Abstract:
A secondary winding of a horizontal flyback transformer of a horizontal deflection circuit develops a horizontal retrace pulse voltage. A secondary winding of a second transformer is coupled in series with a vertical deflection coil of a vertical deflection circuit. An R-C filter is coupled between the secondary winding of the flyback transformer and a primary winding of the second transformer. Horizontal parallelogram errors are corrected by a horizontal rate current injected in a current path of the vertical deflection coils. The R-C filter prevents the vertical deflection current from being parasiticaly coupled to the horizontal deflection circuit.

Description:
This Application claims Benefit of provisional Application Ser. No. 60/115,709 filed Jan. 12, 1999. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to raster correction circuits of a video display. 
     BACKGROUND 
     In a cathode ray tube (CRT) of a video display, a raster is formed by deflecting an electron beam across a phosphor screen. Each electron beam is deflected in a horizontal direction by a magnetic field produced by in a horizontal deflection coil by a horizontal-rate sawtooth current. Likewise, the electron beam is simultaneously deflected in a vertical direction by a magnetic field produced by a vertical deflection coil by a vertical-rate sawtooth current. The result is a negatively-sloped, or “downhill”, scan line as the electron beam is deflected from left to right to form the CRT&#39;s raster. In a typical cathode ray tube used in a color television receiver and having, for example, a screen width of approximately 723 mm and a screen height of approximately 538 mm, a horizontal scan line may drop a distance of approximately 2.4 mm from a perfectly horizontal position in one field. This downhill scan effect introduces both orthogonality and parallelogram errors into the raster. 
     In a perfectly rectangular raster, horizontal and vertical center lines are orthogonal, or perpendicular, to one another. Downhill scanning does not produce a perfectly rectangular raster and hence results in a non-orthogonal relationship between the horizontal and vertical center lines of the raster. Orthogonality error is a quantitative measure, expressed in units of radians or degrees, of the extent to which the horizontal and vertical center lines of a raster depart from orthogonality. The orthogonality error may be magnified at the left and right edges of the raster because the deflection sensitivity increases near the edges of the raster. As a result, the edges of the raster may tilt such that the raster has a generally parallelogram shape. errors in a raster can be obtained by providing a horizontal-rate modulation of a vertical deflection current for substantially offsetting the downhill scan effect caused by vertical deflection of the electron beam. In one of the circuits shown 
     Elimination of both orthogonality and parallelogram errors in a raster can be obtained by providing a horizontal-rate modulation of a vertical deflection current for substantially offsetting the downhill scan effect caused by vertical deflection of the electron beam. A winding of a horizontal flyback transformer can be used to apply a horizontal retrace pulse voltage to a primary winding of a transformer. A secondary winding of the transformer can be coupled to a vertical deflection winding for providing a small horizontal rate sawtooth current to be superimposed on a vertical deflection current. 
     Coupling back of the vertical current to the horizontal deflection circuit is reduced by the relatively large leakage of the transformer. Nevertheless, the residual vertical rate current, during vertical retrace, can still produce a disturbance at the top of the screen, immediately after vertical retrace. It may be desirable to further reduce the coupling back of the vertical current to the horizontal deflection circuit. 
     In carrying out an inventive feature, an R-C filter is interposed in a current path between the transformers. The R-C filter attenuates the coupled back vertical deflection current. Thereby, the addition of the R-C coupling filter prevents the vertical deflection current from affecting the horizontal deflection circuit. 
     SUMMARY OF THE INVENTION 
     A video display deflection apparatus, embodying an inventive feature, includes a first deflection circuit for generating a first deflection current at a first deflection frequency in a first deflection winding to vary a position of an electron beam in a first direction. A second deflection circuit is used for generating a second deflection current in a second deflection winding at a second deflection frequency to vary the position of the electron beam in a second direction. A filter couples the second deflection circuit to the first deflection winding to generate a corrective current in a current path formed by the first deflection winding at a frequency related to the second deflection frequency for providing raster error correction. The filter significantly attenuates parasitic signal coupling in an opposite direction, from the first deflection circuit to the second deflection circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an arrangement for correcting orthogonality and parallelogram errors in a raster, including a filter, in accordance with an inventive feature; 
     FIGS. 2 a  and  2   b  illustrate waveforms useful for explaining the operation of the deflection system shown in FIG. 1, when the filter is employed; and 
     FIGS. 3 a  and  3   b  illustrate waveforms useful for explaining the operation of the deflection system shown in FIG. 1, when the filter is removed. 
    
    
     DETAILED DESCRIPTION 
     A deflection system  100  of FIG. 1 provides deflection for a cathode ray tube, not shown, of a television receiver or a video display terminal. A B +  voltage is coupled to a conventional horizontal deflection circuit  20  through a primary winding L PRI  of a flyback transformer IHVT. A damper current I D  flows through a damper diode D 1  to deflect an electron beam from a left edge of a raster to a center of the raster. A horizontal output transistor Q 1  conducts a current I HOT  to deflect the electron beam from the center of the raster to a right edge of the raster. A horizontal deflection current I H  flowing through a horizontal deflection winding L H  may have a peak-to-peak amplitude of approximately 12 A. A trace capacitor C S , coupled in series with deflection winding L H  provides S-correction for the horizontal deflection current I H . 
     A secondary winding L SEC  of flyback transformer IHVT is coupled via an R-C filter  40 , embodying an inventive feature, to a primary winding  42  of a raster correction transformer  41 . Transformer  41  has a secondary winding  43 . Transformer  41  is wound on a ferrite slug core 1″ long×0.399″ diameter. Winding  43  has N S =60 turns, 5-strand Litz AWG#30 wire, and winding  42  has N P =180 turns, AWG#29 wire. 
     A horizontal-rate retrace pulse, not shown, produced in a conventional manner in deflection circuit  20 , is transformer-coupled to secondary winding L SEC  of transformer IHVT to develop a horizontal-rate retrace pulse  12 . Retrace pulse  12  is coupled via R-C filter  40 , embodying an inventive feature, to winding  42  of transformer  41 . Transformer  41  steps down a significant portion of horizontal-rate pulse  12  coupled through R-C filter  40  and developed in winding  42  according to transformer  41  turns ratio. Raster correction transformer  41  develops a stepped-down horizontal-rate pulse waveform  11  with a peak-to-peak voltage of approximately 50 Vpp across secondary winding  43 . Similarly, a horizontal raster correction current I CORR  is induced in secondary winding  43 . 
     A direct current (DC) coupled vertical deflection circuit  60  includes a conventional vertical-rate sawtooth generator  61  that provides a vertical-rate sawtooth waveform to a non-inverting input of a conventional vertical output amplifier  62 . Vertical output amplifier  62  may include a push-pull transistor output stage, not shown. Vertical output amplifier  62  drives a vertical deflection windings L V1  and a vertical deflection windings L V2 , coupled in series, with a vertical-rate sawtooth current I V . Current I V  may have a peak-to-peak amplitude of approximately A. (2.6 App) 
     Vertical deflection windings L V1  and L V2  are also coupled in series with winding  43  of transformer  41  and with resistor R 4 . Current-sense resistor R 4  generates a feedback voltage at an inverting input of vertical output amplifier  62  responsive to the vertical deflection current I V . Except for the modulation provided by raster correction current I CORR  induced in secondary winding  43 , vertical deflection circuit  60  generates current I V  in a conventional manner. Horizontal rate raster correction current I CORR  flows through both vertical deflection windings L V1  and L V2  to produce a magnetic field which opposes the aforementioned downhill scan effect. 
     For explanation purposes, assume that filter  40  is not used. Instead, assume that winding L SEC  of high-voltage transformer IHVT is coupled directly in parallel with winding  42  of transformer  41 , as shown by a jumper conductor  40   a.    
     Vertical deflection current I V  flows through secondary winding  43  of transformer  41 . During vertical retrace, a vertical pulse voltage V V  of FIG. 3 b , developed across windings L V1  and L V2  of FIG. 1, produces a vertical rate current component in a current  142  of winding  42  of transformer  41 . Vertical rate modulation of current  142  of FIG. 3 a , during the retrace portion of vertical pulse voltage V V  of FIG. 3 b , shifts the average value of current  142  in a vertical rate. Similar symbols and numerals in FIGS. 1,  3   a  and  3   b  indicate similar items or functions. 
     The vertical rate current component in current  142  of FIG. 1 may be coupled back to horizontal deflection circuit  20  via transformer IHVT and, disadvantageously, may initiate ringing in horizontal deflection winding L H . A resulting width disturbance can become visible on the display screen, not shown. 
     In carrying out an inventive feature, the coupling back from the vertical to the horizontal is reduced or eliminated by the addition of R-C filter  40  between winding L SEC  of transformer IHVT and winding  42  of transformer  41 . This situation is demonstrated, when jumper conductor  40   a  in FIG. 1 is removed and filter  40  is interposed. Capacitor C of filter  40  forms a low impedance for horizontal rate current component of current  142 . Therefore, Capacitor C of filter  40  does not attenuate the horizontal rate current component of current  142 . On the other hand, for the vertical rate current component of current  142 , capacitor C forms a high impedance and acts as an attenuator. Thereby, coupling back, is advantageously, attenuated significantly. 
     The waveform of primary current  142  when R-C filter  40  is in circuit is shown in FIG. 2 a . In contrast to the waveform in FIG. 3 a , vertical deflection current I V  of FIG. 2 b , during vertical retrace, advantageously, does not produce any significant vertical rate current component in current  142  of FIG. 2 a . Similar symbols and numerals in FIGS. 1,  3   a ,  3   b ,  2   a  and  2   b  indicate similar items or functions. The elimination of the parasitic, back coupling effect in current  142  of FIG. 2 a  from current I V  of FIG. 2 b , advantageously, eliminates the width artifact at the start of vertical scan. 
     A damping circuit  60  is formed by a resistor R 1  and a capacitor C 1 , coupled in series. Circuit  60 , is coupled between a center tap  21 , approximately in the midpoint of vertical deflection windings L V1 , and a center tap  21 , approximately, in the midpoint of vertical deflection windings L V2 . 
     The effectiveness of the injection of parallelogram/orthogonality error correction current ICORR by winding  43  at an end terminal  43   a  of the vertical deflection windings Lv V1  and L V2 , that is remote from amplifier  62 , is facilitated by installing damping circuit  60  formed by resistor R 1  and capacitor C 1 . Damping circuit  60  increases the sensitivity of windings L V1  and L V2  to correction current I CORR . Consequently, single ended drive is sufficient.