Abstract:
A method for generating display correction waveforms for a CRT display comprises the steps of selecting one of a plurality of trace portions for forming part of a correction waveform, the trace portions having different average values. Completing each of the correction waveform by combining each selected trace portion with a respective retrace portion such that all completed correction waveforms have a predetermined average value. The correction waveforms may have vertical and/or horizontal rates.

Description:
This application claims the benefit under 35 U.S.C. §365 of International Application PCT/EP00/12464 filed Dec. 9, 2000, which claims the benefit of European Application No. 99125475.6, filed Dec. 21, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates generally to correction waveform generation in an image display, and more particularly to a waveform generator operable at a plurality of display standards. 
     Waveforms comprising multiple frequency components frequently include a DC component which renders the signal essentially unipolar. However, removal of the DC component, for example, by capacitive coupling results in the loss of the unipolar characteristic with the resultant waveform being disposed equally in area, positively and negatively about an average waveform value. This average value is dependent on the waveform shape, thus when AC coupled waveforms with differing shapes produce differing average values with respect to the waveform peaks. Hence the AC peak potentials received by the circuitry following the AC coupling are altered, and vary in accordance with differing waveform shapes. 
     In an exemplary cathode ray tube the distance from the center of electron beam deflection is, in general, shortest to the center of the display screen, with the distance increasing to maximum values in the screen corners. Thus to achieve consistent beam landing or a focused electron beam over the complete screen area requires that a DC focus voltage is combined with a signal waveform comprising multiple frequencies, for example, horizontal and/or vertical frequency parabolic shaped waveforms. Typically this parabolic waveform is generated with low voltages near the system ground potential and added to the high voltage DC focus voltage via an AC coupling. The amplitude of this parabolic signal has a factory determined value, since the distances between all screen locations and the center of electron beam deflection are known and fixed. Thus, a single focus control, which adjusts a DC potential may be provided to allow optimum focus to be obtained, not only in the screen center, but at all screen locations. Such overall optimized adjustment assumes an accurately determined factory set amplitude value for the generally parabolic shaped signals. 
     Although the geometric relationship between the display screen and electron beam are fixed and hence not standards specific, a display device may be capable of operation at multiple display standards with various scanning frequencies and differing retrace and blanking times. Thus a parabolic waveform generator is required which is responsive to the display standard, follows the scanning frequency, is capable of differing phasing relative to a vertical retrace pulse and is responsive to differing blanking durations. Such varieties of waveform shaping and phasing consequently vary the AC peaks with respect to the DC component of the waveform. Hence, when this exemplary waveform is ultimately AC coupled for addition to the high DC voltage for DC focus control, the loss of the DC component of the waveform may necessitate readjustment or optimization of the DC focus control voltage. Hence a display operable at multiple scanning and display standards, may require individual focus control adjustment for each display standard. 
     U.S. Pat. No. 5,471,121 discloses a circuit for generating a dynamic focus voltage. The focus voltage is compared to a reference voltage. Whenever the focus voltage drops below the reference voltage, a switching signal is provided. The switching signal controls a switch, which provides a DC focus voltage during the overscan portions of the trace period. The DC focus voltage has such a value to keep the average value of the focus voltage essentially constant. 
     In EP-A 0 554 836 a parabolic waveform generating apparatus is described. The known apparatus comprises two memory devices, each storing data necessary for generating parabolic waveforms necessary for the images of a specific aspect ratio. For different aspect ratios an arithmetic logic unit processes the output data of the two memory devices to generate new output data values for a desired parabolic waveform. 
     DE-A 197 54 904 provides a method for generating a focus voltage for a CRT. The known method comprises the steps of verifying if reference data have changed, calculating correction data and generating a focus voltage according to the correction data. 
     It is an object of the preset invention to provide a method for generating display correction waveforms for a CRT display. This object is solved by a method according to claim  1 . The inventive method comprises the steps of selecting one of a plurality of trace portions for forming part of a correction waveform, the trace portions having different average values. Completing each of the correction waveform by combining each selected trace portion with a respective retrace portion such that all completed correction waveforms have a predetermined average value. Further advantageous modifications of the inventive method are subject of dependent subclaims. 
     It is another object of the present invention to provide an apparatus having improved focus voltage generation. This object is solved by an apparatus according to claim  1 . 
     It is an advantage of the apparatus according to the invention that independent of the actual waveform modulating the focus voltage a predetermined average value is maintained. In this way a well focused electron beam is yielded over the whole screen area. 
     An embodiment of the inventive apparatus can be equipped with means for superimposing waveforms for vertical and horizontal focus voltage modulation. It is noted that the maintenance of a predetermined average value of the modulating waveform is achieved for vertical and horizontal focus voltage modulation. 
     Further advantageous modifications of the inventive apparatus are subject of dependent subclaims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an exemplary parabolic waveform signal generator coupled to provide dynamic focus in a cathode ray tube. 
     FIG. 2A illustrates a first inventive parabolic waveform. 
     FIG. 2B illustrates AC coupling of FIG. 2A waveform with inventive coefficient V 4 . 
     FIG. 2C illustrates AC coupling of FIG. 2A waveform with inventive coefficient V 4 ′. 
     FIG. 3 illustrates a second inventive parabolic waveform and 
     FIG. 4 shows an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     An exemplary digital parabolic waveform signal generator is illustrated in FIG. 1, coupled, for example, to provide dynamic focus, or electron beam landing correction in a cathode ray tube. The digital parabolic waveform signal generator is depicted in area  100 , which may form part of an inventive integrated circuit, for example, ST Microelectronics type STV2050. The digital waveform generator  100  is coupled to area  200  which includes a differential amplifier and lowpass filter, which is coupled to dynamic focus signal generator  250 , and cathode ray tube CRT. 
     Digital waveform generator  100  includes a digital controller CTRL  105  which controls all functions within generator  100  via data bus  115 , and provides control communication via a data bus  70  to an external microprocessor  75 . A RAM  110  is connected to an external EEPROM memory (PROM  80 ) via a dedicated data bus  85  from which it receives operating data at power on. RAM  110  stores operating data values for an exemplary display device, but in particular, data specific to the generation of a parabolic waveform shape. To achieve good results for the display at different scanning frequencies and display standards, the focus voltage is modulated both in horizontal and vertical direction with regard to the beam landing position on the screen. The focus voltage is modulated by parabolic waveforms wherein the waveforms for vertical and horizontal modulation, respectively, are generated separately. Of course it is also possible to modulate the focus voltage only by either a vertical or a horizontal waveform. 
     At first the vertical parabolic waveform is considered. Data bus  115  supplies vertical parabola specific data from RAM  110  to vertical parabola generator  120 , vertical coefficient data to generator  130  and vertical compensation data to generator  140 . Parabola generator  120  generates a vertical parabolic waveshape Vpar represented by six bit digital values in accordance with specified amplitude values or coefficients, occurring at specific times during trace or active picture time. Compensation generator  140  forms a six bit digital value Vcomp which is coupled as one input to selector switch  150 . Output Vpar from generator  120  is coupled as a second input to switch  150  which is controlled by a vertical rate signal Svrt occurring during a vertical retrace period. Thus switch  150  couples the digital parabolic waveform Vpar to digital to analog converter  160  during active picture, or vertical trace time, and selects digital word Vcomp for digital to analog conversion by DAC  160  during the vertical retrace period. Digital to analog converter  160  generates an analog signal which is coupled to a first input of differential amplifier  170 . 
     In a similar way the horizontal parabolic waveform is generated. Data bus  115  also supplies horizontal parabola specific data from RAM  110  to horizontal parabola generator  121 , horizontal coefficient data to generator  131  and horizontal compensation data to generator  141 . Horizontal parabola generator  121  generates a parabolic waveshape Hpar represented by six bit digital values in accordance with specified amplitude values or coefficients, occurring at specific times during horizontal trace or line period. Compensation generator  141  forms a six bit digital value Hcomp which is coupled as one input to selector switch  151 . Output Hpar from generator  121  is coupled as a second input to switch  151  which is controlled by a horizontal rate signal Shrt occurring during a horizontal retrace period. Thus switch  151  couples the digital parabolic waveform Hpar to digital to analog converter  161  during an active line, or horizontal trace time, and selects digital word Hcomp for digital to analog conversion by DAC  161  during the horizontal retrace period. Digital to analog converter  161  generates an analog signal as a differential output which is coupled to a second input of differential amplifier  170 . 
     Amplifier  170  is configured as a differential input amplifier, with input resistors R 1  and R 2  of similar values to provide improved stability with temperature. However, it is also possible to manipulate the summing of the input signals by selecting different values for the resistors R 1  and R 2 . Then the input parabolic signals are summed in inverse proportion to the value of the input resistors R 1  and R 2 . In this way an individual weighting coefficient can be assigned to each of the inputs of amplifier  170 . The gain of amplifier  170  is determined in part by resistor R 2  and R 3  and capacitor C 1 , which provides frequency dependent negative feedback. The analog signal from DAC  160  supplied to the first input of the amplifier  170  is parabolic in shape comprised of up to 64 discrete amplitude levels where each level, or amplitude value is held constant for a number of line periods. These discrete amplitude values, which describe the vertical parabola, are only permitted to change during horizontal retrace periods. 
     The analog signal from DAC  161  is parabolic in shape as well with a resolution of 64 discrete amplitude levels. The amplitude values change during horizontal trace e.g. at positions corresponding to the horizontal positions of vertical lines of a grid which is used during the setting of convergence correction values. It is possible to adapt the timing of the changes of the focus voltage for horizontal modulation according to the requirements of different modes of operation, for example zoom modes. 
     The changes in parabolic signal values or steps generate transients which are removed by lowpass filtering resulting from feed back capacitor C 1  of amplifier  170  and lowpass filtering at the amplifier output provided by series connected resistor R 4  and shunt connected capacitor C 2 . 
     The lowpass filtered, vertical rate parabolic signal Vpar, depicted in FIG. 2A, is coupled via resistor R 5  to amplifier  180  of area  250 . As is well known, negative feed back from the amplifier output via resistor R 7  forms a low, or virtual earth input impedance. Amplifier  180  also provides voltage gain such that the output signal has an amplitude in the range of approximately 600 V which is coupled via capacitor C 3  to the wiper of focus potentiometer Rf. Thus the summed vertical and horizontal rate parabolic signals form a focus modulation signal Fm which is added to the DC focus voltage Vf, for example 8.5 kV, generated by potentiometer Rf and applied as waveform Vfm to the focus electrode of cathode ray tube CRT. 
     Coefficient generator  130  forms vertical parabola amplitude determining coefficients as three digital words V 1 , V 2  and V 3  which set the amplitude of the parabola at specific time intervals to be generated by generator  120 . The coefficients are independent of one another but have fixed positions or line counts relative to each other within the period of a field. For example, in FIG. 2A, the time between ordinates V 1  and V 2  is the same as that between ordinates V 2  and V 3 . A field repetition rate parabola is illustrated in FIG. 2A, with a maximum amplitude defined by 6 bits, giving 64 possible amplitude values. The parabola position or phase within the field period is also adjustable, for example by offsetting a starting point of a counter which determines the time between ordinates V 1 , V 2  and V 3 . The vertical position adjustment of phasing of the parabolic waveform may be performed by an exemplary remote control RC 73  which communicates with microprocessor  75  via an infra red receiver IRRX,  72 , or during factory setup by a direct data bus connection to microprocessor  75  (not shown). 
     Generator  120  performs calculations which cause the generation of a parabola that passes through the three user defined amplitude values. The general form of equation for a parabolic waveform generation is, 
     
       
         Parabola= ax   2   +bx+c,   (1)  
       
     
     where variables a, b, c and Z are calculated as follows from the user defined values for V 1 , V 2  and V 3 , 
     
       
           a= 1 /Z   2 *(2 V 3−4* V 2+* V 1),  (2)  
       
     
     
       
           b= 1 /Z *(− V 3+4* V 2−3* V 1),  (3)  
       
     
     
       
         c=V1,  (4)  
       
     
     
       
           Z= 12*( VGD+ 1),  (5)  
       
     
     where VGD, vertical grid distance, represents a vertical image dimension measured in scan lines, which may have values between 11 and 63. During setup the parabola amplitude coefficients V 1 , V 2  and V 3  are adjusted in conjunction with focus control Rf to achieve optimum overall CRT focus. 
     Data representing coefficient V 4  is read from RAM  115  and formed into digital word V 4  by compensation data generator  140 . Data switch  150  provides selection between parabola data from generator  130  and compensation data representative of a fixed or DC value from generator  140 . Switch  150  is controlled by a vertical rate signal Svrt to select DC compensation data during the vertical retrace period and parabolic waveform data for the active part of the field period. The function coefficient V 4  value will be explained with reference to FIGS. 2A,  2 B and  2 C. 
     Coefficient generator  131  forms horizontal parabola amplitude determining coefficients as three digital words H 1 , H 2  and H 3  which set the amplitude of the horizontal parabola at specific time intervals to be generated by generator  121 . The coefficients are independent of one another but have fixed positions relative to each other within the period of a line. For example, the time between ordinates H 1  and H 2  is the same as that between ordinates H 2  and H 3 . The parabola position or phase within the line period is also adjustable, for example by offsetting a starting point of a counter in the same way as explained for the vertical parabola. The horizontal parabola itself is calculated in a similar way as the vertical parabola by replacing V 1 , V 2 , and V 3  with H 1 , H 2 , and H 3 , respectively in equations (1) to (4) given above. The parameter Z is selected to be 
     
       
         Z=14  (5′).  
       
     
     However, it is also possible to select a different value and the invention is not limited to a specific value. It is preferred to utilize values that are easily deductible from the horizontal timing that is required to synchronise the focus voltage modulation with the deflection of the electron beam. 
     Data representing coefficient H 4  is read from RAM  115  and formed into digital word H 4  by compensation data generator  141 . Data switch  151  provides selection between parabola data from generator  131  and compensation data representative of a fixed or DC value from generator  141 . Switch  151  is controlled by a horizontal rate signal Shrt to select DC compensation data during the horizontal retrace period and parabolic waveform data for the active part of the line. The function of coefficient H 4  is similar to that of coefficient V 4 . The explanation will be given below with reference to FIGS. 2A,  2 B and  2 C. 
     In FIG. 2A parabolic signal Vpar is illustrated with coefficient V 4  having two different values namely V 4  and V 4 ′ (shown with a dashed line). FIGS. 2B and 2C depict signal Vpar, of FIG. 2A, coupled via amplifier  180  and capacitor C 3  to form of focus waveform Vfm. However, since the horizontal component of waveform Vfm is approximately double that of the vertical component, in the interest of drawing clarity FIGS. 2B and 2C show only the vertical rate parabolic component of signal Fm. 
     The AC coupling of signal Fm by capacitor C 3  results in the loss of the waveform DC component, which consequently causes signal Fm to be disposed symmetrically, in terms of waveform polarities, about the DC focus voltage Vf. Thus, as described, with the amplitude of signal Fm being factory determined and preset, exemplary focus control Rf may be adjusted to achieve optimum CRT focus at the screen center by means of peak voltage value Vfc, with focusing at the screen top and bottom being determined by cusp voltages Vft and Vfb respectively. In actuality, if waveform Vfm is appropriately shaped by means of horizontal and/or vertical coefficient value manipulation, optimum focus may be achieved over the whole CRT display surface. 
     However, as has been described previously, changes in the vertical parabolic signal shape, for example as depicted in FIG. 2A by coefficient V 4 ′ shown with the dashed line, cause the mean value to be different. In FIGS. 2B and 2C the parabolic waveforms Vparab are identical in both shape and amplitude. For example, in FIG. 2B if the waveform amplitude is measured relative to the mean or average value by the addition of values Vft+Vfc, this value is equal to the corresponding signal amplitudes Vft′+Vfc′ of FIG.  2 C. However, since the mean values of the waveforms shown in FIGS. 2B and 2C are different, the optimized center screen focus of FIG. 2B, resulting from the addition of exemplary peak signal amplitude Vfc and DC value Vf is no longer optimum for the waveform depicted in FIG. 2C as a consequence of the diminished peak amplitude of signal Vfc′ relative to the mean value of the waveform. In fact the whole screen is defocused as a result of the differing mean values which necessitates readjustment of focus control Rf to restore overall optimum focus. 
     FIGS. 2B and 2C illustrate that coefficient V 4 , which is selected during the vertical retrace period and thus plays no CRT electrode control, may advantageously provide compensation for changes in the mean value of focus modulation waveforms generated for differing display or deflection standards. For example, differing display standards may be considered with reference to FIG. 2A, which indicates a field period comprising a vertical retrace or vertical blanking interval Tvrt and active scan period 2T. In the NTSC television signal format, the field period comprises 262.5 horizontal line periods with interval Tvrt representing approximately 20 line periods, thus the ratio of retrace or vertical blanking interval to the field period is approximately 1:13 or 8%. However, in the ATSC 1080I high definition television standard or ANSI/SMPTE standard 274M, a frame comprises 1125 lines with 1080 active line periods. Thus there are 45 lines of non-active picture per frame which in the interlaced format would be distributed between each field comprising 562.5 horizontal line periods. The non-active picture or blanking and vertical retrace interval Tvrt representing approximately 22.5 line periods. Hence the ratio of retrace or vertical blanking interval to the field period is approximately 1:25 or 4% which is approximately half that of the NTSC format. This ratiometric difference in waveform shape or timing may be obviated by advantageous use of coefficient V 4  which has differing preset standard specific values, selected to maintain optimum beam landing or focus by compensating for differences in the average value of vertical rate correction waveforms. 
     FIG. 3 depicts a parabolic waveform shape, for example, generated in accordance with a display image having a vertical blanking width different to that of the signal for which the parabolic signal of FIG. 2A was generated. The waveform depicted in FIG. 3 is shaped in accordance with the values of ordinates V 12 , V 22  and V 32 , where ordinate V 12  is delayed or phase shifted by TΦ relative to the start of the vertical retrace interval Tvrt. In addition the waveform shape may be considered to represent a parabola superimposed on a field rate ramp or sawtooth signal as depicted by broken line S. Advantageously compensation data word V 42  provides an adjustable signal component which allows differing waveform shapes to have substantially similar DC components, thus facilitating operation at multiple display standards without focus readjustment or multiple focus values. 
     The same explanation is applicable to the coefficient H 4  which is selected to maintain optimum beam landing or focus by compensating for differences in the average value of horizontal rate correction waveforms. 
     FIG. 4 shows an alternative embodiment of the present invention. The main difference compared to the embodiment in FIG. 1 is that the values of the horizontal and vertical parabolas Hpar, Vpar and the compensation coefficients H 4  and V 4  are added at first in adder stage  162 . Subsequently, the sum value is converted into an analog value by DAC  160  and supplied to the first input of differential amplifier  170 . The second input of amplifier  170  is connected to a fixed reference potential Uref. The output of amplifier  170  is processed in the same way as described for the previous embodiment. Evidently the embodiment shown in FIG. 4 yields the same effects and advantages as set out above requiring only a single digital to analog converter.