Color projection television system with apparatus for correcting misconvergence and misgeometry which calculates coefficients of equations representing deflection correction waveforms

A projection television convergence method and apparatus allow rapid and simple correction of low-order misconvergence and misgeometry problems. A hybrid system employs digital programmable attenuators and a digital control system, making it possible to eliminate misconvergence and misgeometry using simple local adjustment on a limited number (e.g., 9) of locations in the picture. The values of the coefficients in the correction waveforms, which are represented by the settings on a number of programmable attenuators, are determined by simultaneously solving the convergence governing equations, and are stored in memory. Thus, the prior art tedious individual adjustment of typically more than 30 potentiometers, each with global effects, by a factory worker, service-engineer or the consumer is obviated. An alternate embodiment includes a simple black and white camera to eliminate the human adjuster. The system detects areas of misconvergence or misgeometry in the composite image by measuring the luminace reflected from the screen during the projection of complementary test patterns of equal luminance in the separate image sources. The system treats areas of luminace discrepancy as misconverged areas, and causes an appropriate level of correction signals to be fed to the misconverged CRTs.

BACKGROUND OF THE INVENTION 
The present invention relates generally to convergence methods and 
apparatuses used in projection television (PTV) systems. More 
particularly, the present invention relates to a method and apparatus 
which enable simple, accurate and rapid adjustment of the convergence in a 
projection television system. 
Convergence in the context of the present invention may be defined as the 
alignment and matching of the images produced by the image sources in a 
color PTV system. Typical PTV systems include, inter alia, a rear 
projection screen for receiving a composite image to be displayed and 
three separate cathode ray tubes (CRTs), each for projecting a different 
color component--red, green or blue--of the composite image. As the three 
CRTs cannot be physically located in the same place, two are usually 
located off the axis which is perpendicular to the plane of the projection 
screen (the normal). This non-alignment of the CRTs is a major source of 
misconvergence since the images from the three CRTs do not impinge upon 
the projection screen identically. Other misconvergence problems are due, 
for example, to inherent differences between the two or more CRTs. 
To compensate for distortions caused by the non-alignment of all of the 
CRTs with the normal, and other distortions caused by the geometry of the 
screen and internal distortions of the CRTs, corrective waveforms are 
applied to convergence dipoles of each CRT. The coefficients of the 
waveforms are usually determined by separate potentiometers. 
For proper convergence, prior art PTV systems require the careful 
adjustment of more than 30 (typically 42) potentiometers, each controlling 
a global aspect of the composite image such as size, overall shift, 
linearlity and trapezium distortion to name a few. This requires skill and 
patience normally beyond that of the average user/consumer. Additionally, 
PTV systems often need to be re-converged after shipping or major changes 
in climate. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a PTV convergence method and 
apparatus requiring fewer adjustments needed for correction of low-order 
misconvergence and misgeometry problems. 
It is also an object to reduce the time required for proper convergence. 
Another object is to increase the convergence accuracy obtainable in PTV 
systems. 
A further object of the present invention is to satisfy the above-mentioned 
objects using a simple embodiment with which the average user can properly 
effect convergence. 
Yet another object is to cause misconvergence and misgeometry errors to be 
automatically corrected on demand by the user without the need to rely on 
the user's ability to make visual measurements. 
Some of the aforementioned objects of the invention are realized by 
introducing digitally programmable potentiometers (or attenuators) in the 
analog convergence system by introducing simple digital control circuitry. 
This will be referred to as a "hybrid" system. The adjustment of each CRT 
is accomplished with simple local adjustments at a small number of 
locations in the composite image (e.g., 9). The correct values of the 
correction currents at the local adjustment points are stored in memory. 
After the adjustment procedure, all of the coefficients in the corrective 
waveform are automatically determined, and the attenuators are adjusted by 
the digital circuitry by the rapid solving of simultaneous equations 
governing the convergence of the images. These calculations can be done 
cyclically to compenate for any drift in the analog waveform. 
In an alternate embodiment a simple, inexpensive black and white camera is 
included to measure the luminance of light reflected from the screen. 
Between two CRTs to be converged, a checkerboard pattern of alternating 
colored and black checks is applied to one CRT, and a complementary 
pattern is applied to the other CRT, the luminance of the two separate 
images as seen by the camera being equal. In case of a well converged 
composite image the camera sees a homogeneous luminance field. The 
processing system treats deviations from homogeneity as misconvergence or 
misgeometry, and calculates from these deviations, the magnitude of the 
correction signal to be fed to one of the CRTs until the misconvergence or 
misgeometry are minimized. These calculations are based on five 
measurements at, for example, 9-256 areas of the composite image. All of 
the coefficients of the correction waveforms are again automatically 
calculated and stored in memory. 
In this alternate embodiment the correction signal can be generated by a 
fully digital waveform generator.

DETAILED DESCRIPTION OF THE INVENTION 
The projection scheme 100 for a conventional color PTV system is shown in 
FIG. 1. A rear projection screen 102 receives a composite image from the 
three CRTs 104, 108 and 112. Each CRT has an associated group of 
deflection coils (not shown) for applying a deflection current for 
deflecting an electron beam made to strike a phosphorous surface, and 
convergence dipoles (106, 110 and 114) for applying correction waveform 
currents for proper convergence. The CRT 104 projects all of the red light 
of the composite image, while the CRTs 108 and 112 project all of the 
green and blue light, respectively, to produce a full color composite 
image for viewing. 
Because the CRTs 104 and 112 project images from an angle .alpha. away from 
the normal 116, distortions in the red and blue images result, typically 
as shown in FIG. 2. For example, the green CRT 108 may produce an image 
202 with various forms of distortion which give it a non-rectangular 
shape. The red CRT 104 may produce an image with the contours of shape 
204, and the blue CRT 112 may produce an image with the contours of shape 
206. 
The aforementioned distortions result not only in misalignment of colors 
which lead to false composite colors on the viewing surface 102, but also 
lead to both improper image sizes as well as well as other abnormalities 
sometimes noticeable to the viewer. 
Therefore, the partial images must be corrected both to form rectangular 
shaped partial images of the proper dimensions, and to insure that the 
three partial images properly coincide. To eliminate the various 
distortions mentioned above, correction current waveform are fed to the 
convergence dipoles. 
The production of these correction waveforms typically is controlled by a 
group of attenuators, each corresponding to a different aspect of 
convergence--for example, the trapezoidal distortion due to the oblique 
projections of the images from the red and blue CRTs onto the screen 102. 
Each CRT is fed a horizontal and a vertical correction waveform with the 
coefficients of the terms in the equations representing the waveforms 
being determined by a separate attenuator. Listed below are typical 
vertical and horizontal waveforms, Cx(X,Y) and Cy(X,Y), respectively. 
(1) Cx(X,Y)=Cdcx+CorxY+CsxX+CkxXY+CtxX.sup.2 +CbxY.sup.2 +CpcxXY.sup.2 
+CacxX.sup.2 Y+CcsxX.sup.2 Y.sup.2 
(2) Cy(X,Y)=Cdcy+CoryX+CsyY+CkyY.sup.2 +CtyXY+CbyX.sup.2 +CpcyX.sup.2 
Y+CacyXY.sup.2 +CcsyX.sup.2 Y.sup.2 
The coefficients are as follows: 
Cdc=overall shift ("disparity") 
Cor=orthogonality deviation 
Cs=size difference 
Ck=keystone distortion due to oblique projection in the vertical direction 
Ct=trapezoidal distortion due to oblique projection of red and blue images 
sources in horizontal direction 
Cb=line-low 
Cpc=pincushion distortion due to curvature of the faceplate 
Cac=asymmetric corner distortion 
Ccs=corner shift 
In prior art systems, adjustment of each coefficient/attenuator 
simultaneously causes changes in the geometry and convergence of the image 
at a large number of locations. Thus, the convergence of prior art PTV 
system is a tedious one. 
The present invention greatly facilitates the ease of convergence. It is 
accomplished by simple local adjustments of convergence at only a limited 
number of locations--9 in the preferred embodiment. FIG. 3 shows a display 
screen 302 with the 9 conveniently chosen local adjustment locations or 
points 304. 
Without specifying the values, the cartesian coordinates of the local 
adjustment points are as follows 
______________________________________ 
(-x, +y) (O, +y) (+x, +y) 
(-x, O) (O, O) (+x, O) 
(-x, -y) (O, -y) (+x, -y) 
______________________________________ 
As a result of applying the current through the convergence dipoles for 
proper convergence at the local adjustment points, there are 9 solutions 
for the 9 unknown coefficients for each of the horizontal and vertical 
directions. 
In the X-direction: 
Cdcx=Cx(O,O) 
Corx=[Cx(O,Y)-Cx(O,-Y)]/(2Y) 
Csx=[Cx(X,O)-Cx(-X,O)]/(2X) 
Cbx=[Cx(O,Y)+Cx(O,-Y)-2Cx(O,O)]/(2Y.sup.2) 
Ckx=[Cx(X,Y)+Cx(-X,-Y)-Cx(-X,Y)-Cx(X,-Y)]/(4XY) 
Ctx=[Cx(X,O)+Cx(-X,O)-2Cx(O,O)]/(2X.sup.2) 
Cpcx=[Cx(X,Y)+Cx(X,-Y)-Cx(-X,Y)-Cx(-X,-Y)-2Cx(X,O)+2Cx(-X,O)/(4XY.sup.2) 
Cacx=[Cx(X,Y)+Cx(-X,Y)-Cx(X,-Y)-Cx(-X,-Y)-2Cx(O,Y)+2Cx(O,-Y)]/(4X.sup.2 Y) 
Ccsx=[Cx(X,Y)+Cx(-X,Y)+Cx(-X,-Y)+Cx(X,-Y)+Cx(-X,-Y)+Cx(X,-Y)-2Cx(X,O)-2(Cx( 
O,Y)-2Cx(-X,O)-2Cx(O,-Y)+4Cx(O,O)]/(4X.sup.2 Y.sup.2) 
In the Y-direction: 
Cdcy=Cy(O,O) 
Cory=[Cy(X,O)-Cy(-X,O)]/(2X) 
Csy=[Cy(O,Y)-Cy(O,-Y)]/(2Y) 
Cby=[Cy(X,O)+Cy(-X,O)-2Cy(O,O)]/(2X.sup.2) 
Cky=[Cy(O,Y)+Cy(O,-Y)-2Cy(O,O)]/(2Y.sup.2) 
Cty=[Cy(X,Y)+Cy(-X,-Y)-Cy(-X,Y)-Cy(X,-Y)]/(4XY) 
Cpcy=[Cy(X,Y)+Cy(-X,Y)-Cy(-X,-Y)-Cy(X,-Y)-2Cy(O,Y)+2Cy(O,-Y)]/(4X.sup.2 Y) 
Cacy=[Cy(X,Y)+Cy(X,-Y)-Cy(-X,Y)-Cy(-X,-Y)-2Cy(X,O)+2Cy(-X,O)]/(4XY.sup.2) 
Ccsy=[Cy(X,Y)+Cy(-X,Y)+Cy(-X,-Y)+Cy(X,-Y)-2Cy(X,O)-2Cy(O,Y)-2Cy(-X,O)-2Cy(O 
,-Y)+4Cy(O,O)](4X.sup.2 Y.sup.2) 
The manually adjustable potentiometers of typical PTV systems are replaced 
by a system with digitally programmable attenuators that are controlled by 
a microprocessor system. 
FIG. 4 shows a convergence control panel 400 for the preferred embodiment. 
The various buttons displayed may contain light emitting diodes (LEDs) so 
that a button is illuminated when depressed. The "TV" button 402 is 
illuminated during normal operation of the PTV system--i.e., when no 
convergence adjustments are being made 
When convergence is altered in any way, the user may recall the previous 
digitally stored convergence values for the case when the geometry and 
convergence were correct by depressing the "RCL" button 404. However, the 
former self-adjusted or factory values of the coefficients may prove to be 
incorrect due to the changes in the conditions, such as temperature, 
humidity, and changes in physical dimensions caused by moving, dropping or 
applying vibratory forces to the PTV system. 
In this case, the "PR" button 406 is depressed. Spots or small images 
(partial images) from each of the three CRTs are projected onto the 9 
locations, one location at a time, to determine if the color of the sum of 
the partial images is white. If a spot color is other than white, 
readjustment at the location is required. 
When readjustment is required, the "S" button 408 is depressed, enabling 
adjustment of the overall horizontal and vertical shifts of the images 
from the three CRTs. The system first generates spots with the red and 
green CRTs 104 and 108 at central locations in these images. The spots are 
superimposed by depressing one or more of the buttons 412-418, with the 
direction of shift in the images being indicated by the arrows. When the 
spots of the red and green CRT are superimposed the "ST" button 420 is 
depressed, and the adjusted values of the correction waveforms are stored 
in memory. The system then projects centrally located spots from both the 
green and blue CRTs 108 and 112 for superposition as described above. When 
the spots are superimposed, and the "ST" button is again depressed, the 
values of the correction waveform are again stored in memory. 
The system then resumes projecting spots in succession at the remaining 8 
local adjustment points for evaluation. If all the spots are white, proper 
convergence is indicated, and the "TV" button may be depressed. However, 
if all of the spots are not white, fine tuning adjustment is necessary, as 
the problems of misconvergence are beyond simple horizontal and vertical 
image shifts. 
The "A" button 410 is depressed in this case, so that the system generates 
spots in succession at each of the local adjustment points. At each 
location the red and green spots are converged, and then the green and 
blue spots are converged. The "ST" button 420 is depressed after each 
superposition. 
After superposition occurs and the values of the correction waveform are 
stored at a given location, the system generates the next set of spots, 
and the above routine is continued until all of the spots at the 9 
locations are superimposed. At this point, the system calculates the 
correct values of the correction waveform coefficients by substituting the 
stored values into the equations 1 and 2. The value of the correction 
waveforms are then rapidly determined and applied to the convergence 
dipoles 106, 110 and 114. The system generates spots in succession from 
all three CRTs simultaneously so that the user may determine whether 
convergence has occurred at the local adjustment points. 
If the convergence is not satisfactory to the user, the "PR" button 406 is 
depressed, and the routine is repeated. When convergence is satisfactory 
the TV button 402 is depressed, and normal TV operation starts. 
In the "TV" mode, the system periodically measures the correction currents 
at the 9 locations and readjusts the attenuators to the stored correction 
waveform values if necessary. 
A separate button not indicated may recall the factory settings when 
desired. 
FIGS. 5 and 6 in combination show a hardware implementation for the hybrid 
convergence system of the present invention. FIG. 5 shows a programmable 
waveform generator for only one of the CRTs. A separate programmable 
waveform generator sharing the same analog waveform generator is provided 
for the red and blue CRTs in the preferred embodiment. In an alternate 
embodiment there is also a digitally programmable waveform generator for 
the green CRT. An analog waveform generaor 502 generates an adjustable DC 
value, and the following values corresponding to the location in the image 
via a bus 504: +x, +y, +x.sup.2, +y.sup.2, +xy, +x.sup.2 y, +xy.sup.2 and 
+x.sup.2 y.sup.2. Several programmable attenuators with the "A" 
designation are included to realize the actual values of the coefficients 
in the correction waveforms. The attenuators 506, 508, 510, 512, 514, 516, 
518, 520 and 522 represent the coefficients for the horizontal correction 
waveform, while the attenuators 524, 526, 528, 530, 532, 534, 536 and 538 
represent the coefficients for the vertical correction waveform. 
The optional switches 507 and 525 are for temporary presentations of the 
adjustable DC values during the adjustment procedure. The values from the 
attenuators are transferred to a corresponding adder 540 or 542 to 
complete the correction waveforms. The correction waveforms are applied to 
voltage-to-current converters 544 and 548, and finally to the horizontal 
convergence dipole 546 and the vertical convergence dipole 550, 
respectively. A switch 552 selects lines for monitoring the convergence 
dipole currents, including those of the other CRTs not shown in the 
figure. 
The micro-controller 602 (FIG. 6), containing a microprocessor 604, 
monitors, using an analog-to-digital converter 603, the convergence of the 
images. The horizontal and vertical synchronization pulses H and V, are 
used to synchronize the analog and digital processing signal generation to 
the incoming video signals. The signal SCL is a clock signal controlling 
the operation of the attenuators, while the signal SDA selects an 
attenuator and supplies it with a setting. The signals S select the 
convergence dipole current to be measured. The signal C.sub.oo, produced 
by a sampling pulse generator 610, controls the operation of the switches 
507 and 525. 
The convergence adjustment procedure is initiated at the control panel 606, 
with interfaces with the micro-controller 602. The values of the 
coefficients are calculated by the microprocessor 604 and stored in the 
memory 608. The memory 608 is of a nonvolatile, electrically erasable 
type, such as an EEPROM. 
During operation of the PTV system in the "TV" mode, the micro-controller 
602 periodically monitors the circuit I from the various convergence 
dipoles of the 9 positions. When these currents differ from the adjusted 
currents, the programmable attenuators can be re-programmed. The video 
signal generator 612 generates the video signals needed during the 
reconvergence procedure. 
The projection scheme for an alternate embodiment of the present invention 
is shown in FIG. 7. The embodiment in FIG. 7 is for automatic convergence 
of the PTV system without the need for comparison of the partial images or 
spots at the 9 local adjustment points by the user. The scheme in FIG. 7 
is identical to that of FIG. 1, except that a solid state black and white 
camera 702 has been added. The solid state camera 702 is for measuring the 
luminance reflected from the screen 102, and may be for example, a 
two-dimensional ("array") sensor. 
The hardware implementation is shown in FIG. 8. The system contains a 
digital measurement system 802, a correction system 804, which may be 
hybrid or fully digital and an output control 806 all coupled to a 
microprocessor system 808 as shown in the figure. 
The concept behind the automatic convergence version of the present 
invention is illustrated in FIG. 9. In that figure a checkerboard pattern 
is generated in the red CRT 104, and an inverted checkerboard pattern is 
generated in the green CRT 108 to correct misconvergence. Misgeometry can 
be corrected by generating a checkerboard pattern on the green CRT 108 and 
an inverted electronic checkerboard pattern that is added to the camera 
output signal. In the areas where the red and green CRTs are well 
converged, there is no overlap of the red and green checks, and the solid 
state camera 702 "sees" constant luminance in those areas. However, in 
areas where misconvergence occurs, the camera sees areas of higher 
luminance (the yellow regions) and dark areas where the colored checks are 
not contiguous. By outputting the images from the red and green CRTs at 
the same luminance level as seen by the camera, the digital processing 
system 808 can detect and process those areas of luminance discrepancy, so 
that the convergence system can correct the images for convergence. 
The size of the shaded areas shown in FIG. 10 compared to the check size is 
an indication of the amount of misconvergence or misgeometry. 
Correction signals are fed to one of the CRTs based on the calculated 
values for minimum misconvergence. These areas are measured for m.times.n 
areas of the picture (see FIG. 11). This is done for 5 conditions with 
different values for the overall horizontal and vertical shift, S.sub.H 
and S.sub.v, respectively. In the preferred embodiment, (S.sub.H, 
S.sub.V)=(O, O), (O, S.sub.V), (O.sub.1 -S.sub.V), (S.sub.H,O), and 
(-S.sub.H,O). A requirement is that S.sub.H and S.sub.V are smaller than 
half the checksize. From these measurements the S.sub.H and S.sub.V can be 
calculated, resulting in minimum or zero misconvergence. 
Variations and modifications to the present invention are possible given 
the above disclosure. However, variations and modifications which are 
obvious to those skilled in the art are intended to be within the scope of 
this letters patent. For example, the number of local adjustment points 
may be other than 9. Additionally, the correction waveforms may be other 
than those disclosed, and formulas for calculation of the coefficients may 
differ. Also, a different video signal may be used to adjust convergence. 
The programmable attenuators may be employed only for the convergence of 
the red and blue CRTs, and/or replaced by multiplying digital-to-analog 
converters.