Abstract:
A device and method for generation of a dynamic focus correction signal for use with a CRT that includes an analog scanning processor for generating a dynamic focus correction signal that is proportional to Kx 2 +(1−K)x 4 , where x is the distance from a mid point of a viewing surface of the CRT, and K is a real number in the range 0.00 to 1.00. Embodiments of the invention find particular use in CRTs having generally flatter, squarer configurations.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a scanning processor for use with Cathode Ray Tubes (CRTs) as used in television and computer monitor displays and, more particularly, to CRTs having display surfaces with a profile that is more flat and square than existing CRTs. 
   2. Description of the Related Art 
     FIG. 1  shows the effect of the curvature of the CRT on the distance between the inner phosphor surface of the CRT and the electron gun. If the display surface  100  of the CRT were configured to follow the circumference of an imaginary circle  140  centered at the output of the electron gun  110 , then each point on the inner surface of the display surface  100  would be equidistant from the electron gun  110 . 
   However, in general, the display surfaces of CRTs are made to be as flat as possible, within the manufacturing constraints of the vacuum bulb.  FIG. 1  shows a typical configuration of a display surface  100  superimposed on the imaginary circle  140 . 
   It can be seen from  FIG. 1  that the distance between a point at the center of the display surface is significantly closer—as shown by a first arrow  120 —to the electron gun  110 , than a point at an outer edge of the display surface. The greater distance is represented by a second arrow  130 . 
   The difference in distance between the different points of the display surface poses problems in beam brightness and focusing. Points closer to the gun require a different focal length than to points further away. Closer points will also tend to be brighter than further away points. 
     FIG. 2  shows an enlarged view of the display surface of a CRT affected by the described focusing problems. The display surface should be displaying a single horizontal line of uniform thickness. However, due to the differing focal length across the horizontal axis of the display screen, the image of the line tends to taper at the extreme edges of the display surface. The effect is shown in a deliberately exaggerated fashion for clarity, but particularly on larger screens it is noticeable to the human eye. 
   These problems are exacerbated by recent developments in CRT technology which have yielded so-called minineck tubes having a reduced depth i.e., distance between display surface and electron gun. 
   To overcome problems associated with the increasing flatness and squareness of CRTs, manufacturers of televisions and monitors attempt to configure their products to dynamically adapt the focusing and brightness for different points on the display surface. Prior art solutions include supplying focus and brightness adjust circuits arranged to accept one or more signals configured to alter focus and brightness characteristics of the displayed image. 
   However, prior solutions are limited in the amount of correction that they are able to supply. Prior solutions are limited to being able to supply parabolic correction signals, i.e., signals characterized by the mathematical expression x 2 . The correction signals are applied to bias or modulate the focusing and/or brightness signals. Such correction signals have been acceptable with older types of vacuum tubes, but are not found to adequately remedy the problem with the flatter and squarer tubes now becoming available. 
   A typical graph showing the dynamic focus voltage required for a particular point at a distance x from the screen center is shown in  FIG. 3  for both horizontal  200  and vertical  210  deflections. In the vertical direction, the particular curve  210  shown in  FIG. 3  is defined by the relationship:
 
V dyn     —     focus     —     v ∝x 2  
 
   In the horizontal direction, the relationship is different, as the tube is not symmetrical in the horizontal and vertical directions. The relationship defining horizontal dynamic focus is given by:
 
V dyn     —     focus     —     h ∝x 2.6  
 
   These figures are exemplary only, and different tubes from different manufacturers can have dynamic focus voltages defined as:
 
V dyn     —     focus ∝x n  
 
where n is generally in the range 2-2.6, although further improvements in tube design may lead to progressively higher values of n.
 
   Prior art systems for dynamic focus correction are limited to correcting for the case where the correction voltage is proportional to the square of the distance from the center point. Since most tubes have a different relationship as defined previously, this approximation results in poor focus and brightness performance, particularly at the extremities of the screen, where the value of x is larger and so exacerbates the problem. Also, since different tubes have different characteristics, each type of tube must be individually optimized, leading to increased design work for each end product. 
   BRIEF SUMMARY OF THE INVENTION 
   The disclosed embodiments of the present invention provide an analog scanning processor for generation of a dynamic focus correction signal for use with a CRT, the dynamic focus correction signal characterized in that it is proportional to Kx 2 +(1−K)x 4 , where x is the distance from a mid point of a viewing surface of the CRT, and K is a real number in the range 0.00 to 1.00. 
   Preferably, the dynamic correction signal is a horizontal or vertical dynamic focus correction signal. 
   Preferably, the processor is arranged to generate a plurality of dynamic correction signals. 
   Preferably, the processor includes means for generating a dynamic brightness correction signal. 
   Preferably, the dynamic correction signal for use in a horizontal direction is different to the dynamic correction signal for use In a vertical direction. 
   Preferably, the processor includes a shape adjustment circuit arranged to receive as inputs a sawtooth waveform at the deflection frequency; a shape control signal; and an amplitude control signal, the shape adjustment circuit is arranged to produce a signal which approximates closely to the sawtooth input waveform raised to a power n, where n is a real number. Ideally, the value of n is in the range 2.00 to 4.00. 
   In accordance with another embodiment of the invention, a CRT monitor is provided that includes an analog scanning processor for generation of a dynamic correction signal for use with a CRT monitor. The analog scanning processor is configured to generate a correction signal that is proportional to Kx 2 +(1−K)x 4 , where x is the distance from a mid point of a viewing surface of the CRT, and K is a real number in the range 0.00 to 1.00. 
   In accordance with another embodiment of the invention, generation of the correction signal includes use of a shape adjustment circuit that receives as inputs a sawtooth waveform, preferably parabolic, as well as a shape control signal and an amplitude control signal, and wherein two output signals are produced, Out 1  and Out 2  according to the following:
 
Out 1   =H   amp   ×H   phasesize   2   ×[H   shape +(1 −H   shape )× H   phasesize   2   ]V   amp   ×V   sawtooth   2  
 
Out 2   =V   bright   ×V   sawtooth   2  
 
where:
 
   H sawtooth  is a sawtooth waveform at the horizontal deflection frequency (normalized and centered); 
   V sawtooth  is a sawtooth waveform at the vertical deflection frequency including vertical size and position information; 
   V bright  is an amplitude adjustment for the dynamic brightness control; 
   V amp  is the vertical amplitude control; 
   H amp  is the horizontal amplitude control; 
   H phase  is the horizontal phase control; 
   H shape  is the horizontal shape control; 
   H size  is the horizontal size control; and 
   H phasesize =(H sawtooth +H phase )×(1+H size ). 
   A method of generating a dynamic correction signal for a CRT monitor is also provided, the method including generating the correction signal that is proportional to Kx 2 +(1−K)x 4 , where x is the distance from a mid point of a viewing surface of the CRT, and K is a real number in the range 0.00 to 1.00. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Representative embodiments of the present invention are hereinafter described, by way of example only, with reference to the appended drawings in which: 
       FIG. 1  shows the effect of curvature of a CRT on beam distances; 
       FIG. 2  shows an exaggerated view of the effect of a variation of focus distance on the presentation of a single horizontal line; 
       FIG. 3  shows the dynamic focus voltage required for horizontal and vertical deflections in a typical CRT; 
       FIG. 4  shows a top level view of a dynamic correction circuit; 
       FIG. 5  shows a detailed view of a dynamic focus system: 
       FIG. 6  shows a detailed view of the shape adjustment circuit; and 
       FIG. 7  shows a composite horizontal and vertical dynamic focusing signal. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following description, the dynamic correction circuit will be described as it applies to dynamic focus correction. However, one skilled in the art will appreciate that similar techniques may be applied to dynamically adjust the brightness. 
     FIG. 4  shows a top level view of the dynamic correction circuit  300 . In order to dynamically alter the focus, a circuit  306  is provided which varies the horizontal and vertical deflection waveforms and applies an AC voltage to the focus electrode. 
   Dynamic brightness adjustments may be performed by modulating the ABL voltage which is normally applied at the video pre-amplification stage. 
   For horizontal dynamic focus adjustments, the circuit  300  includes a Phase Locked Loop (PLL)  302  that is locked to an incoming horizontal sync input  310 . The PLL fixes the phase and also coarse tunes the internal Automatic Gain Circuit (AGC) which generates the horizontal dynamic focus correction signal. The output signals from the PLL to circuit  306  are Frequency  312  and gross phase  314 . 
   For vertical dynamic focus adjustments, the circuit includes an AGC  304  which is locked to a vertical sync input  320 . The output  322  from the AGC circuit is a squared sawtooth waveform V sawtooth   2 . 
   Other inputs are provided to the circuit  306  which are used to fine tune the output signals. The other input signals are:
         Horizontal amplitude control (H amp )  330     Vertical amplitude control (V bright )  340     Vertical amplitude control (V amp )  345     Horizontal phase control (H phase )  350     Horizontal size control (H size )  360     Horizontal shape control (H shape )  370         

   The circuit  300  has two outputs  380 ,  390 . The first output  380  is a composite output for horizontal and vertical focus correction. The second output  390  is the vertical dynamic brightness control signal. The outputs may be represented:
 
Out 1 (380)= H   amp   ×H   phasesize   2   ×[H   shape +(1 −H   shape )× H   phasesize   2   ]V   amp   ×V   sawtooth   2  
 
Out 2 (390)= V   bright   ×V   sawtooth   2  
 
where:
 
   H sawtooth  is a sawtooth waveform at the horizontal deflection frequency (normalized and centered); 
   V sawtooth  is a sawtooth waveform at the vertical deflection frequency including vertical size and position information; 
   V bright  is an amplitude adjustment for the dynamic brightness control; 
   H phasesize =(H sawtooth +H phase )×(1+H size ). 
     FIG. 5  shows the internal structure of shape adjustment circuit  306 . The primary input  322  to the circuit is the output from the AGC circuit  304 . The input  322  is a squared parabolic waveform. The signal  322  is split into two components, which are separately processed to produce the two output signals  380 ,  390 . 
   In the first branch, the input signal  322  is applied to a variable gain amplifier  412 . The gain of this amplifier is adjustable by signal  340  which is a Vertical, amplitude control signal. This signal is provided to allow user adjustment of the vertical control signal  390 , and it may be adjusted through an on screen menu system. 
   The output of amplifier  412  passes to amplifier arrangement  426  which includes a transistor and a current source. The output signal  390  is derived from the emitter of the transistor. 
   In the second branch, the input signal  322  is applied to a variable gain amplifier  410 , where the gain is adjustable by signal  345 , which allows user adjustment of the composite adjustment signal  380 . The output of amplifier  410  is applied to a summer circuit  420 . The other input to the summer circuit  420  is derived from a number of input signals. 
   Signals  312  and  314 , which are output from the PLL  302  and represent frequency and gross phase information respectively, are applied to the AGC circuit  402 . Circuit  402  acts to generate a ramp signal at the frequency determined by input  312 . The ramp signal is applied to phase circuit  404 . Circuit  404  receives a second input  350  which is a horizontal phase control signal, allowing separate adjustment of the phase of the output signal  380 . 
   The output of circuit  404  is applied to a variable gain amplifier  406 , whose gain is adjustable by signal  360 . Signal  360  is a horizontal size control signal, which is provided to allow a user to control the horizontal size of an image displayed on the CRT. Again, this option may be accessed via an on-screen menu system. 
   The output of amplifier  406  is applied to circuit  408 , which also has as an input  370  for controlling the shape of the output signal  380 . The operation of circuit  408  is described later with reference to  FIG. 6 . The output of circuit  408  is a shape-adjusted version of the input to circuit  408 , the shape adjustment being performed in accordance with the value of the shape adjustment signal  370 . The output of circuit  408  is applied to variable gain amplifier  418 , whose gain is adjustable in accordance with the signal  330  which controls the amplitude of the signal presented to the summer circuit  420 . 
   The output of the summer circuit  420  is applied to the output amplifier arrangement  428  which includes a transistor and a current source. The composite output signal  380  is derived from the emitter of the transistor. 
     FIG. 6  shows the internal structure of shape adjustment circuit  408 . The purpose of this circuit is to alter the shape of its input waveform, H sawtooth , so that it more closely resembles the waveform required for dynamic focus correction, i.e., it is altered to resemble a waveform characterized as X n , where n is a real, non-integer value, generally in the range 2-4. 
   Analogue implementation of circuits that can raise an incoming signal to an arbitrary real (non-integer) power are complex and can generally only be created for a specific value of n. In order to provide a more generic solution that may be tailored by a designer to operate with a specified CRT, it is desirable to provide a correction circuit which may be configured to assume a desired value of n. 
   However, such circuits are complex, and it is found that a good approximation of x n  is provided by the following mathematical expression:
 
 x   n   ≈K   1   x   2   +K   2   x   4  
 
where K 1 =H shape  and K 2 =(1−H shape )=(1−K 1 )
 
   Typical current CRTs require a value of n=2.6. Of course, the above expression is only ever 100% accurate for the case where n is equal to 2 or 4, but the extent of the inaccuracy is never more than ±1.5% in the range n:2→4. 
   Compared to the prior art approach of using a parabolic waveform (n=2), where a more appropriate correction signal would set n=2.6, the error can be as much as 9.5% over the same range. Therefore, embodiments of the present invention offer a much improved performance. 
     FIG. 6  shows the internal operation of shape adjustment block  408 . The input to multiplication block is H sawtooth , which is squared by being multiplied by itself. The resultant squared signal is split into two paths, with the signal on the first path being further multiplied by (1−H shape ) in multiplication block  460 , before being added to H shape    370  in adder  470 . The result of the addition is then multiplied by the signal on the other path in multiplier  480 . The result of this operation may be expressed mathematically as:
 H amp .[(H shape )H sawtooth   2 +(1−H shape )H sawtooth   4 ] 
   This can be seen to have the form required i.e., KX 2 +(1−K)x 4 . 
   For a typical CRT having n=2.6, H shape  may be set to 0.59 to achieve the desired result. Of course, different CRT configurations will require different values of n, and H shape  may be adjusted accordingly. 
   The signal provided for the vertical dynamic focus control is a parabolic control signal. This is found to be adequate in most cases, as the vertical dimension is shorter than the horizontal dimension, reducing the extent of the problem. However, if necessary, a similar approach to vertical dynamic focus as is used for horizontal dynamic focus may be adopted. 
   The vertical dynamic focus signal is superimposed on the horizontal dynamic focus signal to yield the composite output signal  380 . The form of this waveform is shown in  FIG. 7 . The horizontal component of the signal is designated  500 , and the vertical component is designated  510 . The composite signal is applied to the focus electrode of the CRT. 
   The dynamic brightness correction signal  390  may be applied to the video preamplification stage as has been described already. 
   Use of embodiments of the invention allow a TV or monitor manufacturer to offer users an opportunity to further customize the setup of their equipment through use of on-screen menus permitting focus and/or brightness to be altered. 
   The present invention includes any novel feature or combination of features disclosed herein either explicitly or any generalization thereof regardless of whether or not it relates to the claimed invention or mitigates any or all of the problems addressed. 
   All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety. 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.