External magnetic field compensator for a CRT

Light sensors with red filters are located at four corners of a CRT out of view of the operator. A red raster is generated in the corners of the CRT, the pattern of the raster corresponding to the area covered by the sensors. A pair of coils is located about the face of the CRT in a Helmholtz configuration and are driven by a current generator in response to the sensor output to maximize sensor output, thereby generating a compensating magnetic field which cancels external axial fields affecting the CRT.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates generally to the automatic cancellation of external 
magnetic fields and, in particular, to apparatus for compensating for an 
external magnetic field affecting the operation of a cathode ray tube 
(CRT) display. 
2. Description of the Prior Art 
Frequently it is necessary to operate a CRT display in the presence of a 
five gauss axial external magnetic field. With only a degaussing network 
in conjunction with a shield around the CRT, a 1.5 to 2 gauss axial field 
may be effectively cancelled. It has been suggested that Hall effect 
devices and fluxgate transformers may be used in conjunction with active 
circuitry to cancel external magnetic fields. However, these methods are 
very complex and high in cost. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an apparatus for cancelling an 
external magnetic field affecting a CRT. 
The invention comprises means for generating a reference display of a given 
color in a predetermined area on the face of the CRT during a field 
displayed on the CRT. Means for generating a compensating magnetic field 
about the CRT is provided. Means for controlling the strength of the 
compensating magnetic field functions in response to means for sensing an 
optical characteristic of the reference display and generating an output 
signal representative thereof. 
For a better understanding of the present invention, together with other 
and further objects, reference is made to the following description, taken 
in conjunction with the accompanying drawings, and its scope will be 
pointed out in the appended claims.

DETAILED DESCRIPTION OF THE INVENTION 
In order to completely negate the effects of an external axial magnetic 
field on a high resolution color CRT, simple shielding methods can be 
employed. However, the shield must protrude past the plane of the CRT face 
by a distance which is a function of the amplitude of the external 
magnetic field. For fields of the order of 5 gauss, the shield must 
protrude a distance somewhat greater than three inches. This results in a 
tunnel effect for the operator of the CRT and is unacceptable. 
FIG. 1 illustrates CRT 1 in combination with a block diagram of circuitry 
according to the invention for cancelling external magnetic fields 
affecting CRT 1. As in the prior art, CRT 1 is provided with a degaussing 
coil 2 having a damped sine wave 2a of current at approximately 350 Hz 
applied during a time interval of one field (16.67 milliseconds) every ten 
seconds. This prior art degaussing function fully corrects for horizontal 
and vertical external magnetic fields and partially corrects for axial 
external magnetic fields. 
Operating simultaneously with degaussing coil 2, alternately wound coils 3, 
4 are activated by current generator 5 to create a linear axial magnetic 
field, such as oppositely wound coils in a Hemholtz configuration. Current 
generator 5 eliminates interaction between degaussing coil 2 and coils 3, 
4 which may give rise to incomplete degaussing of the display. Coils 3, 4 
are connected in series and are located within the shield 7 of CRT 1 which 
minimally projects beyond the face 10 of CRT 1. An axial field is created 
between coils 3 and 4 which "bucks" or cancels any axial magnetic field 
which is not counteracted by degaussing coil 2. Current generator 5 is a 
high impedance device and applies a current to coils 3, 4 in response to 
maximizing circuit 6. 
The invention includes the use of sensors 8 on the face 10 of CRT 1, 
preferably, out of view of the operator and in the four corners of the 
face 10. A raster is generated in the areas where the sensors 8 are 
located and the sensors 8 respond to the light created by the raster. The 
sensors 8 monitor an optical characteristic of CRT 1 such as light 
intensity or color purity. Maximizing the output of the light from the 
raster generated under each sensor coincides with an essentially pure, or 
single color, field over the entire CRT face 10. In color monitors, the 
red raster is most sensitive to purity changes. Therefore, sensors 8 with 
red filters 9 are preferred for use with a generated red raster. 
Specifically, sensors 8 are located in the four corners of the face 10 of 
CRT 1 with red filter 9 located between sensor 8 and face 10. Red raster 
generator 11 is used to input information to red raster gun input port 12. 
During each field, generator 11 creates a red raster at the locations of 
the face 10 of CRT 1 which include sensors 8 and filters 9. Maximizing 
circuit 6 senses the output of sensors 8 and controls generator 5 to 
maximize sensor output. Therefore, the red gun of CRT 1 is energized 
within the field of view of sensors 8 which generate output signals which 
are a function of the "redness" of the reference area on CRT face 10 
associated with each sensor. As CRT 1 is exposed to external magnetic 
fields, sensor outputs will change as a function of the amount of red 
light detected in a manner described in greater detail hereinafter. By 
maximizing the average "redness" observed by the sensors 8, external 
magnetic fields are negated. 
Referring to FIG. 3, the red raster 13 generated within the field of view 
of the sensor 8 has approximately the same cross-sectional area and shape 
as the sensor 8 and filter 9 associated therewith. However, red raster 
area 13 may be larger or smaller. 
Red raster generator 11 may be a line counter 14 and a pulse generator 15. 
Counter 14 counts the horizontal lines during each field of display of CRT 
1 and activates pulse generator 15 during the intervals of the lines which 
are within the fields of view 13 of sensors 8. Pulse generator 15 is 
configured to provide pulses to red raster gun input port 12 so that the 
red gun is "on" while it is within the field of view of sensors 8. For 
example, consider a substantially square CRT display of 525 lines with 
262.5 lines per field having 240 lines of display. Assume that the sensors 
8 are located in the outermost corners of the CRT display and that each 
sensor covers 1% of the total area of the CRT face. Therefore, the fields 
of view 13 of the sensor 8 are as follows: the first 10% and the last 10% 
of the first 24 lines (10%) of display and of the last 24 lines (10%) of 
display of each field. As a result, pulse generator 15 would be configured 
to provide an "on" pulse during the first 10% and the last 10% of lines 
1-24 and lines 217-240 of each field. 
As the display of CRT 1 is exposed to external magnetic fields, the output 
of sensors 8 will change as a function of the amount of red light 
observed. This is because the external magnetic field will deflect the 
electron beam emitted by the red electron gun causing color impurities. 
The outputs of the sensors 8 are maximized by maximizing circuit 6 which 
may include peak detectors 16 for detecting the peak excitation of each 
sensor 8 during each field. Summing circuit 17 sums these detected peaks 
to provide an output signal which represents the average "redness" of the 
display for that field. Synchronization of the circuitry is accomplished 
by using the vertical synchronization signal pulse VS (16.67 millisecond). 
Upon receipt of a VS pulse, the peak detectors 16 are zeroed. During the 
next 16.67 millisecond field, each peak detector is charged to an 
amplitude representative of the red light observed by its corresponding 
sensor 8. Signals representing the observed light are summed by summing 
means 17 and stored in first sample and hold circuit 18. 
At this point in time, current generator 5 is slightly altered. 
Specifically, current generator 5 may be an N-bit counter 19 responsive to 
a delayed VS pulse and connected to an N-bit digital-to-analog converter 
20 where N=8, 12 or any other value depending on desired sensitivity. The 
output of converter 20 may be amplified by a current amplifier 22. Counter 
19 is changed by one least significant bit in response to the delayed VS 
pulse thereby causing the current provided by converter 20 to slightly 
change. 
The next VS pulse resets the peak detectors 16 and the process is repeated. 
However, during this field the output of summing means 17 is stored in 
second sample and hold circuit 21. Comparator 22 now compares the data in 
the first sample and hold circuit 18 with the data in the second sample 
and hold circuit 21. If the data in the second sample and hold circuit 21 
is greater than the data in the first sample and hold circuit 18, the 
up/down count line remains unchanged because greater data in circuit 21 
implies that the change in current generator 5 increased the average 
"redness" of the display. If the data in circuit 21 is less than the data 
in circuit 18, the up/down count line is reversed by comparator 22. At 
this period in time, the delayed VS pulse provided to counter 19 again 
changes the count by one least significant bit causing the current 
generated by converter 20 to slightly change. This sequence is continually 
repeated during each field. 
A 12-bit counter and a 12-bit converter may be used as the current 
generator depending on the size of the sensors and the accuracy desired. 
In addition, not all bits of the counter need be used. For example, only 9 
bits of a 12-bit counter may be used. 
The invention, therefore, functions to maximize the average light output of 
the sensors 8. In maximizing these outputs, it maximizes the red light 
output in the fields of view 13 of the sensors 8. This maximizing 
corresponds to negation of the remaining effects of an external magnetic 
field which is not compensated for by degaussing coil 2. FIGS. 4-7 are a 
simplified series of illustrations exemplifying the sequence of correction 
according to the invention. In FIG. 4, fields of view 13a, 13b, 13c, 13d 
defined by the solid lines do not coincide with red rasters 23a, 23b, 23c, 
23d defined by the dotted lines. The shaded area indicates the overlap. 
Assume that the invention is initially activated and results in creating a 
magnetic field which adds to rather than compensates for the external 
magnetic field. As illustrated in FIG. 5, the fields of view would then 
coincide even less after such a correction. As a result, the correction 
provided by current generator 5 would be reversed by reversing the up/down 
line of counter 19 resulting in initial compensation of the external 
magnetic field as illustrated in FIG. 6. Since this initial compensation 
would increase the average "redness" of the display, current generator 5 
would remain in the same condition and increase its current generation to 
further compensate for the external magnetic field as shown in FIG. 7. 
While there have been described what are at present considered to be the 
preferred embodiments of this invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the invention and it is, therefore, aimed 
to cover all such changes and modifications as fall within the true spirit 
and scope of the invention.