Abstract:
An improved driving system and method for shifting a discharge spot from a given discharge cell to an adjacent discharge cell in an A.C. gas discharge, or plasma display, panel, having opposed sets of electrodes respectively covered with corresponding dielectric layers thereby insulated from the discharge gas space. One set of electrodes comprises common electrodes extending in parallel relationship and defining the direction of propagation of the shifted discharge spots. The other set of electrodes, spaced apart from the first set by the gas space, extends in parallel relation transversely to the first common electrodes, and comprise the shift electrodes. The shift electrodes are arranged in groups of a predetermined number in each group and a corresponding number of buses permits selective application of shift pulses to the respectively associated shift electrodes in sequence for each group and simultaneously for the successive groups. An improved operating margin for the shift function is achieved by utilizing the priming discharge effect of a given cell currently discharging, to reduce the necessary firing voltage at an adjacent discharge cell to which the current discharge spot is to be shifted, while minimizing the probability of misfiring at corresponding, remote cells energized in the same phases. An overlap pulse is applied to a discharge cell at which a discharge spot currently is established, to provide a priming discharge for the adjacent cell to which the spot is to be shifted. A shift pulse applied to the adjacent cell terminates after termination of the overlap pulse and thereby produces a lateral field between the two adjacent discharge cells whereby the space charge generated by the priming discharge is attracted to and reduces the necessary firing voltage at the adjacent cell. The duration of the overlap pulse in relation to the amplitude of the shift pulse defines an operating margin for the shift operation which is optimized over a preferred range of the overlap pulse duration.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an improved system and method for shifting a discharge spot in an A.C. type discharge panel and, more specifically, to such system and method for improving the shift operation, including maximizing the shift operating margin. 
     2. Description of the Prior Art 
     Gas discharge panels known in the prior art as Self-Shift Plasma Display Panels, provide the function of shifting the discharge spot from a given cell to an adjacent cell. An example of such a panel is described in detail in U.S. Pat. No. 3,944,875 -- Owaki et al. assigned to a common assignee herein. Briefly, the self-shift plasma display panel described in this patent has an electrode configuration corresponding to that shown herein in FIG. 1. 
     In the prior art panel arrangement of FIG. 1, common electrodes y1 to y5, extending in a horizontal direction, are connected to a common bus yc in turn connected to a terminal Y. Although not illustrated in FIG. 1 to permit clarity of illustration of the electrode structure, the common electrodes y1-y5 are deposited on a substrate and have extending thereover a dielectric layer to insulate same from the discharge space. The panel further includes a second set of shift electrodes arranged in groups and illustrated in FIG. 1 by a1-d1 comprising a first group, a2-d2 comprising a second group, . . . and an-dn comprising an nth group. The corresponding electrodes a of each group are connected in common to a bus BA connected to a terminal A. Similarly, buses BB, BC, and BD are connected to the corresponding electrodes &#34;b&#34;, &#34;c&#34;, and &#34;d&#34;, and in turn are connected to corresponding terminals B, C and D, respectively. As is apparent, the shift electrodes a, b, c, and d of all of the groups extend in generally parallel relation transversely to the common electrodes y and are formed on a second substrate as well including a dielectric layer overlying the electrodes and insulating same from the gas discharge space. The intersections of the shift electrodes and the common electrodes define discharge cells. Sequential application of shift voltages to the terminals A, B, C, and D causes shifting of a discharge spot from a given discharge cell in sequence to the respectively next adjacent cells as defined by the corresponding shift electrodes extending along the associated common electrode. Due to the respective, sequential application of the shift voltages to the terminals A to D, the shift voltages are frequently identified as being of phases A to D. Hence, the corresponding electrodes of each group receive the same phase shift voltage simultaneously. 
     The panel of FIG. 1 additionally includes a set of write electrodes w1 to w5 connected to corresponding terminals W1 to W5, respectively, which are disposed closely adjacent the initial shift electrode a1 of the first group of the set of shift electrodes. The write electrodes w1 to w5 respectively correspond to the common electrodes y1 to y5. The write electrodes would be provided on the same substrate and insulated from the gas discharge space by the same dielectric layer as that employed for the shift electrodes. 
     The panel, as is well known, is sealed and filled with an ionizable gas. 
     Accordingly, as above noted, each of the intersections of the write electrodes and the shift electrodes with the various common electrodes defines a discharge cell. Once a discharge, or discharge spot, is initiated in a given discharge cell, it can be shifted along the direction of the associated common electrode by the sequential application of shift pulses to the successive shift electrodes of each group, and for the successive groups of shift electrodes. Thus, the write electrodes are used to create initial discharges corresponding to information desired to be displayed and those discharges are shifted to desired positions in the panel or, in some cases continuously shifted throughout the length of the panel. New information may be written in for display as the previously written information is advanced, by shifting across the panel. 
     Where display of the information written into the panel is desired to be maintained at a given panel position, the shift voltage pulses may be supplied continuously to the shift electrodes at which the discharge spots displaying that information currently have been shifted, or in accordance with a further technique, a shift voltage may be applied alternately to two adjacent ones of the successive shift electrodes by application of alternate shift voltage pulses to the corresponding buses associated with those shift electrodes. In this regard, the shift voltage pulses may act as sustain voltage pulses to maintain the display. 
     The shifting of the discharge spot in a gas discharge panel, as is well known, is achieved by making use of the so-called priming effect. Specifically, when a discharge spot is generated at a given discharge cell, that discharge has a primary current, or priming effect, on the adjacent discharge cell due to the space charge created by the existing discharge, that space charge being formed of electrons, ions, and metastable atoms generated by the existing discharge at the given discharge cell. This space charge or priming effect serves to lower the firing voltage at an adjacent discharge cell below that level of firing voltage which otherwise would be necessary to create a discharge at the adjacent discharge cell. As a result, the lower limit, or minimum value, of the shift voltage required in a shift operation as above described is determined by this reduced value of firing voltage at the adjacent discharge cell resulting from the priming effect of the current discharge at the given cell. Conversely, this priming effect also sets an upper limit, or maximum value, of the permissible shift voltage pulse level, as determined by the firing voltage of a remote discharge cell receiving the same phase shift voltage pulse, and thus at the same time as the referenced adjacent cell intended to receive the discharge, by virtue of the common bus arrangement, to prevent misfiring, and to assure cancellation of the wall charge of the given discharge cell from which the discharge spot is shifted. 
     Specifically, with reference to FIG. 1, if the given cell illustrated at the circle P1 in FIG. 1 is currently undergoing a discharge and thus maintained in a &#34;ON&#34; condition, holding the discharge spot, the magnitude of the shift voltage pulse to be applied to the adjacent discharge cell defined by the circle P2 to which the discharge spot is to be shifted must be selected to be of a voltage level in accordance with the following conditions. First, the lower limit of that voltage level of the shift pulse must be greater than the firing voltage Vf1 at the discharge cell P2; as noted, the priming effect of the discharge cell P1 will cause a relative reduction in the normal firing voltage level required to create a discharge at the cell P2 in the absence of such priming discharge. Conversely, the shift pulse applied to cell P2 must be less than a level at which a discharge would be created at the cell P2&#39; defined by the corresponding electrode b2 of the adjacent group (2) of shift electrodes. Specifically, the cells P2 and P2&#39; simultaneously receive the same shift pulse phase through their respective shift electrodes b2 and b3 from the common bus BB. The level of the shift voltage which could create a discharge at cell P2&#39; may be defined as Vf3. 
     Thus, under the condition of a given cell P1 undergoing discharge, the shift voltage for shifting that discharge to the adjacent cell P2 requires that the shift voltage be of a level exceeding the minimum firing voltage Vf1 at the adjacent cell P2 to which the discharge is to be shifted, but must be lower than the level Vf3 such as would create a discharge at the remote cell P2&#39; which corresponds to the cell P2 but is in a different group of the shift electrodes. The difference of these firing voltage levels, (Vf3-Vf1) as defined by the adjacent, and the corresponding, remote discharge cells P2 and P2&#39; (i.e., the &#34;corresponding&#34; cells being those energized from a common phase shift voltage over the same shift bus) defines the shift operating margin of the panel. 
     The shift operating margin of the panel thus is a critical condition in prior art shift-type plasma display panels, introducing corresponding difficulties in the manufacture and operation of such panels, including the driving circuitry therefor. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide a shift-type plasma display panel having a substantial improvement, or increase, in the operating margin for the shift operation of the discharge spot. 
     Another object of the present invention is to provide an improved method for shifting a discharge spot which affords stable and reliable shift operations. 
     A still further object of the present invention is to provide a highly effective system and method for shifting a discharge spot from one discharge cell to an adjacent discharge cell defined by two adjacent, successive shift electrodes and a transversely extending, common electrode. 
     Yet another object of the present invention is to provide an improved method and system for an A.C. gas discharge panel having an array of discharge cells defined by spaced electrodes and particularly for a self-shift type of plasma display panel. 
     The present invention accordingly provides an improved method and system for shifting discharge spots in an A.C. type gas discharge panel. Such a panel typically includes one set of common electrodes extending in parallel and disposed on a corresponding substrate and insulated from a gas discharge space by a suitable dielectric layer. A second set of parallel, shift electrodes extends in parallel relation transversely to the first set of common electrodes and likewise is disposed on a corresponding substrate and insulated from the gas discharge space by a suitable dielectric layer. The two transversely related sets of electrodes, thus separated by the gas discharge space and insulated therefrom by the corresponding dielectric layers, define at their intersections, discharge cells. A discharge formed in a given cell between a given one of the common electrodes and one of the shift electrodes then is advanced or shifted to a successive, adjacent cell by the application of a shift voltage of the corresponding, next phase, to that adjacent cell. 
     In accordance with the invention, the priming effect of the currently discharging cell due to its space charge is utilized to advantage in the shift operation for establishing a discharge at the adjacent cell; moreover, an undesired priming effect on a corresponding discharge cell along that same common electrode, but defined by the corresponding shift electrode of a different group of the shift electrodes, is minimized. Specifically, the shift electrodes are arranged in groups of a predetermined number, corresponding ones of the electrodes of each such group being connected to a common shift bus for receiving shift voltages applied thereto. Thus, a shift pulse of the proper phase for shifting the discharge from a given to the adjacent cell is applied simultaneously to the corresponding electrodes of each of the groups, and thus to the remote cells corresponding to that adjacent cell to which the discharge is to be shifted, as defined by corresponding shift electrodes of the other groups. 
     Specifically, in a shift cycle of operation, a shift pulse is applied to the adjacent cell to which the discharge is to be shifted. During the time duration of that shift pulse, an &#34;overlap&#34; pulse is applied to the given cell which currently is discharging and from which the discharge is to be shifted. That overlap pulse is controlled to terminate prior to termination of the shift pulse. There results a lateral field effect between the adjacent cell and the given cell, thereby attracting electrons from the currently discharging cell to the adjacent cell which is to receive the shifted discharge, and promoting the firing, or discharging, at the adjacent discharge cell which is to receive the discharge spot. This termination of the overlap pulse prior to that of the shift pulse also minimizes the undesired priming effect of the existing discharge on the corresponding remote discharge cells and as well assists in the elimination of the wall charge at the given cell from which the discharge is being shifted, thereby to assist in preventing misfires and in general to improve the operating margin of the shift function. 
     The invention will be better understood as to the above and other objects thereof, and as to its details from the following detailed description of one preferred embodiment thereof, taken with reference to the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a typical electrode arrangement of a self-shift plasma display panel of the prior art, and to which the method and system of the present invention may be applied for improving the shifting operations therein; 
     FIGS. 2A and 2B, respectively, show in diagrammatic fashion cross-sectional views of a plasma display panel with respect to a given cell at which a discharge is occurring and an adjacent cell to which the discharge is to be shifted, to facilitate an explanation of the principal of the method of the invention; 
     FIG. 3A, is a waveform plot of the discharge cell potentials and FIG. 3B is a corresponding waveform plot of the pulse voltages OP and SP applied to the shift electrodes and a ground level voltage applied to the common electrode, for the illustrative cells of FIGS. 2A and 2B; 
     FIG. 4 is a plot of the pulse width or duration of the overlap pulse OP with respect to the amplitude of the shift pulse SP for illustrating the relationship therebetween in defining the shift operating margin; 
     FIG. 5A are waveform plots of pulse voltages applied to various electrodes; 
     FIG. 5B is a plot of the cell voltages resulting from the electrode pulse voltages of FIG. 5A, and illustrate the shift operation method of the invention as applied to a self-shift plasma display panel as shown in FIG. 1; 
     FIG. 6 comprises enlarged waveforms corresponding to cell voltages an-y, bn-y, and cn-y of FIG. 5B; and 
     FIG. 7 is a block diagram of a system for driving a plasma display panel in accordance with the method of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Whereas FIG. 1 illustrates a prior art plasma display panel with which the method and system of the present invention may be employed, the invention will be better understood with reference initially to FIGS. 2A and 2B which show in cross section two adjacent cells A and B of a plasma display panel of the type of FIG. 1, for illustrating important discharge conditions occurring during a shift operation in accordance with the method of the present invention. Reference will also be made concurrently to FIG. 3A which illustrates the waveforms of the cell potentials occurring within adjacent cells and to FIG. 3B which illustrates the waveforms of the pulse voltages for producing those cell potentials as applied to the shift electrodes and as well a steady straight ground potential applied to the common electrode with which those discharge cells are associated, in a shift operation. 
     In FIGS. 2A and 2B, a plasma display panel includes a first substrate 10 to which there are applied electrodes x1 and x2 and a covering dielectric 11 and a second substrate 12 to which there are applied a common electrode y with a covering dielectric 13, the dielectric layers 11 and 13 bordering on and defining therebetween a gaseous discharge space, all in accordance with known prior art configurations of such devices. Common electrode y corresponds to any of the common electrodes y1 to y5 of FIG. 1 and electrodes x1 and x2 correspond to any of the adjacent write (w1 to w5) and shift (a1 to d1, . . . an to dn) electrodes of FIG. 1. 
     In FIG. 3B, the pulse voltage OP is applied to the electrode x1 and the pulse voltage SP is applied to the electrode x2, the common electrode y being held at ground potential &#34;0&#34;. Although other arrangements may be employed, it is sufficient that these pulse voltages OP and SP be applied as positive voltages to the shift electrodes x1 and x2 while the common electrode y is clamped to ground potential. There results cell potentials of corresponding waveform configurations as shown in FIG. 3A and specifically cell potentials OP and SP corresponding to the cells A and B as labelled in FIGS. 2A and 2B. 
     As shown in FIG. 3A, pulse voltage OP applied to cell A has a duration or pulse width τ1 and an amplitude or voltage value of V01, while the shift pulse SP applied to the discharge cell B has a pulse duration or width τ2, greater than τ1, and an amplitude level Vs. As later described, the pulse OP need not be initiated simultaneously with the pulse SP; however, for purposes of the present invention, it is important that the pulse OP terminates prior to termination of the pulse SP. 
     Where time t is defined by t 0  &lt;t&lt;t 1 , then, during the common or ovelap period of the two pulses OP and SP, common electric fields are established in the cells A and B as illustrated by the arrows in FIG. 2A. In this condition, and prior to any shift of a discharge spot to cell B, cell A undergoes discharge and information is stored on the dielectric layers associated with cell A in the form of a wall charge. 
     When the time t has advanced to the relation: t 1  &lt;t&lt;t 2 , the potential of the electrode x1 has been reduced to ground level in accordance with the termination of the OP pulse at time t1 = τ1, and there results a lateral field between the shift electrodes x2 and x1 as indicated in FIG. 2B. Cell A which is currently undergoing discharge creates a space charge and the lateral field acts thereupon to attract electrodes from this priming discharge of cell A to the positive potential existing on the electrode x2 by virtue of the positive value of the shift pulse SP relative to the potential of electrode x1 as seen in FIG. 3A. Thus, electrons are attracted from the space charge produced in cell A into the discharge cell B. Thus, the firing voltage of discharge cell B is considerably reduced and, as a result thereof, the discharge spot is shifted readily by the shift voltage pulse SP applied to the electrodes associated with cell B thereby to generate quickly and maintain, stably, the discharge spot shifted from cell A at the adjacent discharge cell B. Thereafter, the wall charge associated with cell A can be removed or eliminated by applying an erase pulse to the discharge cell A. Simultaneously, the discharge spot now shifted to cell B can be maintained or, alternatively, the discharge spot can be shifted sequentially to the next successive discharge cell adjacent cell B (not shown in FIG. 2B), by performing a similar shift operation as above described. 
     Whereas the above shift operations have been described in relation to FIGS. 3A and 3B as being performed using positive going overlap and shift voltages OP and SP, negative such pulses may be employed in the alternative. In this instance, however, the result or effect of the pulse voltages is not as distinctive as that obtained when employing positive pulses. Specifically, in the case of negative pulses, the lateral field formed after the termination of the overlap pulse OP must function to promote the attraction into cell B of positive ions from the space charge generated by the priming discharge in cell A, for example, but the ions thus generated have less mobility than electrons. As a result, a delay in the shift of the discharge spot from cell A to cell B is likely to be encountered. 
     The overlap pulse OP employed in this invention has a pulse width τ1 and a voltage value V01, which values are selected to be sufficient for causing cell A to undergo a priming discharge and thereby to serve as a charge source cell for the shifting operation with respect to the adjacent cell B. 
     From a qualitative view point, the overlap pulse OP must be so selected that its pulse width τ1 is comparatively narrow and is in such relation to the timing that it falls in advance of the termination, or fall, of the shift voltage pulse SP. This serves to suppress the intensity of the priming discharge at cell A and thereby to minimize any undesired and unnecessary priming effect at remote discharge cells corresponding to that adjacent discharge cell B to which the discharge spot is to be shifted; corresponding cells are those associated with the corresponding shift electrodes in other groups thereof which, accordingly, receive simultaneously the same phase of the shift pulse voltage as that applied to the specific adjacent cell B which is to receive the discharge. An undesired priming effect extending to such remote, corresponding cells can cause misfiring, i.e., unintentional firing of those remote cells. Moreover, the relative times of termination of the overlap pulse OP and the shift pulse SP are such as to enhance the priming effect as to the adjacent cell which is to receive the shifted discharge spot, due to the lateral field created by the continuation of the shift pulse SP following termination of the overlap pulse OP. 
     Whereas these qualitative results are significant, in practical operation, quantitative parameters of the respective pulses must be set so as to maximize the shift operating margin as actually experienced in a practical panel, thus taking into account the discharge gap length, the composition of the gas mixture and the gas pressure in the panel shift operation to be driven, and other such characteristics as can be defined and confirmed empirically, i.e., experimentally, or in actual use. 
     FIG. 4 comprises a plot of the overlap pulse width τ1 as a function of the amplitude or level of the shift pulse voltage Vs, and has plotted therein characteristic curves established empirically in experiments for determining the minimum and maximum shift voltage levels Vs min  and Vs max , respectively, the region encompassed therebetween thus defining the shift voltage operating margin. Specifically, the pulse width τ1 of the overlap pulse OP is measured along the X axis while the amplitude, or level, of the shift pulse SP is measured along the Y axis. In this case, the overlap pulse OP and the shift voltage pulse SP are selected in such relation to each other as to rise, or initiate simultaneously and to have the same voltage amplitude value (V01 = Vs); moreover, the pulse width τ2 of the shift voltage pulse SP is selected to be 9μsec. The gas discharge panel subjected to this testing has dielectric layers comprising magnesium oxide (MgO) and has a discharge gap length of 120μm, the gas filling this space being a mixture of 0.1 % Xe with residual Ne, and the Pd value, i.e., the product of gas pressure P and the discharge gap length d, is about 4 Torr.cm. 
     As is shown in FIG. 4, the shift operating margin comprising that range or region between the minimum shift voltage Vsmin and the maximum shift voltage Vsmax is dependent on the pulse width τ1 of the overlap pulse OP, that margin having a distinctive increase at its lower limit. In the specific panel tested, and in view of the experimental conditions indicated, it was observed that the maximum shift operating margin is achieved utilizing an OP pulse width τ of a value in the range of from 2 to 3.5μsec and more preferably about 3μsec, that maximum shift operating margin being about 50% greater than the margin outside of these preferred ranges. 
     It moreover has been determined that the optimum pulse width for the overlap pulse OP does vary as a function of discharge gap length and gas pressure, and particularly that the optimum width increases with an increase of the discharge gap length of the panel and decreases, i.e., becomes more narrow, with an increase of the gas pressure. Thus, for a variety of different panel design conditions, the optimum pulse width τ 1 of the overlap pulse OP has been observed to extend over a range of from 0.3 to 5μsec. 
     Lower limits do exist on the usable overlap pulse width. Specifically, if a pulse width becomes too short, relative to the time delay of establishing a discharge, the probability of achieving a discharge is reduced, and hence, the reduced priming effect reduces the operating margin; conversely, if the overlap pulse width becomes too great, charges generated by the discharge are attracted to the dielectric layers, reducing the amount of space charge, and hence, the beneficial priming effect with respect to the adjacent discharge cell to which the discharge spot is to be shifted. 
     In a panel employed for experiments, a desirable time duration for the application of a pulse to assure establishment of a proper wall charge is from 5 to 6μsec. Therefore, the pulse width τ2 of the shift voltage pulse SP is selected to be of a value greater than 5μsec. 
     Practical operating conditions employed in the experiment included setting the pulse width of the shift voltage pulse SP at 9μsec. and employing a narrow width erase pulse EP having the same voltage value as the shift pulse SP but a pulse width of 2μsec, the erase pulse EP being applied to the discharge cell after completion of the shift of the discharge spot to the adjacent cell. Typically, the erase pulse width is selected to be less than 2μsec. and a desirable range is from 1 to 2μsec. Therefore, in some circumstances, the optimum pulse width of the overlap pulse OP is or may be the same as the pulse width of the erase pulse. 
     A feature of the present invention is that of suppressing excessive priming discharge at the discharge cell from which a discharge spot is to be shifted, while at the same time intensifying the priming effect at the adjacent cell which is to receive the shifted discharge spot, by means of the lateral field. In achieving these desirable results, the invention permits the initiation, or rising, of the overlap pulse, OP to either precede or succeed, or coincide with, the initiation of the shift pulse SP, so long as the termination of the overlap pulse OP precedes that of the shift pulse SP. 
     Moreover, whereas it is desirable to select a voltage value of the overlap pulse OP which is the same as that of the shift pulse SP, especially for practical drive circuit considerations, the present invention also permits reducing the intensity of the priming discharge by employing a voltage value V01 for the overlap pulse OP which is less than that for the shift pulse SP. 
     FIG. 5A comprises a plot of voltage pulses to be applied to the various of the electrodes of the panel in FIG. 1, VW being applied to any selected one of the write electrodes w1 to w5 through their respective terminals W1 to W5, voltage VA-VD being applied to the terminals A-D of the shift buses BA-BD, respectively, and voltage VY being applied to the terminal Y of the common bus Y c  associated with the common electrodes y1 to y5. FIG. 5B illustrates the waveforms of the cell voltages, or potentials, resulting from the voltage pulses of FIG. 5A and specifically with respect to the cells associated with the write electrode shown at w-y, and those associated with the successive groups of shift electrodes as shown at an-y through dn-y, respectively. As to each of FIGS. 5A and 5B, the shift voltage pulses SP and, as well, the associated overlap pulses OP are applied in successive phases to the successive shift electrodes. Moreover, as is apparent from FIGS. 5A and 5B, the write voltage pulses WP, the overlap pulses OP (1, 2, . . . ), the shift voltage pulses SP (1,2, . . . ), the latter also used as the sustaining voltage pulses, and the narrow erase pulses EP are employed for driving the panel. 
     In operation, the write voltage pulse WP supplied to a given one of the write electrodes w1-w5 produces a discharge spot in the cell defined between that selected write electrode and the respectively associated one of the common electrodes y1-y5. Following the write operation, the ensuing overlap pulse OP1 in the VW waveform of FIG. 5A operates in association with the shift pulse SP1 in waveform VA to shift that discharge spot from the cell associated with the write electrode to the adjacent cell along the respectively associated common electrode y and the first shift electrode 21. As shown, the next or succeeding operation is a stabilization cycle in which the same shift pulse SP1 is supplied to that same shift electrode and simultaneously an erase pulse EP is supplied to the write electrode. 
     In the next cycle, overlap pulse OP2 of waveform VA and shift pulse SP2 of waveform VB serve to shift the discharge to the next adjacent discharge cell. The aforedescribed cycles of operation then continue in this manner, sequentially shifting the discharge spot to successive, next adjacent cells along the length of the common electrode. In view of the grouping of the shift electrodes and pulsing thereof through the respective common buses, it is convenient to refer to the overlap and the shift pulses as occurring in successive phases. 
     The cell voltages of FIG. 6 correspond to those identically labelled an-y, bn-y, and cn-y in FIG. 5B, and are enlarged to facilitate additionally showing the wall charge voltages Wca, Wcb and Wcc in the corresponding cells. The overlap pulses OP2 and OP3 are seen to produce a decreased wall charge compared to that produced by the shift pulses such as SP2 and SP3 (or the stabilizing pulses of like amplitude which follow the shift pulses in the cycle following a shift operation). The duration of the overlap pulses OP must be sufficient to produce the priming discharge, but need not produce a wall charge and may, in fact, produce a minimum value or even no wall charge. 
     After the overlap pulse terminates, and during the second half-cycle of the given phase, a sustain pulse Vs applied to the common (y) electrode extinguishes the wall charge. The sustain pulse Vs may or may not produce a discharge, depending on the amount of wall charge produced by the preceding priming discharge pulse OP2 of an-y in FIG. 6. As shown however, the wall charge has extinguished at Vs. Should any wall charge remaining after Vs, it is extinguished by the following EP of the next phase of an-y. 
     It is to be recognized that the above waveforms of the cell voltages primarily are illustrative and represent essentially idealized theoretical characteristics and are presented to assist in better understanding of the invention. 
     FIG. 7 is an illustrative block diagram of a driving circuit in accordance with the system of the invention. The four drivers DVA, DVB, DVC, and DVD are respectively connected to the shift bus terminals A, B, C, and D of the gas discharge panel SSP which may take the form of FIG. 1. These drivers DVA-DVD supply driving pulse trains such as shown at VA through VD, respectively, in FIG. 5A under control of the signal multiplexing circuit MPX which generates control pulses a to d having four different phases and serving correspondingly to shift a spot to the (four) successive discharge cells (an to dn) defined by the successive (four ) shift electrodes of each group (1 to n) thereof. The grouping of electrodes by four (4) and the corresponding use of four phases is desirable and practical, but not limiting. The multiplexing circuit MPX is responsive to the respective outputs of a timing control circuit CNT, a phase switching circuit PHS, and the clock source CL. The timing control circuit CNT outputs basic timing signals, st, ot, and et defining the timing functions (duration and synchronization) of the shift pulses SP, the overlap pulses OP, and the erase pulses EP. The phase switching circuit PHS generates gating signals for phase switching which thus define the phases in which appropriate ones of the shift, overlap and erase pulses are supplied by the multiplexer MPX to the drivers DVA to DVD. Each of the circuits CNT and PHS functions in response to a clock pulse train received from the clock pulse generator CL. 
     The timing control circuit CNT also supplies an output yst to the common electrode driver DVY, the output of which is supplied to the terminal Y for the common electrodes and specifically for generating the pulse train shown at VY in FIG. 5A. 
     The write electrodes w1 to w5, corresponding to the shift channels and thus the respectively associated common electrodes y1 to y5, are connected at their corresponding terminals W1 to W5 (shown collectively in FIG. 6 by the single terminal W) to the write drivers WD1 to WD5. The write drivers are controlled in operation by the multiplexing circuit MPW which generates a multiplex signal of the write timing signal wt, the overlap timing signal ot, and the erasing timing signal et supplied from the timing circuit CNT in accordance with data output signals from the character generator CG. Specifically, the information to be displayed in accordance with the outputs from the character generator CG control the multiplexer MPW to enable the appropriate ones of the write drivers WD1 to WD5 to generate the write pulse WP for application to the corresponding write electrodes, as required for display of the information supplied by the character generator CG. That character generator information, of course, is supplied at appropriate timed intervals under control of the multiplexer. Moreover, the shift output sh from the multiplexer is supplied through an AND gate 20, also receiving the write timing signal wt from the timing control circuit CNT, through an OR gate 22 to the multiplexer MPW, the OR gate 22 also supplying the timing signals ot and et therethrough to MPW. 
     In a preferred operation, the write pulse WP is selectively supplied to the appropriate write discharge cell for initiating a discharge during the cycle or interval of operation in which the shift electrode farthest from the write electrode is being activated. For the present illustrative embodiment having the shift electrodes arranged in groups of four, each group designated a, b, c, and d, the write voltage pulse WP would be applied while a shift pulse SP is applied to the d electrode; this alternatively can be expressed as the write pulse WP being applied during the D phase of the shift voltage SP, corresponding to the occurrence of the shift voltage SP in the pulse train VD simultaneously with VW, in FIG. 5A. 
     Similarly, the application of an overlap pulse OP to the write electrode is timed to correspond to that phase in which a shift voltage pulse SP is applied to the shift electrode closest to the write electrode. In the illustrative example herein, this occurs when the shift voltage pulse SP is applied to the a electrode as may be seen from the waveforms VW and VA of FIG. 5A. 
     Whereas a useful and practical embodiment of the present invention has been set forth hereinabove, it will be apparent to those of skill in this field that various modifications and combinations can readily be made. For example, the system and method of the invention may be used not only with a self-shift panel of the type shown in FIG. 1, but also to a gas discharge panel having a parallel electrode configuration as disclosed in U.S. Pat. No. 3,775,764 to J. P. Gauer, entitled &#34;MULTI-LINE PLASMA SHIFT REGISTER DISPLAY&#34; and to a gas discharge panel having a crossed electrode configuration in a special pattern particularly as is shown in FIG. 10 of U.S. Pat. No. 3,704,389 to W. B. McClelland, entitled &#34;METHOD AND APPARATUS FOR MEMORY AND DISPLAY&#34;. 
     It will be understood, of course, that for purposes of explanation of the present invention, FIG. 1 has illustrated a panel having only a few representative common, write, and shift electrodes; clearly, this is not limiting, and in any panel of any desired size and number of such electrodes could be employed. Likewise, the invention is not restricted to use with a four-phase system having the shift electrodes arranged in groups of four but instead is applicable to other panels having different such phasing or grouping arrangements. 
     Accordingly, the present invention provides a method and system of operation for shifting a discharge spot which is highly effective for increasing the shift operating margin of such a gas discharge panel and, moreover, affords a noticeable and significant advantage, or improvement, in attaining stable, accurate, and high speed shift operations. Thus, it is intended by the appended claims to cover all such modifications and adaptations which fall within the true spirit and scope of this invention.