Shadow mask color system with calligraphic displays

A display system is disclosed utilizing a cathode ray display device utilizing three electron beams which are directed through a shadow mask to excite three-color phosphor dots, and which is incorporated in a system to alternately provide raster scan display signals and calligraphic display signals to obtain the benefits of both raster scan and calligraphic displays (dot or line drawings). A deflection unit is disclosed with the X and Y deflection coils each driven by two pairs of transistor circuits. The transistor circuits are controlled so that alternatively one transistor in each set passes current to provide bi-directional currents at desired levels to obtain specific beam displacement. A resistive element between individual deflection coils provides an indication of deflection current, which indication is processed by a differential amplifier to indicate displacement. In accordance with the instant position of the beam, a convergence unit is controlled to reflect spherical considerations. Specifically, the convergence of each color beam is varied in accordance with first and higher orders of X and Y displacement. Focus is controlled to avoid certain effects and obtain others.

BACKGROUND AND SUMMARY OF THE INVENTION 
Electronic color display systems have come into widespread use in a variety 
of applications. In addition to the widespread use of commercial 
television, such systems are also employed widely for simulation, study, 
information transfer, design, and so on. To consider a specific exemplary 
application, such display systems are employed to provide the visual 
simulation in aviation pilot training units. In such an application, any 
of a variety of conditions or patterns can be simulated to afford a pilot 
flying experiences that might otherwise be very costly, difficult to 
obtain, or dangerous. For example, in addition to providing routine 
training in specific aircraft operating over specific terrain, simulators 
can give pilots the experience of such operations as a landing with 
retracted landing gear. Of course, the value of the experience is related 
to its realism. Accordingly, considerable effort has been made to 
accomplish stark realism, particularly with respect to the visual 
presentation which is perhaps the area of greatest human concentration. 
Prior video systems for use in aircraft simulators have utilized both the 
calligraphic and raster scan modes of operation. Generally, the raster 
scan mode of operation is in widespread use, as in color television, and 
is effective to display landscape and scenes. The raster scan display is 
also relatively convenient to tilt or rotate with respect to an artificial 
horizon, which is inherently necessary for the display of an aircraft 
simulator. Generally, the raster scan mode of operation has attained a 
degree of excellence in the use of shadow mask cathode ray tubes. However, 
the raster scan mode is not without limitations in various applications as 
the visual system of an aircraft simulator. Specifically, the raster scan 
mode tends to reproduce lines rather poorly if they are offset from the 
horizontal and the vertical. That is, diagonally extending lines in a 
raster scan image tend to reveal a staircase or staggered appearance. 
Also, in a shadow mask display, tilting the raster scan image from the 
horizontal tends to produce moire patterns which detract from the realism 
of the scene. In addition to these drawbacks, raster scan displays 
reproduce lights rather poorly. For example, in simulating an evening 
aircraft landing (a very desired simulation), the lights of the airport 
and surrounding area provide critical reference points. However, raster 
scan displays characteristically do not simulate such lights with the 
desired degree of realism. 
The presentation of lights and lines in an electronic display is 
considerably improved in the calligraphic mode of operation. Therein, the 
beam is deflected from point to point to produce lines or dots as 
disclosed in the book Principles of Interactive Computer Graphics 
published in 1973 by McGraw-Hill Book Company, authored by Newman and 
Sproull. Such display systems have also been called "stroke writing 
systems", a form of which is disclosed in U.S. Pat. No. 3,775,760 entitled 
Cathode Ray Tube Stroke Writing Using Digital Techniques. While 
calligraphic modes of operation are effective for producing lights, as 
dots and lines, such displays involve serious color limitations and are 
complicated to formulate for depicting complete scenes. Consequently, both 
raster scan displays and calligraphic displays have involved substantial 
compromises in prior-art systems. 
The foundation of the present invention is premised on the discovery that a 
shadow mask television display system can be alternatively driven with 
raster scan display signals and calligraphic display signals to accomplish 
a considerably improved visual display. In general, the effective 
realization of such a system required the solution of several inherent 
problems. For example, calligraphic display systems require effective 
control of beam deflection for movement from point to point. However, 
raster scan display systems require a high speed beam deflection pattern 
which is consistent and simply sweeps across the screen. In the operation 
of raster scan display systems, using a shadow mask, the individual beams 
are converged to pass through holes in the shadow mask, then diverge to 
individually excite specific color phosphor dots. In view of the repeating 
raster pattern, the convergence of the beams can be simply adjusted in 
accordance with the deflection in either the X or Y component direction. 
In some instances, the pattern of the shadow mask is varied to compensate 
for the spherical effects on the convergence of the beam. However, in the 
calligraphic mode of operation, the convergence is variously affected by 
hysteresis as well as the spherical effects with the result that 
previously known convergence techniques have not been adequate to enable 
calligraphic operation in a shadow mask cathode ray tube. 
In addition to the above problems, certain special-effect problems have 
been recognized as somewhat inherent in cathode ray tube display systems. 
For example, difficulty has been experienced in providing the 
characteristics of realism for simulated lights, as in causing them to 
scintillate as well as to grow with perspective as the viewpoint is 
changed. The present system effectively solves such problems as well as 
the problems attendant combining calligraphic and raster displays along 
with the problem of moire patterns in a shadow mask cathode ray display. 
In general, the present invention incorporates a cathode ray display device 
utilizing a shadow mask and a multiplicity of electron guns for providing 
color-associated electron beams to impact upon a target screen after 
passing through holes in the shadow mask. A deflection means is provided 
in the form of a controlled switching device which facilitates 
bi-directional currents through the deflection coils and affords fine beam 
positioning. A convergence system is provided to enable the use of the 
shadow mask for calligraphic displays and further for improving color 
presentations by considering higher order effects of displacement in 
either the X or Y component direction as related to each color beam. 
Finally, a focus control apparatus avoids certain undesirable effects and 
accomplishes certain desired effects including a variation in the beam 
focus depending upon the current mode of operation of the system.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT 
As indicated above, a detailed illustrative embodiment of the invention is 
disclosed herein. However, display systems may be embodied in accordance 
with various forms, some of which may be detailed rather differently from 
the disclosed embodiment. Consequently, the specific structural and 
functional details disclosed herein are merely representative, yet in that 
regard they are deemed to provide the best embodiment for purposes of 
disclosure and to provide the basis for the claims herein which define the 
scope of the present invention. 
Referring initially to FIG. 1, a display is depicted on a screen 10 housed 
in a cabinet 12. In general, the system of the present invention may be 
variously embodied with the utilization of image splitters and various 
optical systems to attain the desired realism as viewed from a particular 
location as in a simulator cockpit. However, in the interests of 
simplicity, the system is depicted in FIG. 1 with the image appearing on 
the screen 10 at one side of the cabinet 12. 
In the exemplary application of the present system, the image on the screen 
10 simulates a view that is presented to a pilot in the course of landing 
an aircraft. In that regard, the pilot is presented a representation of a 
runway 14, a landscape including mountains 16, and lights 18. In 
accordance with the present invention and as disclosed in greater detail 
below, the lights 18 are provided in a calligraphic mode while the runway 
14 and the mountains 16 are imaged by a raster scan mode. The image 
depicted on the screen 10 is accomplished by the excitation of phosphor 
dots of component colors by separate electron beams acting through a 
shadow mask. 
Considering the system in somewhat greater physical detail, the screen 10 
comprises the face of a cathode ray tube 20 (FIG. 2) as generally well 
known in the prior art. The tube 20 incorporates a shadow mask 22 and 
electron beam forming elements 24 (generally indicated) for providing 
three electron beams each of which is associated with a primary light 
color, e.g. red, green, blue. 
The cathode ray tube 20 is also equipped with a deflection yoke 26 for 
scanning the beams over the screen 10 either in a raster or calligraphic 
patterns. Finally, the tube 20 is also fitted with a convergence yoke 28 
which is somewhat modified from the traditional form of such apparatus. 
Specifically, as disclosed in detail below, the convergence yoke 28 
includes structure for displacing the blue beam laterally. 
In the operation of the tube 20, the electron beams are driven to excite 
phosphors on the screen 10 alternately in two modes of operation. That is, 
the beams are deflected in a raster pattern during one scansion of the 
screen 10 during which the runway 14 (FIG. 1) and mountains 16 are 
developed. During a following interval, the beams in the tube 20 (FIG. 2) 
are driven in a calligraphic mode to excite phosphors on the screen 10 to 
depict the lights 18 (FIG. 1) of the depicted scene. Thereafter, the mode 
would again involve a raster scan with the sequence continuing as a 
dynamic presentation indicative of the changing scene. In that regard, it 
is to be appreciated that simulation systems are well known in the field 
of computer graphics for developing signals to accomplish either a raster 
scan display or a calligraphic display (sometimes referred to as a 
line-drawing display). Such processors are described in the 
above-referenced book, Principles of Interactive Computer Graphics. 
Turning now to the components of the system of FIG. 2, separate signal 
sources 30 and 32 (FIG. 2 left) are illustrated to provide raster display 
signals and calligraphic display signals respectively. Of course, in 
practice, these signals may be provided by a single computer facility as 
well known in the prior art and as described in the above-referenced book, 
Principles of Interactive Computer Graphics however, not for use in the 
same system. 
The signals from the sources 30 and 32 are applied to a sequencer and 
separator 34 which sequences the alternate mode of operation and 
segregates the signals into various separate components. That is, the mode 
alternates between raster scan and calligraphic display. Accordingly, a 
switch for the alternate signals is required. Specifically, video signals 
are supplied to a video amplifier 36 which in turn supplies signals to 
modulate the beams of the electron guns in the tube 20 as symbolically 
represented collectively by elements 38. Thus, just as in a conventional 
and traditional broadcast television receiver, the structure of the 
sequencer and separator 34 functions in a well known manner to provide 
separate signals for deflecting a cathode ray beam and for modulating the 
intensity of that beam. Traditionally, the deflection signals are ramp 
voltages while the modulating signals (video) are analogs of the dissected 
image. With respect to the well known calligraphic mode of operation as 
illustrated in the above-referenced book, Principles of Interactive 
Computer Graphics (page 26), digital data defining a specific display is 
reduced to deflection signals (X and Y) along with an unblanking signal 
for the control grid, elements 38. Such well known calligraphic display 
signals are provided from the source 32. 
The conventional television signals and the calligraphic display signals, 
as described above, are simply sequenced by the sequencer and separator 34 
so that the tube 20 is alternately actuated by the two types of display 
signals in alternate modes. 
The sequencer and separator 34 provides deflection signals (X and Y 
components) to a deflection processing circuit 40 which in turn supplies 
the component signals to individual amplifier and driver circuits. 
Specifically, the X component is provided to a driver 42 while the Y 
component is provided to a driver 44. The X and Y drivers 42 and 44 are 
connected to the deflection yoke 26. Accordingly, the yoke 26 displaces 
the electron beam as to raster scan the beam over the screen 10 during 
raster scan operation and to move the beam in the desired vector or line 
pattern during a calligraphic display. Commensurate with such beam 
displacement or deflection, the video amplifier 36 controls the elements 
38 to modulate the color beams and thereby accomplish the desired color 
display on the screen 10 as generally well known in the prior art. 
As indicated above, a difficulty attendant the use of a shadow mask 22 in a 
system of calligraphic display has involved the problem of providing 
sufficient accuracy in deflecting the beam. The recognition and solution 
of that problem will be more apparent from a consideration of the detailed 
disclosure set forth below relating to the drivers 42 and 44. 
Other problems, also as indicated above, attendant such operation have 
involved convergence and focus control. In that regard, the system of FIG. 
2 includes a convergence unit 46 and a focus control circuit 48. The 
convergence unit 46 receives signals indicative of beam deflection (X and 
Y) through lines 50 and 52 from the drivers 42 and 44 respectively. From 
such signals, the convergence unit 46 develops special convergence signals 
for the individual electron beams. Specifically, convergence signals for 
the beams "red" and "green" are developed along with two convergence 
signals for the beam "blue" as described in detail below. The convergence 
unit 46 is also coupled to the control circuit 48 through a cable 54. 
The focus control circuit 48 receives signals from the front end of the 
system, specifically including the sources 30 and 32 and the separator 34. 
From the received signals, the circuit 48 provides dynamic focus signals 
as indicated to be applied to the tube focusing elements 56 and also 
provides a power signal to the tube 10 as indicated through a conductor 
58. The structural and operating details of the focus control circuit 48 
are considered below. 
Turning now to the deflection drivers 42 and 44, a representation of these 
similar circuits is illustrated in detail in FIG. 3 which will now be 
considered. Specifically, the X-component circuit is disclosed in FIG. 3 
to which an input signal X.sub.i is indicated to be applied at deflection 
amplifier circuits 60. The signal X.sub.i is amplitude modulated to 
indicate a desired positive or negative displacement in the X-component 
direction for points of impact by the beams on the display screen. 
Preliminary processing of the signal X.sub.i (as pin-cushion effect 
compensation) is performed in the circuits 60 to provide two pairs of 
output control signals. These signals manifest the desired direction and 
amplitude of current flow through the coils 62 and 64 which comprise the 
deflection yoke 26 (FIG. 2). The actual current X.sub.c in the coils 62 
and 64 may be either positive or negative, as illustrated in FIG. 3, to 
accomplish displacement to the right or left from a center point on the 
screen. Of course, the amplitude of the current X.sub.c establishes the 
degree of beam displacement. 
To accomplish variable bi-directional current flows through the coils 62 
and 64, two pairs of semi-conductor control paths are provided. Such paths 
would normally be provided by parallel sets of transistors to accommodate 
the desired currents. However, for simplicity of illustration herein, 
single transistor paths are depicted. Specifically, from a junction point 
66 (left) connection is provided to a pair of transistors 68 and 70. 
Actually, the junction point 66 is connected through a resistor 72 to the 
emitter of the transistor 68 which has its collector connected to supply 
voltage and is controlled at the base electrode through a resistor 74. In 
a similar configuration, the junction point 66 is connected through a 
resistor 76 to the emitter of the transistor 70, which has its collector 
connected to ground and its base connected to receive a control signal 
through a resistor 78. 
In the operation of the deflection driver system, one or the other of the 
pair of transistors (68 or 70) is conductive with the result that current 
flow is accommodated either to or from the junction point 66, either from 
the potential source applied at terminal 80 or to ground as indicated at 
82. Thus, for a positive flow (+X.sub.c) through the coils 62 and 64, 
current is provided through the transistor 68. For a negative current flow 
(-X.sub.c), current flow is provided through the transistor 70. Of course, 
these currents are accommodated at the other side of the coils 62 and 64 
by a similar pair of semiconductor control paths. Specifically, a junction 
path 84 (right from the coils 62 and 64) is connected through resistors 86 
and 88 respectively to the emitters of transistors 90 and 92 (oppositely 
poled). The collector of the transistor 90 is connected to receive 
positive potential from the terminal 80, while the collector of the 
transistor 92 is connected to reference potential ground 82. 
The base of the transistor 90 is connected through a resistor 94 to provide 
an input signal while the base of the transistor 92 is connected through a 
similar resistor 96 to perform a similar function. The complete flow paths 
for the currents -X.sub.c and +X.sub.c will now be apparent. Specifically, 
the current +X.sub.c flows through the transistor 68, the coils 62 and 64, 
then through the transistor 92. The opposite current -X.sub.c flows 
through the transistor 90, the coils 62 and 64 and the transistor 70. As 
indicated above, due to the current capacity limitation of devices, the 
transistors 68, 70, 90, and 92 each comprise a parallel set of transistors 
in practice to accommodate the desired levels of deflection current. 
A resistor 98 is provided between the coils 62 and 64 for manifesting the 
actual current flow to a differential amplifier 100, thereby providing an 
indication of the actual deflection in the form of a signal X.sub.s. 
The transistors 68, 70, 90, and 92 are controlled by four deflection 
control amplifiers 102, 104, 106, and 108 respectively. Although these 
amplifiers accomplish small maintenance currents in the driver 
transistors, in the main, for any deflection, current is carried either by 
the transistors 68 and 92 or the transistors 70 and 90, depending upon the 
direction or sign of the deflection. Thus, one transistor in each pair 
conducts to provide the desired deflection current through the coils 62 
and 64. In the conventional type deflection amplifier, only one transistor 
would be used to provide current in either direction, thus the entire 
power supply voltage would be applied to each transistor. In this circuit, 
the power supply voltage is divided equally across each of the conducting 
transistors 68 and 92 or 70 and 90. This technique allows using 
conventional type transistors to provide the necessary deflection voltage 
for shadow mask cathode ray tubes. Control of the deflection current is 
accomplished by four signals provided from the deflection amplifier 
circuits 60 to the similar individual amplifiers 102, 104, 106, and 108. 
Considering the amplifier 102 as representative, a signal is applied 
through a resistor 110 to the base of a transistor 112. The collector of 
the transistor 112 is connected to receive positive potential from the 
terminal 80 while the emitter of the transistor 112 is connected to 
provide an input to the transistor 68. Specifically, the emitter of the 
transistor 112 is connected to a junction point 114 which is connected to 
the resistor 74 and referenced through a resistor 116 to the junction 
point 66. 
The amplifiers 106, 104, and 108 are similar to the amplifier 102, each 
incorporating a transistor as the control element. Specifically, the 
amplifiers 104, 106, and 108 incorporate transistors 118, 120, and 122 
respectively. The amplifiers share some bipass elements. Specifically, an 
RC circuit 124 is provided between the bases of the transistors 112 and 
118; and a capacitor 125 is connected between the emitters of the 
transistors 112 and 118. Similar elements are provided with respect to the 
transistors 90 and 92. Specifically, the emitters of the transistors 120 
and 122 are connected to the junction 84 through resistors 127 and 129 
respectively and together through a capacitor 131. The bases are 
interconnected through an RC circuit 133 and to the circuit 60 through 
input resistors 135 and 137. 
Recapitulating to some extent, in the operation of the deflection driver as 
depicted in FIG. 3, the amplifier circuits 60 may provide certain well 
known correction, as for avoiding pin-cushion effects. Such corrections 
and apparatus therefor are well known in the prior art. The circuits 60 
provide four output signals. To specify a desired deflection, at any given 
time, two of the signals are active while the other two signals are nil. 
That is, to specify a deflection to the right, a positive current X.sub.c 
is to be generated by turning on the transistors 68 and 92. Consequently, 
the amplifiers 102 and 108 are signaled to command the desired current 
flows through the transistors 68 and 92 to accomplish the desired current 
through the coils 62 and 64 and accordingly accomplish the desired 
deflection. 
Conversely, a displacement of the beam to the left is accomplished by a 
negative current (-X.sub.c) which is provided by signaling the amplifiers 
104 and 106 to turn on the transistors 90 and 70 to provide the desired 
current through the coils 62 and 64. Thus, fine control of deflection is 
accomplished without the momentum effect generally inherent in oscillatory 
deflection circuits. Further stability is provided in the driver by 
utilizing the current deflection signal X.sub.s applied as a negative 
feedback through a conductor 126 to the deflection amplifier circuits 60. 
The signal X.sub.s is also very important to provide accurate deflection 
information which is utilized in the operations of focusing and 
convergence as treated below. 
Preliminary to considering the convergence unit 46 (FIG. 2), some 
preliminary definitions of signals will be helpful. Basically, a plurality 
of signals are developed which utilize the deflection signals X.sub.s and 
Y.sub.s (referred to hereinafter simply as X and Y) along with signal 
information developed therefrom indicative of the designated quadrant and 
hemisphere designated by the deflection. That is, in accordance with 
convention, the display screen is divided into four quadrants, Q.sub.1, 
Q.sub.2, Q.sub.3, and Q.sub.4. Furthermore, the screen is divided 
horizontally and vertically into hemispheres; specifically, a top 
hemisphere T, a bottom hemisphere B, a left hemisphere L, and a right 
hemisphere R. The quadrant and hemisphere information takes a binary form 
indicating that the signal either exists or does not exist. On the other 
hand, the manifestations of displacement X and Y take the form of analog 
signals. For example, a signal XYQ.sub.1 manifests the product of X and Y; 
however, exists only during the interval when the deflection is in the 
first quadrant Q.sub.1. In view of the above explanation, the following 
chart will summarize the signals generated for use in the system. 
______________________________________ 
Signal Designation 
Explanation 
______________________________________ 
-XYQ.sub.1 The negative product of X and Y 
effective only when deflection is 
in the first quadrant. 
-XYQ.sub.2 The negative product of X and Y 
effective only when deflection is 
in the second quadrant. 
-XYQ.sub.3 The negative product of X and Y 
effective only when deflection is 
in the third quadrant. 
-XYQ.sub.4 The negative product of X and Y 
effective only when deflection is 
in the fourth quadrant. 
XYQ.sub.1 The positive product of X and Y 
effectively only when deflection is 
in the first quadrant. 
XYQ.sub.2 The positive product of X and Y 
effectively only when deflection is 
in the second quadrant. 
XYQ.sub.3 The positive product of X and Y 
effectively only when deflection is 
in the third quadrant. 
XYQ.sub.4 The positive product of X and Y 
effectively only when deflection is 
in the fourth quadrant. 
X.sup.2 L The square of X effective only 
when deflection is in the left 
hemisphere. 
X.sup.2 R The square of X effective only 
when deflection is in the right 
hemisphere. 
Y.sup.2 B The square of Y effective only 
when deflection is in the bottom 
hemisphere. 
Y.sup.2 T The square of Y effective only 
when deflection is in the top 
hemisphere. 
-XL The negative value of X effective 
only when deflection is in the 
left hemisphere. 
-XR The negative value of X effective 
only when deflection is in the 
right hemisphere. 
XL The positive value of X effective 
only when deflection is in the 
left hemisphere. 
XR The positive value of X effective 
only when deflection is in the 
right hemisphere. 
-YT The negative value of Y effective 
only when deflection is in the 
top hemisphere. 
-YB The negative value of Y effective 
only when deflection is in the 
bottom hemisphere. 
YT The positive value of Y effective 
only when deflection is in the 
top hemisphere. 
YB The positive value of Y effective 
only when deflection is in the 
bottom hemisphere. 
______________________________________ 
The signals as indicated in the above chart are employed in association 
with a plurality of constants for each convergence signal, i.e. red 
convergence, green convergence, blue convergence, and blue lateral 
convergence. Essentially, the convergence correction involves a formula of 
constants applied to the signals which may be stated mathematically as: 
##EQU1## 
The values of K are tuned constants associated with the signal values as 
indicated by the subscripts. The values X and Y are the deflction 
manifestations. 
Thus, each of the convergence or correction signals is developed on the 
basis of the position of the beam (indicated in components of X and Y) and 
also on the basis of highest orders of such deflection, e.g. X.sup.2, XY, 
and so on. The need for higher orders of correction stems from the 
spherical departure of the target configuration and the fact that 
corrections vary nonlinearly. However, it is also noteworthy that 
displacements along one coordinate impact upon correction in the other 
coordinate. 
In addition to the correction based on linear and compound orders of 
displacement, correction is also provided for hysteresis effects. That is, 
correction is provided depending upon the immediately prior deflection of 
the beam in order to compensate for hysteresis in the deflection. To 
consider the structural details of the convergence unit, reference will 
now be made to FIG. 4. 
The deflection-indicating signals X and Y are applied to a plurality of 
signal generators embodied in a block 128 (upper left). Essentially, the 
generators comprise digital and analog circuits as well known in the prior 
art for formulating signals as set forth in the above chart. Specifically, 
for example, the signal -XYQ.sub.1 is timed to provide a nil value except 
during the interval when the deflection falls in the first quadrant and 
during that time, the amplitude of the signal manifests a negative value 
of the product of the X and Y component deflections. Thus, the chart 
signals are developed as indicated by their legends. 
The individual signals from the signal generators 128 are applied through a 
symbolically represented cable 130 to individual convergence units. 
Specifically, the signals are applied to a red convergence unit 132 (shown 
in detail), a green convergence unit 134, a blue convergence unit 136, and 
a blue lateral convergence unit 138. The convergence units are similar 
with the consequence that only the unit 132 is shown in structural detail. 
Within the convergence unit 132, there are two basic types of circuit 
connections. Specifically, ground-referenced adjustment circuits receive a 
single input signal while opposing signal-referenced circuits receive two 
signal inputs. The ground-referenced adjustment circuit 140 for the signal 
X.sup.2 L is illustrated in detail to include a ground-referenced 
potentiometer 142 providing a tapped signal through a resistor 144. 
Opposing signal-adjustment circuits are similar as represented for the 
signals XL and -XL. Specifically, such signals are applied to a circuit 
146 across a potentiometer 148 from which a signal is tapped to be 
supplied through a resistor 150. The remaining adjustment or control 
circuits in the red convergence unit 132 are similar and are simply 
represented by a block and bear the label "control". These circuits each 
take the form of one or other of the circuits 140 or 146, depending upon 
whether a single input signal is provided or two input signals are 
provided. 
The outputs from all of the adjustment or control circuits, e.g. circuits 
140 and 146, are applied to a summing junction 152 which provides the 
input to a red convergence output amplifier 154 shown in detail and 
incorporating hysteresis correction. The convergence signals for the other 
colors are developed by similar structures represented by blocks. 
Specifically, the green convergence circuit 134 provides an output which 
is processed by a green output amplifier 156, while the blue convergence 
unit supplies an amplifier 158, and the blue lateral convergence unit 
supplies a blue lateral amplifier 160. 
Considering the details of the output amplifiers as collectively 
represented by the red output circuit 154, the input signal is applied to 
an operational amplifier 162. The differential input to the amplifier 162 
is ground referenced through a resistor 164 and feedback paths are 
provided through a capacitor 166 connected in parallel with a pair of 
series resistors 168 and 170. The junction point between the resistors 168 
and 170 is connected to three parallel RC circuits 171, 172, and 173 which 
are referenced to ground potential. The three RC circuits 171, 172, and 
173 simulate the hysteresis or B-H curve to correct the convergence signal 
based upon its recent history. The individual corrections are summed by 
the amplifier 162 and adjusted in accordance with the prior state of the 
signal to reflect hysteresis correction. Consequently, the output from the 
amplifier 154 is provided in a conductor 175 to manifest the red 
convergence signal. 
From the above description, it may be seen that a number of factors are 
included in the determination and provision of the convergence signal, 
including nonlinear or higher order factors as X.sup.2, XY, and so on. The 
constants for varying the application of such factors are provided by the 
potentiometers in the adjustment or control circuits (e.g. 140 and 146) of 
the convergence units. Specifically, one constant for X.sup.2 is used 
while deflection is to the left of center and is provided by the 
potentiometer 142 (FIG. 4). Similarly, the constant for the positive or 
negative value of X while the signal is to the left of center is provided 
by the potentiometer 148. Thus, in essence, the deflection is tuned for 
each quadrant of deflection by adjusting each of the potentiometers in the 
individual adjustment or control circuits within the convergence units for 
each of the beams. In that regard, note that the blue convergence involves 
two separate output signals which results from the fact that traditionally 
blue convergence facilitates no lateral adjustment. The added deflection 
structure with respect to the blue beam is illustrated in FIG. 5 and will 
now be considered. 
Referring to FIG. 5, the neck 176 of the tube 20 (FIG. 2) is depicted in 
section to illustrate the beam convergence elements. The symbolically 
represented elements 178 afford convergence control for the green beam 
while the elements 180 accommodate the red beam. Additionally, the 
elements 182 provide convergence for the blue beam; however, 
conventionally since the blue beam is the center reference, only vertical 
displacement (Y) is provided. 
In accordance with the present invention, wherein calligraphic or line 
vector drawings are developed through a shadow mask, lateral convergence 
correction of the blue beam is provided. Accordingly, a blue lateral 
deflection coil 184 is mounted contiguous to the neck 176 of the tube 20 
just exterior of the blue elements 182. The coil 184 is connected to 
receive the blue lateral convergence signal formulated as described with 
reference to FIG. 4. Accordingly, both X and Y convergence correction is 
provided for each of the three color beams. 
In the operation of a system of the present invention, initial tuning 
involves adjusting the potentiometers (e.g. potentiometers 142 and 148 
FIG. 4) for each of the convergence units, specifically convergence units 
132, 134, 136, and 138. Such adjustment is accomplished by deflecting the 
beam to a quadrant or hemisphere in which the signal is active, then 
tuning the associated potentiometer to attain the desired color purity. Of 
course, once such convergence tuning has been accomplished, the system can 
be expected to maintain consistent operation for a substantial period of 
time. The important consideration to be noted is the fact that higher 
order phenomena of beam displacement is important in the convergence 
correction. 
As indicated above, the system of the present invention is capable of 
eliminating certain special effects previously considered detrimental and 
accomplishing certain other special effects previously considered 
desirable. Specifically, reference is made to moire patterns in raster 
scan operation, light scintillation, perspective growth, and so on. An 
initial consideration involves the fact that a different focus is desired 
for raster scan operation than is desired for calligraphic display 
operation. Accordingly, depending upon the mode of operation (calligraphic 
or raster scan), the focus of the individual beams in the tube 20 should 
be modified. More specifically, in the raster scan mode of operation the 
beams should be under focused, the lack of sharp focus serving to blend 
individual scanning lines together, improving the composite image. 
Conversely, during the calligraphic mode, when pinpoint lights and lines 
are simulated on the screen, the beams should be overfocused; however, 
more sharply focused to obtain the desired effects. Consequently, 
depending upon the mode of operation the focus of the beams is modified 
from a generally overfocused state during calligraphic operation to a 
generally underfocused state during raster scan operation. 
During angular rotation with raster scan operation, it has been determined 
that moire patterns can be significantly reduced by varying the focus of 
the beams. These patterns usually detract from the image during roll angle 
or angular offset between the raster pattern and the shadow mask. In 
general, critical angles of raster rotation involve tuning. In that 
regard, the beam is generally further underfocused at angles of rotation 
which tend to present moire patterns. 
Other effects which relate to focusing are the brightness or intensity of a 
current image component and the perspective growth of an image component. 
For example, dimming lights depicted in an image should be progressively 
more defocused to eliminate the consequence that eventually such lights 
become so dim and small (due to decreasing beam size) that they begin to 
scintillate excessively. A similar criteria exists with respect to the 
remoteness of the lights from the simulated viewpoint. That is, very 
remote lights tend to become exceedingly dim and small and accordingly 
begin to scintillate and requiring that they be progressively more 
overfocused during the calligraphic mode of operation. Since light 
scintillation is a real world phenomenon, proper control of focus to 
produce some scitillation provides a unique feature. 
These various considerations are combined in the system of the present 
invention to provide an output indicative of the desired focus of the 
beams, which special effect focus is then combined with focus corrections 
based upon the square (X.sup.2) of the abscissa deflection and the square 
(Y.sup.2) of the ordinate deflection to compensate spherical offset. 
Preliminary to considering the structure of the focusing control circuit in 
detail, reference will be made to the diagram of FIG. 6. A color dot triad 
is represented including a red dot 186, a blue dot 188, and a green dot 
190. A hole 192 in the shadow mask (offset from the dots) is also 
represented in FIG. 6 suggesting the manner in which angularly offset 
color beams pass through the shadow mask to selectively excite a 
particularly color dot. It may be seen from the figure that depending upon 
the focus of the beams, the color dots may radiate light over a varying 
surface area (more or fewer dots excited). That is, the more sharply 
focused the beams are at the point of impingement on the screen, the 
smaller the area of excitation and illumination. Accordingly, it may be 
seen that in simulating lights, dimmer and more remote lights should be 
progressively defocused as also should also occur in raster presentation 
with roll angles to avoid moire patterns. It will thus be apparent that 
defocusing has the effect of diminishing the line scanning pattern of 
raster scan operation. 
Considering the structure of the focusing circuit 48, signals indicative of 
beam intensity, mode of operation, depth of view and roll angle, are 
available as described herein or well known in the art and all applied to 
an effects focus signal generator 194 (FIG. 7). The generator 194 may take 
a variety of forms varying from rather elaborate digital-analog simulation 
means to a simple signal scaler for combining the individual received 
components to accomplish a desired compromise i.e. a circuit for summary 
voltages. That is, in accordance with conventional terminology, the scaler 
changes the signal representations by a factor then the signals are simply 
combined additively. 
First, the signal applied at the terminal 196 is binary in nature 
commanding substantial underfocusing for a raster mode of operation and 
substantial overfocusing for calligraphic mode of operation. As indicated 
above, the signal is available from the sequencer and separator 34 (FIG. 
2). A signal indicative of beam intensity from the same source is applied 
at the terminal 198 being continuously variable to provide an effect upon 
the focusing. Again, brighter display commands a higher degree of focus. 
Somewhat similarly, a continuous signal is provided at a terminal 200 
indicative of the viewing distance to the simulated object. The signal is 
the so-called Z signal from the source 32 as well known in the art. As the 
distance increases, the focusing signal defocuses. Finally, a selective 
range of signals may be provided at the terminal 202 to indicate critical 
areas of moire patterns which should cause defocusing. As indicated in 
FIG. 2, such signals are available from the source 30. 
The signals applied at the terminals 198, 200, and 202 may be combined 
either positively or negatively with the basic mode signal which commands 
either an underfocused or overfocused state. Accordingly, the individual 
signals ae summed within the generator 194 to provide an output to a 
ground-referenced potentiometer 204 for supplying one component input to 
an amplifier 206. Two other inputs are provided to the amplifier 206, from 
potentiometers 208 and 210 which receive the signals X.sup.2 and Y.sup.2 
(development explained above) from the convergence unit 46 (FIG. 2). These 
signals adjust the focus to compensate for the fact that the beam travels 
a progressively greater distance with greater deflections (the screen 
radius not being the same as the beam deflection radius) and thereby 
accomplishing true compensation in accordance with the square of the 
deflection. 
The individual inputs are summed by the amplifier 206 (FIG. 7) and applied 
to an operational amplifier 212 which is in turn connected to drive a 
power supply 214. Thus, the focusing energy is provided from the power 
supply 214 to accomplish the desired effects with operating conditions and 
the squares of the displacement or deflection components. 
Considering the operation of the system in view of the above structural 
details, it may now be seen that the display on the screen 10 (FIG. 1) is 
provided alternatively by a raster scan mode of operation and a 
calligraphic display operation. During the raster scan pattern of 
operation, the electron beams ae somewhat underfocused and convergence 
correction is fully applicable to accomplish improved color registration. 
Following such operation, during a calligraphic display mode, the 
deflection is from point to point with the deflection circuits affording 
the necessary point-to-point control. At such points, the lights 18 are 
provided in the raster-developed scene including the runway 14 and the 
mountains 16. For the random pattern movements (encountered in 
calligraphic operation) the convergence is maintained by the convergence 
unit as set forth in considerable detail above affording the desired color 
purity. Again, the focusing considerations are also applicable. Thus, by 
enabling the combination of a calligraphic display along with a raster 
scan display in a color mask system, the present invention affords 
effective displays of landscapes and the like which may be conveniently 
tilted or rotated and which can be effectively simulated and processed in 
accordance with well known computer graphics techniques. Furthermore, the 
system affords realistic light displays which are possible by use of 
calligraphic display techniques in a shadow mask structure. The 
consequence is a particularly effective system for the simulation of 
graphics, including the simulation of visual displays for aircraft 
simulators. Essentially, a full range of color presentation is available 
with a relatively high degree of color registration and without undue 
pattern or moire effects. Relatively complicated scenes may be provided 
with a reasonable computation facility, the various display elements being 
effectively and compatibly merged into a realistic display. Recognizing 
the widely varying possibilities for utilization of the structures and 
techniques disclosed herein, the scope hereof is understood to be 
determined in accordance with the claims as set forth below.