GB2085631A - Gas discharge display apparatus and a method of operating the apparatus - Google Patents

Abstract

In order to reduce possible flicker effects when displaying black characters on an orange background on a single layer gas discharge display panel comprising a row-column matrix of cells, cathode electrodes extending along the rows of cells, priming electrodes extending along the columns of cells and display anode(s) defining groups of cells, each group comprising a plurality of adjacent columns of cells; corresponding cells in each group are primed synchronously in a sequence which has a predominantly row component. Consequently an entire row of cells across a panel can be addressed in the time taken to prime the cells in one row of a group, in practical terms this means substantially instantaneously.

Description

SPECIFICATION Gas discharge display apparatus and a method of operating the apparatus The present invention relates to a gas discharge display apparatus and to a method of operating the apparatus.

European Patent Application 79200028.3, published under Serial No. 0 003 1 57 (PHB 32606),discloses a gas discharge display apparatus comprising a panel formed by a single layer matrix of cells. Each row of cells has its own cathode and each column has its own priming or scanning electrode. The cells are arranged in groups with each group having a separate display input connected by a conductor common to a display anode or display anodes associated with the group. This specification discloses a number of closed and open loop scanning sequences whereby the cells are primed. These sequences are predominantly orientated along the columns of each group of cells, that is having scanned down (or up) one column, the scanning sequence transfers laterally by one cell and continues up (or down) the adjoining column.By energising the display anode contemporaneously with the priming of selected cells then the low current, barely visible breakdown of a cell is taken over such that the current drain between the display anode and cathode increases with the consequent increase in light output. In order to avoid the worst effects of flicker, the cells in each group are scanned at a frequency of the order of 50 Hz. This has been found satisfactory when presenting information in orange (the colour of the glow discharge) characters e.g. alpha-numeric characters, on a black background. However, when presenting information as black characters (switched-off cells) on an orange (switched-on cells) background some observers have noticed a slight flicker which is believed due to the observer blinking and in so doing his eye registers an apparently blank column of cells.In reality the column of cells have been energised but what is believed to have happened is that the time taken to blink, which time is of the order of several milliseconds, corresponds to the time taken to scan down a column of cells. Whilst this problem could be resolved by increasing the frame rate of scanning a group of cells this would be at the expense of light output which in turn is critical particularly when displaying orange characters on a black background. Another way of solving this problem would be to have a higher scanning frequency when displaying black characters on an orange background and a lower scanning frequency when displaying orange characters on a black background. However, this would involve a duplication of the driving circuits which would be expensive.

Accordingly, it is an object of the present invention to be able to display black characters on an orange background and orange characters on a black background on a gas discharge display panel without any noticeable flicker.

According to one aspect of the present invention there is provided a gas discharge display apparatus comprising a gas discharge display panel comprising a row-column matrix of gas discharge cells, each row of cells having its respective cathode electrode and each column of cells having its respective priming electrode, the cells in adjacent columns being arranged in repeating groups and each group having display' anode(s) connected to a separate input by a common conductor; a source of cathode address pulses coupled to the cathode electrodes, a source of priming pulses coupled to the priming electrodes, and means for controlling the sequence of application of the cathode address and priming pulses so that corresponding cells of each of the groups are primed synchronously in a sequence having a predominantly row component.

According to another aspect of the present invention there is provided a method of operating a gas discharge display panel comprising a rowcolumn matrix of gas discharge cells, each row of cells having its respective cathode electrode and each column of cells having its respective priming electrode, the cells in adjacent columns being arranged in repeating groups and each group having display anode(s) connected to a separate input by a common conductor, the method comprising applying cathode address pulses and priming pulses to their respective electrodes such that corresponding cells of each of the groups are primed synchronously in a sequence having a predominantly row component.

The scanning of the cells in each group may be open- or closed-loop and may be interlaced.

Cathode or display anode blanking may be applied in order to ensure that each of the cells in a row sharing the same display anode can be turned-on as required. Because corresponding cells in each group are addressed in synchronism then the display anodes may be addressed in accordance with a line dumping sequence, that is an entire row of cells is addressed in four address periods and at the commencement of scanning the next row of cells then the address information relating to the previous line or row of cells is changed (or dumped) so that information relating to the present row of cells can be supplied.

British Patent Specification 1,317,221 suggests that an entire display panel may be scanned in a generally horizontal zig-zag fashion.

However, assuming an address period per cell of 100,aS and a field rate of 50 Hz then the maximum number of cells each panel can have is 200. However there is no suggestion in that specification of combining a plurality of such panels in a single envelope and scanning the panels in synchronism so that a row of cells across the entire width of an envelope can be addressed substantially simultaneously.

The present invention will now be explained and described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 shows diagrammatically a portion of a gas discharge display panel having separate priming electrodes and display anodes, Figure 2 is a section on the line Il-Il of Figure 1, Figure 3 illustrates a closed loop, interlaced scanning sequence suitable for use with the panel shown in Figures 1 and 2, Figure 4 illustrates an open loop, interlaced scanning sequence suitable for use with the panel shown in Figures 1 and 2, Figure 5 illustrates an open loop, ordinary scanning sequence suitable for use with the panel shown in Figures 1 and 2, and Figure 6 is a block schematic circuit of an addressing system suitable for providing the interlaced scanning sequence of Figure 3.

Referring to Figures 1 and 2, the illustrated panel corresponds to that shown in Figures 9 and 10 of European Patent Publication Serial No.

0 003 1 57 and is intended to illustrate one of a number of gas discharge display panel constructions which can be included in the apparatus made in accordance with the present invention. The different panel constructions are incorporated herein by way of reference to European Patent Publication Serial No.

0003157.

Referring initially to Figure 2 the panel comprises a substrate 65 of an insulating material on which cathode electrodes K1 ... K48 are thick film printed. An apertured plate 66 is superposed on the cathode electrodes so the rows of apertures in the plate are aligned with respective ones of the cathode electrodes. The plate 66 may be of an electrically insulating material or of an electrically conductive material having an insulating surface thereover, including the surface of the apertures. An optically transparent cover plate 67 is disposed over the apertured plate 66 and is spaced therefrom by spacer buttons 68 inserted into additional apertures 69 located between the rows and columns of apertures forming the gas discharge cells. The spacer buttons 68 may comprise ballotini which have been softened and deformed under pressure into.

the apertures 69. By way of example the pitch between the apertures 69 corresponds to the distance between twelve cell forming apertures.

On the underside of the cover plate 67 transparent priming electrodes S1 to S4 of say tin and indium oxides are formed by thin film processes. The priming electrodes S1 to S4 are aligned with respective columns of cells. Thin film printed display anodes 60 and 62 are located between pairs of priming electrodes. A gas such as mixture of argon and neon preferably with mercury vapour at a sub-atmospheric pressure of 400 Torr (1 Torr = 133.32 Pa) fills the cells and a planar chamber 70 formed between the apertured plate 66 and the cover plate 67. In so doing the gas contacts all the electrodes in the panel. A glaze 71 seals the edges of the panel and prevents the loss of gas.

As shown in Figure 1 each of the cathode electrodes K1, K2 ... K48 connects all the cells in one row and each column of cells 40 has its own priming electrode. The display anodes are arranged with one between each pair of columns of cells. The panel will normally inlcude a keep-alive cell (not shown) which will have its own anode and cathode.

In order to effect priming, the column of cells are divided into groups with 4 columns per group and in the case of 480 columns of cells there are 120 groups.

The display anodes 60 and 62 of each group of cells are connected by way of a resistance 64 of 56 KQ to a respective external connection D1, .2... D120 (not shown). A thick-film printed resistance 52 of 1 I\/IQ is connected to each priming electrode S1 to S4. Conveniently each thick film reistance 52 is printed directly onto its associated priming electrode.The priming electrodes of the first column in each group, that is columns 1, 5, 9 and so on counting from the left in Figure 1 are connected to a first common priming bus rail P 1 ,the second electrodes in each group, that is columns 2, 6, 10 and so on are connected to a second common bus rail terminal P2 and in a similar fashion the third electrodes, that is columns 3, 7, 11 and so on and the fourth electrodes, that is columns 4, 8, 1 2 and so on, are connected respectively to third and fourth common priming bus rails P3, P4.

By suitable addressing circuitry (not shown), corresponding cells in each group are primed at the same time.

By providing the planar chamber 70, metastable atoms produced by the breaking down of a cell using priming (or scanning) pulses, drift to adjacent cells including the one which will be addressed next. Further the planar chamber 70 enables an increased pumping rate to be achieved when evacuating and degassing the panel.

The construction of the panel enables a number of different scanning or priming sequences to be achieved. In order to improve the viewing of black characters on an orange background it has been found preferably to use sequences in which there is a greater emphasis on horizontal or row scanning rather than vertical or column scanning.

By way of example three such sequences are illustrated in Figures 3 to 5. To avoid confusion the priming electrodes have been individually referenced S1, S2, S3, S4, S5, S6 and so on. The priming electrodes per group of cells are connected to corresponding priming electrodes of the other groups of cells via their respective common priminmg bus rails P 1, P2, P3, and P4 as shown in Figure 1. For the sake of clarity, the display anodes have been omitted. A keep-alive cell 40 with its separate cathode and anode electrodes 42,44 is shown in the top left hand corner of each of Figures 3, 4 and 5.

Figure 3 illustrates an example of a closed loop interlace scanning sequence. The keep-alive cell 40 provides free ions which enable the cell K 1 S 1 to break down when a potential difference is applied across the cell. Typically the potential difference is constituted by 100 yS cathode pulses and 20,uS priming pulses, the cell K1S1 being broken down during the last 10 ,uS of the priming pulse and in so doing the cell emits a light at a barely visible level. If the cell is to emit a bright light than a 100 yS display pulse is applied contemporaneously to the display anode (not shown).The direction of priming proceeds along the entire length of the row of cells in the group, that is K1S2, K1S3, K1S4, then it continues in the opposite direction along the cells associated with the third cathode, that is K3S4, K3S3, K3S2, K3S1, and so on until it reaches K47S1 whereupon the scanning takes place along the even numbered rows of cells in a group commencing at K48S1 and finishing at K2S1.

Thereafter the interlace scanning sequence repeats beginning at K1S1. A block schematic circuit diagram of an addressing circuit will be described later with reference to Figure 6.

All the groups of cells are scanned in sequence once they have been primed by a ripple-through effect across the panel. If desired at the initial switch-on of the panel a starter circuit (not shown) energises all the cells associated with the cathode K1 thereby providing free ions to permit each group of cells to be scanned.

Using an address period per cell of 100,uS.then in the case of a group comprising four columns of cells, each row of cells in a group is scanned in 400 S, in other words practically simultaneously when the speed of response of a human eye is considered. As corresponding rows of cells in each group are addressed at the same time then the overall effect will be that an entire row of cells associated with a particular cathode is addressed in 400 S which from the point of view of the response of the eye is substantially instantaneously. Consequently when a viewer blinks, the effect will be almost the same as for a line dump display system in which the entire row is displayed substantially instantaneously.One slight drawback to this closed loop interlace scanning sequence is that geometric irregularity occurs because for example the rows of cells associated with cathodes K47, K48 are primed and if necessary displayed one after the other whereas in the centre of the group nearly 10 mS (assuming a 50 Hz frame scan rate) occurs between the displaying of the rows of the cells associated with the cathodes K23 and K24.

This geometric irregularity can be overcome by the open loop interlace scanning sequence illustrated in Figure 4. Assuming a 50 Hz frame scan rate and a cell address period of 100 yS, then for the first 100 MS address period of a first field all the cells across the entire panel associated -with the cathode K1 are primed by all the priming electrode bus rails being pulsed. Thereafter the scanning of the even numbered rows of cells commences beginning in the case of the first group at K2S1 and continues as indicated by the full line drawn arrows to K48S1 At the commencement of the second field, once again all the cells associated with the cathode K1 are primed during the first 100 MS address period.

Thereafter commencing at K3S1 all the odd numbered rows are scanned in zig-zag or serpentine fashion, as shown by the broken line drawn arrows, ending up with K47S4. The next frame commences again with the priming of the cells associated with the cathode Ki. Compared to the closed loop arrangement shown in Figure 3, as each field commences in the top left hand corner of each group of cells then approximately 10 mS elapses between the scanning of cells in adjacent rows and thereby the geometric irregularity of the Figure 3 scanning sequence is avoided.

Figure 5 illustrates an open loop row by row scanning sequence. At the commencement of each frame it is necessary to prime all the cells associated with the cathode K1. Thereafter the scanning commences at K2S1 and as shown by the arrows proceeds row by row in zig-zag fashion to the cell K48S4, with each cell priming an adjacent cell.

An advantage of an interlaced scan over an open loop scan is that the frame scane rate can be reduced from the generally accepted minimum rate of 50 Hz to 40 Hz, consequently the on-time of each cell may be greater which in turn means that a slightly lower display anode-cathode voltage difference will be required for the same overall light output which lower voltage difference will extend the life of a panel. However a disadvantage of operating a panel having one or more common display anodes to each group of cells with a predominantly row scanning sequence is that those case where two horizontally adjacent cells are to be turned-on using the some display anode, then the first cell primed will stay-on for both address periods whereas the second cell will not be switched-on. This can be overcome by means of cathode or display anode blanking.In either case such blanking involves terminating the cathode or display anode address pulse 10 MS before the end of the address period thereby ensuring that the first cell is turned-off by the time that the second, adjacent cell breaks down.

Alternatively the display pulse can be blanked for the first 10 yS of the address period giving a more favourable duty ratio as the addressed cells are not on during this period due to a delay in time for breakdown.

Although three examples of a predominantly horizontal or row scanning sequences have been described, it is intended that the scope of the appended claims should cover other, nondescribed scanning sequences.

Figure 6 is a block schematic circuit diagram of am embodiment of an addressing circuit for a gas discharge display panel 100 of the type shown in Figures 1 and 2 and which is operated in accordance with the closed loop, interlaced scanning sequence shown in Figure 3.

For the sake of explanation, it will be assumed that the panel 100 is a matrix comprising 96 (columns) x 48 (rows) of cells. The columns of cells are grouped in fours with the priming electrode of the first column in each group being connected to one input, the priming electrodes of the second column in each group being connected to a second input and so on. For convenience the priming electrode inputs have been shown collectively as 102. Each of the twenty-four groups of columns has its own display input shown collectively as 104. The forty-eight cathode inputs are collectively referenced as 106.In order to scan all 1 92 cells in each group fifty times a second it is necessary to complete a scan in approximately 20 mS thereby making it necessary to apply address pulses of approximately 400 tS to the cathode inputs 106 and 20 yS priming pulses having a pulse repetition frequency of 10 KHz to the priming electrodes.

The pulses for the cathode inputs 106 and priming electrode inputs 102 are derived from a common clock oscillator 108 which produces a clock frequency of 960 KHz. The clock frequency is first divided by six in a divider 110 to produce a reduced frequency of 1 60 KHz which is divided again by sixteen in a character counter 112. The output frequency from the character counter 112 is 10 KHz which is applied to the primer scanner 116 for scanning the priming electrodes of the panel 100. The primer scanner 11 6 includes a resettable eight position counter. Every fourth pulse, that is outputs from the counter positions one an five, is applied to a cathode scanner 114 which comprises a programmable counter.The scanner 114 is connected to the cathode inputs 106 and applies a 400 ,uS wide pulse to each cathode in accordance with the programme, in the case of the interlaced scanning sequence shown in Figure 3, firstly to the odd numbered cathodes K1, K3, K5 ... K47 and then to the even numbered cathodes K48, K46,K44... K2, the sequence repeating commencing at the cathode K1.

In order to display data in this illustrated example, it will be assumed that the forty-eight cathode rows of the panel 100 is divided into six lines of characters eight rows high. Further it will be assumed that only the middle four of the six lines will be used for the message which will comprise alphanumeric characters of 7 x 5 format with one cell gap between the characters and rows.

The data source comprises an address memory (RAM) 120 which is capable of storing several pages of message in say ASCII coded form. Each page consisting of four lines of sixteen characters.

Thus each page is read as corresponding rows of cells in each group of four columns is scanned by the cathode and priming electrode pulses. In order to read the information in the RAM 120 in the correct sequence outputs from the character counter 112 and the cathode scanner 114 are connected to it. In the case of the output from the cathode scanner, which output comprises a row address, this is incremented after every n/2 counts, where n is the number of cathodes per character row. In the present example n=8. The information in the form of the character code from the RAM 120 is supplied to a character generator in the form of a read only memory (ROM) 122. The ROM 122 also receives a signal indicative of the row number from the cathode scanner 114, this row number must be interlaced in agreement with the cathode scan.

A parallel to serial register 124 is connected to the output of the ROM 1 22. The register 124 in turn feeds data to a serial to parallel data dump register 126. At any instant only one cell in a corresponding position in every group is being scanned, consequently every fourth bit of data from the register 124 is loaded into the dump register 126. The bits which are to be loaded into the dump register 126 depend upon which of the priming electrodes is currently active. Proper synchronisation is achieved using a comparator 128 which receives inputs from the priming electrode scanner 116 via a decoder 132 and from a modulo 4 counter 130 which is connected to the clock generator 108. The output of the comparator 128 comprises a signal of a frequency of 240 KHz which is applied as a clock signal to the dump register 126.

The data reaches the display panel 100 one row late relative to the logic circuits because the data dump register presents, at the occurrence of a strobe pulse, one row of data to the display anode drivers (not shown) while filling with the data related to the next row. The decoder 132 has to allow for the one row delay in the dump register 126 and accordingly it derives the priming electrode number for the next set of data to be dumped.

The illustrated circuit can be readily adapted for an eighty character wide display panel by having a larger data dump register 126 to take into account that there will be 120 display anodes, increasing the clock frequency to 4.8 MHz which in turn means the clock signal to the data dump register becomes 1.2 MHz and changing the character counter 112 to a modulo 80 one.

Although the data source has been described as being an address memory it may also comprise a keyboard or a PROM.

Claims (11)

1. A gas discharge display apparatus comprising a gas discharge display panel comprising a row-column matrix of gas discharge cells, each row of cells having its respective cathode electrode and each column of cells having its respective priming electrode, the cells in adjacent columns being arranged in repeating groups and each group having display anode(s) connected to a separate input by a common conductor; a source of cathode address pulses coupled to the cathode electrodes, a source of priming pulses coupled to the priming electrodes, and means for controlling the sequence of application of the cathode address and priming pulses so that corresponding cells of each of the groups are primed synchronously in a sequence having a predominantly row component.

2. A method of operating a gas discharge display panel comprising a row-column matrix of gas discharge cells, each row of cells having its respective cathode electrode and each column of cells having its respective priming electrode, the cells in adjacent columns being arranged in repeating groups and each group having display anode(s) connected to a separate input by a common conductor, the method comprising applying cathode address pulses and priming pulses to their respective electrodes such that corresponding cells of each of the groups are primed synchronously in a sequence having a predominantly row component.

3. A method as claimed in Claim 2, wherein the scanning sequence is such that for each group of cells the cells in a first row are primed in turn proceeding in a first direction, then the cells in a second row are primed in turn proceeding in a second direction opposite to the first direction, then the cells in a third row are primed in turn proceeding in the first direction, and so on, the number of rows shifted in the column direction between successive row scans in the opposite directions being less than the number of cells in a row.

4. A method as claimed in Claim 2 or 3, wherein the sequence comprises a closed loop interlace scan and wherein the priming proceeds in a zig-zag fashion on alternate rows proceeding in one column direction and returning in the other column direction.

5. A method as claimed in Claim 2 or 3, wherein the sequence comprises an open Ipop interlace scan comprising two fields, each field comprising a different plurality of alternate rows of cells which are scanned in a zig-zag fashion.

6. A method as claimed in Claim 5, wherein at the commencement of each field, all the cells associated with one cathode are primed.

7. A method as claimed in Claim 2 or 3, wherein the sequence is an open loop one preceeding in zig-zag fashion along successive rows, and wherein at the commencement of each frame, all the cells associated with one cathode are primed.

8. A method as claimed in any one of Claims 2 to 7, wherein when at least two adjacent columns share a display anode, cathode or display anode address blanking is used.

9. A method as claimed in any one of Claims 2 to 8, wherein the display anodes are addressed in accordance with a line dumping sequence.

1 0. A method of operating a gas discharge display panel, substantially as hereinbefpre described with reference to the accompanying drawings.

11. A gas discharge display apparatus substantially as hereinbefore described with reference to the accompanying drawings.