Method for adjusting the overall luminosity of a bistable matrix screen displaying half-tones

In a method for adjusting the overall luminosity of at least a part of a matrix screen, each row of the part is processed several times non-periodically to display half-tones. Each processing consists of a semi-selective operation followed by a selective operation. A delay is planned between the selective operation and the semi-selective operation, this delay being proportional to a weighting factor that is adjustable as a function of the desired overall luminosity and proportional to the time interval between the beginning of the treatment in progress and the beginning of the next treatment.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a method for adjusting the overall
 luminosity of a bistable matrix screen displaying half-tones. It also
 relates to a display device that uses the method.
 The invention can be applied to screens of the type having an internal
 memory. A screen with an internal memory is a screen whose cells, which
 form the pixels, preserve the "written" state or the "extinguished" state
 after the end of the signal activating the "written" state or the
 "extinguished" state as is the case notably with plasma panels and
 especially with alternating type plasma panels.
 The screens to which the invention applies comprise elementary cells
 arranged in rows and columns in matrix form.
 The use of display screens in a very wide variety of luminous environments
 may lead to the adjusting of their overall luminance as a function of the
 ambient luminosity in which they are used. In fact, it is recommended that
 the overall luminance of the screen should be comparable to that of the
 environment, otherwise unnecessary fatigue will be created for the user.
 The conditions of illumination around the screen may vary by a factor of
 about 1000 (from some tens of lux indoors with attenuated illumination to
 some tens of thousands of lux outdoors in sunlight).
 The description shall be made in the case of a alternating type of plasma
 display panel. However, the invention can be applied to other types of
 bistable display panels, for example liquid-crystal display screens.
 2. Description of the Prior Art
 The working and structure of alternating-type plasma panels is well known.
 These panels, are for example, of the crossed-electrode type defining a
 cell as described in the French patent FR-2 417 848. The addressing of a
 given cell is achieved by the selection of two crossed electrodes to which
 appropriate voltages are applied at a given instant so that the difference
 in potential prompts a writing discharge or an erasure discharge between
 these electrodes.
 A standard method of addressing consists of a row-by-row operation. In this
 case, all the cells of a given row simultaneously receive a command, by
 means of a semi-selective operation, for them to be erased or written on,
 for example to be erased, and this operation is followed by a selective
 operation during which at least one of the cells of the row is written on.
 The semi-selective operation followed by the selective operation is
 accomplished, for each row, with a time lag from one row to the other
 corresponding to the duration of a row cycle.
 Generally, the addressing by semi-selective operation and selective
 operation is done by a method in which addressing square-wave signals are
 overlaid on basic square-wave signals as explained, for example, in the
 patents FR-2 635 901 and FR-2 635 902.
 These basic square-wave signals are applied simultaneously to all the cells
 for a period constituting an addressing stage and the addressing
 square-wave signals are overlaid on these basic square-wave signals only
 for the rows of cells addressed with, from one row to the other, the time
 lag corresponding to the duration of a row cycle T1; this means that the
 starting points of two consecutive addressing stages are separated by the
 duration of the row cycle.
 Generally, in each row cycle, the addressing stage is followed by a
 sustaining stage during which the cells in the written state are
 activated, i.e. they produce light. Indeed, in this sustaining stage,
 sustaining signals are applied simultaneously to all the cells and prompt
 sustaining discharges that provide the essential part of the light
 emission perceived by an observer.
 The sustaining signal is an alternating signal formed by voltage square
 waves that succeed one another with opposite polarities: each change in
 sign of the alternating signal (leading edges or trailing edges) generates
 a discharge in the gas or an emission of light in the cell or cells
 concerned. Thus, the quantity of light emitted by a cell in the
 illuminated state, namely the written state, is substantially proportional
 to the number of edges corresponding to polarity changes and,
 consequently, to the frequency of the sustaining signal.
 It must be noted that in the addressing stage, as regards both recording
 and erasure, the basic square-wave signals have substantially one and the
 same amplitude as the sustaining signals and, consequently, they too may
 generate discharges comparable to the sustaining discharges, with light
 emission. Consequently, it may be assumed that the addressing stages
 contain at least one sustaining cycle.
 To adjust the overall luminosity of an alternating type plasma panel, there
 is a known way of causing variation in the frequency of the sustaining
 signals. By making this frequency adjustable, the overall luminance of the
 panel is adjusted.
 There is also a known way, described in the patent FR-2 662 292, of
 separating the selective (recording) operation from the semi-selective
 operation by an adjustable period that is substantially equal to a
 fraction of an image frame period, this fraction representing a percentage
 of the maximum luminosity. It may be recalled that the image frame period
 corresponds to the time needed to display an image.
 It is increasingly being sought to display images in half-tones. In this
 type of display panel, each cell has several levels of illumination. The
 French patent FR-2 536 565 has proposed the processing, of all the rows of
 the panel several times and non-periodically in order to have several
 illumination periods for each cell.
 This method uses several scans that are interleaved.
 This method cannot be used to adopt the method for adjusting the overall
 luminosity which consists in separating the selective recording operation
 from the semi-selective operation since it is already necessary to
 distribute several commands for the recording and erasure of a row during
 a frame period.
 Furthermore, it is difficult to adapt the method for adjusting the overall
 luminosity by variation of the sustaining frequency to the systems of
 half-tone displays using the above method since the processing rates of
 each row are imposed.
 Up to now, no half-tone display method has been proposed enabling an
 adjustment of the overall luminosity.
 SUMMARY OF THE INVENTION
 The present invention proposes a method for adjusting the overall
 luminosity of at least a part of a half-tone display screen.
 The method according to the invention consists in processing each row of
 the part of screen several times, non-periodically, in a semi-selective
 operation followed by a selective operation, a delay being planned between
 the selective operation and the semi-selective operation, said delay being
 proportional to a weighting factor k(o&lt;k&lt;1) adjustable as a function of
 the desired overall luminosity and proportional to the time interval
 between the start of the processing operation in progress and the start of
 the next processing operation.
 This method is simple to implement and makes it possible to obtain a
 dynamic range of adjustment for a constant number of half-tones wherein
 the greater the number of rows, the greater is this dynamic range of
 adjustment.
 The present invention also relates to a display device to which the method
 for the adjusting of overall luminosity can be applied.

MORE DETAILED DESCRIPTION
 FIG. 1 is a graph giving a view, in time, of the instants of processing of
 a row 1 of a bistable screen by the known method of half-tone generation
 without adjustment of luminosity. Each row is processed N times to display
 an image. In the non-restrictive example shown, N=4.
 Tb represents the time taken to process all the rows of the screen once.
 The time needed to display an image or the frame time is therefore T=N.Tb.
 The row 1 is processed at the successive instants 0, a, b, c, 1, 1+a, 1+b,
 1+c, 2 . . . with 0&lt;a&lt;1, 0&lt;b&lt;1, 0&lt;c&lt;1 and a&lt;b&lt;c
 The processing operations applied at the instants 0, a, b, c, are designed
 to keep or modify the written or extinguished state of the cells of the
 row 1.
 N scans per image make it possible to obtain 2.sup.N different tone levels
 (generally grey levels) for each cell if the instants a, b, c, etc. are
 judiciously chosen. It is possible, for example, to choose them so as to
 obtain a geometric progression. We then have:
EQU a-0=2.sup.0 /2.sup.N -1
 b-a=2.sup.1 /2.sup.N -1
EQU c-b=2.sup.2 /2.sup.N -1
EQU 1-c=2.sup.3 /2.sup.N -1
 In other words, the time intervals between two successive processing
 operations for the same row increase proportionally by the power of two.
 In the graph of FIG. 1, the following values have been chosen:
EQU a=1/2.sup.N -1=1/15
EQU b=3/2.sup.N -1=3/15
EQU c=7/2.sup.N -1=7/15
EQU 1=15/2.sup.N -1=15/15
 The table of FIG. 2 gives a view, for a cell controlled by the known
 half-tone display method, of the possible tone levels, the state of the
 cell at the instants 0, a, b, c and its period of illumination or
 luminance. The first column shows the tone level encoded in binary mode.
 The first bit is the least significant bit and the last bit is the most
 significant bit. The next column shows the commands to be applied to the
 cell at the instants 0, a, b, c. At the instant 0, the first bit is used.
 At the instant a, the second bit is used, at the instant b the third bit
 is used and at the instant c the last bit is used. If the bit equals 0,
 the cell is erased E and if the bit equals 1, the cell is illuminated A.
 The last column gives the period of illumination of the cell as a function
 of the frame time T=NTb for each tone level. The illumination times may
 thus take 16 different values ranging from 0 to NTb.
 Let us take the example of a conventional plasma panel screen comprising
 480 rows of cells processed 50 times per second with a frame time of 20
 ms. If it is desired to display an image with four half-tones on this
 screen, all the rows of the screen need to be processed twice in 20 ms.
 The row cycle is equal to: Tl=20/(480.2)=20.8 .mu.s.
 This period Tl is just enough to carry out an addressing phase. No
 sustaining phase is added. This period corresponds to the time taken to
 carry out a semi-selective operation followed by a selective operation.
 In this method, the scans are interleaved. FIG. 3 shows the succession of
 scans and the rows processed with the method. Four interleaved scans are
 performed. They are referenced B1, B2, B3, B4. The time taken to process a
 row is Tl.
 The period Tl of the first scan B1 which processes an n-order row in of the
 screen is followed by another period Tl of the second scan B2 which
 processes a p (p.noteq.n) order row lp, then another period Tl of the scan
 B3 which processes a q-order row lq and then again, during another period
 Tl, the scan B4 which processes an r-order row lr. The operation is then
 resumed with the scan B1 which processes the row ln+1, then the scan B2
 which processes the row lp+1, etc. The image is displayed when all the
 rows have been processed once by each scan. Thus, each scan processes the
 panel row by row in an ordered way. Each row 1 will have been scanned four
 times by the scans B1, B2, B3, B4 at successive instants corresponding to
 the graph of FIG. 1.
 FIG. 4 shows an exemplary view of the operations for scanning a screen of a
 plasma panel having eight rows, on which it is desired to display eight
 half-tones by the known method. The choice of the same number of rows and
 half-tones is but a coincidence. Three interleaved scans are made. The
 time axis is divided into 24 periods corresponding to 24 row cycles Tl
 numbered 1 to 24. Each processing of a row comprises a semi-selective
 operation E (erasure for example) followed by a selective operation l
 (recording for example). The two operations take place for one and the
 same row during the same row cycle Tl.
 This figure does not show the basic square-wave signals which are applied
 simultaneously to all the cells but only the addressing square-wave
 signals which correspond to the semi-selective and selective operations.
 It can be ascertained that, for one and the same row, the time intervals
 between two successive processing operations increase in geometric
 progression. For the first row for example, the interval between the first
 two processing operations corresponds to one-seventh of the frame time T.
 The next interval is 2T/7 and the interval that follows is 4T/7.
 FIG. 5 shows the distribution in time of the operations for processing a
 row with the method according to the method, this method enabling the
 adjusting of the overall luminosity in the case of a half-tone display.
 According to the invention, for a row each processing operation comprises
 a semi-selective operation followed by a selective operation. For at least
 one given processing operation, the selective operation is separated from
 the semi-selective operation by a time interval that is weighted with
 respect to the time interval between the beginning of this processing
 operation and the beginning of the next processing operation.
 It has been assumed in FIG. 5 that for the row considered the
 semi-selective operations always take place at the instants 0, a, b, c, 1,
 1+a, 1+b, 1+c, etc.
 The selective operations then take place respectively at the instants:
EQU 0'=k(a-0)
EQU a'=k(b-a)
EQU b'=k(c-b)
 c'=k(1-c)
 Where k is a constant ranging from 0 to 1, used as a weighting parameter to
 adjust the overall luminosity of the screen of the panel. The value of k,
 in this example, determines the time intervals during which the cells are
 forced into the extinguished state.
 The delay between the selective operation and the semi-selective operation
 is proportional to this parameter k and to the time interval between the
 beginning of the semi-selective operation of the processing operation in
 progress and the beginning of the semi-selective operation of the next
 processing operation. The different values of the delay therefore increase
 in a geometrical progression as the intervals between the beginning of the
 different processing operations.
 FIG. 6 assembles, in one table and for each tone level (16 possibilities),
 the state of a cell at the instants 0', a', b', c' and the period of
 illumination or luminance of the cell, this cell being controlled by the
 method according to the invention. It can be verified that the progression
 of the periods of illumination is kept. The overall luminosity of the
 screen of the panel is modified in a ratio of 1-k.
 FIG. 7 resumes the example of an eight-row screen displaying eight
 half-tones to which there is applied a method for the adjusting of the
 overall luminosity according to the invention. Interleaved scans are used
 to address the cells of the screen.
 However, in this case to obtain 2.sup.N half-tones, there are N scans
 needed, each scan being formed by two sub-scans. N sub-scans B1, B2, B3, .
 . . , BN of a first group carry out semi-selective operations and N
 sub-scans B'1, B'2, B'3, . . . , B'N of a second group carry out selective
 operations. There is a decorrelation between the sub-scans that generate
 the erasure and the sub-scans that generate the recording. It is assumed
 that a row cycle corresponds to the time taken to carry out a
 semi-selective operation followed by a selective operation.
 FIG. 9 shows a view, in time, of the succession of sub-scans that process
 the rows of the screen with the method according to the invention.
 During the first half of the first row cycle Tl, the first sub-scan B1 of
 the first group achieves a semi-selective operation on the n-order row ln.
 During the second half, the first sub-scan B'1 of the second group
 achieves out a selective operation on the m (m.noteq.n) order row lm.
 During the first half of the second row cycle Tl, the second sub-scan B2 of
 the first group achieves a semi-selective operation on the p (p.noteq.n)
 order row lp and during the second half of the second row cycle Tl, the
 second sub-scan B'2 of the second group achieves a selective operation on
 the q (q.noteq.m) order row lq. The succession of the sub-scans is carried
 out in this way until the last sub-scan B'N of the second group which
 carries out a selective operation on the s-order row ls. Then, during the
 first half of the following row cycle, the first sub-scan B1 of the first
 group achieves a semi-selective operation on the n+1 order row ln+1. The
 image is displayed when each sub-scan has processed all the rows at least
 once.
 In the example shown in FIG. 7, during the first half of the first row
 cycle Tl, the row 11 is erased by the sub-scan B1 and during the second
 half of the first row cycle Tl the row 17 is written on by the sub-scan
 B'1. During the next cycle Tl, the row 18 is erased by the sub-scan B2 and
 then the row 11 undergoes recording by the sub-scan B'2. During the third
 row cycle Tl, the row 16 is erased by the sub-scan B3 and then the row 15
 undergoes recording by the sub-scan B'3. During the next cycle Tl, the
 sub-scan B1 erases the row 12 and then the sub-scan B'1 achieves a
 recording on the row 18. During each row cycle, a row is addressed
 semi-selectively and then another row is addressed selectively. Each
 sub-scan achieves an ordered processing, either in erasure or in recording
 mode, of all the rows of the screen.
 In FIG. 7, the weighting parameter k is chosen to be equal to 0.3. This
 gives an overall luminance of 70% of the maximum luminance.
 The dynamic range of the adjustment is limited by the smallest possible
 variation in luminosity .DELTA.l.
 ##EQU1##
 N is the number of scans
 and NL is the number of rows.
 The dynamic range of adjustment is equal to the ratio of the maximum
 luminance to the minimum luminance. The minimum luminance is approximately
 equal to the product of the maximum luminance and of deltal.
 In a panel with NL=480 and N=2, giving four half-tones:
 ##EQU2##
 The dynamic range of adjustment is equal to 160. In other words, the method
 according to the invention makes it possible to obtain 160 different
 levels of luminance.
 In a screen with NL=512 and N=8 giving 256 half-tones:
 ##EQU3##
 The dynamic range of adjustment is equal to about 2.
 In the above description, it has been assumed that the method according to
 the invention is applicable to all the rows of the screen. It is of course
 possible to apply it only to a part of the screen, for example to a
 half-screen. Only the rows of this part will be processed by the method
 that has been described.
 FIG. 8 gives a schematic view, by way of a non-restrictive example, of an
 alternating plasma panel 1 to which the method according to the invention
 can be applied. This panel 1 has column electrodes X1 to X8 orthogonal to
 the row electrodes Y1 to Y8. Each intersection between a column electrode
 and a row electrode defines a cell C which represents a pixel. The panel 1
 has eight rows (L1 to L8) and eight columns (C1 to C8) giving 64 cells C.
 There could be many more or far fewer cells C. The row electrodes Y1 to Y8
 are connected to an addressing device 2. This device superimposes
 addressing square-wave signals on a sustaining signal made in the form of
 basic square-waves that are always present on all the rows, for the
 semi-selective erasure command or for the selective recording command
 applied to the addressed row or rows.
 The column electrodes X1 to X8 are also connected to an addressing device 3
 which makes a selective application, during the recording command, of
 masking pulses solely to the columns which correspond to the cells C that
 do not have to be subjected to writing.
 The synchronization between the signals applied to the row electrodes Y1 to
 Y8 and to the column electrodes X1 to X8 is symbolized in the figure by a
 control and synchronization device 4 which is connected to the two
 addressing devices 2 and 3.
 The control and synchronization device 4 receives firstly the number of the
 row to be erased from a generator 5 of the sequencing of the
 semi-selective operations (erasure) and secondly the number of the row to
 be written on from a generator 6 of the sequencing of the selective
 operations (recordings).
 These two sequencing generators may be formed by read-only memories as
 shown in FIG. 8. At least one sequencing is memorized in the sequencing
 generators. For example, a single sequencing may be memorized in the
 generator 5 for the sequencing of the erasures and several different
 sequencing operations may be memorized in the generator 6 of the
 sequencing of the recordings, each sequencing corresponding to a different
 level of overall luminosity. It is enough to choose the desired level of
 luminosity by means of a luminosity control device 7 placed at the
 disposal of the user. This control device may be a selector switch or any
 other equivalent system. It is connected to the generator of the
 sequencing of the recordings 6. This control device makes a selection, in
 the read-only memory of the generator 6, of the zone in which the
 sequencing of the rows to be written on is stored in order to obtain the
 desired level of luminosity. It is of course possible, conversely, to
 provide for only one sequencing memorized in the generator 6 of the
 sequencing of the recordings and several sequencings memorized in the
 generator 5 of the sequencing of the erasures. The luminosity control
 device 7 would then be connected to the generator of the sequencing of the
 erasures.
 In all the examples described, it has been assumed that the semi-selective
 operation corresponds to an erasure and that the selective operation
 corresponds to a recording. It has thus been possible to adjust the
 luminosity of the written information on the display panel by adjusting
 the weighting parameter k. It would of course be possible for the
 semi-selective operation to correspond to a recording and the selective
 operation to an erasure. Thus, the luminosity of the background of the
 display panel screen would be adjusted by adjusting the weighting
 parameter k.
 The examples described relate to alternating type plasma panels. The method
 according to the invention can also be applied notably to liquid crystal
 panels or certain electroluminescent panels. Liquid crystal panels do not
 themselves produce light but work in transmission and modulate the light
 of a source before which they are placed. By applying the method according
 to the invention to these panels, the transmission time of the light is
 adjusted in order to adjust the overall luminosity.