Abstract:
An apparatus for generating a dynamic focus signal appropriate for a flat-type CDT, or flat-screen CRT has a first multiplier for receiving a horizontal sawtooth wave and outputting the square component of the wave; a second multiplier for receiving a vertical sawtooth wave and outputting the square component of the wave; a third multiplier for multiplying the outputs of the first and the second multipliers; and a mixer for mixing the outputs of the first through the third multipliers and outputting the result as a dynamic focus signal. The apparatus is able to perform optimal focus control appropriate for a flat-screen CRT.

Description:
CLAIM OF PRIORITY  
         [0001]    This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C §119 from an application entitled Apparatus For Generating Dynamic Focus Signal earlier filed in the Korean Industrial Property Office on Aug. 14, 2001, and there duly assigned Serial No. 2001-0049035 by that Office.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an apparatus for generating a dynamic focus signal in a display apparatus using a Cathode Display Tube (CDT) or, and more particularly, to an apparatus for generating a dynamic focus signal appropriate for a flat-type CDT, or flat-screen Cathode Ray Tube (CRT).  
           [0004]    2. Description of the Related Art  
           [0005]    Since distances from the electron gun to each area of the screen are different in a display apparatus using a CDT or CRT, dynamic focus for adjusting the focus of an electron beam with respect to an area of the screen is needed so that an electron beam generated in the electron gun is landed with a correct focus.  
           [0006]    That is, since the distance between the electron gun and the center of the screen is short, an electron beam directed to the center of the screen is needed to correspond to a short focal length. However, the distance between the electron gun and the right or left area of the screen is longer than that between the electron gun and the center of the screen, an electron beam directed to the right or left area of the screen is needed to correspond to a longer focal length. As this, adjusting the focal length of an electron beam with respect to an area of the screen is referred to as dynamic focus.  
           [0007]    The focus of an electron beam is controlled by a focus signal applied to the focus electrode of the electron gun.  
           [0008]    [0008]FIG. 1 is a waveform diagram of exemplary focus signals. The focus signals of FIG. 1 show the waveforms of horizontal focus signals and a vertical focus signal in one vertical cycle. Referring to FIG. 1, each horizontal focus signal has high levels at the beginning and end of a horizontal scanning period, and has a low level in the middle area ofthe horizontal scanning period. According to this, in one horizontal cycle, the focus voltages of the left and right areas in the screen are different from the focus voltage of the center of the screen.  
           [0009]    Meanwhile, in the waveforms of FIG. 1, in one vertical cycle, the waveform of each horizontal focus signal is uniform. This means that horizontal dynamic focus controls for the center and corners of the screen are done in a uniform way.  
           [0010]    Though with the focus signals shown in FIG. 1, correct focus may be achieved to a degree in an ordinary non-flat-screen CRT, precise focus cannot be achieved in a flat-screen CRT which is used now or will be used in the future.  
           [0011]    The reason is because the distance between the electron gun and the corners of the screen (the top left-hand corner, top right-hand corner, bottom left-hand corner, and bottom right-hand corner) of the flat-screen CRT is longer than that of the conventional non-flat-screen CRT s. Therefore, with the dynamic focus signal having the same amplitude (V2) of the horizontal focus signal for each corner of the screen as the amplitude (V1) of the horizontal focus signal for the center of the screen, as shown in FIG. 1, it is difficult to adjust focuses for corner areas of the screen.  
         SUMMARY OF THE INVENTION  
         [0012]    To solve the above problems, it is an objective of the present invention to provide an apparatus for generating a dynamic focus waveform appropriate for a flat-type CDT, or flat-screen CRT.  
           [0013]    To accomplish the objective of the present invention, there is provided an apparatus for generating a dynamic focus signal appropriate for a flat-screen CRT, the apparatus having a first multiplier for receiving a horizontal sawtooth wave and outputting the square component of the wave; a second multiplier for receiving a vertical sawtooth wave and outputting the square component of the wave; a third multiplier for multiplying the outputs of the first and the second multipliers; and a mixer for mixing the outputs of the first through the third multipliers and outputting the result as a dynamic focus signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    A more complete appreciation of the present invention, and many of the attendant advantages thereof, will become readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:  
         [0015]    [0015]FIG. 1 is a waveform diagram of exemplary dynamic focus signals;  
         [0016]    [0016]FIG. 2 is a waveform diagram of focus signals appropriate for a flat-screen CRT;  
         [0017]    FIG. 3  is a block diagram of a dynamic focus circuit adopting an apparatus for generating a dynamic focus signal according to the present invention;  
         [0018]    [0018]FIG. 4 is a detailed block diagram of a dynamic focus signal oscillator of FIG. 3;  
         [0019]    [0019]FIG. 5 is an example of dynamic focus voltage needed in a flat-screen CRT; and  
         [0020]    [0020]FIG. 6 is an example of a dynamic focus signal generated by the apparatus for generating a dynamic focus signal according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    [0021]FIG. 2 is a waveform diagram of a focus signal appropriate for a flat-screen CRT. Compared to the waveform of FIG. 1, the waveform of FIG. 2 has a horizontal focus signal, of which waveform is not uniform, in a vertical cycle.  
         [0022]    That is, the amplitude (V2) of each horizontal focus signal of the beginning area and ending area of a vertical cycle is greater than the amplitude (V1) of the horizontal focus signal of the middle area of the vertical cycle.  
         [0023]    According to this difference, the focus voltage of each corner of the screen is higher than that of the center of the screen so that precise focus control is enabled.  
         [0024]    [0024]FIG. 3 is a block diagram of a dynamic focus circuit adopting an apparatus for generating a dynamic focus signal according to the present invention.  
         [0025]    The apparatus of FIG. 3 includes a horizontal and vertical processor  302 , a dynamic focus amplifier  304 , a Fly-Back Transformer (FBT)  306 , a flat-screen CRT  308 , and a microprocessor  310 .  
         [0026]    The horizontal and vertical processor  302  includes a dynamic focus signal oscillator  402  for generating a dynamic focus signal, a first gain adjusting unit  404  for adjusting the gain of the dynamic focus signal generated in the dynamic focus signal oscillator  402 , a second gain adjusting unit  406  for adjusting the gain of each horizontal focus signal in a vertical cycle, and a key stone  408  for adjusting the gain difference of the back porch and the front porch of the dynamic focus waveform.  
         [0027]    The microprocessor  310  controls the gain adjusting units  404  and  406  and the key stone  408  in the horizontal and vertical processor  302 . Control data in the microprocessor  310  is provided to the gain adjusting units  404  and  406  in the horizontal and vertical processor  302  in serial communications.  
         [0028]    Meanwhile, the dynamic focus amplifier  304  amplifies a dynamic focus signal generated in the horizontal and vertical processor  302 , and the FBT  306  mixes the amplified dynamic focus signal and a DC focus voltage and provides the mixed signal to the electron gun of the CDT  308 .  
         [0029]    [0029]FIG. 4 is a detailed block diagram of the dynamic focus signal oscillator  402  of FIG. 3. The oscillator of FIG. 4 includes first, second and third multipliers  502 ,  504  and  506 , first, second and third gain controllers  508 ,  510  and  512 , and a mixer  514 .  
         [0030]    The first multiplier  502  receives a horizontal sawtooth wave (HDF) having a horizontal frequency, and the second multiplier  504  receives a vertical sawtooth wave (VDF) having a vertical frequency.  
         [0031]    The first multiplier  502  and the second multiplier  504  are 2ND order multipliers that output the square components of the horizontal sawtooth wave (HDF) and the vertical sawtooth wave (VDF), respectively. That is, each of the first multiplier  502  and the second multiplier  504  outputs the result of quadratic equation calculation of the input signal.  
         [0032]    The multiplication results of the first multiplier  502  and the second multiplier  504  are a parabolic HDF signal and a parabolic VDF signal, respectively.  
         [0033]    Meanwhile, the third multiplier  506  multiplies the outputs of the first multiplier  502  and the second multiplier  504  and then outputs the resulting product.  
         [0034]    The first through third gain controllers  508  through  512  adjust the outputs of the first through third multipliers  502  through  506 , respectively. The mixer  514  mixes the outputs of the first through third gain controllers  508  through  512 , and outputs the mixed result. The gains of first through third gain controllers  508  through  512  are controlled by the microprocessor  310  in response to the vertical and horizontal sync signals. The control signals from the microprocessor  310  are transmitted serially to the gain controllers  508  through  512 . The dynamic focus signal, as shown in FIG. 2, is obtained from the output of the mixer  514 .  
         [0035]    When HDF(x) denotes the horizontal sawtooth wave (HDF) and VDF(x) denotes the vertical sawtooth wave (VDF),  
           Parabolic HDF signal=A×HDF ( x )× HDF ( x )  (1) 
           Parabolic VDF signal=B×VDF ( y )× VDF ( y )  (2) 
           Composite HV dynamic focus signal=A×HDF ( x )× HDF ( x )+ B×VDF ( y )× VDF ( y ) + C×HDF ( x )× HDF ( x )× VDF ( y )× VDF ( y )  (3) 
         [0036]    Here, A, B, and C denote the amplification degrees of the first through third gain controllers  508  through  512 , respectively.  
         [0037]    Accordingly, due to the last term, C×HDF(x)×HDF(x)×VDF(y)×VDF(y), the feature that the maximum value of the focus signal for the center of the screen is different from the maximum values of the focus signal for the outer area inside the screen, which is the purpose of the present invention, is obtained.  
         [0038]    [0038]FIG. 5 is an example of dynamic focus voltages needed in a flat-screen CRT, taking into account horizontal=350 V, vertical=150 V, plus a DC voltage, the center area of the screen should be 6.00 KV, the top center of the screen (E of FIG. 5) should be 6.15 KV, the right center of the screen (F of FIG. 5) should be 6.35 KV, and each of the corners of the screen (A through D of FIG. 5) should be 6.50 KV. However, in each of the corners (A through D) actually needs about 6.6 KV.  
         [0039]    [0039]FIG. 6 is an example of a dynamic focus signal generated by the apparatus for generating a dynamic focus signal according to the present invention.  
         [0040]    In the dynamic focus signal of FIG. 6 , the amplitude (V2) of each of the beginning part and the ending part of the horizontal focus signal in a vertical cycle is greater than the amplitude (V1) of the horizontal focus signal of the middle part in the vertical cycle.  
         [0041]    That is, the amplitude (450 V) of the horizontal focus signal of each of the beginning part and the ending part in the vertical cycle is greater than the amplitude (350 V) of the horizontal focus signal of the middle part in the vertical cycle. Also, the level of the peak value of each horizontal focus signal in the vertical cycle changes forming a semi-circle shape (450 V→440 V→. . . →350 V→350 V→350 V→440 V→450 V).  
         [0042]    As described above, in the apparatus and method for generating a focus signal according to the present invention, by generating a focus signal having different maximum values in the central area and in the outer area inside the screen appropriate for a flat-screen CRT, optimum focus control is performed.