Integrated Circuit Character Generator

Abstract

Deflection and intensity control signals suitable for forming characters on the face of the cathode ray tube are derived from suitably shaped charge pick-up plates capacitively spaced from a sequentially excited conductor grid.

Description

BACKGROUND OF THE INVENTION

This invention relates to character generators and more particularly to an improved electrostatic character generator.

The cathode ray tube is presently finding extensive use for displaying changing information to the public. Numbers and letters from data sources such as computers can be presented more easily and at lower cost on a cathode ray tube than in any other manner. A present widely used approach for displaying information provided in a standard code, such as ASCII, into a character recognizable by a human observer is the dot matrix display. This entails causing the electron beam of the cathode ray tube to be swept in a raster scan similar to that used in television. The beam is gated on and off at appropriate places to form a dot display. The 5 wide .times. 7 high matrix is one industry standard. However, it is conceded that the legability of letters generated by this method is marginal and the aesthetic quality is not very good.

The technical drawback of this approach will become clear from the following. For the usual 10 megacycle or less video bandpass, only 10.sup.7 dots may be written per second with any clarity, allowing for horizontal and vertical spacing between characters and retrace time. This means that in a practical system only a hundred thousand characters may be written per second. In order to prevent serious visible flicker, the display must be refreshed at least 50 times per second. Therefore, a maximum of 2,000 characters may be displayed on a screen at one time using the raster scan dot matrix approach, even with a 7 .times. 5 format.

More legible characters can be generated using a larger number of dots. This permits the display of lower case letters, not possible in a 5 .times. 7 format, but is possible in a 10 .times. 14 format. However, it is clear, that any attempt to increase legability by increasing the number of dots would of necessity reduce the number of displayable characters and/or require a more sophisticated video system at greatly increased cost.

The basic problem with the dot matrix approach is that characters are basically styled after cursive motions of the hand and are inherently not suited to an on/off approach. The eye is very insensitive to small or even large deviations of a character shape from that expected so long as they are essentially plastic deformations of the idealized character. Discontinuities however are very objectionable and of course are part and parcel of any strictly digital technique. It is thus apparent that character generation is fundamentally an analog function which should be selected digitally by a character code rather than a strictly digital synthesis. A partial step in this direction has been used in the trade and is known as the stroke method of character generation. In this technique a character is represented by drawing straight line segments from point to point to give an approximation to the character shape. While such an approach is basically capable of producing high quality characters it has in the past not been possible to implement such an approach at low cost. Typical systems now on the market will generate characters by the stroke method in 5 microseconds, allow strokes only in eight directions, weigh 44 pounds and cost in excess of $5,000.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to provide a low cost character generator which affords a character display having excellent aesthetic quality.

It is still another object of this invention to provide a character generator having a low video bandwidth requirement while providing an aesthetically improved character display.

Yet another object of the invention is the provision of a character generator which can be fabricated using standard integrated circuit technology and therefore is a low cost and low power system.

Yet another object of this invention is to provide a new and novel character generation system.

These and other objects of the invention may be achieved in a character generator wherein a parallel grid of conductors are provided in one plane. These conductors may be charged in parallel with a suitable voltage and then discharged sequentially. For each character desired to be displayed, there are provided capacitively spaced from the conductor grid conductive plates or areas.

One pair of these conductive plates provide horizontal deflection signals hereafter called X deflection. A second pair of these conductive plates provide vertical deflection signals hereafter called Y deflection. A third pair of these plates provide electron beam on/off control signals, hereafter called Z signals. As a result of the capacitive spacing from the wires, a charge induced in the plates as the wires sequentially are discharged. The sequential discharge of the wires effectively causes a sequential scan of the plates and the voltage derived from the plates at any instant is determined by the size of the plate. Changes in the voltages derived from the sequentially scanned plate are controlled by changes in the shape of the plate as it is being scanned. The X and Y voltages are integrated and are applied to the deflection system of the cathode ray tube which provides the pedestal X and Y voltages which determine at which location of the CRT the character is displayed. The Z voltage is applied to the cathode ray electron beam control to gate the beam on and off as required for the particular character or symbol being displayed.

When the word "character" is used herein, it is intended to cover besides a character, a symbol or sign or any free form design for which this invention can be used to generate signals which when applied to a CRT will cause a desired display on the fact thereof.

The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an embodiment of the invention.

FIG. 2 is an isometric view illustrating how a portion of the invention may be fabricated.

FIG. 3 is a circuit diagram illustrating the charging and discharging arrangement for the drive lines.

FIG. 4 is a waveform diagram showing the voltages which occur on the sequence of lines as a function of time, and

FIGS. 5 and 6 are illustrative of forms for pick-up plates which may be used to generate X, Y and Z signals for displaying the letters M and A for example.

Referring now to FIG. 1 of the drawing, a character generator will include, a plurality of drive lines, here designated by reference numerals 1 through 21, by way of illustration. The number of drive lines is determined by the resolution desired for a particular character as well as the time allotted for its generation. These drive lines are preferably all in the same plane. At both ends of these drive lines are "line charge and discharge" circuits respectively 30, 32. The details of these will be shown in FIG. 3 herein.

Capacitively spaced from the drive lines are conductive plates or areas respectively designated as X1, Y1, Z1 and X2, Y2, Z2. There are three of these conductive plates required to provide horizontal deflection signals, (X), vertical deflection signals and electron beam control signals (Z), for each character or symbol desired to be displayed. Actually each plate consists of two conductive areas, details of which will be shown in FIGS. 5 and 6. Thus, plates X.sub.1, Y.sub.1, Z.sub.1 are one set and are assigned to one character. Plates X.sub.2, Y.sub.2, Z.sub.2 are a second set, assigned to a second character. More sets of plates are added for each character or symbol desired to be displayed.

These sets of plates are all connected to a multiplexer circuit which includes, for the X1 plate, for example, two transistors respectively 34, 36, which have their source electrodes connected to the respective portions of the X1 plate and their drain electrodes connected to a pair of X signal busses respectively 38, 40. The gate electrodes of transistors 34 and 36 are connected together and to a decoder 42 having its output terminal designated by the letter A, (assuming that X1, Y.sub.1 and Z.sub.1 will generate signals corresponding to the letter A). The X collecting bus lines 38, 40 are connected to an X differential amplifier 44, whose output is applied to the horizontal deflection circuit of the cathode ray tube upon which the character A is to be displayed.

Transistors 44, 46, have their source electrodes connected in parallel with the respective source electrodes of transistors 34 and 36. Their gate electrodes are connected to ground. Their bases are connected to the decoder 42 by a terminal designated as A.

The Y1 plates are similarly connected to the source electrodes of two transistors respectively 46, 48. The gate electrodes of these two transistors are connected to the two Y deflection bus lines 50, 52. The Y deflection bus lines are connected to a Y differential amplifier 54.

Two additional transistors respectively 56, 58, have their source electrodes connected in parallel with the source electrodes of transistors 46, 48, their drain electrodes connected to ground, and their gate electrodes connected to the A terminal of the decoder 42.

The Z.sub.1 conductive plates are connected to the source electrodes of transistors 60, 62, whose gate electrodes are connected to the "A" terminal of decoder 42 and whose drain electrodes are connected to the Z bus lines respectively 64, 66. The Z bus lines are connected to a current sensitive Z differential amplifier 68, whose output is applied to the cathode ray tube beam control circuit. Transistor 70, 72 have their source electrodes connected in parallel with the source electrodes of transistors 60 and 62, their drain electrodes connected to ground, and their gate electrodes connected to the A terminal of the decoder.

The conductive plates assigned to other characters such as X2, Y2, Z2, are connected to transistors in the multiplexer which are connected to terminals in the decoder such as B, B, in a similar manner as has been described in connection with X.sub.1, Y.sub.1 and Z.sub.1.

In operation, code signals representative of a desired character are applied to the decoder. Before the character is applied to the decoder, all the NOT terminals (A, B, etc.) are high thus effectively connecting to ground the source electrodes of all of the transistors whose drains are connected to the X, Y, and Z bus bars. This grounds all of the bus bars. All of the lines 1 through 21 are charged to a predetermined voltage when the code signals are applied to the decoder by a signal applied to a precharge enable signal source 73, which enables the line charge and discharge circuits 30, 32.

A selected character, such as "A" would cause the A terminal of the decoder to go high and the A to go low while all of the other "NOT" terminals remain high. This unclamps transistors 34, 36, 46, 48 and 60, 62 whereby they can respond to any signal applied to them from the sets of plates such as X1, Y1 and Z1 to which they are connected. When a selection is made via the decoder 42, such as the one just discussed, a strobe circuit 74 is enabled after the lines 1 through 21 have been charged up. This applies a signal to the line charge and discharge circuit 40. The strobe signal causes these lines to sequentially discharge.

In response to the discharging lines, voltages are capacitively induced in the sets of plates X1, Y1, Z1, X2, Y2, Z2, etc. However, in view of the clamping action caused by the transistors connected to the "NOT" terminals of the decoder, the only transistors which can respond to the voltages induced in the plates are those which have been selected by the decoder. Thus, the voltages induced in plates X1, Y1, Z1 are selected by transistors 34, 36, 46, 48 and 60, 62, and are applied to the respective X, Y, and Z pick up bus bars. The outputs of these busses, as previously indicated, are applied to the respective X, Y and Z differential amplifiers 44, 54 and 68, from whence they are applied to the respective horizontal and vertical deflection circuits and the beam control circuit on the cathode ray tube.

From the foregoing it should be clear that any arbitrary function of time may be generated by varying the width of the conductive area over each driven line, and hence the capacitive coupling between that particular driven line and the conducting area.

FIG. 2 is a perspective view illustrating, by way of example, one way of implementing the drive lines and the plates. In FIG. 2, there is shown, by way of example, three drive lines respectively 1, 2, 3, which are deposited on one surface of an insulator 76. Upon the other surface of the insulator there is deposited an X.sub.1 plate designated as X.sub.1 +, and X.sub.1 -. The reasons for this double conductive area for a plate will be provided subsequently herein.

Of course, printed circuit or integrated circuit techniques may be employed for this arrangement.

FIG. 3 illustrates an arrangement for charging and discharging the lines in the grid employed in this invention, P channel, MOS technology may be employed in the arrangement for charging and discharging the lines. The lines are initially precharged to a negative voltage of, for example, 15 volts through transistors 80, 82, 84, and 86, when the gates of these transistors have an enabling signal applied thereto from the precharge enable signal source 73. Current from a source of potential designated as -V, is applied through the respective bus lines to the drain electrodes of the transistors 80, 82, 84 and 86. The source electrode of transistor 80 is connected to one end of line 1, the source electrode of transistor 82 is connected to one end of line 3. The source electrode of transistor 84 is connected to one end of line 2 and the source electrode of transistor 86 is connected to one end of line 4. The other end of line 1 is connected to the gate electrode of a transistor 88 whose source electrode is connected to ground and whose drain electrode is connected through a resistor 90 to the potential source -V. The drain electrode of transistor 88 is also connected to the gate of a transistor 92, whose source electrode is connected to ground and whose drain electrode is connected to the one end of line 2 as well as to the source electrode of transistor 84. The drain electrode of transistor 84 is connected to the -V potential source. The other end of line 2 is connected to the gate electrode of a transistor 94, whose source electrode is connected to ground and whose source drain electrode is connected to a resistor 96 and to the gate electrode of transistor 98. The other end of resistor 96 is connected to the -V potential source. Transistor 98 has its source electrode connected to ground and its drain electrode connected to the one end of line 3 and to the source electrode of transistor 82. The drain electrode of transistor 82 is connected to -V potential source.

The other end of line 3 is connected to the gate electrode of a transistor 100, whose source electrode is connected to ground and whose drain electrode is connected to a resistor 102 and to the gate electrode of a transistor 104. The source electrode of transistor 104 is connected to ground and the drain electrode is connected to the one end of line 4 and also is connected to the source electrode of transistor 86. The drain electrode of transistor 86 is connected to the -V potential source and the other end of resistor 102 is connected to the -V potential source also.

It is believed that the circuitry shown and explained for four lines is adequate to exemplify the structure required for a multiplicity of lines.

The lines are charged in parallel when a "precharge enable" signal from the source 73, which is merely a gate, is enabled to apply an enabling voltage to the gate electrodes of all of the transistors 80, 82, 84, 86, etc. to enable them to apply voltage from the -V potential source to the lines. The precharge enable signal source may be activated in any number of ways such as by the discharging last line, or when a character code is applied to the coder, with the strobe signal being delayed long enough to permit the circuits to settle. These techniques are well known to those skilled in the art.

Once these lines have been charged and the multiplexer has settled, a character code which is applied to the decoder and a strobe signal is applied to the gate electrode of a transistor 79. Transistor 79 has its source electrode connected to ground and its drain connected to one end of line 1 and also to the source electrode of transistor 80, whose drain electrode is connected to the -V voltage source. In response to the strobe signal transistor 79 discharges the voltage of line 1 to ground. When the voltage of line 1 drops to the threshhold of transistor 88, transistor 88 is turned off and the voltage at its drain electrode begins to rise. When the voltage has risen to the threshhold of transistor 92, it is turned on and commenced to discharge line No. 2 to ground. The voltage on line No. 2 drops until transistor 94 is rendered non-conductive. The voltage at the drain of transistor 94 rises until it attains a value which turns on transistor 98. Transistor 98 discharges line 3.

From the foregoing it will be seen that the lines are charged in parallel and are sequentially discharged in response to the strobe input. The voltage on the lines plotted as a function of time as is shown in FIG. 4. This is shown for six lines. It is clear from FIG. 4, that the time of rapid voltage change for each line (and consequently the time when capacitive current is flowing to the overlying conducting areas) is later for higher numbered lines than for the low numbered lines, being delayed by the propagation delay through the drive electronics of each line. Thereby the sequential nature of the deflection of each displayed stroke which the character generator creates is assured.

The charge induced in any overlying conducting area is proportional to the capacitance between the driven line and the overlying area and the voltage excursion of the particular driven line. It is independent of the rise or fall time of the particular circuit involved. Since the capacitance between an overlying area and the driven line is proportional to the area overlapping between the two, it is essentially a geometrical quantity and easily controlled as by using the high quality photolithographic technique used to make ordinary integrated circuits.

This invention may be operated in either of two modes. One is the current sensitive mode and the other is the integration mode where charge added to each overlying conducting area is integrated by the total capacitance of the area and by the input stage of the amplifier into which the conducting area feeds. The disadvantage of the current sensitive mode is that the device will be sensitive to the rise time of the drive electronics. The advantage is that the output is directly proportional to the width of the conductive area over a line at a particular time and hence the layout of the plates is easier. Both systems are workable, but the integrated mode is preferred for the X and Y plates, since higher precision may be achieved. The current sensitive mode is preferred for the Z plate.

FIG. 5 shows an arrangement of conductive areas or plates for the integrated mode for producing signals for the letter "M". Two complimentary shaped plates are provided for X, two complimentary shaped plates for Y and two complimentary shaped plates for Z.

Plates 110 and 112 respectively generate -X and +X signals. Plates 114 and 116 respectively generate -Y and +Y signals. Plates 118 and 120 respectively generate "Z off" and "Z on" signals. Originally, the deflection system of the cathode ray tube being used for display positions the electron beam at the lower left-hand corner of character space. The letter M is then formed by holding the X coordinate constant and sweeping the Y coordinate with the passage of time until it reaches the top of the left stroke of the letter M. At that point the X coordinate is swept in the positive direction, (to the right), at a rate somewhat slower than the Y coordinate is swept in the negative direction, (downward). When the beam reaches the center of the letter M, at the cusp, the direction of Y is reversed while the direction of the X sweep is maintained constant. When the beam reaches the upper right-hand corner or right stroke of the letter M, X is maintained constant and Y is swept in a negative direction until it reaches the lower right-hand end of the letter, when the beam is turned off. At this time the electron beam is positioned to the lower left-hand corner of the beginning of the next character and the wires are precharged and the entire cycle can be repeated.

The sequence described is achieved by the widths of the differential X and Y conductive areas. At the commencement of the generation of the letter M, it will be seen that the "Z on" plate 120 is wider than the "Z off" plate 118, as a result of which a larger "Z on" signal is generated whereby the CRT electron beam is turned on. For the first leg of the character, the +Y area 116 is made approximately 10 times as wide as the -Y area 114, resulting in a large differential positive charge on the +Y area relative to the -Y area. At the same time the +X and -X areas are equal.

The numbers 1 through 27 at the bottom of the drawing represent, by way of illustration, 27 drive lines which are sequentially discharged. By the time the 7th drive line has been discharged, the top of the left stroke of the letter M has been reached. At this point the +X area which overlies lines 8 through 19 is made wider than the -X area, six times as wide, by way of illustration, and hence the differential output from these two lines sweeps the CRT electron beam in a positive direction. Over these lines, the -Y area is made much larger than the +Y area hence the charge on the -Y line increases with time relative to that of the +Y area, sweeping the electron beam down to form the downgoing part of the letter M.

At line 14 the +Y area is again made larger than the -Y area thus sweeping the beam in the upward direction while X is still being swept. The +X area is still larger than the -X area and thus the electron beam continues to be swept to the right. Thus, the upward going center section of the M is formed. The upper right-hand corner of the letter M occurs between lines 19 and 20. From this point onward the +X and -X areas are equal again and hence the differential X output voltage remains constant. However, the -Y area is made larger than the +Y area and the electron beam is swept downward to form the right stroke of the letter M.

During the interval described, the output from the two Z plates 118 and 120 are fed into a low input impedance current sensitive differential amplifier. As previously indicated since the "Z on" area is larger than the 37 Z off" area, the output of the differential amplifier will gate the electron beam on. After line 26, the "Z off" area is made larger than the "Z on" area and the current sensitive amplifier will then turn the electron beam off.

The integrating mode of operation just described is one wherein the charge added to each overlying conductive area of a plate in response to the discharging conductor adjacent thereto, is integrated by the total capacitance of the area, the multiplexer electronics and the input stage of the amplifier into which the character generator feeds. The total voltage change of each line incrementally adds a charge to the X, Y and Z lines which changes the voltage sequentially with time, always in the negative direction, hence the voltage will be a monatonically decreasing function of time for all lines.

Referring to FIG. 6, the plates for generating the letter A are shown. As before, in the case of the Z plates the "Z on" plate 122 is larger than the "Z off" plate 124 in order to turn the beam on. Since the beam will be moved from the left bottom end of the letter upward to the top along a diagonal, the +Y plate 126 is larger than the -Y plate 128 to insure upward motion, and the +Y plate 130, is larger than the -X plate 132 to assure some motion to the right. When the electron beam reaches the apex of the letter A, which occurs about line 7, the +Y plate 126 is made small and the -Y plate 128 is made large. The ratio of the +X and -X plates is maintained the same. This causes the electron beam to move downward from the apex and to the right until it reaches the bottom end of the letter A at drive line 15.

It now becomes necessary to draw the stroke of the A. Until the electron beam is moved upward to the position of the stroke, which will be executed from right to left, the beam is turned off. Thus, the "Z off" plate over lines 15 through 18 is made larger than the "Z on" plate. The +Y plate is made larger than the -Y plate in order to elevate the beam from the bottom of the letter A up until the location where the stroke begins. The -X plate is made somewhat larger than the +X plate in order to move the beam backward or to the left to the location where the stroke is to end. From line 18 through line 21, the "Z on" plate is made large again to turn on the electron beam. Y deflection is to be maintained constant over this interval and therefore the +Y and -Y plates are made to have an equal area. X deflection is to occur from right to left, and therefore the -X plate 132 is made much larger than the +X plate. When line 21 is reached the letter A is finished and therefore the "Z off" plate becomes greater than the "Z on" plate turning off the electron beam.

As pointed out previously, the design of the character generator described herein lends itself to integrated circuit technology wherein the entire character generator including the line charge and discharge circuits, the multiplexer circuit, the drive lines and the differential outputs may all be incorporated on a single silicon chip. Characters may be changed by changing chips. The character generator size may be changed by adding chips.

There has accordingly been described and shown herein a novel, useful and improved character generator suitable for manufacture using integrated circuit techniques whereby the costs and size may be minimized.

Claims

What is claimed is:

1. A character generator for generating voltages for application to a cathode ray tube to establish a character display on the face thereof comprising:

a grid of a plurality of parallel spaced substantially coplanar conductors,

means for charging to a predetermined voltage the conductors of said grid,

means for sequentially discharging the plurality of parallel spaced conductors in said grid,

shaped conductive area means capacitively spaced from said grid of conductors,

said shaped conductive area means being shaped for deriving voltages from said grid for forming a character display on the face of a cathode ray tube to which they are applied, and

means for applying the voltages derived by said conductive area means to a cathode ray tube for forming a character display on the face thereof.

2. A character generator as recited in claim 1 wherein said shaped conductive area means comprise a complimentary shaped pair of areas shaped for providing horizontal deflection voltages,

a complimentary shaped pair of areas shaped for providing vertical deflection voltages, and

a complimentary shaped pair of shaped areas for providing electron beam control voltages.

3. A character generator for generating voltages for application to a cathode ray tube to establish a character display on the face thereof comprising:

a grid of a plurality of parallel spaced substantially coplanar conductors,

means for charging up the conductors of said grid,

means for sequentially discharging the plurality of parallel spaced conductors in said grid,

a plurality of sets of shaped conductive plate means capacitively spaced from the wires of said grid,

each set being shaped for deriving voltages from said grid required to establish a different character display when applied to the face of a cathode ray tube, and

means for selectively applying the voltages derived by one of said plurality of sets to a cathode ray tube for forming a character display on the fact thereof.

4. A character generator as recited in claim 3 wherein each of said plurality of sets of shaped conductive plate means includes three pairs of complimentary shaped plates,

a first pair being shaped for providing horizontal deflection voltages,

a second pair being shaped for providing vertical deflection voltages,

a third pair being shaped for providing electron beam control voltages.

5. A character generator as recited in claim 3 wherein said means for sequentially discharging the conductors of said grid includes:

first normally inoperative means connected to one end of each grid wire for discharging said wire when rendered operative,

second normally inoperative means connected to the other end of each grid wire and to the first normally inoperative means of a succeeding grid wire for rendering said first normally inoperative means operative in response to the grid wire to which it is connected being discharged, and

means for rendering operative the first normally inoperative means of a first of said grid wires.

6. A character generator as recited in claim 4 wherein said means for selectively applying the voltages derived by one of said plurality of sets to a cathode ray tube includes for each pair of plates in each set,

a first, second and third pair of bus lines

a first normally inoperative switch means respectively connecting one plate of each first pair of plates in each set to one of said first pair of bus lines,

a second normally inoperative switch means respectively connecting the other plate of each first pair of plates in each set to the other of said first pair of bus lines,

a third normally inoperative switch means respectively connecting one pair of each second pair of plates in each set to one of said second pair of bus lines,

a fourth normally inoperative switch means respectively connecting the other plate of each second pair of plates in each set to the other of said second pair of bus lines,

a fifth normally inoperative switch means respectively connecting one pair of each third pair of plates in each set to one of said third pair of bus lines,

a sixth normally inoperative switch means respectively connecting the other plate of each third pair of plates in each set to the other of said third pair of bus lines,

a first, second and third differential amplifier each having input and output,

means connecting said first and second normally inoperative switch means to said first differential amplifier input,

means connecting said third and fourth normally inoperative switch means to said second differential amplifier input,

means connecting said fifth and sixth normally inoperative switch means to said third differential amplifier input, and

means connecting the output of said first, second and third differential amplifiers to a cathode ray tube, and

means for rendering operative the first, second, third, fourth, fifth and sixth switch means for a set of plates which produce a desired character representation on the face of said cathode ray tube.

7. Apparatus as recited in claim 6 wherein said first and second differential amplifiers are of the integrating type and said third differential amplifier is of the current sensitive type.

8. Apparatus as recited in claim 6 wherein said first, second and third differential amplifiers are of the current sensitive type.

9. Apparatus as recited in claim 6 wherein there is included an operative grounding switch means for each plate in each set of plates,

a ground potential point,

means connecting each plate in each set of plates to said ground potential point through a different one of said ground switch means, and

means for rendering inoperative the ground switch means connected to a set of plates whose inoperative switch means have been rendered operative.

10. A character generator for generating deflection and beam control voltages for application to a cathode ray tube to establish a character display on the face thereof comprising:

a grid of parallel spaced substantially coplanar conductors,

means for charging in parallel to a predetermined voltage the conductors of said grid,

means for sequentially discharging the conductors of said grid,

a first pair of conductive plate means capacitively spaced from said grid,

said first pair of conductive plate means having complimentary shapes for deriving from said discharging wire grid voltages which, when applied to a cathode ray tube constitute the horizontal deflection voltages required for displaying a character,

a second pair of conductive plate means capacitively spaced from said grid,

said second pair of conductive plate means having complimentary shapes for deriving from said discharging wire grid voltages which, when applied to a cathode ray tube constitute the vertical deflection voltages required for displaying a character,

a third pair of conductive plate means capacitively spaced from said grid,

said third pair of conductive plate means having complimentary shapes for deriving from said discharging wire grid voltages which when applied to a cathode ray tube constitute the beam control voltages required for displaying a character, and

means for applying the voltages derived by said first, second and third pair of conductive plate means to a cathode ray tube.