Abstract:
In a deflection yoke for use in a color cathode ray tube receiver, two sets of sextuple pole coils formed of bifilar windings are disposed around the orbits of three electron beams emitted from an electron gun of a cathode ray tube. Horizontal-period parabolic currents produced in a bridge circuit consisting of saturable reactors are caused to flow in such sextuple pole coils. Further the parabolic currents are modulated at the vertical period by the saturable reactor. Then sextuple-pole magnetic fields are generated by the modulated parabolic currents to thereby exert vertical force on the three electron beams, hence realizing proper correction of ΔVCR to consequently optimize the balance between the corners and the center of a screen. Such optimization is conventionally difficult due to some restrictions existing in the winding distribution of a vertical deflection coil and a horizontal deflection coil in relation to convergence and focus of side beams.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a deflection yoke and a color cathode ray tube receiver using such a yoke, and more particularly to those equipped with a convergence corrector for correcting misconvergence of a color cathode ray tube employed in a television receiver, a display monitor or the like. 
     In a color cathode ray tube, a color picture is displayed on its screen by vertically and horizontally deflecting the forward directions of three electron beams emitted from an electron gun. 
     For deflection of electron beams, there is used a deflection yoke having a horizontal deflection coil and a vertical deflection coil. 
     In a cathode ray tube, a deflection yoke is installed in one region termed a cone which is defined from a neck of the tube to a funnel thereof. 
     A horizontal deflection current and a vertical deflection current are caused to flow, respectively, in a horizontal deflection coil and a vertical deflection coil on the orbits of three electron beams emitted from the electron gun, thereby forming deflection magnetic fields. The electron beams are deflected vertically and horizontally by such deflection magnetic fields. 
     In the color cathode ray tube, three electron beams emitted from its electron gun are converged on one point of a fluorescent screen via color selection electrodes of an aperture grill or a shadow mask, whereby a desired color picture is reproduced on the screen. 
     In this case, if there occurs misconvergence where the three electron beams fail to be converged on one point of the fluorescent screen, it causes some color deviation or color phase irregularity. 
     Generally, in any color cathode ray tube having an in-line type electron gun where a center electron beam G for lighting a green fluorescent layer and side electron beams R, B for lighting red and blue fluorescent layers are arranged in a line, misconvergence shown in FIG. 1 occurs on the screen when the vertical deflection magnetic field is a uniform one. 
     In this example, a red side beam R deviates leftward, while a blue side beam B deviates rightward. 
     It is widely known that this misconvergence can be corrected by forming the vertical deflection magnetic field into a barrel shape. 
     More specifically, there is generally performed a technique of adjusting the winding distribution of a vertical deflection coil to thereby form the vertical deflection magnetic field into a barrel shape . 
     However, if the vertical deflection magnetic field is formed into a barrel shape, there occurs another misconvergence in a vertical direction, as shown in FIG.  2 A. 
     In case such vertical misconvergence is existent, the difference between the average value of the side beams R, B and the center beam G is termed VCR (Vertical Center Raster). 
     It is possible to achieve static correction of this VCR by means of adding, for example, some magnetic member to the electron gun. 
     Practically, however, the VCR is not always fixed in dimension, and there may arise some difference between the VCR at the t op and bottom of the screen along the Y-axis thereof, i.e., at the screen center, and the VCR at the horizontal top and bottom ends of the screen, i.e., at the screen corners. 
     For example, there may remain a pattern of FIG. 2B where the beam G is outside at the screen center, while the beam G is inside at the screen corners. 
     It is supposed here that the difference between the raster VCR at the screen center and the raster VCR at the screen corner is termed ΔVCR. 
     In order to change such ΔVCR, the following two measures may be adopted for example. 
     The first measure is carried out by adjusting the winding distribution of the vertical deflection coil to thereby balance the screen corner and the screen center. 
     The second measure is carried out by utilizing that a horizontal deflection magnetic field that affects the raster VCR at the screen corner. 
     More concretely, the screen corner and the screen center are balanced by adjusting the winding distribution of the horizontal deflection coil. 
     Now a consideration will be given below on the force exerted on the center beam G and the side beams R, B by the vertical deflection magnetic field in a barrel shape. 
     It is supposed that the magnetic fields exerted respectively on the in-line center beam G and side beams R, B are in the directions indicated by arrows in FIG.  3 . 
     FIG. 3A shows an example where the electron beams are deflected upward along the Y-axis of the screen, and FIG. 3B shows another example where the electron beams are deflected toward the upper right end of the screen corner. 
     The horizontal component of the magnetic field exerted on each electron beam, i.e., the magnetic field for vertical deflection, can be changed by adjusting the winding distribution of the vertical deflection coil. 
     It is also possible to change the magnetic fields separately at the screen center and the screen corners to a certain extent. 
     However, if the horizontal component of the vertical deflection magnetic field is changed by adjusting the winding distribution of the vertical deflection coil, the vertical component of the vertical deflection magnetic field is also changed simultaneously therewith. 
     For this reason, if the winding distribution of the vertical deflection coil is altered, the horizontal convergence is affected as observed in the HCR (Horizontal Center Raster) which represents the difference between the average value of the side beams R, B and the center beam G. 
     Also in the case of adjustment by the winding distribution of the horizontal deflection coil, the concept is still the same although the direction of the magnetic field is different, and therefore it is possible to change ΔVCR at the screen corner, but such adjustment affects the vertical convergence. 
     Further, any change of the magnetic fields affects the focus characteristics of electron beams as well as the convergence characteristics thereof. 
     Thus, in either of the first and second measures mentioned above, there exist some restrictions relative to the winding distribution in connection with the convergence or focus of the side beams R and B. 
     It is therefore difficult to optimize ΔVCR by altering the winding distribution of the vertical deflection coil or the horizontal deflection coil. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished in view of the problems described above. It is an object of the invention to provide a deflection yoke and a color cathode ray tube receiver using such a yoke equipped with a convergence corrector which is capable of correcting ΔVCR independently to thereby achieve proper convergence with high precision. 
     According to one aspect of the present invention, there is provided a deflection yoke which comprises parabolic current producing means for producing a horizontal-period parabolic current and then supplying the parabolic current to a convergence correcting coil; sextuple-pole magnetic field generating means disposed around the orbits of three electron beams emitted from an electron gun, and exerting vertical force on the three electron beams by a sextuple-pole magnetic field generated in accordance with the horizontal-period parabolic current supplied from the parabolic current producing means; and saturable reactor means for modulating, by a vertical-period current, the horizontal-period parabolic current flowing in the sextuple-pole magnetic field generating means. 
     This deflection yoke is installed in a cone region of a cathode ray tube employed in a color cathode ray tube receiver. 
     In the deflection yoke having the above structure and a color cathode ray tube receiver using such deflection yoke, a horizontal-period parabolic current produced in the parabolic current producing means is caused to flow in the convergence correcting coil, so that any misconvergence is corrected by a correcting magnetic field generated by the convergence correcting coil. 
     The horizontal-period parabolic current is caused to flow also in the sextuple-pole magnetic field generating means. 
     Consequently, the sextuple-pole magnetic field generating means generates a sextuple-pole magnetic field in accordance with the horizontal-period parabolic current, and exerts vertical force on three electron beams by the sextuple-pole magnetic field. 
     In this case, the saturable reactor means modulates, by the vertical-period current, the horizontal-period parabolic current flowing in the sextuple-pole magnetic field generating means. 
     As a result, the horizontal-period parabolic current modulated at the vertical period is caused to flow in the sextuple-pole magnetic field generating means. 
     The above and other features and advantages of the present invention will become apparent from the following description which will be given with reference to the illustrative accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanatory diagram of horizontal misconvergence induced on the screen of a cathode ray tube when the vertical deflection magnetic field is a uniform one; 
     FIG. 2A is an explanatory diagram of vertical misconvergence induced on the screen of a cathode ray tube when the vertical deflection magnetic field is a barrel one; 
     FIG. 2B shows an exemplary pattern of ΔVCR induced on the screen of a cathode ray tube; 
     FIG. 3A shows magnetic fields impressed to in-line electron beams when the electron beams are deflected toward the upper end of the screen along its Y-axis, illustrating a state seen from the screen side; 
     FIG. 3B shows magnetic fields impressed to in-line electron beams when the electron beams are deflected toward the upper right end of the screen, illustrating a state seen from the screen side; 
     FIG. 4 is a schematic perspective view showing the whole of a color cathode ray tube where the present invention is applied; 
     FIG. 5 is a partly sectional side view of a deflection yoke where the present invention is applied; 
     FIG. 6 is a circuit diagram showing a structural example of a convergence corrector installed in the deflection yoke where the present invention is applied; 
     FIG. 7 shows an exemplary structure of a saturable reactor employed in the present invention; 
     FIG. 8 is a connection diagram showing an exemplary structure of individual coils and a convergence correcting coil which constitute a second bridge circuit employed in the present invention; 
     FIG. 9 is a connection diagram showing a structural example of two sets of sextuple-pole coils and two saturable reactors employed in the present invention; 
     FIG. 10 is an explanatory diagram for explaining the principle of operation of a saturable reactor including a first bridge circuit employed in the present invention; 
     FIGS. 11A to  11 E are waveform charts for explaining the operations of two sets of sextuple-pole coils and two saturable reactors employed in the present invention; 
     FIG. 12 is a waveform chart showing the polarity of a parabolic current in the present invention; 
     FIG. 13 shows a sextuple-pole magnetic field generated according to a positive parabolic current in a correcting circuit employed in the present invention, illustrating a state seen from the front of a cathode ray tube; 
     FIG. 14 shows a sextuple-pole magnetic field generated according to a negative parabolic current in the correcting circuit employed in the present invention, illustrating a state seen from the front of the cathode ray tube; and 
     FIG. 15 shows changes of the positional relation caused between a center beam G and side beams R, B by the correcting circuit employed in the present invention, illustrating an exemplary pattern observed on the entire screen. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter some preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 4 perspectively shows the whole of a color cathode ray tube where the present invention is applied. 
     In FIG. 4, a panel  12  having a fluorescent screen on its inner face is attached to the front portion of a picture tube  11 , and an electron gun  13  for emitting electron beams therefrom is enclosed in a rear end portion of the picture tube  11 . 
     Further a cone-shaped deflection yoke  14  for deflecting the electron beams emitted from the electron gun  13  is attached to a neck of the picture tube  11 . 
     FIG. 5 is a partly sectional side view of the deflection yoke  14  according to the present invention. 
     As obvious from FIG. 5, the deflection yoke  14  is equipped with such component members as a horizontal deflection coil  15 , a vertical deflection coil  16 , a coil bobbin  17 , a core  18  and a ring magnet  19 . 
     The horizontal deflection coil  15  and the vertical deflection coil  16  serve to deflect the electron beams, which have been emitted from the electron gun  13 , leftward/rightward (in horizontal direction) and upward/downward (in vertical direction), respectively. 
     These deflection coils  15  and  16  are installed in the cone-shaped coil bobbin  17 . 
     More specifically, the horizontal deflection coil  15  is positioned on the inner peripheral side of the coil bobbin  17 , while the vertical deflection coil  16  is positioned on the outer peripheral side of the coil bobbin  17 . 
     The core  18  is composed of ferrite, and is so installed as to cover the deflection coils  15  and  16  for further enhancing the efficiency of magnetic fields generated from the deflection coils  15  and  16 . 
     The ring magnet  19  is provided in the neck of the deflection yoke  14  for correcting any assembly error of the electron gun  13 . 
     FIG. 6 is a circuit diagram showing a structural example of a convergence corrector installed in the deflection yoke  14 . 
     In FIG. 6, series-connected coils L 1 , L 2  and similar series-connected coils L 3 , L 4  are bridge-connected in parallel to each other to thereby constitute a first bridge circuit  20 . 
     Out of these two sets of coils, the coils L 1 , L 4  and the coils L 2 , L 3  constitute a saturable reactor  26 , as shown in FIG.  7 . 
     Now the structure of this saturable reactor  26  will be described below with reference to FIG.  7 . 
     The coils L 1 , L 4  and the coils L 2 , L 3  are wound around two drum cores  21 ,  22 , respectively. 
     These coils may be so wound as to form bifilar windings. 
     If a plurality of wires are wound simultaneously to form bifilar windings, the winding states of the coils L 1 , L 4  and the coils L 2 , L 3  are mutually equalized so that substantially equal magnetic characteristics can be achieved in such two pairs of coils. 
     The coils L 1 , L 4  and the coils L 2 , L 3  are wound in different directions so as to generate magnetic fields of mutually reverse directions. 
     Two permanent magnets  23 ,  24  are disposed outside the two drum cores  21 ,  22  in such a manner that fixed bias magnetic fields are impressed from the two permanent magnets  23 ,  24  to the coils L 1 , L 4  and the coils L 2 , L 3 . 
     In this embodiment, the permanent magnet  23  is so disposed as to operate the drum core  21  as S pole, while the permanent magnet  24  is so disposed as to operate the drum core  22  as N pole. 
     Between the two drum cores  21  and  22 , there is provided another drum core  25  which is similar in shape. 
     A modulating coil L 5  is wound around the drum core  25 . This modulating coil L 5  impresses a magnetic field, which corresponds to the current flowing in the coil L 5 , to the coils L 1  to L 4 . 
     The saturable reactor  26  has the structure mentioned above. 
     As will be described later, the saturable reactor  26  functions as means to generate a horizontal deflection-period parabolic current modulated at the vertical deflection period. 
     In the first bridge circuit  20  consisting of the coils L 1  to L 4  shown in FIG. 6, coils L 6 , L 7  and coils L 8 , L 9  are connected in series, respectively, between output terminals of the bridge circuit  20 , i.e., between a common junction A of the coils L 1 , L 2  and a common junction B of the coils L 3 , L 4 . 
     These four coils, i.e., the coils L 6 , L 7  and the coils L 8 , L 9 , constitute a second bridge circuit  27 . 
     Further, a convergence correcting coil Lc is connected between output terminals of the second bridge circuit  27 , i.e., between a common junction C of the coils L 6 , L 7  and a common junction D of the coils L 8 , L 9 . 
     FIG. 8 shows an exemplary structure of the coils L 6  to L 9  and the convergence correcting coil Lc. 
     The coils L 6  and L 9  are wound around a core  28  which forms a closed magnetic circuit. 
     Further, bias coils Lb 1  and Lb 2  are also wound around the core  28 . 
     And a vertical deflection current flows in the bias coils Lb 1 , Lb 2  via vertical deflection coils LV 1 , LV 2  which will be described later. 
     Meanwhile, the coils L 7 , L 8  are wound around a coil bobbin (not shown) in a manner to form, e.g., bifilar windings. 
     The inductance is rendered variable by shifting a core  29  inward or outward with regard to the bobbin. 
     The convergence correcting coil Lc consists of four split coil members Lc 1  to Lc 4 . 
     These four coil members Lc 1  to Lc 4  are positioned at an angular interval of 90° around the neck N of the color cathode ray tube. 
     In FIG. 6, the convergence correcting coil Lc is shown simply as a single coil. 
     Further, one end of each of sextuple-pole coils  30 ,  31  is connected to the output terminal A of the bridge circuit  20 . 
     The sextuple pole coil  30  consists of six series connected coils L 10  to L 15  and is connected, at an open end of the coil L 10 , to the output terminal A of the bridge circuit  20 . 
     Also the other sextuple pole coil  31  consists of six series-connected coils L 16  to L 21  and is connected, at an open end of the coil L 21 , to the output terminal A of the bridge circuit  20 . 
     As shown in FIG. 9, the respective coils L 10  to L 15  and L 16  to L 21  of the sextuple-pole coils  30  and  31  are disposed in the periphery of the neck N of the color cathode ray tube. 
     More specifically, in the periphery of the neck N of the color cathode ray tube, substantially C-shaped cores  32 ,  33  are disposed in the vertical direction on both sides of the neck N, while substantially I-shaped cores  34 ,  35  are disposed in the horizontal direction on both sides of the neck N. The coils L 10  to L 15  and L 16  to L 21  are wound around such cores respectively. 
     The sextuple-pole coil  30  is so structured that the coils L 10 , L 11  are wound around legs of the core  32 , the coil L 12  is wound around the core  34 , the coils L 13 , L 14  are wound around legs of the core  33 , and the coil L 15  is wound around the core  35  respectively in this order. 
     Similarly, the sextuple pole coil  31  is so structured that the coils L 16 , L 17  are wound around legs of the core  32 , the coil L 18  is wound around the core  34 , the coils L 19 , L 20  are wound around legs of the core  33 , and the coil L 21  is wound around the core  35  respectively in this order. 
     Each component coil of the sextuple-pole coil  30  and each component coil of the sextuple-pole coil  31  are wound to form bifilar windings. 
     The coils L 10 , L 11  and the coils L 16 , L 17  are wound around the core  32  in such a manner as to generate, between the end faces of the legs thereof according to the current directions, magnetic fields in the directions indicated by arrows of solid and dotted lines in the diagram. 
     Similarly, the coils L 13 , L 14  and the coils L 19 , L 20  are wound around the core  33  in such a manner as to generate, between the end faces of the legs thereof, magnetic fields in the directions indicated by arrows of solid and dotted lines in the diagram. 
     Meanwhile, the coils L 12 , L 18  and the coils L 15 , L 21  are wound around the cores  34  and  35  respectively in such a manner as to generate horizontal magnetic fields indicated by arrows of solid and dotted lines in the diagram. 
     In FIG. 9, each arrow of the solid and dotted lines indicates the direction of the relevant magnetic field seen from the front of the color cathode ray tube. 
     The solid-line arrows represent the sextuple-pole magnetic field generated by the sextuple-pole coil  30 , and the dotted-line arrows represent the sextuple-pole magnetic field generated by the sextuple pole coil  31 . 
     Meanwhile, the ends of coils on one side of saturable reactors  36  and  37  are connected to the output terminal B of the bridge circuit  20 . 
     As shown in FIG. 9, the saturable reactor  36  comprises an E-shaped core  38 ; coils L 22 , L 23  wound around the end legs of the core  38  and connected in series to each other; coils L 24 , L 25  wound around the end legs of the core  38  and connected in series to each other; and an I-shaped core  39  attached to the end face of each leg of the core  38 . 
     Similarly to the saturable reactor  36  mentioned above, the saturable reactor  37  comprises an E-shaped core  40 ; coils L 26 , L 27  wound around the end legs of the core  40  and connected in series to each other; coils L 28 , L 29  wound around the end legs of the core  40  and connected in series to each other; and an I-shaped core  41  attached to the end face of each leg of the core  40 . 
     As shown in FIGS. 6 and 9, each open end of the coils L 22 , L 26  in the saturable reactors  36 ,  37  is connected to the output terminal B of the bridge circuit  20 . 
     The open end of the coil L 23  is connected to the open end of the coil L 15  in the sextuple-pole coil  30 , and the open end of the coil L 27  is connected to the open end of the coil L 16  in the sextuple-pole coil  31 . 
     Further, the open end of the coil L 24  is connected to the cathode of a diode D 1 , and the open end of the coil L 28  is connected to the anode of a diode D 2 . 
     The saturable reactors  36 ,  37  of the above structure are so set that, when a current is caused to flow in the modulation-side coils L 24 , L 25  and L 28 , L 29 , the inductance of each of the coils L 22 , L 23  and L 26 , L 27  is reduced. 
     As will be described later, a vertical-period current is supplied to the coils L 24 , L 25  and the coils L 28 , L 29 . 
     The aforementioned sextuple-pole-coils  30 ,  31 , saturable reactors  36 ,  37  and diodes D 1 , D 2  constitute a circuit  42  for correction of ΔVCR. 
     In FIG. 6 again, the cathodes of diodes D 3 , D 4  are connected in common to each other, and the anode of the diode D 1  and the cathode of the diode D 2  are connected in common to the anode of the diode D 3 . 
     Meanwhile, the open ends of the coils L 25 , L 29  in the saturable reactors  36 ,  37  are connected in common to the anode of the diode D 4 . 
     Further, series-connected resistors R 1 , R 2  and series-connected resistors R 3 , R 4  are connected in parallel respectively to the series-connected diodes D 3 , D 4 . 
     The modulating coil L 5  of the aforementioned saturable reactor  26  is connected between the cathode common junction of the diodes D 3 , D 4  and the common junctions of the resistors R 1 , R 2  and R 3 , R 4 . 
     Horizontal deflection coils LH 1 , LH 2  connected in parallel to each other correspond to the horizontal deflection coil  15  in the deflection yoke  14  shown in FIG.  5 . 
     Vertical deflection coils LV 1 , LV 2  connected in series to each other correspond to the vertical deflection coil  16  in the deflection yoke  14  shown in FIG.  5 . 
     A resistor R 5 , a variable resistor VR and a resistor R 6 , which are connected in series to one another, are connected in parallel to the vertical deflection coils LV 1 , LV 2 . 
     The slide contact of the variable resistor VR is connected to the common junction of the vertical deflection coils LV 1 , LV 2 . 
     A horizontal-period sawtooth current (horizontal deflection current) is supplied from a horizontal deflection circuit (not shown) to the horizontal deflection coils LH 1 , LH 2 . 
     Meanwhile, a vertical-period sawtooth current (vertical deflection current) is supplied from a vertical deflection circuit (not shown) to the vertical deflection coils LV 1 , LV 2 . 
     Consequently, a horizontal deflection magnetic field and a vertical deflection magnetic field are formed on the orbits of electron beams, and the electron beams are deflected by such deflection magnetic fields. 
     The horizontal deflection current flows between input terminals of the bridge circuit  20 , which consists of coils L 1  to L 5 , via the horizontal deflection coils LH 1 , LH 2 , i.e., between a common junction E of the coils L 1 , L 3  and a common junction F of the coils L 2 , L 4 . 
     Meanwhile, the vertical deflection current flows between input terminals G, H of the circuit consisting of the modulating coil L 5 , diodes D 3 , D 4  and resistors R 1  to R 4 , via the vertical deflection coils LV 1 , LV 2 . 
     Next, a description will be given on the circuit operation of the convergence corrector having the above-mentioned structure. 
     First, the circuit operation of the saturable reactor  26  including the bridge circuit  20  of coils L 1  to L 4  will be described with reference to an explanatory principle diagram of FIG.  10 . 
     Suppose now that, when a sawtooth horizontal deflection current has been supplied between the two input terminals of the first bridge circuit  20 , i.e., between the common junction E of the coils L 1 , L 3  and the common junction F of the coils L 2 , L 4  via the horizontal deflection coils LH 1 , LH 2 , the current flows into the input terminal E as indicated by a solid-line arrow in FIG.  10 . Then, magnetic fields directionally identical with the fixed bias magnetic field are generated by the coils L 1 , L 4 , while magnetic fields directionally reverse to the bias magnetic field are generated by the coils L 2 , L 3 . 
     In this case, the magnetic fields derived from the coils L 1 , L 4  are increased since the magnetic fields generated in accordance with the horizontal deflection current are directionally identical with the fixed bias magnetic field. 
     Consequently, the magnetic saturation of the core  21  tends to be higher in FIG. 7, thereby reducing the inductances of the coils L 1 , L 4 . 
     Meanwhile, the magnetic fields derived from the coils L 2 , L 3  are decreased since the magnetic fields generated in accordance with the horizontal deflection current are directionally reverse to the fixed bias magnetic field. 
     Consequently, the magnetic saturation of the core  23  tends to be lower in FIG. 7, thereby increasing the inductances of the coils L 2 , L 3 . 
     As a result, the current delivered via the input terminal E comes to flow into one coil of the smaller inductance. 
     More specifically, in case the deflection current is delivered via the input terminal E as indicated by a solid-line arrow in FIG. 10, this current first flows through the coil L 1  and then flows from the output terminal A into the second bridge circuit  27  consisting of coils L 6  to L 9 . 
     Subsequently this current flows through the bridge circuit  27  and, after flowing out from the output terminal B, the current further flows out to an external device from the other input terminal F via the coil L 4 . 
     Meanwhile, in case the deflection current flows into the input terminal F as indicated by a dotted-line arrow in FIG. 10, magnetic fields directionally reverse to the fixed bias magnetic field are generated by the coils L 1 , L 4 , while magnetic fields directionally identical with the bias magnetic field are generated by the coils L 2 , L 3 . 
     In this case, the magnetic fields derived from the coils L 1 , L 4  are decreased since the magnetic fields generated in accordance with the horizontal deflection current are directionally reverse to the fixed bias magnetic field. 
     Consequently, the inductances of the coils L 1 , L 4  are increased. 
     On the other hand, the magnetic fields derived from the coils L 2 , L 3  are increased since the magnetic fields generated in accordance with the horizontal deflection current are directionally identical with the fixed bias magnetic field. 
     Consequently, the inductances of the coils L 2 , L 3  are decreased. 
     As a result, the current delivered via the input terminal F comes to flow into one coil of the smaller inductance, as in the foregoing case. 
     More specifically, in case the deflection current is delivered via the input terminal F as indicated by a dotted-line arrow in FIG. 10, this current first flows through the coil L 2  and then flows from the output terminal A into the second bridge circuit  27  consisting of coils L 6  to L 9 . 
     Subsequently this current flows through the bridge circuit  27  and, after flowing out from the output terminal B, the current further flows out to an external device from the other input terminal E via the coil L 3 . 
     In this manner, the current flows in the same direction (indicated by the arrow in the diagram) in the second bridge circuit  27  of four coils L 6  to L 9 , regardless of the direction of the current flowing in the bridge circuit  20  of coils L 1  to L 4 . 
     Therefore, the waveform of this current is rendered approximately parabolic. 
     That is, the first bridge circuit  20  consisting of the coils of the saturable reactor  26  shown in FIG. 7 generates a horizontal parabolic current in compliance with a flow of the horizontal-period sawtooth current. 
     This horizontal parabolic current flows through the bridge circuit  27  of coils L 6  to L 9 . 
     Meanwhile, when the vertical deflection current flows in the bias coils Lb 1 , Lb 2  via the vertical deflection coils LV 1 , LV 2  in FIG. 8, the coils Lb 1 , Lb 2  generate, in the core  28 , a bias magnetic field corresponding to the vertical deflection current. 
     Then the inductances of the coils L 6 , L 9  wound around the core  28  are affected and changed by such a bias magnetic field. 
     More concretely, the inductances of the coils L 6 , L 9  are reduced in accordance with an increase of the vertical deflection current. 
     As a result, a difference is induced between the current flowing in the coil L 6  and the current flowing in the coil L 9 , and then the difference current flows in the convergence correcting coils Lc 1  to Lc 4 . 
     In this stage, the current flowing in the convergence correcting coils Lc 1  to Lc 4  is modulated at the vertical deflection period to have a waveform substantially parabolic. 
     That is, this current becomes a parabolic one modulated at the horizontal deflection period and the vertical deflection period. 
     A quadrupole magnetic field is formed by the convergence correcting coils Lc 1  to Lc 4  in accordance with the above parabolic current. 
     The quadrupole magnetic field is generated merely for correction of the misconvergence between the beams R and B, and has no function for correction of ΔVCR. 
     Correction of ΔVCR intended in the present invention is realized by the circuit  42  shown in FIG.  6 . 
     Now a description will be given on the circuit  42  below. 
     As shown in FIG. 6, the circuit  42  is connected to the output terminals A and B of the bridge circuit  20 . 
     Therefore, the horizontal deflection-period parabolic current produced in the saturable reactor  26  flows also in the sextuple-pole coils  30 ,  31  and the saturable reactors  36 ,  37 . 
     Meanwhile, a current Iv 1  rectified by the diode D 1  is supplied from a vertical deflection circuit (not shown) via vertical deflection coils LV 1 , LV 2  to the coils L 24 , L 25  of the saturable reactor  36 . 
     FIG. 11A shows the waveform of this current Iv 1 . 
     In the saturable reactor  36 , the inductances of the coils L 22 , L 23  are modulated due to a flow of the current Iv 1  in the coils L 24 , L 25 . 
     In this configuration, the sextuple-pole coil  30  is connected in series to the coils L 22 , L 23 , and the inductances of these coils L 22 , L 23  are modulated by the current Iv 1 , so that the horizontal parabolic current flowing in the sextuple-pole coil  30  is also modulated by the current Iv 1 . 
     FIG. 11B shows the waveform of the horizontal parabolic current IA thus modulated. 
     Meanwhile in the saturable reactor  37 , a current Iv 2  rectified by the diode D 2  is supplied to the coils L 28 , L 29 , as in the foregoing saturable reactor  36 . 
     FIG. 11C shows the waveform of such current Iv 2 . 
     The inductances of the coils L 26 , L 27  are modulated due to a flow of the current Iv 2  in the coils L 28 , L 29 . 
     Consequently, the horizontal parabolic current flowing in the sextuple-pole coil  31  is modulated by the current Iv 2 . 
     FIG. 11D shows the waveform of the horizontal parabolic current IB thus modulated. 
     Since the coils L 10  to L 15  and L 16  to L 21  of the sextuple-pole coils  30 ,  31  are formed of bifilar windings, the horizontal parabolic current IA of the waveform shown in FIG. 11B flows in the sextuple-pole coil  30 , while the horizontal parabolic current IB of the waveform shown in FIG. 11D flows in the sextuple pole coil  31 . 
     Accordingly, a composite current (IA-IB) thereof becomes a sextuple-pole current having the waveform of FIG.  11 E. 
     When this sextuple-pole current flows in the sextuple-pole coils  30 ,  31 , sextuple-pole magnetic fields are formed in the neck N by the sextuple-pole coils  30 ,  31 , as shown in FIG.  9 . 
     Regarding the correlation between the waveform of FIG.  11 E and the screen, the current at the top of the screen corresponds to the left end of FIG. 11E, and the polarity of the parabolic current is assumed to be such as shown in FIG.  12 . 
     FIGS. 13 and 14 show sextuple-pole magnetic fields generated in accordance with the polarity of the parabolic current, as viewed from the screen side of the cathode ray tube. 
     Since a sextuple-pole current of the waveform shown in FIG. 11E flows in the sextuple-pole coils  30 ,  31 , downward force in FIG. 13 is exerted on the side beams R. B by the horizontal magnetic field at the left and right ends of the screen top. At the screen center, the current polarity is inverted as shown in FIG.  12 . Accordingly, upward force is exerted on the side beams R, B reversely to the above. 
     Consequently, the side beams R, B are lowered at the left and right ends of the screen top while being raised at the screen center. 
     Meanwhile the current at the screen bottom corresponds to the right end of FIG. 11E, so that the waveform of the parabolic current becomes reverse to the above. 
     Therefore, as shown in FIG. 14, upward force is exerted on the side beams R, B by the horizontal magnetic field at the left and right ends of the screen bottom. At the screen center, the current polarity is inverted as shown in FIG.  12 . Accordingly, downward force is exerted on the side beams R, B reversely to the above. 
     Consequently, the side beams R, B are raised at the left and right ends of the screen bottom while being lowered at the screen center. 
     Since the side beams R, B are changed as described by the sextuple-pole magnetic fields produced by the sextuple-pole coils  30 ,  31 , the side beams R, B are shifted inward at the screen corners and outward at the screen center, as shown in FIG. 15 which represents the entire screen. 
     This signifies that the pattern shown in FIG. 2B is corrected. Thus, it becomes possible to correct ΔVCR independently. 
     The shifts of the side beams R, B shown in FIG. 15 can be reversed with facility by inverting the direction of the sextuple-pole current or by changing the winding direction of the sextuple pole coils  30 ,  31 . 
     According to the present invention, as described above, a horizontal deflection-period parabolic current modulated at the vertical deflection period can be caused to flow in a sextuple-pole magnetic field generating means which exerts vertical force on three electron beams. 
     More specifically, ΔVCR can be corrected independently by the sextuple-pole magnetic field generating means. 
     Further, in determining the winding distribution of a vertical deflection coil or a horizontal deflection coil, it becomes possible to eliminate the necessity of taking ΔVCR into consideration. 
     That is, both the focus characteristic and the convergence characteristic are rendered compatible due to the enhanced degree of freedom in the winding distribution of the deflection coil. 
     Although the present invention has been mentioned hereinabove with reference to some preferred embodiments thereof, it is to be understood that the invention is not limited to such embodiments alone, and a variety of other changes and modifications will be apparent to those skilled in the art without departing from the spirit of the invention.