Abstract:
The present invention provides a tension mask having a frequency distribution with improved vibration damping. The tension mask includes a center portion between two edge portions. The tension mask also has a parabolic frequency distribution between the edge portions whereby the center portion has a central frequency distribution value and the edge portions have a relatively lower peripheral frequency distribution value characterized in that the range of variation between the center and edge portions frequency distribution value is in the closed interval of about 8 Hz≦Δ≦12 Hz.

Description:
This invention generally relates to cathode ray tubes and, more particularly, to a tension mask having a frequency distribution with improved vibration damping. 
     BACKGROUND OF THE INVENTION 
     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 comprises an array of elements of three different color emitting phosphors. An aperture mask is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. The aperture mask is a thin sheet of metal, such as steel, that is contoured to somewhat parallel the inner surface of the tube faceplate. An aperture mask may be either formed or tensioned. 
     The aperture mask is subject to vibration from external sources (e.g., speakers near the tube). Such vibration varies the positioning of the apertures through which the electron beams pass, resulting in visible display fluctuations. Ideally, these vibrations need to be eliminated or, at least, mitigated to produce a commercially viable television picture tube. 
     SUMMARY OF THE INVENTION 
     The present invention provides a tension mask for a cathode-ray tube having a center portion between two edge portions and a parabolic frequency distribution between the edge portions. The center portion has a central frequency distribution value and the edge portions have a relatively lower peripheral frequency distribution value characterized in that the range of variation between the center and edge portions frequency distribution value is in the closed interval of about 8 Hz≦Δ≦12 Hz 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a side view, partly in axial section, of a color picture tube, including a tension mask-frame-assembly according to the present invention; 
     FIG. 2 is a plan view of the tension mask-frame-assembly of FIG. 1 according to an aspect of the invention; 
     FIG. 3 is a graph depicting modal shapes for various tension distributions; 
     FIG. 4 depicts a bar graph showing mask tension ranges as limited by scan frequencies; 
     FIG. 5 depicts a graph showing mask stress vs frequency; 
     FIG. 6 depicts a graph showing total frame load vs frequency; and 
     FIG. 7 depicts a graph comparing a prior art tension mask frequency distribution to a tension mask frequency distribution according to the present invention. 
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     FIG. 1 shows a cathode ray tube  10  having a glass envelope  12  comprising a rectangular faceplate panel  14  and a tubular neck  16  connected by a rectangular funnel  18 . The funnel  18  has an internal conductive coating (not shown) that extends from an anode button  20  to a neck  16 . The panel  14  comprises 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 the inner surface of the faceplate  22 . The 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. A tension mask  30  is removably mounted in a predetermined spaced relation to the screen  28 . An electron gun  32  (schematically shown by the dashed lines in FIG. 1) is centrally mounted within the neck  16  to generate three in-line electron beams, a center beam and two side beams, along convergent paths through the mask  30  to the screen  28 . 
     The tube  10  is designed to be used with an external magnetic deflection yoke, such as the yoke  34  shown in the neighborhood of the funnel to neck junction. When activated, the yoke  34  subjects the three beams to magnetic fields that cause the beams to scan horizontally and vertically in a rectangular raster over the screen  28 . 
     The tension mask  30 , shown in greater detail in FIG. 2, is interconnected with a peripheral frame  39  that includes two long sides  36 ,  38  and two short sides  40 ,  42 . The two long sides  36 ,  38  of the tension mask  30  parallel a central major axis, X, of the tube. The tension mask  30  includes an apertured portion that contains a plurality of metal strips  44  having a plurality of elongated slits  46  therebetween that parallel the minor axis of the tension mask  30 . 
     Specifically, the apertured portion of tension mask  30  illustrated in FIG. 2 is a tie bar or webbed system. The tension mask  30  has a center portion  50 , mask edge portions  52  about 0.5 in. from the edge of the short sides  40 ,  42  and mask edge portions  51  about 1.0 in. from the edge of the long sides  36 ,  38 . The two mask edge portions  52  are parallel to the tube  10  central minor axis, Y. The two mask edge portions  51  are parallel to the tube  10  central major axis, X. Two mask edge portions  51  are attached to the peripheral frame  39  along the two long sides  36 ,  38 . 
     The natural frequency distribution across any complete horizontal (central major axis, X) dimension of the tension mask  30  provides a useful way of comparing any tube to any other tube, regardless of size. Effectively, the natural frequency distribution, which is a function of the respective tension distribution and the vertical dimension of the tension mask  30 , is a universal metric that dictates microphonic behavior of tubes. 
     In the preferred embodiment, the natural frequency distribution is a substantially parabolic function that is substantially smooth and continuous. The natural frequency distribution comprises a central frequency distribution for the center portion  50  and peripheral frequency distributions for the edge portions  52 , wherein the values of central frequency distribution are constructively greater than the values of the peripheral frequency distribution. The difference between the maximum of central frequency distribution and the minimum of the peripheral frequency distribution is about 10 Hz. 
     When the center portion  50  is under greater tension than the mask edge portion  52 , the condition is called a mask ‘frown.’ A mask ‘frown’ has a fundamental mode of vibration that principally involves the edge portion  52  of the mask  30 . Border damping systems (BDS), i.e., vibration dampers, can effectively damp vibrational energy because the BDS are triggered by vibrations in the edge portion  52  of the mask  30 . 
     When the center portion  50  is under less tension than the mask edge portion  52 , the condition is called a mask ‘smile.’ As such, the values of the central frequency distribution are less than the values of peripheral frequency distribution. For a ‘smile’ condition the damping of vibrations tend to be poor because the vibrating mask  30  has a fundamental mode dominated by the motion of the center portion  50  and does not trigger the BDS. 
     When the natural frequency distribution is even or flat, the values of the central frequency distribution and the peripheral frequency distribution are substantially similar. This example is difficult to implement. In addition, a slight change in tension distribution caused during manufacture of the tension mask  30  or during cathode ray tube operation could produce a ‘smile,’ which is undesirable. 
     FIG. 3 is a graph  300  depicting modal shapes for various tension distributions. The graph  300  is defined by normal displacement (axis  302 ) and major axis location (axis  304 ). Specifically, the graph  300  shows which portion of the tension mask  30  is excited by vibrations for a flat, ‘smile’ or ‘frown’ tension. The tension mask with a ‘smile’ (plot  306 ) shows considerably more vibration in the center portion  50  than a tension mask  30  with a ‘frown’ (plot  308 ). Additionally, there is more vibration in the center portion  50  of a tension mask  30  having an even tension distribution (plot  310 ) than for a tension mask  30  having a ‘frown.’ 
     A tension mask  30  having a ‘frown’ has resonant frequencies that are more broadly spaced than a tension mask  30  having a ‘smile’ or flat distribution. Thus when there is a vibration, energy from the first mode of the disturbance does not feed the second mode, thereby not prolonging the vibrational effect. 
     A tension distribution in accordance with the present invention producing a parabolic ‘frown’ in about an 80 Hz to 90 Hz range, the frequency at a given mask location can be represented by equation:                f        (   x   )       =       -       Bx   2       L   2         +   A             Expression                 1                                
     The preferred embodiment has the following provisions: 
     
       
         92≧ A≧ 88  Expression 2  
       
     
     
       
         12≧ B≧ 8  Expression 3  
       
     
     
       
         12≧ f ( x   max )− f ( x   min )≧8  Expression 4  
       
     
     where f(x) represents the frequency distribution over x, L represents one-half of the total length of tension mask  30  along the major axis, and x represents a major axis position from −L to +L, wherein the absolute value of L is normalized to 1. f(x max ) and f(x min ) represent the peak value of the frequency distribution at the center portion  50  and the minimum value the frequency distribution at the edge portion  52 , respectively. It is preferred that at least 8 Hz differential be maintained between the frequency distribution at the center portion  50  and edge portion  52  is maintained. 
     When the mask frequency vibrations occur at a scan frequency or at a harmonic, a beating effect would result, wherein low amplitude modulation become perseptable. FIG. 4 provides some guidance in constructing tension masks with good microphonics performance. The bar graph  400  in FIG. 4 shows mask tension ranges as limited by scan frequencies (axis  402 ). Specifically, different bars occupy certain scanning frequencies with about a 20 HZ cushion. Excessive vibration (bar  404 ) occurs in the frequency range of 0 Hz to about 40 Hz. The 50 Hz European television broadcast format 1 H Phase Alternate Line (PAL) (bar  406 ) excludes the frequency range from about 40 Hz to about 60 Hz. The 60 Hz American television broadcast format 1 H (NTSC) (bar  408 ) excludes the frequency range from about 50 Hz to about 70 Hz. The 100 Hz European broadcast format 2 H PAL (bar  410 ) excludes the frequency range from about 90 Hz to about 110 Hz. The 120 Hz American broadcast format 2 H NTSC (bar  412 ) excludes the frequency range from about 110 Hz to about 130 Hz. To utilize the frequency range from about 130 Hz to about 200 Hz, an excessive frame weight would be required because only such a frame could tension a mask enough to reach these higher frequencies. The graph  400  shows that there is a narrow 20 Hz window (space  416 ) between 70 Hz and 90 Hz where the mask frequencies are adequately separated from standard scan frequencies and their harmonics. 
     Furthermore, because vibration amplitude is inversely proportional to mask tension  30 , it is desirable to have overall mask tension as high as possible. The 10 Hz edge-to-center differential prescribed in Expression 4 provides a desirable solution to minimizing vibration while preserving the necessary ‘frown’ tension distribution. 
     FIG. 5 depicts a graph  500  showing mask stress (axis  502 ) vs frequency (axis  504 ). Specifically, the graph  500  shows the mask stress (axis  502 ) vs frequency ( 504 ) for different size cathode ray tubes. By varying the stress on the tension mask  30  for various sized tubes, the desired frequency can be attained. The present invention can be practically achieved on all current tube sizes. More specifically, graph  500  depicts a hierarchical relationship among the various size tubes, wherein smaller tubes can achieve the desired frequency distribution with lower mask stress loads than larger tubes. For example, graph  500  shows that an A90 (plot  514 ) 36 inch size tube experiences greater mask stress (axis  502 ) at a particular frequency (axis  504 ) than an A80 (plot  512 ) 32 inch size tube. The A80 (plot  512 ) 32 inch size tube experiences greater mask stress (axis  502 ) than an A68 (plot  510 ) 27 inch size tube at a particular frequency (axis  504 ). The A68 (plot  510 ) 27 inch size tube experiences greater mask stress (axis  502 ) than a W76 (plot  508 ) 30 inch cinema screen tube at a particular frequency (axis  504 ). Finally, the W76 (plot  508 ) 30 inch cinema screen tube experiences greater mask stress (axis  502 ) than a W66 (plot  506 ) 26 inch cinema screen tube at a particular frequency (axis  504 ). 
     FIG. 6 depicts a graph  600  showing total frame load (axis  602 ) versus frequency (axis  604 ) for different size cathode ray tubes. The total frame load (axis  602 ) is the resultant force the tension mask  30  experiences as the two long sides  36 ,  38  of the peripheral frame  39  apply equal and opposite outward forces, thereby tensioning the center portion  50  and edge portions  52  of the tension mask  30 . FIG. 6 shows an A90 36 inch size tube (plot  612 ) experiences greater total frame load (axis  602 ) at any frequency (axis  604 ) compared to an A80 32 inch size tube (plot  610 ). The A80 32 inch size tube (plot  610 ) experiences greater total frame load (axis  602 ) at any frequency (axis  604 ) compared to an A68 27 inch size tube (plot  608 ) and W76 30 inch cinema screen tube (plot  608 ). Finally, the A68 and W76 tubes (plot  608 ), in turn, experience greater total frame load (axis  602 ) at any frequency (axis  604 ) as compared to a W66 26 inch cinema screen tube (plot  606 ). 
     FIG. 7 depicts a graph  700  comparing a prior art tension mask frequency (axis  702 ) and location on major axis (axis  704 ) to a tension mask frequency (axis  702 ) and location on major axis (axis  704 ) according to the present invention. Specifically, the prior art frequency distributions do not follow the frequency distribution of equation 1. More specifically, one prior art frequency distribution (plot  708 ) approximates the shape of a high order polynomial (plot  706 ). A second prior art frequency distribution (plot  712 ) approximates the shape of another high order polynomial (plot  710 ). A frequency distribution (plot  714 ) according to the present invention has a parabolic shape and is within the preferred range. 
     As the embodiments that incorporate the teachings of the present invention have been shown and described in detail, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings without departing from the spirit of the invention.