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United States Patent 3,614,764
Kolb ,   et al. October 19, 1971

APPARATUS FOR PROVIDING GRAPHICAL IMAGES ON A RADIANT-ENERGY-RESPONSIVE SURFACE

Abstract

A phototypesetting system wherein characters may be displayed on a cathode-ray tube includes a character-positioning means for locating accurately each character horizontally on the face of the tube in response to the status of an accumulator means. The character-positioning means includes an output circuit means having a substantially linear response to the status of the accumulator to provide a substantial portion of the character-positioning signal, and a compensating means, also responsive to the status of the accumulator means, to supply only that portion of the character-positioning signal which is necessary to compensate for nonlinearity in the characteristics of the cathode-ray tube so that the characters of consistent size may be accurately positioned regardless of the horizontal position of the characters relative to the face of the tube.


Inventors: Kolb; Edwin R. (University Heights, OH), Drake; Herbert E. (Willowick, OH)
Assignee: Harris-Intertype Corporation (Cleveland, OH)
Appl. No.: 04/710,350
Filed: March 4, 1968


Current U.S. Class: 345/13 ; 346/33MC; 396/550
Current International Class: B41B 27/00 (20060101); B41B 27/28 (20060101); B41B 19/00 (20060101); B41B 19/01 (20060101); G09G 1/00 (20060101); G06f 003/14 ()
Field of Search: 340/324.1,324A 95/4.5 315/26,31

References Cited

U.S. Patent Documents
3422305 January 1969 Infante
3435278 March 1969 Carlock et al.
3403289 September 1968 Garry
3422304 January 1969 Thorpe
Primary Examiner: Caldwell; John W.
Assistant Examiner: Trafton; David L.

Claims



What is claimed is:

1. Apparatus for providing graphical images on an energy-responsive surface with an energy beam and having beam deflection means for deflecting said beam along perpendicularly related coordinate axes to form images on said surface and comprising:

image position command means registering digital representations respectively representative of desired major positions along one of said axes at which images are to be formed;

first deflection signal means for providing linear deflection signals for application to said deflection means to deflect said beam along said one axis in accordance with the magnitude of said linear deflection signals, said linear deflection signals being derived from said digital representation and being of magnitudes respectively linearly related to said digital representations to effect the majority portion of the beam deflection required to position the beam to each of said desired positions;

second deflection signal means for providing, for each of said desired positions, a nonlinear deflection signal which is independently derived from said digital representations for application to said deflection means to effect further positioning of said beam along said one axis to an associated one of said desired positions to compensate for nonlinear beam

responsive characteristics. 1. Apparatus for providing graphical images on an energy-responsive surface with an energy beam and having beam deflection means for deflecting said beam along perpendicularly related coordinate axes to form images on said surface and comprising:

image position command means registering digital representations respectively representative of desired major positions along one of said axes at which images are to be formed;

first deflection signal means for providing linear deflection signals for application to said deflection means to deflect said beam along said one axis in accordance with the magnitude of said linear deflection signals, said linear deflection signals being derived from said digital representations and being of magnitudes respectively linearly related to said digital representations to effect the majority portion of the beam deflection required to position the beam to each of said desired positions;

second deflection signal means for providing, for each of said desired positions, a nonlinear deflection signal which is independently derived from said digital representations for application to said deflection means to effect further positioning of said beam along said one axis to an associated one of said desired positions to compensate for nonlinear beam responsive characteristic.

2. Apparatus for providing graphical images on an energy-responsive surface with an energy beam and having beam deflection means for deflecting said beam along perpendicularly related coordinate axes to form images on said surface and comprising:

image position command means registering digital representations respectively representative of desired major positions along one of said axes at which images are to be formed;

first deflection signal means for providing linear deflection signals for application to said deflection means to deflect said beam along said one axis in accordance with the magnitude of said linear deflection signals, said linear deflection signals being derived from said digital representations and being of magnitudes respectively linearly related to said digital representations to effect the majority portion of the beam deflection required to position the beam to each of said desired positions;

second deflection signal means for providing, for each of said desired positions, a nonlinear deflection signal which is independently derived from said digital representations for application to said deflection means to effect further positioning of said beam along said one axis to an associated one of said desired positions to compensate for nonlinear beam responsive characteristics;

said second deflection signal means includes a first portion having compensating circuit means for providing a first portion of each said nonlinear deflection signal as a function of an associated one of said desired positions and an interpolation circuit means for providing the remaining portion of each said nonlinear deflection signal dependent at least in part on the said associated one of said desired positions.

3. Apparatus as set forth in claim 2 wherein said compensating circuit means includes reference means for providing reference signals having values respectively dependent upon associated ones of said desired positions, said interpolation means providing said remaining portions of said nonlinear deflection signals dependent at least in part on the value of said reference signals.

4. Apparatus for providing graphical images on an energy-responsive surface with an energy beam and having beam deflection means for deflecting said beam along perpendicularly related coordinate axes to form images on said surface and comprising:

image position command means registering digital representations respectively representative of desired major positions along one of said axes at which images are to be formed;

first deflection signal means for providing linear deflection signals for application to said deflection means to deflect said beam along said one axis is in accordance with the magnitude of said linear deflection signals, said linear deflection signals being derived from said digital representations and being of magnitudes respectively linearly related to said digital representations to effect the majority portion of the beam deflection required to position the beam to each said desired positions;

second deflection signal means for providing, for each of said desired positions, a nonlinear deflection signal which is independently derived from said digital representations for application to said deflection means to effect further positioning of said beam along said one axis to an associated one of said desired positions to compensate for nonlinear beam responsive characteristics;

said second deflection signal means provides reference signals having values dependent upon the said desired positions, and configuration forming means for controlling the formation of configurations on said surface dependent in part on the value of said reference signals.

5. Apparatus for providing graphical images on an energy-responsive surface with an energy beam and having beam deflection means for deflecting said beam along perpendicularly related coordinate axes to form images on said surface and comprising:

image position command means registering digital representations respectively representative of desired major positions along one of said axes at which images are to be formed;

first deflection signal means for providing linear deflection signals for application to said deflection means to deflect said beam along said one axis in accordance with the magnitude of said linear deflection signals, said linear deflection signals being derived from said digital representations and being of magnitudes respectively linearly related to said digital representations to effect the majority portion of the beam deflection required to position the beam to each of said desired positions;

second second deflection signal means for providing, for each of said desired positions, a nonlinear deflection signal which is independently derived from said digital representations for application to said deflection means to effect further positioning of said beam along said one axis to an associated one of said desired positions to compensate for nonlinear beam responsive characteristics;

said energy-responsive surface is the display face of a cathode-ray tube and said second deflection signal means provides focus signals for said tube which vary in value dependent upon the said desired positions.
Description



RELATED APPLICATIONS

Reference is hereby made to U.S. application Ser. No. 591,734 , filed Nov. 3, 1966, entitled TYPESETTING SYSTEM; and U.S. application Ser. No. 710,349, filed Mar. 4, 1968 entitled PHOTOTYPESETTING METHOD AND APPARATUS, both assigned to the assignee of this application.

BACKGROUND OF THE INVENTION

It is now well known in the computer industry to generate characters for display on a cathode-ray tube, and it has also been proposed to use a cathode-ray tube as a projection device in a phototypesetting system where the characters formed on the face of the tube are recorded on photographic material. The characters may be formed in a variety of ways, including scanning a limited portion of the tube and illuminating only those areas which form a part of the character, moving the cathode-ray tube beam from spot to spot within a limited area of the tube to form the character, or by directing a shaped beam representing the character onto the face of the tube. Each of these methods of character generation in a phototypesetting system requires that the character be accurately positioned horizontally with respect to the face of the tube.

It is understood by those skilled in the art that the deflection currents necessary to move the cathode-ray tube beam, and thus the characters, in equal increments across the tube face are not necessarily equal themselves. The amount of variation depends on the quality and type of tube used as well as upon the characteristics of the deflection amplifiers and deflection coils. Within the tube, the nonlinear characteristics are due to the varying length of the beam between the deflection elements and the tube face as it moves horizontally across the tube face and to the change in the angle at which the beam strikes the tube face. Thus, a nonlinear deflection current must be supplied to the deflection coils surrounding the tube if equal increments of spacing are to be created on the face of the tube. It has also been recognized that the voltage necessary to focus the beam is also dependent upon the horizontal position of the bema with respect to the tube face.

In the above mentioned U.S. application Ser. No. 591,734, characters on a cathode-ray tube beam are positioned in response to the status of an accumulator means which selects one of 64 circuits each providing the deflection potentials for a limited horizontal segment of the tube. An interpolation means is also included to further locate the beam between those positions determined by the output from the selected one of the 64 circuits. Each of the 64 circuits supplies the proper voltage to compensate for nonlinearities in the cathode-ray tube, as well as appropriate focusing and intensity voltages to permit equal spacing of the beam on the face of the tube in response to equal changes in the status of the accumulator. This arrangement, however, requires that substantially all of the deflection potential be supplied by a single selected circuit, and therefore it was necessary that the circuit components be designed to function at these high voltage or current levels without long term drift or variations in their output voltage.

The present invention is an improvement over the nonlinearity compensating circuit described in the above mentioned U.S. application Ser. No. 591,734, and utilizes a single circuit to provide a majority of the deflection signals for positioning the cathode-ray tube beam. This circuit has a substantially linear output in response to an accumulator means, and further, utilizes a compensating circuit to supply the necessary correcting signals to the output of the linear circuit so that equal spacing will occur on the face of the cathode-ray tube for equal changes in the status of the accumulator means. Each compensating circuit also supplies reference voltages to the character generating means to insure that the characters formed are of proper size independent of their horizontal location on the tube face.

It is therefore an object of this invention to provide an improved phototypesetting system employing a cathode-ray tube for displaying characters on the face thereof, including a character-positioning means comprising an output circuit means having a substantially linear response to the status of an accumulator means which represents the horizontal locations at which the characters are to be displayed, the output circuit means supplying a substantial portion of the character-positioning signal to the deflection elements of the cathode-ray tube, and a compensating means, also responsive to the status of the accumulator means, to supply an additional character-positioning signal which compensates for the nonlinear characteristics of the cathode-ray tube so that characters of consistent size may be accurately positioned on the tube, and so that equal changes in the status of the accumulator means will cause equal changes in the horizontal spacing on the face of the tube regardless of the horizontal position of the characters relative to the face of the tube; and to provide means supplying the proper reference voltages to a character generating means so that characters of consistent size may be generated regardless of their horizontal position relative to the face of the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a system employing a cathode-ray tube to display characters;

FIG. 2 is a detailed block diagram of a phototypesetting system incorporating means compensating for the nonlinear characteristics of a cathode-ray tube thereby permitting characters of typesetting quality to be formed having consistent size and spacing independent of their horizontal location on the tube face;

FIG. 3 is an electrical schematic diagram of the output circuit means which provides a major portion of the deflection voltage to position the characters horizontally on the face of the cathode-ray tube;

FIG. 4 is a detailed block diagram of a means compensating for the nonlinear characteristic of the cathode-ray tube;

FIG. 5 is an electrical schematic diagram of one of the 48 individual compensating circuits which comprise the compensating means of FIG. 4; and

FIG. 6 is an electrical schematic diagram showing a typical amplifier and offset means used in conjunction with the compensating means of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 which is a simplified block diagram of a cathode-ray tube system for displaying characters, a character memory 10, which may be a rotating disc onto which the character information is optically recorded, a magnetic disc, a core matrix memory, or other similar type of memory device, supplies the necessary information for generating a font of characters through the character selection circuit 15 to the character generator circuit 20. Character-generating signals representing a selected character are applied along lines 21 and 22 to the horizontal and vertical deflection amplifiers 25 and 26, respectively, which amplifiers in turn modify the current through deflection coils 27 and 28 surrounding the neck of the cathode-ray tube 30 to deflect the beam to the appropriate positions and form the character.

The individual characters are positioned relative to the face of the tube in response to a command from a control circuit 35 which supplies character-positioning signals along lines 36 and 37 to the horizontal and vertical amplifiers 25 and 26. A control record 40, containing such information as character selection and interword spacing, is read by a tape reader 41 and that information supplied as an input to the buffer 42. While the control record 40 is shown in FIG. 1 as a magnetic tape, other types of control means such as the output from a computer, may be used to select and position the characters and to supply other functional data.

The buffer 42 accepts the information from the control record at its optimum readout rate and then transfers that information to the decoder circuit at a rate which can be accepted by the remainder of the phototypesetting system. Different control functions will take differing amounts of time, and even the generation of some characters in some cases will take longer than the generation of other characters. One output of the decoder circuit is therefore applied as a character selection signal to the character selection circuit 15 and other outputs are directed to the control circuit 35.

In the preferred embodiment of this invention, the cathode-ray tube 30 displaying the characters uses electromagnetic deflection means, and both the character-positioning signals and the character-generating signals are variable voltages applied to the horizontal and vertical deflection amplifiers 25 and 26 which modify the current flowing through the deflection coils 27 and 28. It has been found that supplying both the character-positioning and the character-generating signals to the same amplifier is preferred, especially when generating characters of typesetting quality. It is understood, however, that this invention is not limited to cathode-ray tubes having electromagnetic deflection means.

The magnitude of the character-positioning signals from control circuit 35 appearing on lines 36 and 37 are determined by the instructions from the control record 40 and in response to information received from the character generator circuit 20. In the present invention, the cathode-ray tube beam is first positioned by the character positioning signals and thereafter moved to various positions within a character field in response to the character-generating signals. The character generator circuit 20 also functions to modulate the beam intensity so that it may illuminate the appropriate spots on the tube face as the beam is moved within the character field so that a character of typesetting quality may be formed.

The character generator circuit 20 then supplies the necessary information to the control circuit 35 to modify the character-positioning signals once the character-generating operation for that character is finished in order to reposition the starting location of the cathode-ray tube beam for the next character. The point size and width of the character will, of course, determine the magnitude of the change in the character-positioning signals. Interword spaces supplied by the control record 40 also function to modify the character-positioning signals so that a justified line of type may be formed on the cathode-ray tube face. In the preferred embodiment of the invention, only a single line of characters at a time appears on the cathode-ray tube.

Since the cathode-ray tube 30 is inherently a nonlinear device, the proper voltages must be applied to the deflection amplifiers in order to provide the proper spacing between characters, to provide the proper focusing voltage for the cathode-ray tube beam and to generate characters having consistent size. Without correcting for these nonlinear characteristics, characters formed at the edge of the tube will have a size and spacing different from those characters formed near the center of the tube. Since lines of characters of typesetting quality are desired, means are provided to compensate for this nonlinear characteristic. This is done by modifying the voltages applied to the deflection amplifiers in accordance with the horizontal position of the characters as they appear on the face of the tube.

FIG. 2 is a more complete block diagram showing the general arrangement of the various circuits and structural components which make up the complete phototypesetting system. As mentioned previously, the control record is read by the reader 41 which converts the information stored thereon into electrical signals which are directed into the buffer storage device 42.

The control record 40, among other things, selects the point size of the characters to be displayed, the type or style of font to be displayed, interword spaces, the amount of leading, and other similar phototypesetting functional commands. The control record also selects the characters to be displayed in sequence, and when used in a phototypesetting system, the control record supplied the proper interword spaces to create justified lines of type.

One output from the decoder 43 is applied to the character generator 20 to select the information representing one of the plurality of characters supplied by the character memory 10 for display on the cathode-ray tube 30. Another output from the decoder 43 is applied to the control circuit 35 where the proper interword spaces for justified lines of type are stored in the space memory circuit 45, and where the size of the character to be displayed is placed into the character size circuit 46. Also, leading instructions are stored in the leading control circuit 47, and vertical rule commands may be applied to the vertical rule generator 48.

The characters formed on the face of the tube may be photographically recorded by projecting the image of the characters through lens 50 onto photographic material 51 which is positioned relative to the face of the tube by a motor 52. The motor is controlled by one output of the leading control circuit 47. The other output of the leading control circuit is connected to the leading digital to analogue converter or D/A 53 which positions the characters vertically with respect to the face of the tube allowing interpolation between the angular positions of the stepping motor 52 and permits simultaneous leading of the film and generation of the characters.

In the embodiment of the invention described herein, characters may be generated by moving a cathode-ray tube beam from spot to spot and energizing the beam to illuminate the spots and thus form the character. This character generator is more completely described in the above-mentioned copending U.S. application Ser. No. 710,349. It is to be understood, however, that other types of character-generating means may be used without departing from the scope of this invention, such as that described in copending U.S. application Ser. No. 591,734.

As mentioned previously, the point size of the character is determined by an instruction from the control record 40, and when a character is selected for display, a signal representing the width of that character is supplied by the character generator 20 to the character size circuit 46 on line 55. The character size circuit supplies several outputs, one modifying the status of the horizontal scale register 56, another establishing a status of the vertical scale register 57, a third output is applied on line 58 to the beam control circuit 59 and determines the duration of time that the beam remains on at each spot to control the intensity of the character display, and a fourth output represents the width of the character display which is applied to the horizontal accumulator circuit 60 at the completion of the character-generating operation.

By way of illustration, assuming that the cathode-ray tube beam is initially positioned at the extreme left of the writing surface on the cathode-ray tube face, the character generator circuit 20 will supply the necessary signals to the horizontal and vertical deflection amplifiers to form the character, and once character generation has been completed, the control circuit 35 will reposition the beam to a new starting location preparatory to the generation of the next character. Since each character normally has a different width, as for example in FIG. 1 where the letter a has a width value W.sub.a and the letters i and m have width values W.sub.i and W.sub.m, respectively, the spacing of each character is determined by a signal from the character memory which, when multiplied by the point size to be displayed as stored in the character size circuit 46, results in a digital input which is applied to the horizontal accumulator 60 after completion of character generation. The horizontal accumulator 60 controls other circuits which supply the proper character positioning signal to the horizontal deflection amplifier.

Referring to the generation of a character, the output of the character generator 20 is applied to the horizontal character generating digital to analogue circuit or D/A 62 and the the vertical character generating D/A 63. These circuits convert the digital D/A 76. supplied by the character generator into analogue voltages which are applied as inputs to the horizontal and vertical deflection amplifiers 25 and 26. The magnitudes of these analogue voltages are determined by reference voltages supplied to these components by the horizontal scale D/A 65 and the vertical scale D/A 66, respectively.

The reference voltages supplied by the horizontal scale D/A 65 and the vertical scale D/A 66 are determined in part by the horizontal position of the character to be generated and by the status of the horizontal scale register 56 and the vertical scale register 57, respectively, these registers having previously been set to a digital value representing the character size by the character size circuit 46 under the direction of the control record 40.

The horizontal position of the characters displayed on the tube face is determined by the status of the horizontal accumulator 60 consisting of 14 registers, the outputs of which, in the preferred embodiment, are connected to three circuit means for positioning the characters, including an output circuit means or major horizontal linear D/A 70, a compensating means or the major horizontal nonlinear D/A 75, and the interpolation D/A 76. The outputs from each of these D/A's is combined and supplied as a voltage input to the horizontal deflection amplifier 25.

The six registers in the horizontal accumulator 60 control both the output circuit means 70 and the compensating means 75, while the eight registers representing the least significant digits in the accumulator 60 control the interpolation D/A 76.

As mentioned previously, deflection voltages necessary to position the beam in equal incremental steps horizontally across the face of the tube are not necessarily equal, but will vary depending upon the horizontal location of the beam. This is due in part to the fact that the beam path becomes shorter as it moves from the edges of the tube toward the center and the angle which the beam makes with the face of the tube approaches 90.degree.. Thus, it is necessary to supply a nonlinear deflection current to the deflection coil surrounding the tube if equal increments of spacing are to be created on the face of the tube. Furthermore, the proper focusing voltage will depend on the horizontal position of the beam.

In this invention, the horizontal character-positioning signal is supplied by three circuits, the first being a linear circuit supplying a major portion, from 90 to 100 percent, of the character-positioning signal, the second being a nonlinear circuit supplying a signal which depends upon the horizontal position of the beam, and the third circuit interpolating between the locations of the beam established by the first two circuits. In this embodiment of the invention, the character-generating signal is added to the output of these three circuits to position the beam at a series of predetermined locations relative to the starting position, as defined by the character-positioning signal thus to form the selected character.

A major portion of the horizontal character-positioning signal is supplied to the horizontal deflection amplifier 20 by the output circuit means or major horizontal linear digital to analogue circuit or D/A 70, a detailed schematic of which is shown in FIG. 3. This circuit provides substantially equal changes in the magnitude of its output signal in response to equal changes in the status of those registers in the accumulator means 60 to which it is operably connected. Therefore, the output circuit means 70 supplies a voltage which is a linear function of the status of the accumulator means 60.

The six registers representing the most significant digits stored in the accumulator means are shown at the left in FIG. 3. In the preferred embodiment of this invention, the register 60a representing the most significant digit stored in the accumulator functions to move the beam 5.668 inches from its last position when it is placed in the set state. The next most significant digit register 60b, when set, will cause the beam to move 2.834 inches (or one-half that of register 60a), and the remainder of the registers, 60c through 60f each move the beam a distance which is half that caused by the previous register. By combining the outputs from several of these registers, the beam may be directed to a location within 0.177 inch anywhere within the writing surface on the face of the tube.

The output circuit means 70 includes an operational amplifier OA1 having a plurality of current summing resistors R1 through R6 connected to its input. The output on line L1 from the operational amplifier, constituting a major portion of the horizontal character positioning signal, is applied as one input to the horizontal deflection amplifier 25.

One end of each current summing resistor R1 through R6 is connected directly to the input of the operational amplifier OA1. This input is maintained at "virtual ground," or a constant voltage, through the operation of a feedback network FB1, consisting of a plurality of resistances, with potentiometer R7 determining the total excursion of the output signal on line L1 and thus fixing the limits of the writing surface on the face of the cathode-ray tube. As mentioned previously, the writing surface for a 10-inch cathode-ray tube is approximately 81/2 inches.

The current summing resistors R1 through R6 are made electrically effective in the input circuit of the operational amplifier by a first circuit means, shown generally at 90, which senses the status of the accumulator means 60. This is accomplished by electrically connecting the selected ones of the current summing resistors to a reference current source maintained at a predetermined voltage level. This source of current is obtained from operational amplifier OA2, and is found on the reference voltage line 91. The reference voltage output of amplifier OA2 is established by Zener diode Z1 in its input circuit.

The first circuit means 90 includes a transistor switch for each register in the accumulator being monitored, and each transistor switch controls the current through a pair of diodes. Each transistor switch is biased into the conducting state when the corresponding register in the accumulator is in the reset state. With the transistors conducting, a -0.5-volt bias is applied to the anode of each diode, reverse biasing these diodes to cutoff, and isolating the associated current summing resistor from the source of current obtained from reference voltage line 91. When any register in the accumulator is placed in the set state, the corresponding transistor switch will be biased into the nonconducting state, and a positive potential from line 92 will appear on the anode elements of the diodes through a dropping resistor. The diodes will be forward biased and conduct, and the associated current summing resistor will be electrically connected to the reference voltage line 91.

Since each of the current summing resistors R1 through R6 has the same voltage applied thereto due to the clamping action of the reference voltage from operational amplifier OA2, and the "virtual ground" established by operational amplifier OA1, each resistor will operate independently of any other resistor, and the input current to the operational amplifier OA1 will be a function only of the number of current summing resistors in the circuit and their resistance.

Current summing resistor R1, for example, has a value of 4,000 ohms, and current summing resistor R2 has a value of 8,000 ohms. Thus, exactly twice the current will be supplied through resistor R1 to the operational amplifier OA1, as supplied through resistor R2. Therefore, when only accumulator register 60a is set, the beam will move to the 5.668-inch position relative to the starting location of the writing surface, and when only accumulator register 60b is set, the beam will move to the 2.834-inch position or half the distance. Resistors R1 and R2 each consist of a network of four resistances, one of which is adjustable for the purpose of obtaining a precise value of resistance, since these two elements provide a substantial proportion of the character-positioning signal and any variation in the value of these elements would cause a substantial error in the positioning of the characters on the face of the tube. The remainder of the current summing resistors are not so compensated since they contribute less to the absolute deflection of the beam than resistors R1 and R2, and any errors in their resistance value may be easily corrected by the compensating means.

With the circuit shown in FIG. 3, no current summing resistor will be connected as an input to the operational amplifier if all of the registers were placed in the reset state and therefore no output current will appear on line L1. Since the cathode-ray tube beam will normally be centered when there is no current applied to the deflection coils, the output circuit means 70 includes means to supply an offset voltage to the operational amplifier, thus to position the beam to one end of the writing surface. This offset voltage is obtained from Zener diode Z1 and is applied through resistors R7 and potentiometer R8 to the input of operational amplifier OA1. Potentiometer R8 controls the center position of the writing surface with respect to the tube face.

Thus, the output circuit means 70 supplies a major portion, between 90 and 100 percent, of the horizontal deflection signal with the magnitude of that signal varying linearly in accordance with the status of the accumulator means 60. Since this circuit does not include precision adjustment means, it can be designed to give a stable output at high current levels over a long period of time.

Since the cathode-ray tube is a nonlinear device, a compensating means or major horizontal nonlinear digital to analogue (D/A) circuit 75 such as that shown in FIG. 4 is provided to supply a further portion of the horizontal character positioning voltage to be added to that signal supplied by the output circuit means 70. The compensating means 75 includes 48 individual compensating circuit elements 95a through 95vv, each having an output providing a further portion of the horizontal character-positioning signal. A second circuit means or decoder 100 senses the status of the six most significant digits in the accumulator means 60 and selects the one of the 48 compensating circuits to combine its output with the output signal from the output circuit means 70 so that equal changes in the status of the registers in the accumulator means 60 causes equal changes in horizontal spacing on the face of the cathode-ray tube independently of the horizontal position of the characters relative to the face of the tube.

Each of the 48 individual circuits has four outputs, one of which provides a further portion of the horizontal character-positioning signal and is applied as an input to the deflection operational amplifier and offset circuit 105, another output supplies the correct focusing voltage for the cathode-ray tube within each of the 48 horizontal areas controlled by the selected circuit and this voltage is applied to the focus operational amplifier and offset circuit 106, and the third and fourth outputs provide the proper vertical size and horizontal size voltages to the operational amplifiers and offset circuits 107 and 108, respectively. As mentioned previously, the vertical size and horizontal size voltages are modified by the horizontal and vertical scale D/A's 65 and 66 and then applied as reference voltages to the character generator circuit so that characters of consistent size are formed and displayed on the face of the cathode-ray tube regardless of their horizontal position relative to the face of the tube.

The second circuit means 100 is divided into two separate circuits. Decoder circuit 100a senses the status of the three most significant digit registers in the accumulator 60 and provides an enabling voltage on one of six output lines 110a through 110f. It is recognized that a three-digit binary code is cable of producing eight outputs, however, the two highest codes in the register are not used since the writing surface on the face of the tube is divided into only 48 segments in the embodiment where an 8 1/2-inch writing surface is employed. However, if a larger writing surface were needed, the decoder 100a could be expanded to provide a full eight outputs, with a corresponding increase in the number of the individual compensating circuits from 48 to 64. Thus, in the embodiment described, each of the six outputs from decoder circuit 100a is applied as an enabling voltage to eight individual compensating circuits, as for example, output line 110a connected to compensating circuits 95a through 96h.

Decoder circuit 100b senses the status of registers 60d through 60f in the accumulator means and provides an output on one of eight lines 115a through 115h each line supplying an enabling voltage to six individual compensating circuits. Only the single compensating circuit which has an enabling voltage from both the decoder 100a and decoder 100b will be effective to supply the compensating voltage to the horizontal character-positioning signal, the correct focus voltage for the horizontal position of the beam as represented by the status of the accumulator means, and the horizontal and vertical size voltages which are ultimately directed to the character-generating means.

Reference is now made to FIG. 5 which is a detailed electrical schematic diagram of a typical one of the 48 individual compensating circuits comprising the compensating means 75. Each of these circuits is substantially identical with only minor variations in the component values, especially in resistors R15 through R22, and output voltages being necessary to supply the correct compensating or reference outputs.

Only when the enabling signal is applied to both of the diodes D1 and D2 from the decoders 100a and 100b , respectively, will transistor Q1 conduct and bias switching transistor Q2 into the conducting state. At this time, diodes D3 through D10, which were biased to cutoff by the -2 -volt potential on line 120, are now allowed to conduct through the action of the positive biasing potential supplied through transistor Q2. The thirty volt reference or clamping voltage on line 125 is then applied to a resistance network where that voltage is modified and applied as the compensating or reference voltage for that horizontal position of the beam to an associated amplifier and offset circuit.

FIG. 6 is an electrical schematic diagram of a typical amplifier and offset circuit. The voltage from one of the outputs of the selected individual compensating circuit appearing on line 130 is applied as an input to the operational amplifier OA3. The gain of this amplifier is determined by the feedback circuit FB2 and by the setting of potentiometer R10 in that circuit. An offset voltage is provided by the resistance network shown generally at 140.

Referring again to FIG. 2, the interpolation D/A 76 is shown sensing the status of the accumulator means 60 and supplies the remaining portion of the horizontal character-positioning voltage. This circuit is similar in design to the major horizontal linear D/A 70 circuit shown in FIG. 3, except it does not include means to supply an offset voltage. The interpolation D/A 76 is provided with a reference voltage from the horizontal size output of the selected one of the individual compensating circuits in the compensating means 75 and modifies that voltage in accordance with the status of the least significant digits stored in the accumulator 60 as sensed by a third circuit means, similar to circuit means 90 in FIG. 3.

Thus, the output circuit means 70 and the compensating means 75 divides the writing area of the tube into 48 segments, and the interpolation D/A divides each of these segments into 255 smaller segments. Accordingly, for a 10 -inch cathode-ray tube having an 8 1/2 -inch writing surface, the beam may be positioned within approximately 0.000692 inch by the combined output of these three character-positioning circuits.

While the form of apparatus herein described constitutes a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

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