CATHODE RAY TUBE DYNAMIC CHARACTER CORRECTION CIRCUIT
United States Patent 3757162
A low cost cathode ray tube display system includes major vertical and horizontal deflection circuits for providing movement of the beam in a horizontal and vertical direction respectively and further includes minor vertical and horizontal deflection driver circuits which provide for rapid vertical and horizontal trace and retrace of the beam within a selected area within a character position. A correction circuit is associated with the minor vertical deflection circuit for applying a periodic linear triangular waveform synchronized with the operation of the horizontal deflection circuits whose magnitude is adjustable as a function of character line position so as to provide a uniform character height for each of the characters written along a given line.
US Patent References:
Distortion correction system for flying spot scanners
Gray - September 1968 - 3403289
DISPLAY SYSTEM USING CATHODE RAY TUBE DEFLECTION YOKE NON-LINEARITY TO OBTAIN CURVED STROKES
Criscimagna et al. - November 1970 - 3540032
DEFLECTION AMPLIFIER
Yanishevsky - November 1969 - 3479553
Distortion correction system for flying spot scanners
Gray - September 1968 - 3403289
DISPLAY SYSTEM USING CATHODE RAY TUBE DEFLECTION YOKE NON-LINEARITY TO OBTAIN CURVED STROKES
Criscimagna et al. - November 1970 - 3540032
DEFLECTION AMPLIFIER
Yanishevsky - November 1969 - 3479553
Application Number:
05/214938
Publication Date:
09/04/1973
Filing Date:
01/03/1972
View Patent Images:
Export Citation:
Assignee:
Honeywell Information System Inc. (Waltham, MA)
Primary Class:
Other Classes:
315/394, 315/391, 348/E03.045
International Classes:
G09G1/18; H04N3/233; G09G1/14; H04N3/22; H01J29/70
Field of Search:
315/27TD,27R,276D,18,22,24
Primary Examiner:
Quarforth, Carl D.
Assistant Examiner:
Potenza J. M.
Claims:
Having described the invention, what is claimed as new and novel is
1. Correction apparatus for use in a cathode ray tube display system including a cathode ray tube having major horizontal deflection and major and minor vertical deflection driver circuits operative to generate major and minor positioning waveforms for driving major horizontal and major and minor vertical coils coupled to said tube for deflecting the electron beam along a horizontal line and a vertical direction wherein said minor vertical deflection driver circuits generate a high frequency pulse waveform for producing character writing strokes for said line, said correction apparatus comprising:
2. The correction apparatus of claim 1 wherein said generator means includes:
3. The correction apparatus of claim 2 wherein said comparator means is arranged to operate as a zero-crossing detector circuit and produces said square wave waveform having a period corresponding to said time position waveform.
4. The correction apparatus of claim 2 wherein said integrator means includes:
5. The correction apparatus of claim 3 wherein said generator means further includes:
6. The correction apparatus of claim 4 wherein said adjustable slope correction means includes:
7. The correction apparatus of claim 1 wherein said input means includes:
8. In a display system including a wide deflection angle cathode ray tube, horizontal, vertical and character positioning coils and means for driving said horizontal, vertical and character positioning coils respectively with horizontal, vertical and character pulse positioning signals for establishing the location of character segments for a line of characters to be displayed by said tube wherein said character positioning signals causes a plurality of ramp strokes for a number of time intervals for each character segment, a correction apparatus for compensating for distortion in the characters formed at the extremities of the face of the tube of said line larger in height than the characters formed at the center by differences between the centers of deflection said vertical and character positioning coils, said correction apparatus comprising:
9. In a display system including a wide deflection angle cathode ray tube, horizontal, vertical and character positioning coils and means for driving said horizontal, vertical and character positioning coils respectively with horizontal, vertical and character pulse positioning signals for establishing the location of character segments for a line of characters to be displayed by said tube wherein said character positioning signal causes a plurality of ramp strokes for a number of time intervals for each character segment, a correction apparatus for compensating for distortion in the characters forms at the extremities of the face of the tube of said line larger in height than the characters formed at the center by differences between the centers of deflection said vertical and character positioning coils, said correction apparatus comprising:
10. In the system of claim 9 wherein said comparator means is arranged to operate as a zero-crossing detector circuit and produces said square wave waveform having a period corresponding to said time position waveform.
11. In the system of claim 9 wherein said integrator means includes:
12. In the system of claim 10 wherein said function generator means further includes:
13. In the system of claim 11 wherein said adjustable slope correction means includes:
14. In the system of claim 10 wherein said input means includes:
15. A cathode ray tube display system including a cathode ray tube having major horizontal deflection and major and vertical deflection driver circuits operative to generate major and minor positioning waveforms for driving major and minor horizontal and vertical control circuits coupled to said tube for deflecting the electron beam along a horizontal line and in a vertical direction wherein said minor vertical deflection driver circuits generate a high repetition frequency pulse waveform for producing character writing strokes for said line, said horizontal deflection circuits produce a low frequency sawtooth waveform as said major horizontal positioning waveform and said major vertical deflection circuits produce a staircase waveform as said major positioning waveform, said system further including correction apparatus comprising:
16. A cathode ray tube display system including a cathode ray tube having major horizontal deflection and major and minor vertical deflection driver circuits operative to generate major and minor positioning waveforms for driving major and minor horizontal and vertical control circuits coupled to said tube for deflecting the electron beam along a horizontal line and in a vertical direction wherein said minor vertical deflection driver circuits generate a high repetition frequency pulse waveform for producing character writing strokes for said line, said horizontal deflection circuits produce a low frequency sawtooth waveform as said major horizontal positioning waveform and said major vertical deflection circuits produce a staircase waveform as said major positioning waveform, said system further including correction apparatus comprising:
17. The system of claim 16 wherein said predetermined amplitude of said variable voltage source is adjusted to establish a minimum value for said signal applied to said minor vertical control circuit.
1. Correction apparatus for use in a cathode ray tube display system including a cathode ray tube having major horizontal deflection and major and minor vertical deflection driver circuits operative to generate major and minor positioning waveforms for driving major horizontal and major and minor vertical coils coupled to said tube for deflecting the electron beam along a horizontal line and a vertical direction wherein said minor vertical deflection driver circuits generate a high frequency pulse waveform for producing character writing strokes for said line, said correction apparatus comprising:
2. The correction apparatus of claim 1 wherein said generator means includes:
3. The correction apparatus of claim 2 wherein said comparator means is arranged to operate as a zero-crossing detector circuit and produces said square wave waveform having a period corresponding to said time position waveform.
4. The correction apparatus of claim 2 wherein said integrator means includes:
5. The correction apparatus of claim 3 wherein said generator means further includes:
6. The correction apparatus of claim 4 wherein said adjustable slope correction means includes:
7. The correction apparatus of claim 1 wherein said input means includes:
8. In a display system including a wide deflection angle cathode ray tube, horizontal, vertical and character positioning coils and means for driving said horizontal, vertical and character positioning coils respectively with horizontal, vertical and character pulse positioning signals for establishing the location of character segments for a line of characters to be displayed by said tube wherein said character positioning signals causes a plurality of ramp strokes for a number of time intervals for each character segment, a correction apparatus for compensating for distortion in the characters formed at the extremities of the face of the tube of said line larger in height than the characters formed at the center by differences between the centers of deflection said vertical and character positioning coils, said correction apparatus comprising:
9. In a display system including a wide deflection angle cathode ray tube, horizontal, vertical and character positioning coils and means for driving said horizontal, vertical and character positioning coils respectively with horizontal, vertical and character pulse positioning signals for establishing the location of character segments for a line of characters to be displayed by said tube wherein said character positioning signal causes a plurality of ramp strokes for a number of time intervals for each character segment, a correction apparatus for compensating for distortion in the characters forms at the extremities of the face of the tube of said line larger in height than the characters formed at the center by differences between the centers of deflection said vertical and character positioning coils, said correction apparatus comprising:
10. In the system of claim 9 wherein said comparator means is arranged to operate as a zero-crossing detector circuit and produces said square wave waveform having a period corresponding to said time position waveform.
11. In the system of claim 9 wherein said integrator means includes:
12. In the system of claim 10 wherein said function generator means further includes:
13. In the system of claim 11 wherein said adjustable slope correction means includes:
14. In the system of claim 10 wherein said input means includes:
15. A cathode ray tube display system including a cathode ray tube having major horizontal deflection and major and vertical deflection driver circuits operative to generate major and minor positioning waveforms for driving major and minor horizontal and vertical control circuits coupled to said tube for deflecting the electron beam along a horizontal line and in a vertical direction wherein said minor vertical deflection driver circuits generate a high repetition frequency pulse waveform for producing character writing strokes for said line, said horizontal deflection circuits produce a low frequency sawtooth waveform as said major horizontal positioning waveform and said major vertical deflection circuits produce a staircase waveform as said major positioning waveform, said system further including correction apparatus comprising:
16. A cathode ray tube display system including a cathode ray tube having major horizontal deflection and major and minor vertical deflection driver circuits operative to generate major and minor positioning waveforms for driving major and minor horizontal and vertical control circuits coupled to said tube for deflecting the electron beam along a horizontal line and in a vertical direction wherein said minor vertical deflection driver circuits generate a high repetition frequency pulse waveform for producing character writing strokes for said line, said horizontal deflection circuits produce a low frequency sawtooth waveform as said major horizontal positioning waveform and said major vertical deflection circuits produce a staircase waveform as said major positioning waveform, said system further including correction apparatus comprising:
17. The system of claim 16 wherein said predetermined amplitude of said variable voltage source is adjusted to establish a minimum value for said signal applied to said minor vertical control circuit.
Description:
BACKGROUND OF THE INVENTION
1. Field of Use
This invention relates to cathode ray tube display systems and more particularly to correction apparatus for use in such systems.
2. Prior Art
Numerous techniques have been developed for compensating for the nonlinearity in the vertical and horizontal deflection circuits for cathode ray tubes to improve and linearity of deflection. In some instances, these circuits are introduced primarily for correcting forms of distortion which are known in optics as "pincushion" or "barrel" distortion. For example, U.S. Pats. Nos. 2,831,145; 3,403,289; and 3,512,039 illustrate circuitry for eliminating these types of distortion.
As mentioned, some cathode ray tube display systems include major and minor driving circuits. Conventionally, the minor vertical driver circuit operates to produce small, incremental high velocity displacement of the CRT beam for character writing at high speeds. Stated differently, the minor Y driver circuit is operative to produce character deflection currents in the minor vertical yoke for generating the necessary strokes for tracing out each character. The minor vertical or Y yoke is positioned behind the major vertical yoke. In such arrangements, the major and minor yokes have centers of deflection which are displaced from one another.
With the above arrangements, it has been noted that the differences in centers of deflection between the main yoke and minor yoke produce non-linearities at the face of the CRT display. These linearities are negligible when viewed by an operator in instances where the angle of deflection is small (e.g., 70°). However, in low cost display systems, it is desirable to utilize a CRT tube having a much larger angle of deflection (e.g., 90°). In such systems, the nonlinearities become extremely noticeable and when the display is viewed by an operator, the characters at either end of the display much shorter in height than the characters in the center of a given line. Such distortion has been termed "cigaring" by the prior art.
To avoid such distortion, it has been the practice to specify for use in system CRT tubes with smaller deflection angles or to utilize arrangements other than those which use rear mounted minor Y deflection yokes. However, this results in an increase in the cost of the system.
Another prior art approach is to position magnets around the CRT display tube to compensate for such distortion much the same way magnets are used to compensate for the effects of pin-cushion and barrel distortion.
These arrangements however have a disadvantage of not being able to adjust for differences in the amount of distortion caused by variations in the operating characteristics of the CRT tubes.
Accordingly, it is an object of the present invention to provide a low cost CRT display system which can be constructed using wide angle deflection tubes and minor vertical deflection yokes.
It is a further object of the present invention to provide an improved low cost display system which provides a correction circuit for overcoming distortions produced by differences between the centers of deflection for the major vertical deflection yoke and the minor vertical deflection yoke.
It is still a further object of the present invention to provide a correction circuit for use in a low display system which can adjust the heights of characters so as to produce a uniform display notwithstanding nonlinearities produced by differences in centers of deflection between major and minor magnetic deflection circuits.
SUMMARY OF THE INVENTION
The foregoing objects are achieved in a preferred embodiment of the present invention which provides a correction circuit which is operative in response to a ramp waveform generated in synchronism with the horizontal deflection circuits to produce a triangular voltage waveform for controlling the height of a given line the characters whereby the waveform provides a value of deflection current at either end greater than the deflection current established for the characters which are to be written at the center of the display. The waveform decreases from an established maximum value linearly at a predetermined rate until it reaches a nominal value which corresponds to those character positions which do not require height correction. At the same predetermined rate, the waveform increases linearly until again it reaches the same maximum value. Accordingly, by adjusting the waveform voltage to provide a greater value of current for those character positions at either end, results in a line of characters all of which have the same height.
In greater detail, the correction circuit of the preferred embodiment couples to the horizontal deflection circuits of the display system and includes a comparator circuit which is responsive to the positive and negative going excursions of ramp voltage developed from the horizontal deflection circuits to produce a positive going square wave waveform whose repetition frequency corresponds to the repetition freuqency of the ramp waveform. The square waveform is applied to an inverting input of an operational amplifier circuit included with the correction circuit which is operative to produce a symmetrical sawtooth or triangular waveform as an output signal. By adjusting the height of the square waveform at the input of the amplifier, a desired rate of correction is obtained. Additionally, the integrator circuit includes an adjustment for establishing the slope of the resultant triangular waveform which in turn establishes the amount of correction required for certain characters within a line of characters.
The triangular voltage waveform produced by the amplifier circuit is then applied to the minor yoke driver stage which includes an adjustable voltage source whose output level is set to establish the level at which point the triangular voltage waveform is to be limited or "clipped." The DC voltage level at which the such limiting takes place defines the character positions along the given line where little or no correction of character height is required.
In summary, the function generator circuit is operative to produce a waveform for accurately controlling the height of each of the characters along a line so that all of these characters appear the same in height. The arrangement in accordance with the present invention for the most part utilizes signals generated by other portions of the display system to produce the waveform having the desired characteristics. Accordingly, the invention minimizes the need for additional circuits. Further, the arrangement of the invention with minimal changes in the existing system can be incorporated therein to alleviate the aforementioned type of distortion.
The above and other objects of this invention are achieved in an illustrative embodiment described hereinafter. The novel features which are believed to be characteristic of the invention both as to its organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings. It is to be expressly understood, however, that these drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the type of character distortion encountered in a prior art system;
FIG. 2 illustrates in block diagram form a CRT display system utilizing the correction apparatus of the present invention;
FIG. 2a illustrates in greater detail the function generator circuits of FIG. 2;
FIG. 2b illustrates in greater detail the minor yoke deflection driver circuits of FIG. 2;
FIG. 3a illustrates several waveforms to be used in explaining the operation of the function generator circuit of FIG. 2a; and,
FIG. 3b illustrates several waveforms also used in describing the opertion of the system of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 2 shows in block form a cathode ray tube system arranged to include the correction apparatus of the present invention. The system inclusive of the apparatus of the invention is conventional in design. In particular, the system includes horizontal ramp generating circuits 200 and minor horizontal circuits 210 which provide a horizontal sweep ramp waveform signal and horizontal character ramp positional signals respectively which are applied to a horizontal deflection summing amplifier 220. The amplifier 220 is a direct coupled amplifier arranged in a feedback arrangement and which converts the input voltage signals into a current applied via a pair of transistor driver circuits 240 through the horizontal deflection yoke for deflecting the cathode ray tube beam in a horizontal direction. The conversion is accomplished by the amplifier's 220 comparison of the voltage V1 developed across a resistor in series with the yoke to the input voltage and producing an output signal until the voltage difference between the input signals is zero volts.
Associated processing apparatus, not shown, provides the digital control timing signals EOL and FC010 which time the generation of the signals produced by the circuits 200 and 210. The signals EOL is a negative going signals which is timed by the processing apparatus to switch state at the end of every line of characters so as to initialize the horizontal ramp generator during an inteval corresponding to the retrace portion of the ramp waveform. The signal FC010 is a positive going signal which is timed by the processing apparatus to switch state at the end of every character stroke. Thus, the circuits 200 provide ramp signals for moving the beam in a horizontal direction for a line of characters while circuits 210 provide ramp signals for moving the beam in a horizontal direction for each character position.
In a similar fashion, a group, a group of vertical positioning circuits 260 generate a line positioning staircase waveform which provides for movement of the beam to selected line positions of the cathode ray tube 250 while the minor vertical (Y) driver circuits 340 provide ramp waveforms for rapid vertical trace and retrace deflection of the beam within a character position. The vertical circuits 260 applied the signals to a vertical summing amplifier 280, identical in construction to amplifier 240, which converts the voltage waveform into a current, in a manner described above, applied via transistor driver circuits 300 through the vertical deflection yoke for deflecting the beam in a vertical direction. The operation of circuits 260 and 340 are timed by the EOL control signals which as mentioned switches state at the end of each line, an EOP control signal which switches state at the end of every page (i.e., after a predetermined number of lines) and a control signal FCROO which switches state at the end of every character stroke.
Also, as seen from FIG. 2, the system includes a video amplifier 230, conventional in design, which applies the video signals to the control grid of the tube 250 for turn on and off of the beam current for illumination of the CRT screen. The bias circuits 254 and high voltage supply circuits 252, conventional in design, are arranged to supply the requisite intensity, focus and anode voltage for operating the tube 250.
The deflection yokes of the system of FIG. 2 are arranged so that the minor vertical or Y yoke is located at the rear of the major or main deflection yokes. As mentioned, the minor Y drive circuits 340 drives the minor Y yoke which produces a magnetic field which modulates the electron beam as it is moved across the screen horizontally so as to trace out the necessary strokes for each character. Character spacing is controlled by varying the speed of the electron beam as it is being deflected a selected number of times for each character. In the preferred embodiment, the number of strokes is six, five for the character, and one for character retrace.
While, the deflection arrangement is easy to control and can be low in cost when a large deflection angle tube (i.e., 90°) is selected for tube 250, the differences in the center of deflection between the minor Y yoke and major yokes distort a line of characters in the manner illustrated in FIG. 1. As seen from the Figure, the difference in deflection centers cause a variation in character height in a given line of characters from the center of the screen to the end of the line. This distortion which is very apparent in wise angle cathode ray tubes is termed "cigaring" in that the shape of an envelope form by a line of characters has the cigar-like shape shown.
To correct this distortion a function generaor 320 is added to the system of FIG. 2 to provide a waveform as a further input to the minor Y driver circuits 340 so as to condition the circuits 340 to deflect by a greater amount the "end" characters of FIG. 1 more than the "center" characters which results in a line of characters having the same height.
The function generator 320 is illustrated in FIG. 2a in greater detail. It is seen from FIG. 2 that the generator 320 includes a comparator circuit 322, a height control circuit 324, an integrator circuit 326 and a buffer circuit 328. The comparator circuit 322 includes a high gain differential input, single ended output amplifier 322-1 connected in a feedback arrangement as shown. The amplifier is conventional in design and may take the form of amplifier circuits disclosed in the publication entitled "The Operation and Use of A Fast integrated Circuit Comparator," by R. J. Widlar, published by Fairchild Semiconductor, Copyright 1966. The comparator amplifier 322-1 is operative to compare the signal V1 applied to an inverting terminal 322-2 with a reference voltage applied to a non-inverting terminal 322-4 and produce an output signal representative of a binary ONE (e.g., three volts) or ZERO (e.g., zero volts) at an output terminal 322-6 when one signal is higher than the other. In the arrangement shown, an input resistor 322-5 is connected to ground which establishes the reference voltage applied to terminal 322-4 as zero volts. Initially, the voltage level applied to output terminal 322-6 is representative of a binary ONE and when the voltage level applied to terminal 322-2 is more positive than zero volts, the amplifier 322-1 switches the state of the output signals from a binary ONE to a binary ZERO. The feedback of an output voltage via a resistor 322-8 enhances the switching of amplifier 322-6. When the signal applied to terminal 322-2 becomes more negative than zero volts, the amplifier 322-1 switches back to its original state (i.e., to a binary ONE). Accordingly, the comparator 322 can be seen to function as a zero crossing detector.
The height control circuit 324 is direct coupled to the output terminal 322-6 and includes a PNP transistor stage 324-1 connected as shown.
A voltage divider including a pair of series connected resistors 324-2 and 324-4 divides a voltage of +15 volts to a predetermined voltage which is applied to the base electrode of transistor 324-1 and establishes the level at which the signal applied to the emitter electrode of transistor stage 324-1 is limited. The circuit 324 applies an output signal at an output terminal 324-6 as one input to integrator 326. The integrator 326 includes an operational amplifier 326-1 conventional in design arranged to have an inverting input terminal coupled to terminal 324-6 in series with a variable resistor 326-2 and fixed resistor 326-4 as shown. A non-inverting terminal of the amplifier 326-1 couples to a voltage divider including a variable resistor 326-6 and fixed resistors 326-8 and 326-10 as shown. The amplifier may take the form of an amplifier circuit disclosed in a publication titled "μA741 Frequency Compensated Operational Amplifier," published by Fairchild Semiconductor Corporation, Copyright 1969.
The variable resistor 326-2 is adjusted so as to establish a desired rate of correction corresponding to the slope of the output signal produced by amplifier 326-1. More specifically, the output voltage applied to an output terminal 326-12 is given by the expression:
Eout = -1/(R 1 C 1 )∫Ein . dt
where R 1 equals the values of resistors 326-2 and 326-4 which correspond respectively to 1 kilohm and 4.7 kilohms, C 1 equals the value of capacitor 326-14 which corresponds to 0.1 microfarads, and Ein equals the maximum voltage of the square wave waveform applied to terminal 324-6 for an interval dt corresponding to the period of the square wave waveform. The variable resistor 326-6 enables the level of the output signal applied to terminal 326-12 to the shifted and is normally set to establish a predetermined reference voltage for the amplifier 326-1.
As shown in FIG. 2a, the integrated signal at terminal 326-12 is applied via the buffer circuit 328 to an output line 328-10. The buffer circuit 328 provides the requisite impedance matching between the generator 320 and circuit 340. The buffer circuit 328 includes PNP and NPN transistors 328-2 and 328-4 respectively each connected in combination with different ones of the resistors 328-1, 328-3, 328-5 and 328-6 in an emitter follower configuration as shown. From output line 328-10, the function generator is viewed as a voltage source.
From FIG. 2, it is seen that the line 328-10 serves as one input to circuits 340. These circuits are shown in greater detail in FIG. 2b. Because the circuits are conventional in design they will only be described to the extent necessary to a complete understanding of the present invention. From FIG. 2b, it is seen that the circuits 340 include an AND gate and inverter stage 342 which has its output ac coupled to a further inverter stage 344 which is operative to control the operation of a transistor switching circuit 346.
The operation of the minor Y driver circuits 340 will be considered now in general. Normally signal EOL is at positive voltage. When the signal FCROO is forced from zero volts to a positive voltage level (e.g., +5 volts) (i.e., during the stroke portion of a character segment), it renders a diode 324-2 nonconductive which forces a junction formed by one end of a resistor 342-2 and a diode 342-1 to a positive voltage (e.g., 3 volts). The positive voltage is applied via a resistor 342-4 and a diode 342-5 to the base of electrode of transistor 342-8 switching the transistor on into saturation. The current through a collector load resistor 342-9 forces the collector electrode to approximately zero volts. The decrease in collector voltage is coupled via a capacitor 342-10 to the base electrode of a transistor 344-1. This decreases the conduction of transistor 344-1 through a collector load resistor 344-3 forcing the voltage at the collector electrode from approximately zero volts to a positive voltage. This voltage,decreased by a voltage drop of conducting diode 346-1, shunted by a speed-up capacitor 346-2, is applied to the base electrode of a switching transistor 346-3. This switches transistor 346-3 into conduction which places a junction 348 at approximately zero volts. The junction 348 connects to one end of the minor Y yoke and therefore the current through the yoke is established by the voltage applied to the other end of the yoke which corresponds to a junction 352 in FIG. 2b. The minor Y yoke is shunted by series connected diode 349 and a resistor 350. The transistor 346-3 is protected from high values of inverse voltage by a pair of zener diodes 346-4 and 346-5 connected as shown. Also, a diode 346-6 clamps the collector voltage of transistor 346-3 so that the collector to base voltage does not exceed the saturation voltage of the transistor.
The voltage applied to junction 352 is established by the function generator 320 and the voltage applied by a voltage source 360 via a low impedance current driver circuit 370. In particular, the voltage V3 is applied from the function generator 320 via line 328-10 to a junction 360-1. Also, a second voltage is applied via the emitter electrode of transistor 360-2 to junction 360-1. This value of voltage is established by a voltage divider including a variable resistor 360-3 and fixed resistors 360-4 and 360-5. That is, the voltage level at junction 360-1 approximates the voltage applied to the base electrode of transistor 360-2 by resistor 360-3 decreased by the base to emitter voltage drop of transistor 360-2. The voltage at the junction 360-1 is appled via a transistor 370-1 to junction 352. This voltage across the minor Y coil produces a ramp of current therethrough whose magnitude is proportional to the value of voltage applied to junction 352.
It can be from the above that when the signal FCROO is forced to zero volts (i.e., during the retrace portion of a character segment), the transistor 346-3 is rendered nonconductive. This places the junctions 348 and 352 at the same voltage and stops current flow through the minor Y yoke and collector to emitter path of transistor 346-3. This causes the current flowing through the minor Y yoke to decrease to zero and the magnetic field produced by the yoke to collapse producing current in the same direction through diode 349 and a resistor 350. The values for a pair of collector load resistors 370-2 and 370-3 are selected to reduce the power dissipation of transistor 370-1 at the value of current required to drive the minor Y yoke. A capacitor 370-4 and a capacitor 360-6 both serve as filter capacitors.
OPERATION OF FUNCTION GENERATOR IN THE SYSTEM OF FIG. 2
With specific reference to FIGS. 2, 2a, 2b, 3a and 3b, the operation of the function generator 320 will now be described. The voltage V1 as seen from FIG. 2, is developed in synchronism with the horizontal sweep signal and is applied as an input to terminal 322-2 of the generator 320. It will be noted that the duration designated TL corresponds to the time required to scan a line of characters as illustrated by FIG. 1. The time waveform V1 is converted by the comparator 322 into a square wave waveform V2, which is applied as an input to height circuit 324. This circuit establishes a maximum voltage of approximately 3 volts for waveform V2. The waveform V2 is applied to the inverting input of amplifier 326-1 which produces the triangular waveform V3.
From FIG. 2b, it is seen that the waveform V3 is applied to the junction 360-1. The resistor 360-3 is adjusted to establish a value of voltage δV for voltage E1 at junction 360-1 which establishes the character height for the center characters of a line of characters which appear the same in height as the end characters. Depending upon the operating parameters of the cathode ray tube being used such as the anode voltage versus the sensitivity of the yoke, the voltage E1 will vary in value. Also, by adjusting the value of resistor 326-2 of the integrator 326, the rate of correction can be varied by changing the slope of the triangular waveform V3 as illustrated in dotted form in FIG. 3a.
With the above adjustments, the appropriate magnitude of current through the minor Y coil for each character stroke of current waveform IL in FIG. 3b is established and varies in accordance with the waveform V3 in addition to being synchronized with the switching of signal FCROO as illustrated in FIG. 3b. For example, during the stroke time Ts of each character segment, occurring during the beginning of the time interval TL the transistor 346-3 of the driver circuits 340 is conductive and the voltage applied in accordance with waveform V3 whose maximum value corresponds to the value VP, establishes the magnitude of current IL flowing through the minor Y yoke. Accordingly, this in turn establishes the height of the character within the character segment. During the retrace time TR of the segment, transistor 346-3 is switched off by signal FCROO which in turn decreases the value for current IL flowing through diode 349 and resistor 350 to zero. Similarly, the voltage characteristics of waveform V3 provide the desired amount of correction for the subsequent character segments of a line of characters with the end result that all characters within the line appear the same in height. At the center of a line of characters, the voltage δV established by the voltage source 360 determines the height for the character segments as illustrated by waveform V3 in FIG. 3a. That is, when the voltage of the triangular waveform falls below the level E1, the voltage source 360 maintains the voltage applied to the current driver 370 at the voltage level E1. During the remainder of the line of characters as illustrated by FIG. 3a, the voltage characteristic of the triangular waveform V3 produces the amount of correction required. During the last portion of the time inteval TL, the waveform V3 in the manner described establishes the height of each of the character segments.
There has been shown a correction circuit which can be incorporated into conventional cathode ray tube display systems with little modification to the system for correcting dynamically,the distortion which results in the shortening of characters near the end positions of a line of characters. While, intended specifically to correct distortion known as "cigaring," the apparatus of the invention can also be used to correct distortion brought about for over-compensating for the effects of other forms of distortion.
While in accordance with the provisions and statutes, there has been illustrated and described the best form of the invention known, certain changes may be made in technique and system described without departing from the spirit of the invention as set forth in the appended claims and that in some cases, certain features of the invention may be used to advantage without a corresponding use of other features.
1. Field of Use
This invention relates to cathode ray tube display systems and more particularly to correction apparatus for use in such systems.
2. Prior Art
Numerous techniques have been developed for compensating for the nonlinearity in the vertical and horizontal deflection circuits for cathode ray tubes to improve and linearity of deflection. In some instances, these circuits are introduced primarily for correcting forms of distortion which are known in optics as "pincushion" or "barrel" distortion. For example, U.S. Pats. Nos. 2,831,145; 3,403,289; and 3,512,039 illustrate circuitry for eliminating these types of distortion.
As mentioned, some cathode ray tube display systems include major and minor driving circuits. Conventionally, the minor vertical driver circuit operates to produce small, incremental high velocity displacement of the CRT beam for character writing at high speeds. Stated differently, the minor Y driver circuit is operative to produce character deflection currents in the minor vertical yoke for generating the necessary strokes for tracing out each character. The minor vertical or Y yoke is positioned behind the major vertical yoke. In such arrangements, the major and minor yokes have centers of deflection which are displaced from one another.
With the above arrangements, it has been noted that the differences in centers of deflection between the main yoke and minor yoke produce non-linearities at the face of the CRT display. These linearities are negligible when viewed by an operator in instances where the angle of deflection is small (e.g., 70°). However, in low cost display systems, it is desirable to utilize a CRT tube having a much larger angle of deflection (e.g., 90°). In such systems, the nonlinearities become extremely noticeable and when the display is viewed by an operator, the characters at either end of the display much shorter in height than the characters in the center of a given line. Such distortion has been termed "cigaring" by the prior art.
To avoid such distortion, it has been the practice to specify for use in system CRT tubes with smaller deflection angles or to utilize arrangements other than those which use rear mounted minor Y deflection yokes. However, this results in an increase in the cost of the system.
Another prior art approach is to position magnets around the CRT display tube to compensate for such distortion much the same way magnets are used to compensate for the effects of pin-cushion and barrel distortion.
These arrangements however have a disadvantage of not being able to adjust for differences in the amount of distortion caused by variations in the operating characteristics of the CRT tubes.
Accordingly, it is an object of the present invention to provide a low cost CRT display system which can be constructed using wide angle deflection tubes and minor vertical deflection yokes.
It is a further object of the present invention to provide an improved low cost display system which provides a correction circuit for overcoming distortions produced by differences between the centers of deflection for the major vertical deflection yoke and the minor vertical deflection yoke.
It is still a further object of the present invention to provide a correction circuit for use in a low display system which can adjust the heights of characters so as to produce a uniform display notwithstanding nonlinearities produced by differences in centers of deflection between major and minor magnetic deflection circuits.
SUMMARY OF THE INVENTION
The foregoing objects are achieved in a preferred embodiment of the present invention which provides a correction circuit which is operative in response to a ramp waveform generated in synchronism with the horizontal deflection circuits to produce a triangular voltage waveform for controlling the height of a given line the characters whereby the waveform provides a value of deflection current at either end greater than the deflection current established for the characters which are to be written at the center of the display. The waveform decreases from an established maximum value linearly at a predetermined rate until it reaches a nominal value which corresponds to those character positions which do not require height correction. At the same predetermined rate, the waveform increases linearly until again it reaches the same maximum value. Accordingly, by adjusting the waveform voltage to provide a greater value of current for those character positions at either end, results in a line of characters all of which have the same height.
In greater detail, the correction circuit of the preferred embodiment couples to the horizontal deflection circuits of the display system and includes a comparator circuit which is responsive to the positive and negative going excursions of ramp voltage developed from the horizontal deflection circuits to produce a positive going square wave waveform whose repetition frequency corresponds to the repetition freuqency of the ramp waveform. The square waveform is applied to an inverting input of an operational amplifier circuit included with the correction circuit which is operative to produce a symmetrical sawtooth or triangular waveform as an output signal. By adjusting the height of the square waveform at the input of the amplifier, a desired rate of correction is obtained. Additionally, the integrator circuit includes an adjustment for establishing the slope of the resultant triangular waveform which in turn establishes the amount of correction required for certain characters within a line of characters.
The triangular voltage waveform produced by the amplifier circuit is then applied to the minor yoke driver stage which includes an adjustable voltage source whose output level is set to establish the level at which point the triangular voltage waveform is to be limited or "clipped." The DC voltage level at which the such limiting takes place defines the character positions along the given line where little or no correction of character height is required.
In summary, the function generator circuit is operative to produce a waveform for accurately controlling the height of each of the characters along a line so that all of these characters appear the same in height. The arrangement in accordance with the present invention for the most part utilizes signals generated by other portions of the display system to produce the waveform having the desired characteristics. Accordingly, the invention minimizes the need for additional circuits. Further, the arrangement of the invention with minimal changes in the existing system can be incorporated therein to alleviate the aforementioned type of distortion.
The above and other objects of this invention are achieved in an illustrative embodiment described hereinafter. The novel features which are believed to be characteristic of the invention both as to its organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings. It is to be expressly understood, however, that these drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the type of character distortion encountered in a prior art system;
FIG. 2 illustrates in block diagram form a CRT display system utilizing the correction apparatus of the present invention;
FIG. 2a illustrates in greater detail the function generator circuits of FIG. 2;
FIG. 2b illustrates in greater detail the minor yoke deflection driver circuits of FIG. 2;
FIG. 3a illustrates several waveforms to be used in explaining the operation of the function generator circuit of FIG. 2a; and,
FIG. 3b illustrates several waveforms also used in describing the opertion of the system of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 2 shows in block form a cathode ray tube system arranged to include the correction apparatus of the present invention. The system inclusive of the apparatus of the invention is conventional in design. In particular, the system includes horizontal ramp generating circuits 200 and minor horizontal circuits 210 which provide a horizontal sweep ramp waveform signal and horizontal character ramp positional signals respectively which are applied to a horizontal deflection summing amplifier 220. The amplifier 220 is a direct coupled amplifier arranged in a feedback arrangement and which converts the input voltage signals into a current applied via a pair of transistor driver circuits 240 through the horizontal deflection yoke for deflecting the cathode ray tube beam in a horizontal direction. The conversion is accomplished by the amplifier's 220 comparison of the voltage V1 developed across a resistor in series with the yoke to the input voltage and producing an output signal until the voltage difference between the input signals is zero volts.
Associated processing apparatus, not shown, provides the digital control timing signals EOL and FC010 which time the generation of the signals produced by the circuits 200 and 210. The signals EOL is a negative going signals which is timed by the processing apparatus to switch state at the end of every line of characters so as to initialize the horizontal ramp generator during an inteval corresponding to the retrace portion of the ramp waveform. The signal FC010 is a positive going signal which is timed by the processing apparatus to switch state at the end of every character stroke. Thus, the circuits 200 provide ramp signals for moving the beam in a horizontal direction for a line of characters while circuits 210 provide ramp signals for moving the beam in a horizontal direction for each character position.
In a similar fashion, a group, a group of vertical positioning circuits 260 generate a line positioning staircase waveform which provides for movement of the beam to selected line positions of the cathode ray tube 250 while the minor vertical (Y) driver circuits 340 provide ramp waveforms for rapid vertical trace and retrace deflection of the beam within a character position. The vertical circuits 260 applied the signals to a vertical summing amplifier 280, identical in construction to amplifier 240, which converts the voltage waveform into a current, in a manner described above, applied via transistor driver circuits 300 through the vertical deflection yoke for deflecting the beam in a vertical direction. The operation of circuits 260 and 340 are timed by the EOL control signals which as mentioned switches state at the end of each line, an EOP control signal which switches state at the end of every page (i.e., after a predetermined number of lines) and a control signal FCROO which switches state at the end of every character stroke.
Also, as seen from FIG. 2, the system includes a video amplifier 230, conventional in design, which applies the video signals to the control grid of the tube 250 for turn on and off of the beam current for illumination of the CRT screen. The bias circuits 254 and high voltage supply circuits 252, conventional in design, are arranged to supply the requisite intensity, focus and anode voltage for operating the tube 250.
The deflection yokes of the system of FIG. 2 are arranged so that the minor vertical or Y yoke is located at the rear of the major or main deflection yokes. As mentioned, the minor Y drive circuits 340 drives the minor Y yoke which produces a magnetic field which modulates the electron beam as it is moved across the screen horizontally so as to trace out the necessary strokes for each character. Character spacing is controlled by varying the speed of the electron beam as it is being deflected a selected number of times for each character. In the preferred embodiment, the number of strokes is six, five for the character, and one for character retrace.
While, the deflection arrangement is easy to control and can be low in cost when a large deflection angle tube (i.e., 90°) is selected for tube 250, the differences in the center of deflection between the minor Y yoke and major yokes distort a line of characters in the manner illustrated in FIG. 1. As seen from the Figure, the difference in deflection centers cause a variation in character height in a given line of characters from the center of the screen to the end of the line. This distortion which is very apparent in wise angle cathode ray tubes is termed "cigaring" in that the shape of an envelope form by a line of characters has the cigar-like shape shown.
To correct this distortion a function generaor 320 is added to the system of FIG. 2 to provide a waveform as a further input to the minor Y driver circuits 340 so as to condition the circuits 340 to deflect by a greater amount the "end" characters of FIG. 1 more than the "center" characters which results in a line of characters having the same height.
The function generator 320 is illustrated in FIG. 2a in greater detail. It is seen from FIG. 2 that the generator 320 includes a comparator circuit 322, a height control circuit 324, an integrator circuit 326 and a buffer circuit 328. The comparator circuit 322 includes a high gain differential input, single ended output amplifier 322-1 connected in a feedback arrangement as shown. The amplifier is conventional in design and may take the form of amplifier circuits disclosed in the publication entitled "The Operation and Use of A Fast integrated Circuit Comparator," by R. J. Widlar, published by Fairchild Semiconductor, Copyright 1966. The comparator amplifier 322-1 is operative to compare the signal V1 applied to an inverting terminal 322-2 with a reference voltage applied to a non-inverting terminal 322-4 and produce an output signal representative of a binary ONE (e.g., three volts) or ZERO (e.g., zero volts) at an output terminal 322-6 when one signal is higher than the other. In the arrangement shown, an input resistor 322-5 is connected to ground which establishes the reference voltage applied to terminal 322-4 as zero volts. Initially, the voltage level applied to output terminal 322-6 is representative of a binary ONE and when the voltage level applied to terminal 322-2 is more positive than zero volts, the amplifier 322-1 switches the state of the output signals from a binary ONE to a binary ZERO. The feedback of an output voltage via a resistor 322-8 enhances the switching of amplifier 322-6. When the signal applied to terminal 322-2 becomes more negative than zero volts, the amplifier 322-1 switches back to its original state (i.e., to a binary ONE). Accordingly, the comparator 322 can be seen to function as a zero crossing detector.
The height control circuit 324 is direct coupled to the output terminal 322-6 and includes a PNP transistor stage 324-1 connected as shown.
A voltage divider including a pair of series connected resistors 324-2 and 324-4 divides a voltage of +15 volts to a predetermined voltage which is applied to the base electrode of transistor 324-1 and establishes the level at which the signal applied to the emitter electrode of transistor stage 324-1 is limited. The circuit 324 applies an output signal at an output terminal 324-6 as one input to integrator 326. The integrator 326 includes an operational amplifier 326-1 conventional in design arranged to have an inverting input terminal coupled to terminal 324-6 in series with a variable resistor 326-2 and fixed resistor 326-4 as shown. A non-inverting terminal of the amplifier 326-1 couples to a voltage divider including a variable resistor 326-6 and fixed resistors 326-8 and 326-10 as shown. The amplifier may take the form of an amplifier circuit disclosed in a publication titled "μA741 Frequency Compensated Operational Amplifier," published by Fairchild Semiconductor Corporation, Copyright 1969.
The variable resistor 326-2 is adjusted so as to establish a desired rate of correction corresponding to the slope of the output signal produced by amplifier 326-1. More specifically, the output voltage applied to an output terminal 326-12 is given by the expression:
Eout = -1/(R 1 C 1 )∫Ein . dt
where R 1 equals the values of resistors 326-2 and 326-4 which correspond respectively to 1 kilohm and 4.7 kilohms, C 1 equals the value of capacitor 326-14 which corresponds to 0.1 microfarads, and Ein equals the maximum voltage of the square wave waveform applied to terminal 324-6 for an interval dt corresponding to the period of the square wave waveform. The variable resistor 326-6 enables the level of the output signal applied to terminal 326-12 to the shifted and is normally set to establish a predetermined reference voltage for the amplifier 326-1.
As shown in FIG. 2a, the integrated signal at terminal 326-12 is applied via the buffer circuit 328 to an output line 328-10. The buffer circuit 328 provides the requisite impedance matching between the generator 320 and circuit 340. The buffer circuit 328 includes PNP and NPN transistors 328-2 and 328-4 respectively each connected in combination with different ones of the resistors 328-1, 328-3, 328-5 and 328-6 in an emitter follower configuration as shown. From output line 328-10, the function generator is viewed as a voltage source.
From FIG. 2, it is seen that the line 328-10 serves as one input to circuits 340. These circuits are shown in greater detail in FIG. 2b. Because the circuits are conventional in design they will only be described to the extent necessary to a complete understanding of the present invention. From FIG. 2b, it is seen that the circuits 340 include an AND gate and inverter stage 342 which has its output ac coupled to a further inverter stage 344 which is operative to control the operation of a transistor switching circuit 346.
The operation of the minor Y driver circuits 340 will be considered now in general. Normally signal EOL is at positive voltage. When the signal FCROO is forced from zero volts to a positive voltage level (e.g., +5 volts) (i.e., during the stroke portion of a character segment), it renders a diode 324-2 nonconductive which forces a junction formed by one end of a resistor 342-2 and a diode 342-1 to a positive voltage (e.g., 3 volts). The positive voltage is applied via a resistor 342-4 and a diode 342-5 to the base of electrode of transistor 342-8 switching the transistor on into saturation. The current through a collector load resistor 342-9 forces the collector electrode to approximately zero volts. The decrease in collector voltage is coupled via a capacitor 342-10 to the base electrode of a transistor 344-1. This decreases the conduction of transistor 344-1 through a collector load resistor 344-3 forcing the voltage at the collector electrode from approximately zero volts to a positive voltage. This voltage,decreased by a voltage drop of conducting diode 346-1, shunted by a speed-up capacitor 346-2, is applied to the base electrode of a switching transistor 346-3. This switches transistor 346-3 into conduction which places a junction 348 at approximately zero volts. The junction 348 connects to one end of the minor Y yoke and therefore the current through the yoke is established by the voltage applied to the other end of the yoke which corresponds to a junction 352 in FIG. 2b. The minor Y yoke is shunted by series connected diode 349 and a resistor 350. The transistor 346-3 is protected from high values of inverse voltage by a pair of zener diodes 346-4 and 346-5 connected as shown. Also, a diode 346-6 clamps the collector voltage of transistor 346-3 so that the collector to base voltage does not exceed the saturation voltage of the transistor.
The voltage applied to junction 352 is established by the function generator 320 and the voltage applied by a voltage source 360 via a low impedance current driver circuit 370. In particular, the voltage V3 is applied from the function generator 320 via line 328-10 to a junction 360-1. Also, a second voltage is applied via the emitter electrode of transistor 360-2 to junction 360-1. This value of voltage is established by a voltage divider including a variable resistor 360-3 and fixed resistors 360-4 and 360-5. That is, the voltage level at junction 360-1 approximates the voltage applied to the base electrode of transistor 360-2 by resistor 360-3 decreased by the base to emitter voltage drop of transistor 360-2. The voltage at the junction 360-1 is appled via a transistor 370-1 to junction 352. This voltage across the minor Y coil produces a ramp of current therethrough whose magnitude is proportional to the value of voltage applied to junction 352.
It can be from the above that when the signal FCROO is forced to zero volts (i.e., during the retrace portion of a character segment), the transistor 346-3 is rendered nonconductive. This places the junctions 348 and 352 at the same voltage and stops current flow through the minor Y yoke and collector to emitter path of transistor 346-3. This causes the current flowing through the minor Y yoke to decrease to zero and the magnetic field produced by the yoke to collapse producing current in the same direction through diode 349 and a resistor 350. The values for a pair of collector load resistors 370-2 and 370-3 are selected to reduce the power dissipation of transistor 370-1 at the value of current required to drive the minor Y yoke. A capacitor 370-4 and a capacitor 360-6 both serve as filter capacitors.
OPERATION OF FUNCTION GENERATOR IN THE SYSTEM OF FIG. 2
With specific reference to FIGS. 2, 2a, 2b, 3a and 3b, the operation of the function generator 320 will now be described. The voltage V1 as seen from FIG. 2, is developed in synchronism with the horizontal sweep signal and is applied as an input to terminal 322-2 of the generator 320. It will be noted that the duration designated TL corresponds to the time required to scan a line of characters as illustrated by FIG. 1. The time waveform V1 is converted by the comparator 322 into a square wave waveform V2, which is applied as an input to height circuit 324. This circuit establishes a maximum voltage of approximately 3 volts for waveform V2. The waveform V2 is applied to the inverting input of amplifier 326-1 which produces the triangular waveform V3.
From FIG. 2b, it is seen that the waveform V3 is applied to the junction 360-1. The resistor 360-3 is adjusted to establish a value of voltage δV for voltage E1 at junction 360-1 which establishes the character height for the center characters of a line of characters which appear the same in height as the end characters. Depending upon the operating parameters of the cathode ray tube being used such as the anode voltage versus the sensitivity of the yoke, the voltage E1 will vary in value. Also, by adjusting the value of resistor 326-2 of the integrator 326, the rate of correction can be varied by changing the slope of the triangular waveform V3 as illustrated in dotted form in FIG. 3a.
With the above adjustments, the appropriate magnitude of current through the minor Y coil for each character stroke of current waveform IL in FIG. 3b is established and varies in accordance with the waveform V3 in addition to being synchronized with the switching of signal FCROO as illustrated in FIG. 3b. For example, during the stroke time Ts of each character segment, occurring during the beginning of the time interval TL the transistor 346-3 of the driver circuits 340 is conductive and the voltage applied in accordance with waveform V3 whose maximum value corresponds to the value VP, establishes the magnitude of current IL flowing through the minor Y yoke. Accordingly, this in turn establishes the height of the character within the character segment. During the retrace time TR of the segment, transistor 346-3 is switched off by signal FCROO which in turn decreases the value for current IL flowing through diode 349 and resistor 350 to zero. Similarly, the voltage characteristics of waveform V3 provide the desired amount of correction for the subsequent character segments of a line of characters with the end result that all characters within the line appear the same in height. At the center of a line of characters, the voltage δV established by the voltage source 360 determines the height for the character segments as illustrated by waveform V3 in FIG. 3a. That is, when the voltage of the triangular waveform falls below the level E1, the voltage source 360 maintains the voltage applied to the current driver 370 at the voltage level E1. During the remainder of the line of characters as illustrated by FIG. 3a, the voltage characteristic of the triangular waveform V3 produces the amount of correction required. During the last portion of the time inteval TL, the waveform V3 in the manner described establishes the height of each of the character segments.
There has been shown a correction circuit which can be incorporated into conventional cathode ray tube display systems with little modification to the system for correcting dynamically,the distortion which results in the shortening of characters near the end positions of a line of characters. While, intended specifically to correct distortion known as "cigaring," the apparatus of the invention can also be used to correct distortion brought about for over-compensating for the effects of other forms of distortion.
While in accordance with the provisions and statutes, there has been illustrated and described the best form of the invention known, certain changes may be made in technique and system described without departing from the spirit of the invention as set forth in the appended claims and that in some cases, certain features of the invention may be used to advantage without a corresponding use of other features.