Field compensation for cathode ray tube monitor

Abstract: In a monitor device which forms a display image by using a cathode ray tube, leakage of electric field from the front side of the cathode ray tube is diminished with a simple construction. A leakage electric field compensation electrode (2) is disposed to surround the display area of the front face of a cathode ray tube, and a leakage electric field compensation signal, the level of which varies according to fly-back pulse, is applied to this electrode, with reverse phase.

Description: This invention relates to a monitoring device, and more particularly to an improvement in monitoring devices which form display pictures by using a cathode ray tube. Conventionally, monitoring devices using cathode ray tubes have electric shields on the lateral and rear sides of the cathode ray tubes for the purpose of reducing unnecessary radiation, thereby maintaining the leakage of magnetic and electric fields to level below specific standard levels. However, the leakage of electric field cannot be completely avoided by mere shielding of the lateral and rear sides of the cathode ray tube. There is a large amount of unwanted radiation from a screen of a cathode ray tube which is not shielded. For instance, an A.C. electric field in the form of fly-back pulses is detected when measured, for example, by a tabular electrode D placed in front of a monitoring device M, as shown in Fig. 1. It is desirable to reduce the leakage of electric field, considering the undesirable effects produced by such electric fields on a person's health. A reasonably effective countermeasure is to provide a conductive coating on the surface of the cathode ray tube, which coating is grounded so as to reduce the leakage of electric field from the front face of the cathode ray tube. This countermeasure, however, requires a special transparent conductive paint, and involves excessive costs when considered in the practicalities of mass production of such monitoring devices. It has also been proposed to use a conductive filter at the front of a CRT in place of the conductive coating. However, this countermeasure uses extra special parts, and also involves problems in regard to practicalities. Still another solution is to use an antenna electrode disposed in the monitoring device and capable of creating an A.C. electric field so that the electric field which would otherwise have leaked is cancelled at the source. This method too is impractical since it involves employing a high voltage to be applied to the antenna electrode, and can, thereby, be dangerous. EP-A-0,523,741, which is prior art by virtue of Article 54(3) EPC only, discloses a compensating device using one or a plurality of discrete radiatprs placed at the sides or corners of the display screen. A ring shaped core is used around which two coils are wound. Through one coil the horizontal deflection current flows. The other coil is coupled to ground and a radiator. EP-A-0,500,349, which is also prior art by virtue of Article 54(3) EPC only, discloses a compensating device which uses a degaussing coil or a loop of wire supplied with a signal phase shifted relative to the fly-back pulse. In view of the foregoing, an aim of this invention is to provide a monitoring device which is simple in construction, but yet is capable of reducing the leakage of electric field from the front of a cathode ray tube. According to the present invention there is provided a monitoring device for displaying images using a cathode ray tube, comprising: an electric field forming electrode in the form of a closed loop arranged, in use, to surround the perimeter of a display area of the front face of said cathode ray tube; generating means for generating a leakage electric field compensation signal, the signal level of which varies in correspondence, and reverse phase, with the drive signal for the horizontal deflection yoke of the cathode ray tube by detecting, in use, a drive signal for a horizontal deflecting yoke of a cathode ray tube from a lead line of the horizontal deflecting yoke through a magnetic coupling means comprising a ring shaped core (12A, 12B) enclosing the lead line (18) and having a winding (16) to generate said leakage compensation signal on the basis of said drive signal; and means for supplying said leakage electric field compensation signal to said electric field forming electrode: whereby, in use, leakage of electric field from said surrounded display area is substantially cancelled by the electric field formed by said electric field forming electrode. The dependent claims describe particular embodiments of the invention. According to the present invention, it is possible to obtain a monitoring device in which leakage of electric field is diminished by a simple structure including a leakage electric field compensation electrode which is arranged to surround the display picture area of a cathode ray tube and which receives a leakage electric field compensation signal the level of which varies in relation to the fly-back pulse. The nature, principle and utility of the invention will become more apparent from the following detailed description, meant by way of example, when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters. In the drawings: Fig. 1 is a schematic diagram for the explanation of a method for measuring leakage electric field; Fig. 2 is a schematic diagram showing an embodiment of a monitoring device in accordance with the present invention; Fig. 3 is a perspective view of a pickup unit of the monitoring device shown in Fig. 2; Figs. 4A to 4C are signal waveform diagrams for the explanation of the operation of the pickup unit; and Fig. 5 is a schematic diagram showing a second monitoring device. Preferred embodiments of this invention will be described with reference to the accompanying drawings. A first embodiment is described with reference to Figure 2. The reference numeral 1 generally designates a monitoring device having a cabinet, with a leakage electric field compensation electrode 2 disposed inside the cabinet near the front side of the latter. The leakage electric field compensation electrode 2 is formed by a lead wire which is laid in loop-like so as to surround the display area of the cathode ray tube 3, at the inner side of the outer frame 4 (this is formed by so-called bezel) on the screen side of the cathode ray tube 3, and is fixed to the outer frame 4 by means of a predetermined lead wire supporting member. Thus, the leakage electric field compensation electrode 2 is held without making direct contact with the cathode ray tube 3, in such a manner as not to interfere with the cathode ray tube 3 when the latter is mounted. The cathode ray tube 3 is covered by a shield plate (not shown in figure) over its portion from the neck portion to the portion near the outer frame so as to be shielded, thus reducing the unnecessary radiation to the level below a predetermined level. Meanwhile, a deflecting circuit 6 generates power for driving the cathode ray tube 3 and drives a horizontal deflecting yoke 8, thereby deflecting an electron beam of the cathode ray tube 3 to display a picture. Furthermore, in this embodiment, the monitoring device 1 has a pickup device 10 which picks up a current flowing into a lead 18 which current is a driving signal of the horizontal deflecting yoke 8, and applies the picked up driving signal after amplifying it to the leakage electric field compensation electrode 2 to cancel the leakage electric field. As shown in Fig. 3, the pickup unit 10 includes a ring-shaped core which is split into two halves 12A and 12B received in halves of a case, 14, The arrangement is such that the core halves 12A and 12B are united to form the ring-shaped core when two halves of the case are brought together to close the case. The pickup unit 10 has a winding 16 of a predetermined number of turns provided on the case half receiving so that a magnetic transformer is made up in which the magnetic flux circulating through the ring-shaped core 12A and 12B are detected by the winding 16. The pickup unit 10 is so arranged that the cold side lead line 18 supplying the driving signal for the horizontal deflecting yoke 8 passes the center of the ring-shaped core 12A and 12B, and the core thus picks up the driving signal for the horizontal deflecting yoke 8. The cold side lead line 18 is provided with a direction indicating seal 21 wound around the lead, in order that this lead line can be discriminated from the hot-side lead line and that the pickup unit 10 can be mounted at a correct position and a correct direction. Then, as shown in Figs. 4A to 4C, the leakage electric field VR radiates in the same pattern as the fly-back pulse at the front side of the cathode ray tube 3 (Fig. 4A), and the level of the driving signal VH picked up from the cold-side lead line of the horizontal deflecting yoke 8 varies in accordance with the fly-back pulse (Fig. 4B), whereby the leakage electric field is cancelled by the driving signal VH applied to the leakage electric field compensation electrode 2 in reverse phase to the leakage electric field VR. More specifically, an experiment showed that, when the number of turns of the winding 16 is set to 20 and the driving signal VH of about 20 (Vp-p) was applied to the leakage electric field compensation electrode 2, the remaining leakage electric field VK is reduced substantially to zero (Fig. 4C). It will be understood that the leakage of electric field is effectively diminished by a simple structure which employs the leakage electric field compensation electrode 2 formed of a wire and arranged inside of the outer frame 4 and the pickup unit 10, without requiring any change in the circuit board and other parts. In general, a characteristic of known monitoring devices is that the level of the leakage electric field is largely changed due to variation in the horizontal deflecting current which is caused by a change in the size of the picture frame in the horizontal direction. In the above described embodiment, since the driving signal for the horizontal deflecting coil is picked up, the compensation electric field generated by the leakage electric field compensation electrode 2 is changed by an amount in proportion to the amount of variation in the leakage electric field caused by a change in the picture frame size, thereby preventing variation without adjusting in the intensity of the leakage electric field. According to this described arrangement, leakage of the electric field from a cathode ray tube can be cancelled by an electric field formed by a leakage electric field compensation electrode which is constituted by a lead wire laid around the display area of the front face of the cathode ray tube and which receives picked up driving signals for the horizontal deflecting coil, thus making it possible to reduce the leakage of electric field from the monitoring device by a simple structure. A second arrangement, housing a further form of leakage electric field compensation electrode that may be used in the present invention is now described with reference to Figure 5 which employ the same reference numerals as those in Fig. 2 to depict corresponding portions. Numeral 20 generally designates another monitoring device which employs a leakage electric field compensation electrode 22 which is formed by processing an aluminum board and sheet into frame-like form, also formed by metal plate plated bezel 4. Instead of the leakage electric field compensation electrode 22, it may be used a structure in which the bezel 4 is plated with metal. It is also to be understood that the shape and type of the leakage electric field compensation electrode can have wide selections, e.g., use of various metallic sheets, although a lead wire and a frame of an aluminum sheet are used in the described embodiments. While there has been described in connection with the preferred embodiments of the invention, it will be clear to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the claims.

Claims: A monitoring device (1, 20) for displaying images using a cathode ray tube (3), comprising: an electric field forming electrode (2, 22) in the form of a closed loop arranged, in use, to surround the perimeter of a display area of the front face of said cathode ray tube (3); generating means (10, 24) for generating a leakage electric field compensation signal (VH), the signal level of which varies in correspondence, and reverse phase, with the drive signal for the horizontal deflection yoke of the cathode ray tube by detecting, in use, a drive signal (HV) for a horizontal deflecting yoke (8) of a cathode ray tube from a lead line (18) of the horizontal deflecting yoke (8) through a magnetic coupling means (12A, 12B, 16) comprising a ring shaped core (12A, 12B) enclosing the lead line (18) and having a winding (16) to generate said leakage compensation signal on the basis of said drive signal through the lead line (18); and means (10, 24) for supplying said leakage electric field compensation signal to said electric field forming electrode: whereby, in use, leakage of electric field from said surrounded display area is substantially cancelled by the electric field formed by said electric field forming electrode. A device according to claim 1 wherein said electric field forming electrode (2) includes a lead wire in a loop to be laid around said display area of the front face of said cathode ray tube (3). A device according to claim 1 or 2 wherein: said electric field forming electrode (2) is formed by a or the lead wire covered with an insulating material. A device according to claim 1 or 2 wherein: said electric field forming electrode (21) is formed by a square metallic board with a frame-like form or a metallic sheet. A device according to any one of the preceding claims wherein said electric field forming electrode is fixed inside the front face of a bezel (4). A device according to claim 1 wherein: said electric field forming electrode is a layer of plate, around the front face of which the bezel is plated.

Inventors: HATADA HIDEO, SASAKI MASAHIKO, HATADA, HIDEO, SASAKI, MASAHIKO, Hatada, Hideo, c/o Sony Corporation, Sasaki, Masahiko, c/o Sony Corporation
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: H01J29
IPC codes: H04N5, H04N3, H01J29, G09G1