 and sent on the one hand to the input Q of each of the comparators 18 and on the other hand to the bus address A of memory 30 (the content of counter 26 therefore corresponds to an address of memory). This memory 30 is a memory of 15 words of 8 bits. The outputs Si of this memory 30 are presented on the initialization bus of the counter 24.

The counter 26 is reset to zero at the start of the line and incremented by a countdown end signal sent by the output B0 of the counter 24. In fact, at the end of each countdown, the output B0 of the counter 24 goes to the state logic 1 and, the output RE of counter 26 being in logic state 1, the input CK of this counter 26 receives a pulse. The counter 24 is decremented by the clock 28 and takes into account the outputs Si of the memory 30 during the transmission of its down-counting signal. This signal in fact corresponds to a transition from the output BO of the counter 24 to the logic state 1 and, as the output of the inverter is in the logic state 1, the input LD of the counter 24 receives a pulse.

The information Si is placed at the address i of the memory and is equal to the number of intervals dt to count to pass from the number of intervals corresponding to the gray level i to the number of intervals corresponding to the gray level i + 1.

To obtain the results indicated in Table I, the content of the memory 30 is as follows: Address 0 1 2 3 4 5 6 7 8 Content 116 30 23 20 18 17 17 16 15 Address 9 10 11 12 13 14 15 Content 15 14 14 14 13 13 -

In this example, we see that the content of address 15 of the memory is indifferent since it is not taken into account.

The means 22 therefore operate in the following manner: at the start of the line, the counter 26 is reset to zero. Its content is then 0. At address 0, the memory 30 includes the number of intervals dt corresponding to the gray level 1. This number is transferred to the counter 24 which is decremented by the clock 28 of frequency 1 / dt. When the counter 24 is at zero, it sends a pulse to the counter 26 which is incremented due to this pulse. The new content of the counter 26 is then 1. At the address 1, the memory 30 includes the additional number of intervals to be counted to reach the number of intervals corresponding to the gray level 2. This additional number is transferred to the counter 24 ... And so on.

When the content of counter 26 reaches its maximum value (15), its RE output goes to logic state 0, which blocks it. A new cycle begins with a new line.

The memory 30 is for example of the PROM type. To make the gray level adjustments mentioned above, which implies modifications to the content of this memory, it suffices to replace it by a device called "PROM emulator", all other things being equal and, once the settings completed, replace this emulator with memory 30 in which the values obtained with this emulator are written. In addition, if these settings require the number N to be varied, it is sufficient to do so to change the clock 28.

In Figure 6, there is shown schematically a second particular embodiment of the device object of the invention, allowing the control of the screen 2 with line differentiation. The device schematically shown in Figure 6 differs from the device which is shown in Figure 5 in that it further comprises a third counter 38 whose incrementation is controlled by the line start pulses (which are sent to the 'clock input CK of counter 38) and whose reset to zero is controlled by an image start signal DI which is supplied by means 13. The number s of output of counter 38 is such that 2 ^s is at least equal to L (number of lines on the screen). In addition, in the device represented in FIG. 6, the memory 30 is replaced by a memory 31 of n 8-bit words, n being at least equal to the product of the number of lines of the screen by the number m, equal to 15 in the example given.

The words presented on the address bus A of the memory 31 comprise a least significant part and a most significant part. The outputs SL of the counter 38 constitute the most significant part of each of these words, the least significant part of which is the word supplied as an output by the counter 26. The addresses of the memory are therefore identified by words of s + 4 bits .

The devices described with reference to Figures 5 and 6 could be used by those skilled in the art for controlling a matrix screen with liquid crystals.

Furthermore, the present invention applies both to the control of a black and white screen as to the control of a color screen. TABLE I i Nile Invention Ii / I15 (%) Invention II / I15 (%) Prior art 0 0 0 0 1 116 6.7 0.1 2 146 13.3 1.1 3 169 20.0 2.5 4 189 26.7 6.4 5 207 33.4 15.8 6 224 40.2 19.8 7 241 47.2 27.4 8 257 54.3 41.1 9 272 60.9 45.4 10 287 67.8 54.0 11 301 74.2 68.9 12 315 80.7 73.2 13 329 87.5 81.7 14 342 93.7 95.7 15 355 100 100

Claims (11)

1. Method for controlling a matrix display screen (2) intended for displaying images having gray levels which are identified by whole numbers increasing from 0 to an integer m at least equal to 1, this screen comprising a plurality of lines (4) and a plurality of columns (6) whose intersections are respectively associated with picture elements (12), method according to which, for each image, these lines are successively activated for a given time T called line time which is the same for all the lines, and, when each line is activated, the columns are respectively controlled by signals intended to activate these columns, each signal being applied for a time which depends on the level of gray of the picture element corresponding to the intersection of the activated line considered and the column controlled by the signal considered,
process characterized in that the line time T is subdivided into N equal time intervals dt, N being an integer at least equal to m, in that each gray level i of each line is associated with an integer Nil chosen from intervals dt, l representing the number of the line considered, the numbers Nil forming, for all l fixed, a strictly increasing sequence of the variable i, of first term NOl zero and of last term Nml less than or equal to N , and in that said time during which said signal is applied is equal to the product of dt by that of the numbers in said sequence which corresponds to said line and to said gray level, said column being deactivated after said time during which said signal is applied , until the next line is activated, the Nil numbers being chosen so that obtain a determined distribution for the light intensities of the different gray levels. 2. Method according to claim 1, characterized in that, for any couple of lines l1 and l2, the sequences of numbers Nil1 and Nil2 are identical. 3. Method according to claim 2, characterized in that the gray levels are adjusted as follows:
at least two zones are formed on the screen (2) corresponding respectively to the gray level 0 and to the gray level m,
the fraction of the line time during which the columns are activated for the image elements at the gray level m is varied, until a desired image quality is obtained on the screen,
- a uniform image is formed on the screen having the gray level m thus defined and the brightness of this uniform image is measured,
- the luminosity which must be obtained for each of the other gray levels 1 to m-1, on the basis of a chosen gray level scale, is calculated from this measured brightness value, and
- For each of these other gray levels, a uniform image is formed on the screen having this other gray level and the number of said sequence which corresponds to it is adjusted so as to obtain the brightness calculated for this other level of Grey. 4. Method according to claim 1, characterized in that for certain lines l1, l2 of the screen, the sequences of numbers Nil1 and Nil2 are not identical. 5. Method according to claim 4, characterized in that the maximum gray levels are adjusted as follows:
- the respective luminosities of all the lines (4) of the screen (2) are measured when these lines are at the maximum gray level and the lowest brightness line is taken which is taken for reference, and
- for each of the other lines l, the number Nml corresponding to the maximum gray level is adjusted so that the resulting brightness is equal to the reference brightness. 6. Method according to claim 5, characterized in that the other gray levels 1 to m-1 are then adjusted as follows:
- the brightness which must be obtained for each of the other gray levels 1 to m-1 is calculated from the reference brightness value, according to a chosen gray level scale, and
- For each of these other gray levels and for each line, the image of this line having this other gray level is formed on the screen and the number of said series corresponding to it is adjusted so as to obtain the brightness calculated for this other gray level. 7. Method according to any one of claims 1 to 6, characterized in that, for all l, Nml is less than N. 8. Control device for a matrix display screen (2) intended to display images having gray levels which are identified by whole numbers increasing from 0 to an integer m at least equal to 1, this screen comprising a plurality of rows (4) and a plurality of columns (6) whose intersections are respectively associated with picture elements (12), this device comprising:
means (8) provided for successively activating the lines for a given time T called line time which is the same for all the lines and this, for each image, and
- means (10) for controlling the columns, designed to generate, during the activation of each row, signals intended to activate the columns respectively, each signal being applied for a time which depends on the gray level of the element image corresponding to the intersection of the activated line considered and the column controlled by the signal considered,
device characterized in that the means (10) for controlling the columns comprise:
- Means (22) which are common to all the columns and which comprise:
. means (28) provided for generating pulses of period dt equal to T / N, N being an integer at least equal to m,
. storage means (30, 31) provided for storing, at least for each non-zero gray level i of each line, information linked to a chosen integer Nil, l representing the number of the line considered, the numbers Nil forming , for all l fixed, a strictly increasing sequence of the variable i of last term Nml less than or equal to N, and
- means (16, 18) provided for applying said signal for a time equal to the product of dt by that of the numbers in said sequence which corresponds to said line and to said gray level, and for deactivating said column after said time during which said signal is applied, until the activation of the next line, the time of application of any signal corresponding to the display of a gray level image element 0 being zero, the numbers Nil being chosen so as to obtain a determined distribution for the light intensities of the different gray levels. 9. Device according to claim 8, characterized in that the means (10) for controlling the columns further comprises a shift register (14) whose number of positions is equal to the number of columns (6) and which receives as input gray level information for the columns, each position being associated with a given column and occupied during the activation of a row, by the gray level information i relating to this column, in that the means provided to apply said signal include for each column:
a register (16) which receives as input the information contained in the corresponding position of the shift register (14) and which is controlled by start of line signals, and
a comparator (18) with two inputs, the first input of which is connected to the output of said register (16) and the output of which controls the activation of the corresponding column by means of amplification (20),
and in that the means (22) common to all the columns are provided for sending to the second input of each comparator (18) information representing whole numbers k, this information varying from 0 to m in an increasing manner over time of row so that the column corresponding to this comparator is activated as long as k is less than i then deactivated and maintained in the deactivated state as soon as k reaches i until the activation of the following line. 10. Device according to claim 9, characterized in that for any couple of lines l1 and l2 and for each gray level i the numbers Nil1 and Nil2 are equal, in that the means (22) common to all the columns further comprise:
- a first counter (24) provided for counting down, and
- a second counter (26) which is reset to zero at the start of a line, which is incremented by an end of counting signal emitted by the first counter, and which sends the information to the second input of each comparator (18) representing the numbers k,
in that this first counter (24) is decremented by the means (28) provided for generating the pulses, in that the storage means (30) comprise at least m registers numbered from 0 to m-1 and a bus address to which the information representing the numbers k is sent, in that the output signals from these storage means (30) control the initialization of the first counter (24) which takes these output signals into account during transmission of its end of counting signal, and in that the information present at the address i of the storage means, i taking any of the values 0 to m-1, is equal to the difference between the numbers N ( i + 1) l and Nil. 11. Device according to claim 9, characterized in that, for certain lines l1, l2 of the screen, the sequences of numbers Nil1 and Nil2 are not identical, in that the means (22) common to all the columns comprise in addition :
- a first counter (24) provided for counting down,
- a second counter (26) which is reset to zero at the start of a line, which is incremented by an end of counting signal emitted by the first counter (24), and which sends to the second input of each comparator (18) the information representing the numbers k, and
- a third counter (38) which is reset to zero at the start of an image and incremented at each start of line,
in that the first counter (24) is decremented by the means (28) provided for generating the pulses, in that the storage means (31) comprise at least mxL registers, L being the number of lines, and a bus d address to which the information representing the numbers k is sent in the form of binary words in two parts, the most significant part corresponding to the output signals of the third counter (38) and the least significant part corresponding to the information representing the numbers k , in that the output signals from these storage means (31) control the initialization of the first counter (24) which takes these output signals into account when sending its end of count signal, and in that that the information present at the address ixl of the storage means (31), i taking any one of the values 0 to m-1 and l taking any one of the values 1 to L, is equal to the difference between the numbers N (i + 1) l and Nil.

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