Abstract:
A dynamic focus voltage generator is provided for a tensioned focus mask of a cathode ray tube of video display. The tensioned focus mask includes spaced apart strands and spaced apart crosswires separated from the strands. A synchronizing signal at a horizontal deflection frequency is used for generating a dynamic focus voltage component that varies in accordance with a position of an electron beam on a screen of the cathode ray tube. A synchronizing signal at a vertical deflection frequency is used for generating a dynamic focus voltage component that varies in accordance with the position of the electron beam on the screen of the cathode ray tube. The time varying voltage components are combined with a direct current voltage component for producing a dynamic focus voltage between the strands and crosswires.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a non-provisional application which claims the benefit of provisional application Ser. No. 60/369,920, filed Apr. 4, 2002. 
     
    
       [0002]     The invention generally relates to the application of a focus voltage to conductors of a focus mask of a color picture tube or a cathode ray tube (CRT).  
       BACKGROUND OF THE INVENTION  
       [0003]     A color picture tube includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the faceplate of the tube and is made up of an array of elements of three different color-emitting phosphors. An aperture mask or a shadow mask is interposed between the electron gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A shadow mask is a thin sheet of metal, such as steel, that is contoured to somewhat parallel the inner surface of the tube faceplate. A shadow mask may be either domed or tensioned.  
         [0004]     A type of tension mask, called a tension focus mask, includes two sets of conductive elements that are perpendicular to each other and separated by an insulator. Generally, in a tension focus mask, a vertical set of conductive lines or strands is under tension and a set of horizontal conductive elements sometimes known as crosswires overlies the strands. Different voltages are applied to the set of crosswires and to the set of strands, respectively. The focus voltage that is the difference between the voltage applied to the crosswires and that applied to the strands, creates a quadrupole focusing lens in each aperture of the focus mask. The mask apertures are rectangular and are formed between adjacent vertical strands and adjacent horizontal crosswires.  
         [0005]     Typically, the distance between the focus mask and the screen measured along the beam path increases as the beam sweeps from the center of the CRT towards the edges. The change in the mask-to-screen spacing along the beam path might lead to an over-focussing of the beam at the periphery of the screen if the focus voltage difference is selected to satisfy the requirements at the center of the screen. For example, in a CRT having 27 inch screen and 110 degrees, the focus voltage difference that produces an acceptable beam spot at the screen center may be different by 30% from that required at the screen edge. It may be desirable to avoid the aforementioned difference in focusing.  
         [0006]     In carrying out an inventive feature, the focus voltage difference is made to vary at a horizontal rate with an amplitude that is modulated at a vertical rate. Thereby, advantageously, over-focusing of the beam is prevented.  
       SUMMARY OF THE INVENTION  
       [0007]     A focus voltage generator, embodying an invention feature, for a tensioned focus mask of a cathode ray tube of video display apparatus has a first plurality of spaced apart strands and a second plurality of spaced apart crosswires separated from the strands. A source of a first signal at a frequency related to a deflection frequency is provided. A waveform generator responsive to the first signal for generating a dynamic focus voltage that varies in accordance with a position of an electron beam on a screen of the cathode ray tube and developed between the strands and crosswires. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a side view, partially in axial section, of a color picture tube, including a tension focus mask assembly;  
         [0009]      FIG. 2  is a perspective view of the tension focus mask assembly of  FIG. 1 ; and  
         [0010]      FIG. 3  is a block diagram of a power supply, embodying an inventive feature, for generating a dynamic focus voltage that is coupled to the tension focus mask assembly of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0011]      FIG. 1  shows a cathode ray tube  10  having a glass envelope  12 . A rectangular panel  14  and a tubular neck  16  are connected by a rectangular funnel  18 . Funnel  18  has an internal conductive coating, not shown, that extends from an anode button  20  to a neck  16 . Panel  14  includes a viewing faceplate  22  and a peripheral flange or sidewall  24  that is sealed to the funnel  18  by a glass frit  26 . A three-color phosphor screen  28  is carried by an inner surface of faceplate  22 . Screen  28  is a line screen with the phosphor lines arranged in triads, each triad including a phosphor line of each of the three colors, red-emitting, green-emitting and blue-emitting phosphor lines, R, G and B. A tension focus mask  30  is removably mounted in a predetermined spaced relation to screen  28 . An electron gun  32 , schematically shown by the dashed lines, is centrally mounted within neck  16 . Gun  32  generates three in-line electron beams red, green and blue, not shown, that form a center beam and two side beams, along convergent paths through mask  30  to the screen  28 .  
         [0012]     A deflection yoke  34  is mounted on funnel. Deflection yoke  34  includes a horizontal deflection winding, not shown, for conducting a horizontal deflection current, not shown, at a horizontal frequency Fh such as, for example, approximately 15,724 Hz and a vertical deflection winding, not shown, for conducting a vertical deflection current, not shown, at a vertical frequency Fv such as 60 Hz. Deflection yoke  34  subjects the three beams to magnetic fields which cause the beams to scan horizontally and vertically in a rectangular raster over screen  28 .  
         [0013]     In deflection yoke  34 , fast scanning occurs in a horizontal direction X and slow scanning occurs in a vertical direction Y. However, the invention is equally applicable to an embodiment, not shown, in which fast scanning occurs in the vertical direction Y and slow scanning occurs in the horizontal direction X.  
         [0014]     Tension mask  30  is shown in greater detail in  FIG. 2 . Similar symbols and numerals in  FIGS. 1 and 2  indicate similar items or functions. Tension mask  30  of  FIG. 2  includes two longs sides  36  and  38  and two short sides  40  and  42 . The two long sides  36  and  38  of mask  30  parallel horizontal major axis, X, of tube  10  of  FIG. 1 .  
         [0015]     Tension mask  30  of  FIG. 2  includes two sets of conductors: strands  44  that are parallel to central minor axis y and to each other; and crosswires  46 , that are parallel to central major axis x and to each other. Strands  44  are flat strips that extend vertically, having a width of about 12 mils, a thickness of approximately 2 mils and a separation or pitch of 0.91 mm. Crosswires  46  have a round cross section, a diameter of about 1 mil and extend horizontally with a separation or pitch of 16 mils. Strands  44  and crosswires  46  are separated from each other in the direction of axis Z of  FIG. 1 , in a well-known manner, not shown, by suitable insulators. The separation between strands  44  and crosswires  46  in the direction of axis Z is in the order of, for example, 0.675 inch. An example of such arrangement is shown in U.S. Pat. No. 5,646,478, in the names of Nosker et al., entitled UNIAXIAL TENSION FOCUS MASK FOR A COLOR CRT WITH ELECTRICAL CONNECTION MEANS (the Nosker et al., patent).  
         [0016]     Strands  44  are electrically coupled to an electrode  20  of  FIG. 1  via a first conductive layer, not shown, formed on an interior surface of the glass of CRT  10 . A voltage V 20  of  FIG. 2  of electrode  20  is applied to each strand  44 . Similarly, crosswires  46  are electrically coupled to an electrode  21  of  FIG. 1  via a second conductor, not shown, formed on an interior surface of the glass of CRT  10 . A voltage V 21  of  FIG. 2  of electrode  21  is applied to each crosswires  46 . An example of such arrangement is shown in the Nosker et al., Patent.  
         [0017]     In a similar way to that explained in, for example, U.S. Pat. No. 4,464,601, entitled CRT WITH QUADRUPOLAR-FOCUSING COLOR-SELECTION STRUCTURE, in the name of Stanley Bloom, voltages V 20  and V 21  form electrostatic quadrupolar-focus lens in each aperture such as, for example, an aperture  72 . Each aperture  72  is bound by an adjacent pair of crosswires  46  and by an adjacent pair of strands  44 .  
         [0018]      FIG. 3  is a block diagram of a power supply  100 , embodying an inventive feature, for generating dynamic focus voltage V 21  that is coupled to crosswires  46  of  FIG. 2 . A high voltage power supply  101  generates focus voltage V 20  at a constant level that is coupled to strands  44  of  FIG. 2 . Similar symbols and numerals in  FIGS. 1, 2  and  3  indicate similar items or functions.  
         [0019]     High voltage power supply  101 , that may have a similar construction to that of a conventional horizontal deflection circuit output stage, not shown, includes a flyback transformer T 1 , a rectifier D 1  and a filter capacitor C 1  for generating direct current (DC) voltage V 20  at a high voltage of, for example, 30 kV that is developed at terminal  20 . A conventional low voltage power supply  102  produces an alternating current (AC) voltage, not shown, that is transformer-coupled via a transformer T 2  to a rectifier D 2  for developing a constant DC voltage VDC in a filter capacitor C 2 . Voltage VDC is summed with voltage V 20  and coupled to a terminal T 3   a   1  of winding T 3   a  of a transformer T 3  to provide a DC voltage component of voltage V 21 .  
         [0020]     A periodic horizontal sync signal Hs and a periodic vertical sync signal Vs having periods H and V, respectively, are coupled from a source that is not shown to input terminals  104   a  and  103   a,  respectively. The source of signals Hs and Vs, not shown, may be conventional and may include a sync separator of a video display that separates signals Hs and Vs from an incoming composite video signals. Separated sync signals Hs and Vs may be time shifted with respect to each other.  
         [0021]     Signal Vs is coupled to a waveform generator  103 . Generator  103  generates from signal Vs a full-wave rectified-sinewave  103   b  at a frequency that is equal to vertical frequency Fv. Signal Hs is coupled to a waveform generator  104 . Generator  104  generates from signal Hs a full-wave-rectified sinewave  104   b  at a frequency that is equal to horizontal frequency Fh. Signals  103   b  and  104   b  are multiplied in a multiplier or modulator  105  and transformer coupled via transformer T 3  to produce a transformer coupled dynamic focus voltage component VDF of voltage V 21 . Transformers T 3  and T 2  isolate modulator  105  and power supply  102 , respectively, from high voltage V 20 . Dynamic focus voltage component VDF is a full-wave-rectified sinewave signal at horizontal frequency Fh having peak amplitude that varies at frequency Fv in a full wave-rectified sinewave manner.  
         [0022]     When an electron beam EB of  FIG. 1  is at a horizontal center of scan line  200  of  FIG. 1 , that is located at a vertical center of screen  28 , the peak value of the sum of voltages VDC and VDF of  FIG. 3  is selected to be at a maximum value, for example, 850V. On the other hand, when electron beam EB is at any of the four corners of screen  28 , such as at the edges of a scan line  201  of  FIG. 1 , at a top of screen  28 , and at the edges of a scan line  203 , at a bottom of screen  28 , the peak value of the sum of voltages VDC and VDF of  FIG. 3  is at a minimum value, for example, 580V.  
         [0023]     In each horizontal line such as, for example, scan line  200  of  FIG. 1 , the peak value of the sum of voltages VDC and VDF of  FIG. 3  is at a maximum value at the horizontal center point, not shown, of scan line  200  of  FIG. 1  and at a minimum value at each of the right side and left side ends, not shown, of scan line  200 . In this way, a difference between voltages V 21  and V 20  of  FIG. 3  decreases as electron beam EB of  FIG. 1  moves away from the center of screen  28 , in either the direction of axis X or in the direction of axis Y. On the other hand, the difference between voltages V 21  and V 20  of  FIG. 3  increases as electron beam EB of  FIG. 1  moves towards the center of screen  28 , in either the direction of axis X or in the direction of axis Y. The difference between voltages V 21  and V 20  is determined by the geometry of tension mask  30  of  FIG. 2 , referred to before. It should be understood that the difference between voltages V 21  and V 20  may be different if different geometry of tension mask  30  was selected.  
         [0024]     Dynamic focus voltage arrangement similar to that described in  FIG. 3  can be used in an embodiment, not shown, in which transposed scanning is implemented. Transposed scanning is described in, for example, an article entitled “Transposed Scanning: The Way to Realize Super Slim CRTs”, in the names of Krijn, et al.,published in SID June 2001 digest. Transposed scanning is also described in U.S. Pat. No. 4,989,092, in the names of Doyle et al., entitled PICTURE DISPLAY DEVICE USING SCAN DIRECTION TRANSPOSITION.