US3609745A

US3609745A - Symbol display apparatus

Info

Publication number: US3609745A
Application number: US721477A
Authority: US
Inventors: William R Lamoureux
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1968-04-15
Filing date: 1968-04-15
Publication date: 1971-09-28
Anticipated expiration: 1988-09-28
Also published as: NL6905592A; DE1918372A1; AT297106B; DE1918372B2; NL163642C; CH479917A; NL163642B; BE731529A; GB1244084A; SE339582B; FR2006224A1

Abstract

A direct view display system adopted to operate with a data processor combines a constant time variable velocity display system with uniform intensity. Symbols are selectively generated as a sequence of strokes from a capacitor read-only storage. By applying these strokes through the deflection control circuitry of the cathode-ray tube at a rate which exceeds the time constant of the high frequency yoke, a quasi-integration of the input staircase is provided resulting in uniform display intensity.

Description

United States Patent William R. Lamoureux TIMING PULSE DISTRIBUTOR ROM STROKE SELECT DECODER GENER A 3,333,147 7/1967 Henderson 340/324 3,381,290 4/1968- BreitenbachetaL. 340 3241 3,434,135 3/1969 Granbergetal...... 340/3241 3,437,869 4/1969 Cobbetal. 340/3241 Primary Examiner.lohn W. Caldwell Assistant ExaminerMarshall M. Curtis l Attorneys-Hanifin and Jancin and Joseph J. Connerton ABSTRACT: A direct view display system adopted to operate with a data processor combines a constant time variable velocity display system with uniform intensity. Symbols are selectively generated as a sequence of strokes from a capacitor read-only storage. By applying these strokes through the deflection control circuitry of the cathode-ray tube at a rate which exceeds the time constant of the high frequency yoke, a quasi-integration of the input staircase is provided resulting in uniform display intensity.

8 BIT X COUNTER SYMBOL DISPLAY APPARATUS FIELD OF THE INVENTION DESCRIPTION OF THE PRIOR ART Among the prior art types of symbol-generating apparatus associated with cathode-ray tube displays, one method of character generation is the full screen scan technique in which the CRT beam scans either horizontally or vertically and is selectively unblanked to define characters either as a raster of dots or as a sequence of horizontal or vertical strokes. Such systems have the advantage that the beam traverses the phosphor at a uniform velocity, providing a constant grid-tocathode voltage change on the cathode-ray tube and a resultant uniform intensity. The disadvantage of this scan method is that the expanded code, character plus video, must be stored for continual refreshing of the display, and the magnitude of the storage problem directly affects the character quality and display cost. Since scan strokes are limited to vertical or horizontal segments or 45 multiples thereof, most characters are generated as a dot raster from a dot matrix. Another method of character generation is the utilization of a character box scan such as employed, for example. in a monoscope character generator. In the monoscope, character images are stored on the target pattern of a first CRT, the selected images are scanned with the electron beam, and the secondary emission current from the scanned image is used to modulate the grid-cathode voltage of the direct view CRT. However, in order to tolerate imperfections or variations of the target image on the monoscope, overlapping scans are required which in turn tend to produce a broad brush or heavy line character on the viewing CRT. Another disadvantage is the reliability loss from the dual CRT machine and the increased scheduled maintenance implied by the synchronization and balancing of two independent analog subsystems. In more sophisticated display systems, absolute stroke character generators are used in which the electron beam is moved as a pencil point between any two defined points on the screen of the CRT. While such systems provide fine line display, the velocity of the CRT beam is variable and without elaborate correction circuitry, the intensity of each vector will vary as inverse function of its length.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a CRT display system which combines the desirable features of the scan and stroke systems without the corresponding disadvantages by operating a variable velocity system in a simulated constant velocity mode. Characters which are initially identified by digitally encoded information from a data processor or an associated storage device are decoded to select a specific character for display. The character code representations for said selected character are then translated into a sequence of short strokes from a read-only storage in which the individual stroke information for each specific character is stored. In accordance with the encoded information for the specified character, a sequence of five-bit digital words are generated under the control of a timing pulse distributor on l25-nanosecond centers. Four of these bits are applied to a deflection yoke in accordance with the identified information, while the fifth bit controls the blank/unblank status of the CRT beam. By generating the strokes and applying them to the deflection yoke at a rate which exceeds the time constant of the yoke, the beam will move continuously and at a uniform velocity upon the initiation of a character draw cycle until the character is completed. The above-described system combines the curves that are possible with a monoscope system, the fine lines achieved with an absolute vector system, uniform brightness for a given grid-to-cathode voltage, plus the inherent digital accuracy of the read-only storage without the adjustment and the drift of the monoscope systems.

Accordingly, a primary object of the present invention is to provide an improved direct view display apparatus.

Another object of the present invention is to provide an improved variable velocity cathode-ray tube character generator adapted to provide uniform display intensity without intensity correction apparatus by simulating a constant velocity operational mode.

Still another object of the present invention is to provide an improved cathode-ray display system wherein uniform intensity is provided by generating character component strokes which are applied directly to the deflection yoke of the CRT at a rate which exceeds the time constant of the deflection yoke to simulate a constant velocity system.

Another object of the present invention is to provide an improved display system wherein a capacitive read-only matrix storage device is employed to generate video signals for a CRT display.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING In the drawings:

FIG. 1 illustrates in block logical form a preferred embodiment of the present invention.

FIG. 2 illustrates in schematic form a preferred embodiment of the read-only storage.

FIG. 3 illustrates an illustrative character generated in accordance with the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawings and more particularly to FIG. 1 thereof, coded character signals are applied from a data source which might comprise, for example, a data processor or an associated storage unit to a character select decoder 21. In a display system constructed in accordance with the subject invention, the coded character signals were stored on a recirculating delay line. In the preferred embodiment of the present invention, a character capability of 2 or 64 characters is contemplated, so that six binary bits are used in the character code. However, it will be appreciated that the character capability of the present invention can be extended in either direction with a corresponding modification of the number of input bits. The character select decoder 21 will select one of 64 characters and energize the selected line as more fully described hereinafter.

Before continuing the ensuing description, reference is made to FIG. 3 to clarify the character generation technique. Each character is composed of a number of individual segments called strokes, the number of strokes being determined by the character configuration, the length of each individual stroke being controlled by the time between pulses from timing pulse distributor 27. In the preferred embodiment of the present invention herein described, a maximum of 40 strokes is required to generate the most complex character contemplated. In the interest of clarity, the time required for 40 strokes is allotted to each character, although the invention could be modified for asynchronous operation. To afford spacing between characters in a horizontal and vertical direction, each character may be considered composed within a l0 6 strobe matrix, positioned within a 12x8 character box. The character will be initially positioned by a main deflection yoke (not shown) to the starting position, which for most characters, is the 00 position in the lower left portion of the character box and represents the reset position of the deflection counters, then generated upwardly as 10 vertical segments to the point 33 where a horizontal deflection is initiated. Since at any point in the construction of a character it may be necessary to move the beam upwardly or downwardly, to the left or right, or at 90 combinations, four binary bits of deflection information are required for each stroke. For purpose of illustration, a binary 1 will be assumed to deflect the beam to the right or up, and a binary O to move the beam to the left or down in the horizontal or vertical directions respectively. The time constant of the high-speed deflection yoke employed in the preferred embodiment of the instant invention is l75 nanoseconds, while the individual vector segments are generated on l25-nanosecond centers, or a ratio of approximatelyfi. Thus, the yoke is never allowed to settle during a character generation cycle. Due to the characteristics of the yoke and specifically the setting time, the deflection will vary as a function of the deflection voltage and when it occurred in the cycle. Referring to H6. 2, the generation of the character B is shown to illustrate the operation of the subject invention. At the initiation of the cycle, the character generator is set to the position shown as point 31. The first vertical command will produce a deflection of about one-half unit, the next vertical deflection coupled with the settling of the first vertical command, will terminate a 1%. units, the third vertical command, summing with the first two, will settle at about 2% units, the fourth will settle at nearly 3 units and the fifth command at nearly four units. However, the position of the beam will never be more than one unit behind its associated timing pulse. Since the vertical bar in the B character is intended to be units high, it requires 11 units of current to obtain 10 units of deflection distance. The settling resulting from steps 8, 9, l0 and ll, if not inhibited, would carry the beam to the point 35 beyond turning point 32 as desired. To compensate for the settling of steps 8, 9, l0 and ll, a negative vertical deflection signal is applied at point 33 to counteract the deflection produced by settling of the yoke at the same time that the first horizontal positive deflection signal is applied. The approximately square root relationship between the stepping time of the deflection signals and the time constant of the high frequency yoke produces a nearly perfect right angle turn. The fact that the yoke is then allowed to settle also provides for angles which are not at the major points of the compass, and produces an apparent fine line curve as shown in FIG. 3b.

Referring back to FIG. 1, the stroke generator comprises in the preferred embodiment capacitor diode read-only memory in which the individual strokes for each character are stored. In a direct view display, since each displayed character must be continuously regenerated to maintain the displayed image, each character position is addressed. In a conventional memory cycle, the address time for character position constitutes a substantial portion of the total cycle time relative to character draw time. For example, if character draw time is 4 microseconds, the address time could be 2 microseconds or more. As a result, the read-only storage system employed in the instant invention does not operate on a normal memory cycle mode. The expanded code (video) behind the address is fetched, and the memory is readdressed for the next cycle. The read-only storage 25 employed in the preferred embodiment addresses a block of words for the next character during the character move time when the beam is being repositioned by the main deflection for the next character. The block of words thus addressed contains all of the strokes within a given ASCII code.

As previously indicated, a considerable number of strokes are used in every character and these strokes are generated at a l25-nanosecond rate. The time constant of the high-speed deflection yoke is 175 nanoseconds, thus preventing the yoke from settling between strokes. As a result, the beam will move continuously upon the initiation of the character draw cycle, thereby simulating and obtaining the uniform intensity advantages of a constant velocity system combined with the inherent digital accuracy of the read-only memory without the adjustment and drift of some monoscope systems. As more fully described hereinafter, the only storage employed in the preferred embodiment is a capacitor-diode-gating arrangement comprising a rectangular array of capacitors which are batch fabricated using double-clad flexible circuitry. lt consists of a capacitor matrix etched on a two-sided copper clad Mylar sheet. The term Mylar is a trademark for a polyester film and more specifically a polyethylene terephthalate film. One set of lines, called time lines, run on one side of the sheet and a second set of lines, called sense lines, run orthogonally on the opposite side. An information bit is contained at the intersection of a time pulse line and a sense line by the pressure or absence of an etched capacity plate on both sides of the Mylar sheet. The intersections containing capacitor plates are arbitrarily defined as binary ones, and those without plates are defined as binary zeros. At any point in the construction of a character, it must be determined whether the electron beam is to be deflected up or down, to the left or right, and whether the CRT beam is to be intensified. Thus, five bits of information are required for each of the possible 40 strokes of the character. These information bits, emanating from read-only memory 25 on lines 81-85, correspond to +X, X, +Y, -Y and Z or intensity, respectively.

Referring briefly to FIG. 2, the operation of the capacitordiode read-only storage employed in the preferred embodiment will be described. FIG. 2 shows the configuration for a specific character including five individual outputs for the +X, X, +Y, Y and Z intensity lines respectively. During the nonselected operating time, a positive reference signal is applied to input terminal 41 and to the biasing

resistors

43, 44, 45, 46 and 47 associated with the five outputs l-X, X, +Y, Y and 2 respectively. The signal level applied to the character select terminal 41 during normal operation is approximately +3 volts. The signal level applied from the timing pulse inputs labeled TP 1 through TP 40 is normally maintained at about a 3 volts level so that during the nonselected operation, the voltage across the capacitors is normally zero. When a character is selected by character select decoder 21, the resultant output applied to character select terminal 41 is approximately zero volts. Each of the five output lines has a diode 51, S2, 53, 54 and 55 associated therewith, and a reference voltage is applied to bias these diodes through 7.5 K. resistor 49. When a character is selected, but before it is generated, the signal level applied to character select terminal 41 drops to approximately zero volts for 2 microseconds, and all of the capacitors of the associated character are correspondingly biased. As a negative timing pulse is of approximately 3 volts and -lO0-nanosecond duration is applied to each of the timing pulse terminals in turn, a signal will be coupled wherever a capacitor exists through the associated output diode 51 through 55 to the associated sense amplifiers (not shown). The remaining inputs to each of the sense amplifiers will be the corresponding stroke signal associated with the remaining 63 characters. Thus, each sense amplifier has 64 inputs, only one of which will be energized at any given time. The intensity control line 67 will generate signals through its associated capacitors to control a latch or any bistable device which in turn controls the blank/unblank condition of the cathode-ray tube such that the beam is intensified only during the character-generating sequence. The subject invention operates in a push-pull environment such that the horizontal and vertical deflection will receive a positive signal in one of the two directions required to drive the yoke. However, it will be appreciated that only one side will be driven at any given time such that the presence of a +X capacitor negates the X and vice versa. Referring to FIG. 3, a timing pulse applied to TP 1 will cause an output to be generated from capacitor 69, 71 and 73 corresponding to the +X, +Y and Z intensity lines, respectively. Likewise, each succeeding timing pulse will produce an output on its associated output lines 5659 and 67 which in turn will be coupled through the associated diodes 51 through 55 to generate its respective signal on lines 81-85 which in turn are detected by the associated sense amplifiers. The specific pattern illustrated in H0. 3 is a random pattern which does not apply to any specific character, but is employed merely to illustrate the principles of the present invention. However, with reference to the character of FIG. 3, it is noted that in generating the character 8, a sequence of 1 1 successive timing pulses will be applied to generate the initial vertical line associated with the character, while on the uppermost portion as previously indicated, a negative vertical deflection signal will be applied to counteract the positive beam movement resulting from the yoke settling. The same technique is used in both axes regardless of direction of movement to counteract the effect of yoke settling. During the vertical line generation, the horizontal condition would remain the same so that no capacitors would be associated with the horizontal sequency during this interval. The resultant output signals on lines 81 through 85 are applied to a sense and control circuit shown as block 87 in FIG. 1. The sense and control circuits shown as block 87 include sense amplifiers and control circuitry for the X and Y counters 91 and 93 and the intensity control circuit 95. These signals are applied as a parallel five-bit word, two bits to each of the X and Y counters 91, 93 and a single intensity control bit to the intensity control circuit 95. A +X signal on line 97 will increment the X counter 91 by one, a -X signal on 99 will decrement the X counter 91 by one. Likewise, a +Y signal on line 101 will increment the Y counter 93 by one, and a -Y signal on line 103 will decrement the Y counter 93 by one. The X and Y counters 91 and 93 are not binary counters since S-megacycle operation of binary counters is not suitable for this application. ln the character format described in the present invention, it was noted that characters were generated in an 8X12 character box. Accordingly, the X counter contains eight bits and the Y counter 12 bits and each counter position requires one latch. While various counters having the above-described characteristics may be employed, a preferred digital counter is shown in copending application Ser. No. 604,900 (IBM Docket Kl866009), Digital Cathode-Ray Deflection System, filed by F. R. Carlock et al. on Dec. 27, l966 and assigned to the assignee of the instant invention. The output of these counters are weighted with

equal weight resistors

107, 109, 111 and 115 which are returned to the emitters of the high

frequency yoke drivers

117, 119, 121 and 123, respectively. Thus, current is stepped up or down at an 8 megacycle rate, and is presented to the yoke 125 in the shape of a staircase (up and down with landings) with steps on 8-megacycle centers. As shown in the drawing, the yoke drivers 117 and 119 drive the X-windings 127 and 129 of yoke 125 in a push-pull mode, while the Y-

current drivers

121 and 123 drive the Y-windings 131 and 133, respectively. The X- and Y-yoke windings are damped by their associated

resistors

137, 139, and 141, 143, respectively, such that the magnetic field is continually changing at a uniform rate via the applied S-megacycle pulses to provide uniform and constant intensity. In the preferred embodiment, a yoke having an inductance of 100 microhenries is employed, while the associated windings have a damping resistor of 2 k. ohms. The same technique could be utilized in electrostatic deflection by using the RC time constant instead of the L/R utilized in the preferred electromagnetic embodiment. In a system constructed in accordance with the principles of the instant invention, the individual segments were 16 mils, or approximately the same as a spot size, and the system was synchronous in that time pulses were allowed for each character. However, it will be appreciated that if desired, an asynchronous system could be readily provided and in its simplest embodiment would require only an additional sense line with a capacitor positioned after the last segment to be generated to provide the control signal indicating the end of the character. Likewise in the above-referred to system, the character selection signal applied to the character select decoder 21 originated in a recirculating delay line memory wherein only the six bit coded signal indicative of the characachieves the benefits of the scan and monoscope systems in that fine lines are achievable as with the absolute vector system, a uniform brightness is provided for a given grid-tocathode voltage, plus the inherent digital accuracy of the read only memory without the adjustment and drift of some of the monoscope systems and without the loss of reliability in a dual CRT machine and the increased scheduled maintenance implied by the balancing of two independent analog subsystems.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In a display system, the combination comprising a. a cathode-ray tube having beam deflection and beam intensity control means, said beam deflection means having a predetermined time constant,

. means responsive to coded symbol identification signals for generating a sequence of substantially unit increment deflection signals corresponding to selected symbols, and

. means for applying said sequence of deflection signals to said beam deflection means at a constant repetition rate having a periodicity less than said predetermined time constant of said beam deflection means whereby characters are generated as a sequence of incremental strokes and whereby said beam is in constant motion at a substantially constant rate of deflection so that said strokes are of substantially constant intensity.

2. A display system according to claim 1 wherein said means for generating said sequence of unit increment deflection signals corresponding to said selected symbols comprises a read-only storage device.

3. A display system of the type claimed in claim 1 wherein said beam deflection means comprises a magnetic deflection system including a magnetic yoke having a predetermined time constant.

4. Apparatus of the type claimed in claim 6 wherein said means for generating said sequence of substantially unit increment deflection signals comprises a read-only storage device.

5. Apparatus of the type claimed in claim 1 wherein said beam deflection means comprises an electrostatic deflection means having a predetermined R.C. time constant.

6. Symbol-generating apparatus for generating symbols as a sequence of incremental strokes for a cathode-ray tube display comprising in combination:

a. means for receiving coded symbol identification signals, b. means for decoding said symbol identification signals to identify selected symbols,

means responsive to said decoded symbol identification means for generating a sequence of substantially unit increment deflection signals unique to said selected symbol, each of said signals in said sequence representing one segment of said selected symbol,

. beam deflection means having a predetermined time constant, and

. means for applying said segment representing signals to said beam deflection means at a constant rate exceeding said beam deflection time constant whereby the resulting symbols are of substantially uniform intensity.

7. Apparatus of the type claimed in claim 1 wherein said beam deflection means comprises a magnetic yoke having a predetermined time constant.

Claims

1. In a display system, the combination comprising a. a cathode-ray tube having beam deflection and beam intensity control means, b. said beam deflection means having a predetermined time constant, c. means responsive to coded symbol identification signals for generating a sequence of substantially unit increment deflection signals corresponding to selected symbols, and d. means for applying said sequence of deflection signals to said beam deflection means at a cOnstant repetition rate having a periodicity less than said predetermined time constant of said beam deflection means whereby characters are generated as a sequence of incremental strokes and whereby said beam is in constant motion at a substantially constant rate of deflection so that said strokes are of substantially constant intensity.

6. Symbol-generating apparatus for generating symbols as a sequence of incremental strokes for a cathode-ray tube display comprising in combination: a. means for receiving coded symbol identification signals, b. means for decoding said symbol identification signals to identify selected symbols, c. means responsive to said decoded symbol identification means for generating a sequence of substantially unit increment deflection signals unique to said selected symbol, each of said signals in said sequence representing one segment of said selected symbol, d. beam deflection means having a predetermined time constant, and e. means for applying said segment representing signals to said beam deflection means at a constant rate exceeding said beam deflection time constant whereby the resulting symbols are of substantially uniform intensity.