Apparatus for compensation for image rotation in a CRT display

Abstract: An image rotation compensation circuit for use with a CRT (14). The compensation circuit includes a winding (22) which encircles the CRT (14) and is oriented transverse to the path of the electron beam (20). A controller (26) is connected to the winding (22) and drives a current through the winding to create a magnetic field. The direction and magnitude of the current may be adjusted by the user to substantially cancel an external magnetic field and thus eliminate any image rotation caused thereby.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to the field of video display devices which include a cathode ray tube and, more specifically, to those display devices which also include a "yoke" deflection apparatus. 2. Discussion of the Prior Art A wide variety of video display devices including computer video terminals and the like incorporate a cathode ray tube, or CRT. A CRT includes an electron gun that emits a beam of electrons, which are directed toward a screen. The electron beam is deflected in a controlled manner to scan across the screen. The intensity of the beam is varied to form an image on the screen. A problem which arises in connection with CRT displays is "image rotation" in which the projected image appears rotated with respect to the horizontal axis of the screen. Image rotation is often caused by the horizontal component of an external magnetic field, one source of which is the earth's magnetic field. The amount of rotation may vary both in direction and magnitude depending on the geographic location of the display, the orientation of the display with respect to the external magnetic field, etc. One solution to the image rotation problem is to simply disassemble the display device and perform a manual adjustment of the device's deflection apparatus. For example, if the display device includes a yoke magnetic deflection apparatus, the yoke may be manually rotated with respect to the CRT, thereby offsetting the rotation caused by the external magnetic field. However, there are several disadvantages associated with this technique. First, a trained technician is usually required to perform the adjustment and must typically be dispatched to the user's location, which is quite costly for the user. Second, the disassembling of the display device carries the inherent risk of damage to the interior thereof. In addition, if the display device is later moved to a different geographic location or is simply reoriented with respect to the external magnetic field, a readjustment may be required due to the corresponding change in the direction or magnitude of the external magnetic field. Another solution to the image rotation problem is to place a shield around the CRT assembly. Shielding is typically quite costly and may create other difficulties. For example, the size of the chassis or housing in which the CRT assembly is disposed must often be increased in order to accommodate the shielding. Moreover, if the CRT assembly requires service or repair, the shielding obstructs access to the interior of the assembly. In some CRT displays, an electrostatic deflection apparatus is used to deflect the electron beam, as opposed to a yoke deflector which deflects the beam magnetically. An electrostatic-type deflector includes one or more pairs of plates to which a time-varying voltage is applied. The electron beam passes between each pair of plates and is deflected by a time-varying electric field that is created by the voltage. Some electrostatic-type CRT displays, such as an oscilloscope, include a conductive winding which is wrapped directly around the CRT. By providing a current through the winding, a magnetic field is created which may compensate for misalignment of the plates, as well as degradation of the plates due to aging. However, the magnetic field created by the winding cannot be adjusted to compensate for image rotation caused by an external magnetic field. German publication DE 28 09 725 A1 teaches an image display device with a cathode ray tube (CRT). This publication teaches that it is known practice to provide shielding in the region of the tube cone and in some cases in the region of the tube neck. The German publication also states that shielding provides "no satisfactory effect against foreign fields". The German publication teaches providing a measurement coil and a compensating coil "surrounding the tube", but does not teach that the compensating coil is a unitary part of any defelection yoke means. U.S. Patent US-A-4,296,359 teaches CRT television display error correction to correct electron gun mount rotation errors and shows a deflection yoke. To this end, a coil of wire is placed in front of the deflection yoke and is powered by a variable D.C. voltage source thereby obtaining a desired magnetic field. Again, the coil of wire is not a unitary part of the deflection yoke in the cited U.S. Patent US-A-4,296,359. The invention in its broad form resides in a video display unit for displaying an image on a screen, said unit being subject to an external mangetic field, said external mangetic field causing a rotation of said image with respect to said screen, said unit comprising: cathode ray tube means, said tube means including an electron gun and a screen said gun for emitting an electron beam along a path toward said screen; deflection yoke means, said yoke means being disposed between said electron gun and said screen, said yoke means operating to deflect said electron beam, thereby forming said image, said yoke means having a first end juxtaposed with said screen and a second end juxtaposed with said electron gun, said yoke means further comprising a conductive winding having first and second ends, said winding being substantially disposed in a recess of the first end of the yoke means, said winding forming part of a unitary structure with said yoke means, whereby said yoke means together with said winding is capable of unitary assembly and disassembly; said winding being oriented substantially transverse to said path, said first and second ends of said winding being coupled to control means, said control means for producing a compensation current which is conducted through said winding, said compensation current producing a compensating magnetic field for substantially cancelling said external magnetic field. SUMMARY OF THE INVENTION The present invention provides a new and improved apparatus for compensating for image rotation in CRT video displays that is caused by an external magnetic field. In brief summary, the apparatus includes a coil or winding which encircles the CRT and which is oriented substantially transverse to the path of the electron beam. A controller is connected to the winding and operates to drive a current through the winding. The controller adjusts the current's direction and magnitude to create a magnetic field which offsets the external magnetic field, thus eliminating the image rotation caused thereby. The controller is easily adjusted by the user without disassembly of the display device. As a result, if the display device is relocated or there is a change in the external magnetic field, the controller may simply be adjusted to compensate therefor. The installed apparatus does not substantially affect the dimensions of the CRT assembly. In addition, the apparatus remains separate from the CRT, thereby permitting the separate replacement of either the apparatus or the CRT. The apparatus is low in cost and simple to construct. BRIEF DESCRIPTION OF THE DRAWINGS This invention is pointed out with particularity in the appended claims. The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which: Figure 1 is a front elevational view of a screen of a video display device showing both a normal image and an image which has been rotated as a result of an external magnetic field; Figure 2 is a side elevational view of a CRT assembly which incorporates a compensation apparatus constructed in accordance with the preferred embodiment of the present invention; Figure 3 is a front elevational view of a yoke assembly included in the CRT assembly depicted in Figure 2; and Figure 4 is a schematic circuit diagram of a controller, which forms part of the compensation apparatus shown in Figure 2. DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT Figure 1 is a front elevational view of a video display unit 2 which comprises a chassis 4 and a cathode ray tube (CRT) 6. The CRT 6 displays an image 8 in a well known manner. When the CRT 6 is operating properly, the image 8 is oriented as indicated by the rectangle in solid lines. However, when the CRT 6 is subjected to an external magnetic field, such as that of the earth, the horizontal component of the external field can cause a rotation of the displayed image, as indicated by the rectangle in broken lines. The direction and amount of rotation depend upon the direction and strength of the external magnetic field. Figure 2 is a side elevational view of a CRT assembly 12 which incorporates a compensation apparatus, constructed in accordance with the present invention, for substantially eliminating image rotation caused by an external magnetic field. A CRT 14 is disposed within a yoke assembly 16. An electron gun 18 is disposed in the rear portion of the CRT 14. The electron gun 18 emits a beam 20 of electrons, which is deflected by the yoke assembly 16 to form an image on a screen 28. A portion of the yoke assembly 16 is broken away to show a wire coil or winding 22 disposed therein. The winding 22 encircles the CRT 14 and lies in a vertical plane which is preferably oriented substantially transverse to the path of the beam 20. The winding 22 has two terminals 24 to which a controller 26 is connected. In general, the controller 26 generates a current which is conducted through the winding 22 to create a magnetic field. The controller 26 may be adjusted by the user to adjust both the magnitude and direction of the current. Thus, if the CRT assembly 12 is subject to an external magnetic field which causes a rotation of the displayed image, the user may simply watch the image on the screen 28 while adjusting the controller 26, until a current of the proper magnitude and direction is produced to substantially offset the external magnetic field and thereby eliminate the image rotation. Figure 3 is a front elevational view of the yoke assembly 16 shown in Figure 2, as seen from the right in Figure 2. A hollow region 32, in which the CRT 14 is normally disposed, extends longitudinally through the yoke assembly 16. A narrow recessed area 30 encircles the forward end of the hollow region 32. The recessed area 30 is adapted to receive the winding 22, so that the forward edge of the winding 22 may be substantially flush with a front collar 34. As a result, the profile and outside dimensions of the yoke assembly 16 are substantially unaffected by the addition of the winding 22. Moreover, the winding 22 is disposed such that it does not interfere with the deflection functions of the yoke assembly 16. Lastly, it will be appreciated that the yoke assembly 16, along with the winding 22, may be removed or replaced separately from the CRT 14. Figure 4 is a detailed schematic circuit diagram of the controller 26 (Figure 2). With reference to Figure 4, an alternating voltage from a transformer 36 is rectified by a diode 42 and filtered by a capacitor 44 to provide a substantially direct current voltage to a node 45. A bridge circuit 47 is also connected to the node 45 and to the winding 22. The bridge circuit 47 serves to control the direction and magnitude of the current which is conducted through the winding 22 to thereby control the direction and magnitude of the magnetic field generated by the winding. In particular, two resistors 46 and 48 form a voltage divider which produces a substantially fixed voltage reference at a node 51. The winding 22 is connected between the node 51 and a variable tap 53 of a potentiometer 50. By varying the tap 53, the user may vary the magnitudes of the resistance between the tap 53 and the node 45, and the resistance between the tap 53 and ground potential, thus varying the voltage level at the tap 53. Since the voltage level at the node 51 is fixed, the voltage difference between the tap 53 and the node 51 is effectively varied by adjusting the position of the tap 53. Thus, if the voltage level at the tap 53 is above that at the node 51, current flows through the winding 22 from the tap 53 to the node 51, with the magnitude of the current being related to the magnitude of the voltage difference and the resistance of the winding 22. Conversely, if the voltage level at the node 51 is above that at the tap 53, the current flows from the node 51 to the tap 53. The present invention is also suitable for other types of CRT, displays, such as those which use an electrostatic In summary, the present invention provides a compensation apparatus. In summary, the present invention provides a compensation apparatus which permits a user of a CRT display to quickly and easily eliminate image rotation caused by an external magnetic field. The compensation apparatus is suitable for both electrostatic and magnetic types of display devices and, when installed, does not substantially affect either the dimensions of the CRT assembly nor the functions of the deflection apparatus. The foregoing description has been limited to a specific embodiment of this invention. It will be apparent, however, that variations and modifications may be made to the invention, with the attainment of some or all of the advantages of the invention.

Claims: A video display unit for displaying an image on a screen, said unit being subject to an external mangetic field, said external mangetic field causing a rotation of said image with respect to said screen, said unit comprising: cathode ray tube means (14), said tube means including an electron gun (18) and a screen (28) said gun for emitting an electron beam along a path toward said screen; deflection yoke means 16), said yoke means being disposed between said electron gun (18) and said screen (28), said yoke means operating to defelect said electron beam, thereby forming said image, said yoke means having a first end juxtaposed with said screen (28) and a second end juxtaposed with said electron gun (18), said yoke means further comprising a conductive winding (22) having first and second ends, said winding being substantially disposed in a recess (30) of the first end of the yoke means, said winding forming part of a unitary structure with said yoke means, whereby said yoke means together with said winding is capable of unitary assembly and disassembly; said winding being oriented substantially transverse to said path, said first and second ends of said winding being coupled to control means (26), said control means for producing a compensation current which is conducted through said winding, said compensation current producing a compensating magnetic field for substantially cancelling said external magnetic field. The video display unit according to claim 1, characterized in that said control means (26) includes means for adjusting the direction and magnitude of said compensation current. The video display unit according to claim 2, characterized in that said means for adjusting comprises a voltage reference means (46, 48) and a variable resistance means (50), said first end of said winding being connected to said voltage reference means, said second end of said winding being connected to said variable resistance means, said voltage reference means for producing a substantially constant voltage at said first end, said variable resistance means for producing a variable voltage at said second end.

Inventors: FITZGERALD JOHN J, SANDS ROBERT M, FITZGERALD, JOHN J., SANDS, ROBERT M.
Applicants: DIGITAL EQUIPMENT CORP, DIGITAL EQUIPMENT CORPORATION
Application year: 1988
Main classification: H04N3
IPC codes: H04N9, H04N3