GB2061589A - Electroluminescent video apparatus - Google Patents

Abstract

Video display apparatus comprising a thin film hysteretic electroluminescent panel 18 has line Y and column X orthogonal sets of matrix electrodes. The Y electrodes receive sustain pulses from 14 cyclically replaced in scanning order by line selection pulses from 16 and erase pulses from 15, all fed via line driver 17. Video signals 6 sampled and digitised 8 are fed via X driver 12 to the X electrodes where they combine with the line selection Y pulses to produce modulated write pulses across elements. To provide a frozen display, both components of the write pulses and also the erase pulses are suppressed at the end of a complete scan, leaving only the sustain pulses. <IMAGE>

Description

SPECIFICATION Video apparatus The present invention relates to video apparatus and more particularly to video apparatus having a display screen comprising a thin-film electroluminescent panel.

Thin-film electroluminescent panels having inherent memory characteristics are wellknown, and details of the construction and operation of such a panel are disclosed in United States Patent No. 3,975,661. It has previously been proposed to use in electroluminescent panel as the display screen of a video apparatus.

In accordance with the present invention there is provided video apparatus having a display screen comprising a thin-film electroluminescent display panel having inherent memory characteristics, and a drive for the display panel, the drive being operable in a moving picture mode to cause the panel to display successively changing images, and in a steady picture mode to cause the panel to maintain the display of a single image.

Arrangements embodying the invention will now be described with reference to the accompanying drawings, in which: Figure 1(a) and 1(b) areafragmentary perspective view and a cross-sectional view of a thin-film ELP used in a TV receiver in accordance with the present invention; Figure 2 is a graph showing the hysteresis properties in the applied voltage vs brightness characteristics of the thin-film ELP; Figure 3 shows the electrode pattern of the thin-film ELP; Figure 4 is a time chart showing the relationship between waveforms of voltages applied to respective electrodes of the thin-film ELP and ones of luminescence; Figure 5 is a graph showing the perception characteristics of the human eye; Figure 6 is a block diagram of one preferred embodiment of the present invention;; Figure 7 is a representative example of a display signal generator and an electrode enabling circuit in the preferred embodiment of Fig. 6; Figure 8 is an example of a field control signal generator within a timing control of the preferred embodiment; Figure 9 is a moving picture and steady picture selector within the timing control of the preferred embodiment; Figure 10 is a block diagram of a pulse width generator used in the production of display signals applied to respective electrodes XI-Xm in the preferred embodiment; Figure 11 is a representation for explanation of an interlaced scanning operation used to scan the matrix ELP of the preferred embodiment; Figure 12 is a time chart showing the relationship between respective write and erase periods during the operation of the preferred embodiment;; Figure 13 is a time chart for explaining how selection from a moving picture mode to a steady picture mode occurs in the preferred embodiment; Figure 14 is a time chart associated with another preferred embodiment of the present invention; Figures 15 and 16 are a block diagram and a time chart for a further preferred embodiment; and Figures 1 7 and 18 are time charts showing how sustain pulses may be applied in the embodiments of the present invention.

Attention is initially invited to the construction and properties of a thin-film ELP. In Figs.

1 (a) and 1(b), a predetermined number of first strip-like transparent electrodes 2 are juxta- posed on a glass support 1. On the transparent electrodes 2 there are disposed a layer 3 of dielectric material, for example Y203, a layer 4 of electroluminescent material doped with active material serving as luminous centers, for example, ZnS:Mn, and a different layer 3' of the same dielectric material as above, by a well known deposition technique such as vacuum evaporation and sputtering, forming the three-layer structure ELP. In addition, a predetermined number of second striplike transparent electrodes 5 are disposed normal to the above mentioned transparent electrodes 2.When a suitable voltage is applied between a selected one of the first electrode family 2 and a selected one of the second electrode family 5, only a very limited area at the intersection releases light (luminescence).

This area corresponds to a picture element.

The ELP will release light with a fixed intensity each time voltage is applied so long as the applied voltage remains constant, whereas the brightness will exhibit hysteresis loop characteristics when the applied voltage is varied. The hysteresis properties will be apparent from Fig. 2. Upon initial application of a pulse with a voltage amplitude V1 as seen from Fig. 2(a) the brightness will be at a level B1 of Fig. 2(b). It will be noted that V, > Vth wherein Vth is a given threshold voltage level.

If a suitable write voltage V2 is applied under these circumstances, then the brightness will suddenly jump to the maximum level B3.

Subsequently, provided that the applied voltage is restored to a sustain voltage level V1, the light intensity will settle at B2, which is higher than BI. Upon subsequent application of an erase voltage V3 the intensity significantly drops. The brightness will return to B1 if the sustain voltage V1 is reached again. The history of the hysteresis loop behaviour can be traced by observing the symbols t, t3... t21 in Fig. 2(c). It is understood that the hysteresis loop of Fig. 2(b) may vary in accordance with the amplitude and width of the write voltage. -This makes it- possible to provide a half tone display using amplitude and/or pulse-width modulation.

In-order to provide TV images through the utilization of an ELP with hysteresis properties, the individual electrodes are disposed in the form of-n lines by columns as seen from Fig. 3 such that one electrode family, e.g., column electrodes X1-Xm is sup-plied with two-valued or multi-tone display signals and the other electrode family, for example, line electrodes Y1 -Y,,, with write control signals, erase control signals, and sustain signals ef -fective to-maintain the display stage.

Figs. 4(a) through 4(c) are time charts pf voltage waveforms V,1, Yyi and Vf, A develop ing at the specific electrodes X" Y- and across the picture element (i, # at their intersection when a display signal is impressed on only that picture element. Fig. 4(d) depicts the luminescence released from the picture ele ment (r, # in response to the applied vóltage.

In fig. 4 the- reference # designates a write pulse, the reference # a sustain pulse, the reference 0 an erase pulse, the reference # a display signal pulse applied to a specific column electrode, and the reference # a line selection pulse. It will be noted that the.write pulse # is one that is developed across the electrodes due to a potential difference be- tween the display signal pulse # and the line selection- pulse .

The- lines Y1-Yn are sequentially scanned using line selection pulses and erase pulses while the write pulses are supplied to the respective lines X1-Xm to provide a display.

There will now be described an embodi- ment of the present invention. In this embodi- ment, the application of the write pulses and the erase pulses to the thin-film ELP occurs during respective write periods- and erase periods. A steady picture is available by suppress -ing: both the write pulses and the erase pulses, while continuing the application of the sustain pulses.

The present embodiment takes advantage of one the vision properties of the human eye, viz., that the relationship between the luminescence intensity B-and the critical fusion frequency CFF where a flicker is percepti ble to the human eye varies with the persis- tence period # of an electroluminescent panel, as illustrated in Fig. 5. The longer the persis tence period # the lower the critical fusion frequency CFF as long as the brightness is constant. The present embodiment provides a moving picture with no or little flicker by making the ratio of the enabling period (i.e.

the period for which a picture element emits light) to the disabling period (i.e. the period for which the element is erased) relatively large.

The preferred embodiment which uses an interlaced scanning operation, will bb de scribed in more detail with reference to the block diagram of Fig. 6.

A comppsitive TV;signal -received at a signal input terminal 6 is separated into a video signal and a.synchronizing signal by a signal separator 7. These signals are sent to a signal processor 8 and a timing control 9, respec- tively. The signal processor-8-comprises a sample/hold circuit and an analog-to-digital converter, whereby the video signal is sequen- tially sampled with clock signals and con- verted into, for rxample, a 3-bit digital signal representative of any of eight brightness levels B0-B1,.. B7. - -- The analog-to-digital converter Within the signal processor circuit -8 develops digital sig nals-of 3 hits A, B and C representative of eight tone levels pursuant to the following table: TA8'LE, brightness level A B C B0 0 0 0 B1 0 0 1 B2 0 1 0 B3 0 1 1 B4 1 0 0 B5 1 1 0 1 B6 1 1 1 0 B7 1 - 1- --1 A signal hold circuit 10 is a-shift register which holds each of. the 3 bit brightness signals for the respective X line electrodes X1Xm of the matrix display panel 18.A display signal pulse generator 11 generates the dis play signal pulses which are modulated in accordance with the above described digital brightness signals. A representative example of the generator 11 is shown in Fig. 7 wherein only that part of the generator associ ated with a single electrode line is shown for the sake of convenience. -A pulse width generator 19 (shwon in more datail in Fig. 10) provides pulses of eight different widths on the respective output lines t0, t1, t2, ... t7. A multiplexer 29 provides at the output terminal a thereof pulses c.orresponding to the con tents of the 3 bit- brightness signals Ai; Bi and Ci. An X electrode driver 12 drives a particu lar one of the electrodes X1, X2 ... Xn of the panel 18 in response to the display signal, pulse Meanwhile, a sustain pulse- generator 14, an erase pulse generator 15 and a line selec- tion pulse generator 16 generate respectively the sustain pulse, the erase pulse and the line selection pulse already discussed with respect to. Fig. 4. Those pulses are supplied to Y electrodes Y1, Y2, ... Yn through a Y column electrode driver 7 upon instructions from the timing control 9. The time -relationship whenthis occurs will be described below. The tim ing control 9 provides various control signals in accordance with the clock signals from the clock generator 13 and the TV synchronizing signals. The timing control 9 includes two major important components, a field control signal generator and a moving picture and steady picture selection control.

The field control signal generator provides sequentially an odd field erase interval signal EFO, an odd field write interval signal WFO, an even field erase interval signal EFE and an even field write interval signal WFE within each frame. A suitable generator for accomplishing this is shown in Fig. 8, and comprises a vertical synchronizing pulse input terminal 21, D type flip-flops 22, 23 and AND gates 24, 25, 26, 27.

The moving picture/steady picture selection control provides a control of the supply of erase pulses and write pulses at a predetermined timing pursuant to the operation of a selector knob; an example of a suitable control is shown in Fig. 9.

This example includes a selection switch 28, an RS type flip flip 29 for storing whether the steady picture mode has been selected, an OR logic gate 30 for sensing whether the erase interval signal EFE or EFO is present, a D type flip flop 31 the output of which is inverted in synchronism with the output of the OR logic gate 30 following the selection of the steady picture mode, an AND logic gate 32 controlling the supply of erase pulses, an AND logic gate 33 controlling the supply of line selection pulses, and an AND logic gate 34 controlling the supply of display signal pulses.

In the following description as to operation, the moving picture mode is called "a dynamic mode (DM)" and the steady picture mode "a static mode (SM)". The DM will be described first.

As the illustrated embodiment carries out interlaced scanning, the electrodes Y1, Y2 ... Yn of Fig. 6 are treated as being divided into the odd electrode lines Y01, Y02, . YON and the even electrode lines YEl, YE2, . . YEN (Fig. 11). In other words, Y1 = Yo1 Y2 = YEl, Y@ 1 = YON, and Y@ = YEN, where N = 2N.

In some conventional TV broadcasting systems, an odd field interval and an even field interval alternate every 1/60 seconds, so that an image is displayed at a rate of 30 frames per second. In contrast, in the present embodiment of the invention, a display rate of 15 frames per second is available and each of the two fields per frame is divided into a write field WF and an erase field EF.

Fig. 12 represents the four fields EFO, WFO, EFE and WFE, and the enabling periods and disabling periods for each line. The symbol 0 denotes when the write pulse is to be applied and the symbol X when the erase pulse is to be applied. The solid line denotes the enabling period TB and the broken line the disabling period TN.

During the first horizontal scanning period (1 H = 63.5 s) in the odd field write interval WFO, the video signals associated with electrodes X,-Xm are sampled in sequence and held by the signal hold circuit 10. After completing the sampling operation for the first horizontal line, the display signal pulses are supplied to the electrodes X1, X2, ... Xm at the same time. Concurrently, the line selection pulse is supplied to the scanning electrode Yot. The result is that the write pulse is applied to the m picture elements on the scanning electrode Y01, permitting those elements to produce electroluminescence at brightnesses corresponding to the video signals.

Subsequent to this, the sampling of the video signals and loading into the signal hold circuit are executed during the next horizontal scanning period so that the picture elements on the scanning electrode Y02 emit light. This is repeated until the scanning electrode YON has been selected.

Once the picture elements have been supplied with the write pulse, they are held in the electroluminescent state by sustain pulses until an erase pulse is applied. After the odd field write field interval WFO the odd scanning electrodes Y01, Y02, . . YON are supplied with the sustain pulses until the next succeeding odd field erase interval EFO which appears 3 X 1/60 seconds after the beginning of the preceding odd field write interval WFO. At that stage the erase operation is carried out successively for each electrode in the order of Y01, Y02, ... Y0N in the same way as in the write mode.Since the elements on the odd scanning electrodes Yo1 Yo2 YON are thereafter caused to emit light in response to the next video signal during the odd field write interval WFO immediately after the erase interval EFO, that is, after 1/60 seconds, all the picture elements belonging to the odd scanning electrodes go on emitting light for 3 X 1/60 seconds and remain disabled for 1 /60-seconds. The above mentioned operation is repeated every 1/15 seconds, to permit movement of the displayed image.

The write mode is carried out for the even scanning electrodes UE1, YE2, . . . YEN during the even field write interval WFE. After those picture elements are held in the enabled state for 3 x 1/60 seconds, they are erased in the even field erase interval EFE and held in the erased state for 1/60 seconds. The next succeeding image is written-in during the next succeeding even field write interval WFE. For all the picture elements, therefore, the threefield enabling period and the one-field disabling period are executed alternately.

If the viewer desires to turn the TV receiver from the DM to the SM, then the switch 28 is turned ON so that the RS type flip flop 29 is immediately set and the D input to the D type flip flop adopts the "H" level. Under these circumstances, when the odd field erase interval signal EFO or the even field erase interval signal EFE appears, the D type flip flop 31 is triggered so that the output Q thereof changes from the "H" level to the "L" level. Therefore, the AND logic gates 32, 33, 34 are disabled, thus preventing the erase pulses and the write pulses from being supplied to the line electrodes Y01-YEN; also, subsequent video signals are prevented from being supplied to the signal hold circuit 10. It will be apparent from Fig. 4 that the display image is held on the matrix panel 18 by the sustain pulses.

Fig. 13(a) shows the sequential events where the mode is about to change from DM to SM in response to the viewer's instructions at time Sz or S2 within the field interval EFO or WFO, and Fig. 13(b) shows the events when changing from DM to SM responsive to the viewer's instructions at time S3 or S4 within the field interval EFE or WFE. When all the picture elements on the odd or even scanning electrodes have been written-in, the display will come to a standstill, ensuring an excellent steady image constantly.

Contrarily, if the mode is to change from SM to DM, it is better that DM is restored just prior to the field interval EFO or EFE. This is because a failure to do so results in disturbing an image even for a very short period of time.

Another reason is that to enable the selection from DM to SM and from SM to DM to be carried out by a common circuit construction.

However, it is not absolutely necessary that the viewer's mode selection from SM to DM be synchronous with the circuit operation because the transition condition lasts only several fens of milliseconds and the resulting disturbance in the displayed image is thus not perceptible to the viewer's eye.

One of the major features of the present embodiment is that the write field and the erase field are repeated alternately and the line scanning operation is carried out using a common circuit configuration for both write and erase operations, ensuring the simplicity of the enabling circuit and the control circuit.

Since the write pulses and the erase pulses are not crowded within the same period, this allows freedom in selecting the number and pulse width of the respective pulses. This facilitates adjustment in image quality. The enabling period can be selected to be much longer than the disabling period, thereby achieving a high brightness TV screen. Moreover, since TV images transmitted at a rate of 30 frames per second can be displayed for a period of 3 X 1/60 seconds, it is possible to avoid flickering effects. In the inventor's experiments, satisfactory TV images were ob- tained without any flickering effects.

As noted already, the ELP is enabled at a duty ratio of 3/4 and with an image display rate of 15 frames per second in the above disclosed embodiment as best seen from Fig.

12.

It is well known in the art that each frame of a TV image consists of a first field and a second field. In another preferred form of the invention, video signals are written in the line sequential manner while performing the interlaced scanning operation. Within the first field of the TV signal, the write mode is carried out on the odd lines of the ELP and at the same time the erase mode is carried out on the even lines thereof. Within the succeeding second field of the TV signal, the erase mode is carried out on the odd lines and the write mode is carried out on the even lines. This is illustrated in Fig. 14. The display state of the ELP is maintained by sustain pulses during the period of time from application of the write pulse until application of the erase pulse. Using such a system it is possible to eliminate flickering effects and attain a higher rate of image display.

In some situations it is preferred that the pulse width of the sustain pulse is different during the Dm from the width during the SM, and Figs. 1 5 and 16 illustrate another preferred embodiment of the present invention wherein the shaded blocks are modified to accomplish this. A circuit 14' provides a moving picture/steady picture selection signal, a circuit 16 provides a selection pulse timing signal, and a circuit 1 5 provides an erase pulse timing signal. A composite circuit 1 8 mixes the selection pulse and the erase pulse together for the line driver. A sustain pulse timing signal generator 14 provides a sustain pulse of which the pulse width is different between in the moving picture mode and in the steady picture mode.The signal from the generator 14 is applied to the line driver 1 7.

The outputs of the line driver are supplied to the line electrode group Y1-Yn.

In the case where the TV screen panel is relatively large, with a consequently large number of electrodes, there is relatively little time for the application of the sustain pulses.

The reasons for that will be apparent from Fig. 16 which shows the waveforms when there are 1 60 vertical scanning electrodes. In Fig. 1 6, the write pulse is applied after each sampling operation, these operations being executed about every third the horizontal scanning period. Fig. 16(a) represents the horizontal synchronizing signal PH, Fig. 16(b) represents the signal PS indicative of the sampling period, and Fig. 16(c) represents the write selection pulse signal. There are approximately 240 horizontal synchronizing signals within each field of the TV signal. Of all those synchronizing signals, 80 signals are extracted from one frame for the odd lines and another 80 signals are extracted from a different frame for the even lines.The vertical scanning electrodes of the ELP are supplied with the scanning pulses in the interlaced manner.

In this instance, the period of time for applying a write pulse is at most approxi mately 1 20 ILsec. If both positive and negative polarity sustain pulses were to be applied between write.pulses, this would take longer than the permissible period PS (approximately 63.5 ILsec), whereas if only the positive or negative polarity sustain pulse is applied this would result in nonuniformity in the bright ness.

In addition, if the write pulse Vw is not applied between the sustain pulses V51 and V, of Fig. 17(a) but between the sustain pulses V52 and V53 as shown in Fig. 17(b), it will prevent a valid sustain pulse from being applied immediately after application of the write pulse Vw.

In another preferred embodiment the pulses are arranged as shown in Fig. 18, in which the sustain pulses are supplied in oppositepolarity pairs Vs1, V, and V,, V,, and the spacing between the sustain pulses V51 and V, and the one between the sustain pulses Vs3 and V, are short, while the spacing between the sustain pulses Vs2 and V, is much longer to provide more room for the write pulse Vw.

The video apparatus described above is also described, and certain aspects thereof claimed, in our co-pending parent application No. 36830/78.

Claims (6)

1. Video apparatus having a display screen comprising a thin-film electroluminescent display panel having inherent memory characteristics, and a drive for the display panel, the drive being operable in a moving picture mode to cause the panel to display successively changing images, and in a steady picture mode to cause the panel to maintain the display of a single image.

2. Apparatus as claimed in claim 1, in which the drive is operable during the moving picture mode to supply to the panel write pulses for writing-in an image, erase pulses for erasing an image, and sustain pulses for maintaining the display state of the panel, the drive in the steady picture mode being prevented from supplying said write and erase pulses.

3. Apparatus as claimed in claim 2, wherein the drive is arranged so that the widths of the sustain pulses when the drive is in the steady picture mode differ from the widths when the drive is not in said mode.

4. Apparatus as claimed in claim 1 or claim 2, wherein the drive is arranged so that it enters the steady picture mode only when a complete image is written-in to said panel.

5. Apparatus as claimed in any preceding claim, in which the drive is operable to cause the panel to display an image having a plurality of brightness levels.

6. Apparatus as claimed in any preceding claim, wherein the drive is arranged to supply pairs of opposite-polarity sustain pulses, the intervals between successive pairs of pulses being longer than the period between successive pulses in each pair, the drive being arranged to supply write pulses in the intervals between successive pairs of sustain pulses.