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'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.

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 FIGURE 1 illustrates an arrangement for correcting orthogonality and parallelogram errors in a raster, including a filter, in accordance with an inventive feature; FIGURES 2a and 2b illustrate waveforms useful for explaining the operation of the deflection system shown in FIGURE 1, when the filter is employed; and FIGURES 3a and 3b illustrate waveforms useful for explaining the operation of the deflection system shown in FIGURE 1, when the filter is removed.

DETAILED DESCRIPTION A deflection system 100 of FIGURE 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 LORI of a flyback transformer IHVT. A damper current ID flows through a damper diode D1 to deflect an electron beam from a left edge of a raster to a center of the raster. A horizontal output transistor Q1 conducts a

current IHOT to deflect the electron beam from the center of the raster to a right edge of the raster. A horizontal deflection current IH flowing through a horizontal deflection winding LH may have a peak-to-peak amplitude of several amperes. A trace capacitor Cs, coupled in series with deflection winding LH provides S-correction for the horizontal deflection current IH.

A secondary winding LSEC 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" (2.54 cm) long x 0.399" (10.13 mm) diameter. Winding 43 has Ns=60 turns, 5-strand Litz AWG#30 (0.2546 mm) wire, and winding 42 has Nu=180 turns, AWG#29 (0.2859 mm) wire.

A horizontal-rate retrace pulse, not shown, produced in a conventional manner in deflection circuit 20, is transformer-coupled to secondary winding LSEC 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 40V p-p across secondary winding 43. Similarly, a horizontal raster correction current ICORR 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 pus-pull transistor output stage, not shown. Vertical output amplifier 62 drives vertical deflection windings Lv1 and LV2, coupled in series, with a vertical-rate sawtooth current Vertical deflection windings LV1 and LV2 are also coupled in series with winding 43 of transformer 41 and with resistor R4. Current-sense resistor R4 generates a feedback voltage at an inverting input of vertical output amplifier 62 responsive to the vertical deflection current tV. Except for the modulation provided by raster correction current ICORR induced in secondary winding 43, vertical deflection circuit 60 generates current IV in a conventional manner.

Horizontal rate raster correction current ICORR flows through both vertical

deflection windings LV1 and LV2 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 LSEC of high-voltage transformer IHVT is coupled directly in parallel with winding 42 of transformer 41, as shown by a jumper conductor 40a.

Vertical deflection current lv flows through secondary winding 43 of transformer 41. During vertical retrace, a vertical pulse voltage Vv of FIGURE 3b, developed across windings Lv, and LV2 of FIGURE 1, produces a vertical rate <BR> <BR> <BR> current component in a current I42 of winding 42 of transformer 41. Vertical<BR> <BR> <BR> <BR> <BR> rate modulation of current I42 of FIGURE 3a, during the retrace portion of vertical<BR> <BR> <BR> <BR> <BR> pulse voltage Vv of FIGURE 3b, shifts the average value of current I42 in a vertical rate. Similar symbols and numerals in FIGURES 1,3a and 3b indicate similar items or functions.

The vertical rate current component in current I42 of FIGURE 1 may be coupled back to horizontal deflection circuit 20 via transformer IHVT and, disadvantageously, may initiate ringing in horizontal deflection winding LH. 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 LSEC of transformer IHVT and winding 42 of transformer 41. This situation is demonstrated, when jumper conductor 40a in FIGURE 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 FIGURE 2a. In contrast to the waveform in FIGURE 3a, vertical deflection current IV of FIGURE 2b, during vertical retrace, advantageously, does not produce any significant vertical rate current component in current 142 of FIGURE 2a. Similar symbols and numerals in FIGURES 1,3a, 3b, 2a and 2b indicate similar items or functions. The elimination of the parasitic, back coupling effect in current 142 of FIGURE 2a from current IV of FIGURE 2b, advantageously, eliminates the width artifact at the start of vertical scan.

A damping circuit 60 is formed by a resistor R1 and a capacitor C1, coupled in series. Circuit 60, is coupled between a center tap 21, approximately in the midpoint of vertical deflection windings LV1, and a center tap 21, approximately, in the midpoint of vertical deflection windings LV2.

The effectiveness of the injection of parallelogram/orthogonality error correction current ICORR by winding 43 at an end terminal 43a of the vertical deflection windings L and L, that is remote from amplifier 62, is facilitated by installing damping circuit 60 formed by resistor R1 and capacitor C1. <BR> <BR> <BR> <P>Damping circuit 60 increases the sensitivity of windings Lvl and LV2 to<BR> <BR> <BR> <BR> correction current ICORR. Consequently, single ended drive is sufficient.