MXPA97002045A - Horizontal deflexion circuit with rast correction - Google Patents

Abstract

The present invention relates to an electron beam tending to tilt downward as it is deflected horizontally to form a box in a video display apparatus. The inclination of the beam can cause geometric errors in the box, for example, orthogonality and parallelogram errors. A box correction circuit compensates for orthogonality and parallelogram errors in a box by modulating a horizontal deflection current in a vertical scan rate. The correction current of the frame is phased in relation to a deflection current of horizontal regime so that the sweep lines in a portion of the upper half of the frame are changed to the right and the sweep lines in a portion of the lower half of the box are changed to the left

Description

HORIZONTAL DEFLECTION CIRCUIT WITH BOX CORRECTION This invention relates generally to the field of box correction circuits, and in particular, to a circuit used to correct orthogonality and parallelogram errors in a frame of a cathode ray tube. video display apparatus. In a cathode ray tube (CRT) of a video display apparatus, a box is formed by deflection of at least one electronic beam through a phosphor screen. Each electronic beam is deflected in a horizontal direction by a magnetic field produced by the excitation of a horizontal deflection coil by means of a horizontal regime saw current. Likewise, each electronic beam is simultaneously deflected in a vertical direction by a magnetic field produced by the excitation of a vertical deflection coil by a vertical rated saw current. The result is a negatively sloped sweep line, or "slope", as the electron beam deviates from left to right to form the CRT box. In a normal cathode ray tube used in a color television receiver and having a screen width of approximately 723 mm and a screen height of approximately 538 mm, a horizontal scanning line may fall a distance of approximately 2.4 mm from a perfectly horizontal position in a field.

This declining sweep effect introduces errors of both orthogonality and parallelogram in the box, as shown in Figure 1. In a perfectly rectangular box, the horizontal and vertical center lines are orthogonal, or perpendicular to one another. The declining scan does not produce a perfectly rectangular box and therefore results in a non-orthogonal relationship between the horizontal and vertical center lines of the box. The orthogonality error is a quantitative measure, expressed in units of radians or degrees, of the degree to which the horizontal and vertical center lines of a box move away from the orthogonality. For a box represented in terms of X and Y coordinates, as described in Figure 2, the orthogonality error can be calculated with the following trigonometric formula: X12-X6 Y3-Y9 tan-1 () + tan- 1 () Y 12-Y6 X3-X9 A conventional slope sweep can produce an orthogonality error in the order of approximately 0.2 °. A normal design tolerance for the orthogonality error in a CRT can be specified as + 0.3 °. The orthogonality error can be increased at the left and right edges of the box since, as is well known, the deflection sensitivity of an electron beam is increased as Mega moves to one edge of the box. As a result, the edges of the box can be tilted so that the box has a generally parallelogram shape. The parallelogram error is a quantitative measurement, expressed in units of radians or degrees, to the degree to which the configuration of a box approaches a parallelogram. For a box represented in terms of X and Y coordinates, as described in Figure 2, the vertical parallelogram error can be calculated with the following trigonometric formula X10-X8 X2-X4 tan- 1 () + tan- 1 () Y10-Y8 Y2-Y4 Y3-Y9 - + tan-1 () 2 X3-X9 The horizontal parallelogram error can be calculated with the following trigonometric formula : Y2-Y10 Y4-Y8 tan- 1 () + tan- 1 () X2-X10 X4-X8 Y3-Y9 tan-1 (). 2 X3-X9 In a conventional slope sweep, a normal orthogonality error can be translated into a parallelogram error that is in the order of approximately 1.5 times the orthogonality error. For example, a conventional slope sweep that produces an orthogonality error of 0.2 ° can also produce a parallelogram error that is equal to approximately 0.3 °. A normal design tolerance for the parallelogram error in a CRT, it can be specified as + 0.5 °. If means are employed to correct pinhole error from side to side or from east to west in a box, the declining sweep effect may cause a misalignment of a pincushion correction current cover with respect to the pincushion curvature in the box. Mitigating this misalignment can result in an increase in the parallelogram error by an amount that can be equal to approximately 80%. Therefore, for a conventional slope sweep that produces a parallelogram error equal to approximately 0.3 °, the use of lateral pincushion correction can increase the parallelogram error to approximately 0.6 °. It is convenient to completely eliminate both orthogonality and parallelogram errors in a box so that a CRT can display the image with the highest quality. A possible solution requires rotation of the horizontal deflection coil in relation to the vertical deflection coil in order to align the inclined centerline of the frame with the horizontal centerline of the CRT. Therefore the declining sweep effect is eliminated, but, however, this approach can be problematic. First, this solution can affect the convergence in the video display apparatus. Second, as the central horizontal line inclined towards the central line of the CRT is rotated, the pin cushion in the box also rotates in order to maintain its original relationship with the inclined horizontal center line. Therefore, while this solution can eliminate the orthogonality error, it is not directed to the parallelogram error component due to misalignment of the pinch correction current cover with respect to the pin cushion curvature in the box. A deflection circuit with box correction according to the inventive arrangement taught herein, provides vertical regime modulation of a horizontal deflection current in a deflection coil in a cathode ray tube to correct orthogonality and parallelogram errors in the frame . Said deflection circuit comprises: a deflection coil for forming a box; and, means for generating a corrective current coupled with the deflection coil to stabilize a substantially orthogonal relationship between the left and right side edges of the frame, respectively, and a horizontal axis passing through a geometric center of the frame. The generating means can establish a substantially orthogonal relationship between the horizontal and vertical axes that pass through a geometric center of the box. The corrective current can have a vertical sweep regime and a substantial saw shape. According to an aspect of an inventive arrangement taught herein, a deflection circuit for a video display apparatus comprises, a deflection coil for generating a frame responsive to a deflection current; and means for modulating the deflection current to laterally change a plurality of scan lines. The deflection current can be modulated at a vertical sweep rate. Those of the plurality of scan lines in an upper portion of the box can be changed to a first side edge of the box, and those of the plurality of scan lines in a lower portion of the box can be changed to a second side edge. of the box opposite the first lateral edge. The change of the plurality of scan lines can result in a substantially orthogonal relationship between the first and second side edges of the box, respectively, and a horizontal axis passing through a geometric center of the box. The change of the plurality of scan lines can also result in a substantially orthogonal relationship between the horizontal and vertical axes passing through a geometric center of the box. The modulation means may comprise first and second active devices, each having first and second terminals coupled, respectively, to the first and second terminals of the horizontal deflection coil, wherein the first and second active devices modulate the deflection current for different portions of each period of vertical sweeping. One of the active devices can be biased to drive completely during a period when the other active devices are modulating the deflection current. The modulation of the deflection current can be achieved by linearly varying a conductivity of the other of the active devices during a portion of a vertical scan interval. A horizontal deflection system for a video display apparatus incorporating an inventive teaching arrangement of the present invention comprises: a horizontal deflection coil for generating a box; a centering network coupled in parallel with the deflection coil for placing an electron beam in a geometric center of the frame, the centering network comprising a series interconnection of a centering coil and a centering capacitor; and, first and second active devices for generating a voltage through the centering capacitor, wherein the voltage obtains a minimum peak magnitude near a vertical center of the box and a maximum peak amount at the top of the lower edges of the box. A time between corresponding peak magnitudes of the voltage can be equal to about a vertical sweep period. The voltage can supply the horizontal deflection coil with a corrective current in one direction to stabilize a substantially orthogonal relationship between the left side edges of the box, respectively, and a horizontal axis passing through a geometric center of the frame. The voltage can also supply the horizontal deflection coil with a corrective current in one direction to establish a substantially orthogonal relationship between the horizontal and vertical axes passing through a geometric center of the frame. The corrective current may have a substantially saw shape. The above and other aspects and advantages of the present invention, they will be evident from the following reading of description together with the attached drawings, in which the reference numbers designate the same elements. Figure 1 shows a box that has orthogonality and parallelogram errors. Figure 2 describes a box of a cathode ray tube in terms of X and Y coordinates. Figure 3 shows a conventional horizontal deflection circuit. Figure 4 shows voltage and current waveforms associated with the conventional horizontal deflection circuit of Figure 3. Figure 5 shows a horizontal deflection system having an inventive arrangement described herein. Figure 6 shows voltage waveforms associated with an inventive arrangement described herein. Figures 7 and 8 show equivalent circuits useful for describing the operation of the horizontal deflection system of Figure 5. Figure 9 shows a voltage waveform associated with an inventive arrangement described herein.

Figure 10 shows useful voltage waveforms to describe an aspect of the inventive arrangement described herein. Figure 11, shows current waveforms associated with an inventive arrangement described herein. A conventional horizontal deflection circuit 100 is shown in Figure 3, and its associated voltage and current waveforms are shown in Figure 4. The current flow is defined as positive in the directions indicated in Figure 3. reference to Figures 3 and 4, a voltage is printed B + of 140 Ved through the correction capacitor-S CS through a primary winding LPRI of a high-voltage transformer T1. As an electron beam is deflected to a corner at the top left of a box, a horizontal output transistor Q 1 does not conduct a current. The energy previously stored in the horizontal deflection coil LH causes a current to flow through the damping diode D1 driven outwards and a horizontal deflection coil LH and into the correction capacitor-S CS. At this point, both the damping current I D and the horizontal deflection current I H obtain their negative peak values. When the electron beam reaches the center of the box, the energy stored in the horizontal deflection coil LH has decayed to zero and the horizontal deflection current I H and the damping current I D are equal to approximately zero. The damping diode D1 is inversely driven and the horizontal oscillator circuit 10 causes the horizontal output transistor Q1 to conduct a current IHOT. The horizontal deflection current IH reverses the direction and the energy fed to the horizontal deflection coil LH by the correction capacitor-S CS, allows the horizontal deflection current I H to be linearly increased. When the electron beam reaches the right edge of the box, the horizontal oscillator circuit 10 causes the horizontal output transistor Q 1 to remain polarized inversely. During this recoil interval, the decaying horizontal deflection current I H flows rapidly in the return capacitor CR. When the horizontal deflection current IH decays to approximately zero, it reverses the direction and is then fed by the back-off capacitor CR. Once the recoil capacitor CR has discharged its stored energy through the horizontal deflection coil LH, the electron beam has returned to the upper left corner of the box and the process is repeated. In a box that has a negative orthogonality error, as shown in Figure 1, the box is usually in the form of a parallelogram, with the lines in the upper half of the box changed to the left and the lines in the lower half of the box changed to the right. A generally rectangular box can be obtained from the generally paralleled box of Figure 1 by suitably changing the scan lines in the upper and lower frame halves. For example, in the box generally in the parallelogram of Figure 1, the lines in the upper half of the box can be changed to the right and the lines in the lower half of the box can be changed to the left. An appropriate change of the lines in the respective halves of a box can be achieved by modulating the horizontal deflection current IH with a correction current of the vertical regimen box IO / P as the electron beam is deflected to sweep the box . Referring to Figures 4 (a), 4 (c) and 4 (d), the portion of the trace of the horizontal deflection current IH is the sum of the current of dampers ID, which flows through the damping diode D 1 as the electron beam goes from the left edge of the box to its center and the I HOT current, which flows through the horizontal output transistor Q1 as the electron beam sweeps from the center of the box to its right edge. Thus, a box generally configured in a parallelogram of the type shown in Figure 1, generally indicates that, in the upper half of the box, the electron beam is disproportionately deflected by the current flowing from the horizontal deflection coil LH in the direction of the damping current I D. Similarly, in the lower half of the box, the electron beam is disproportionately deflected by the current flowing through the horizontal deflection coil LH in the direction of the current I HOT. Therefore, in order to generate a generally rectangular box, the correction current of the IO / P box must flow through the horizontal deflection coil LH in the same direction as the I HOT current in the upper half of the box and must flow through the horizontal deflection coil LH in the same direction as the damping current ID in the lower half of the box. In a horizontal deflection system 300, shown in Figure 5, a box correction circuit 200 may be coupled to the horizontal deflection circuit 100 in order to appropriately change the scan lines in the upper and lower halves of a frame generally in parallelogram to produce a generally rectangular box. Referring to Figure 5, the correction circuit of the frame 200 is coupled to the horizontal deflection coil LH through a frame centering network comprising the LC inductor and the DC capacitor. The LC inductor usually has a higher inductance and therefore conducts a peak-to-peak current, lower than that of the horizontal deflection coil LH. The vertical-rated saw voltage waveforms 210 and 21 1, shown in Figure 6, drive the transistors Q2 and Q3 respectively, so that a vertical rate tracking current IO / P flows into the box correction circuit 200. Sawtooth waveforms 210 and 21 1 may be generated by conventional means not described herein.

Referring to Figures 5 and 6, in a top middle portion of the box, the saw-shaped waveform 210, linearly modulates the short-circuit transistor Q2 to saturation while the saw waveform 21 1 causes the signal to saturate. transistor Q3. The operation of the horizontal deflection system 300 can therefore be explained with reference to an equivalent horizontal deflection system 300 ', shown in Figure 7 (a), where the parallel combination of the transistor Q2 and the resistor R2 is represented by a variable resistor R EQ2, and transistor Q3 is represented by a closed switch SW3. Voltage polarities and current flows are defined as positive in the direction indicated in Figure 7 (a). Referring to Figures 7 (a) and 7 (b), during a negative portion of horizontal deflection current IH, which corresponds to the flow of damping current ID through the horizontal deflection coil LH and, therefore, to the deflection of the electron beam from the left edge to the center of the frame, a negative portion of the centering current IC of horizontal flow flows through the centering inductor LC. The diode D2 is driven in an inverted manner, the diode D3 is driven forward and the horizontally centering current IC loads the correction capacitor-S CS through the diode D3 and the switch SW3. A small positive voltage V'C, subject to approximately the sum of the forward voltage drop of the diode D3 and the saturation voltage of the collector to the emitter of the transistor Q3, is also established through the centering capacitor CC. As the electron beam reaches the center of the box, the horizontal deflection current IH reverses the direction and becomes positive, which corresponds to the flow of current I HOT through the horizontal deflection coil LH and, therefore, , to the deflection of the electron beam from the center to the right edge of the box. The horizontal centering current IC also becomes positive. The diode D2 is now propelled forward, the diode D3 is now driven inversely and a horizontal regime current flows through the variable resistor REQ2 and the diode D2. The centering voltage V'C becomes negative and is equal to approximately the voltage VREQ2 generated through the variable resistor REQ2. The successive negative peaks of the centering voltage V'C decrease in magnitude because the saw waveform 210 drives the transistor Q2 close to saturation as the successive horizontal lines are swept in the upper half of the frame. Therefore, the variable resistor REQ2, and consequently, the magnitude of the voltage generated through the variable resistor REQ2, decreases as the electron beam reaches the center of the vertical interval. This corresponds to the fact that, as shown in Figure 1, the errors of orthogonality and parallelogram become less severe, so that less correction is required, as the electron beam sweeps the upper half of the box. The successive decrease in the magnitudes of the negative peaks of the centering voltage V'C produces a vertical-rate voltage ramp V'ramp, as shown in Figures 7 (b) and 9. The voltage ramp V'ramp, generates the IO / P correction current through the horizontal deflection coil LH in the same direction as the I HOT current. Therefore, the magnetic field induced in the horizontal deflection coil LH by the correction current of the IO / P box advantageously supplements the magnetic field induced in the coil by the current I HOT during the deflection of the electron beam from the center to the edge right of the box. The sweep line thus tends to deviate further to the right. In a portion of the lower half of the box, the roles of transistors Q2 and Q3 are inverted. The waveform of saw 210, causes the transistor Q2 to saturate while the saw waveform 21 1 linearly modulates the short circuit saturation transistor Q3. The horizontal deflection system, therefore, can be represented by a horizontal deflection system 300 ', shown in Figure 8 (a), where the transistor Q2 is represented by a closed switch SW2 and the parallel combination of the transistor Q3. and the resistor R3 is represented by a variable resistor REQ3. Voltage polarities and current flows are defined as positive in the direction indicated in Figure 8 (a).

Referring to Figures 8 (a) and 8 (b), during a negative portion of the horizontal deflection current IH, which corresponds to the flow of damping current ID through the horizontal deflection coil IH and, therefore, Upon deflection of the electron beam from the left edge to the center of the frame, a negative portion of the horizontal flow centering current Cl flows through the centering inductor LC. Diode D2 is driven inversely, diode D3 is driven forward and horizontal centering current IC loads the correction capacitor-S CS through diode D3 and variable resistor REQ3. A positive centering voltage V "C, is established through the centering capacitor CC and is equal to approximately the voltage VREQ3 generated through the variable resistor REQ 3. As the electron beam reaches the center of the box, the current of horizontal deflection IH reverses the direction and becomes positive, which corresponds to the flow of current I HOT through the horizontal deflection coil LH, and therefore, to deflect the electron beam from the center to the right edge of the frame. The horizontal centering current IC is also driven inversely and a horizontal current flows through switch SW2 and diode D2.The centering voltage V "C becomes negative and is set to approximately the sum of the voltage drop to forward of the diode D2 and the saturation voltage of the emitter collector ai of the transistor Q2.

The successive positive peaks of the centering voltage V "C increase in magnitude because the saw current waveform 21 1 drives the transistor Q3 linearly to short circuit as the successive horizontal lines are swept in the lower half of the frame Therefore, the variable resistor REQ3, and consequently, the magnitude of the voltage generated through the variable resistor REQ3, increases as the electron beam reaches the lower part of the vertical interval.This corresponds to the fact that, As shown in Figure 1, the errors of orthogonality and parallelogram become more severe, so more correction is required, as the electron beam sweeps the lower half of the box The successive increase in the magnitudes of the peaks positive of the centering voltage V "C produces a voltage ramp of vertical Vramp, as shown in Figure 8 (b) and 9. The voltage ramp V 'ramp generates IO / P correction current through the horizontal deflection coil LH in the same direction as the current I D. Therefore, the magnetic field induced in the horizontal deflection coil LH by the correction current of the inset IO / P advantageously supplements the magnetic field induced in the coil by the current ID during the deflection of the electron beam from the left edge to the center of the frame. Therefore, the sweep line tends to deviate further to the left.

The diodes D2 and D3 in the correction circuit of box 200 may be used as necessary to protect transistors Q2 and Q3 from damage or destruction due to overvoltage stresses that may occur during the transition of state transistors Q2 and Q3. from driving to non-drivers. The resistors R2 and R3 are components of the variable resistors REQ2 and REQ3, respectively, of FIGS. 7 (a) and 8 (a), respectively. Consequently, the values of resistors R2 and R3 affect the characteristics of vertical voltage voltage ramps V'ramp V'ramp, respectively. A horizontal centering of the electron beam in the geometric center of the frame, can therefore be effected by an appropriate choice for the values of resistors R2 and R3, provided that the value of the centering inductor LC is sufficiently low to supply a current required. The horizontal focusing of the electron beam can also be adjusted by appropriately polarizing the vertical-wave saw waveforms 210 and 21 1 to choose a particular point that crosses to zero, in which transistors Q2 and Q2 reverse the roles. For example, Figure 9 (a) shows saw waveforms 210 and 21 1 arranged to show a point crossing zero. Figure 9 (b) shows the saw waveforms 210 'and 21 1' after applying a negative polarization of de to change a point crossing zero towards the right edge of the box. Figure 9 (c) shows saw waveforms 210"and 21 1" after applying a negative polarization of de to change a point that crosses zero towards the left edge of the box.

If the negative polarization of either the saw waveforms 210 or 211 by an amount exceeding both the peak amplitude -VSAW and + VSAW, respectively, only one of the transistors Q2 or Q3 of the box correction circuit 200 can modulate the correction current of the IO / P box during a vertical scan interval; the other transistor remains in saturation during the entire vertical scan interval. For example, if the saw waveform 211 is a voltage of greater magnitude than + VSA, the transistor Q2 becomes saturated during the entire vertical scan interval and the correction current of the IO / P box is modulated only by the transistor Q3. At the other end, if the saw waveform 210 is a voltage of greater magnitude than -VSAW, the transistor Q3 is saturated throughout the vertical sweep interval and the correction current of the IO / P box is modulated only by transistor Q2. The foregoing description of horizontal deflection system 300 demonstrates the beneficial effect provided by the correction circuit of box 200 for a box having orthogonality and parallelogram errors inherently associated with the declining sweep effect. In a portion of the upper half of the box, the correction current of the IO / P box modulates the horizontal deflection current IH to advantageously extend the deflection of the electron beam further toward the right edge of the box. In a portion of the lower half of the box, the correction current of the IO / P box modulates the horizontal deflection current I H to advantageously extend the deflection of the electron beam further towards a left edge of the box. Referring to Figure 10, the net effect on this modulation on a field is that a total current through a horizontal deflection coil LH is biased as a superposition of the horizontal deflection current IH of larger amplitude and the current of correction of the IO / P box of smaller amplitude.

Claims (10)

1. REVIVAL NAME 1. A deflection circuit for a video display apparatus, said circuit comprising: a deflection coil (LH) to form a box; characterized by: means (200) for generating a corrective current coupled to said deflection coil to establish a substantially orthogonal relationship between the left and right lateral edges of said box, respectively and a horizontal axis passing through a geometric center of said box.
2. 2. The deflection circuit of claim 1, characterized in that said generating means (200) establish a substantially orthogonal relationship between the horizontal and vertical axes passing through a geometric center of said frame.
3. 3. The deflection circuit of claim 1, characterized in that said corrective current has a vertical sweep rate.
4. 4. The deflection circuit of claim 3, characterized in that said corrective current has a substantially saw shape.
5. The deflection circuit of claim 1, wherein said means for generating are characterized by a parallel arrangement of the first (Q2) and second (Q3) active devices coupled in parallel with said deflection coil so that each of said first and second active devices modulates said deflection current during a different portion of said vertical sweeping period to laterally change a plurality of sweeping lines of said frame.
6. The deflection circuit of claim 5, characterized in that said first (Q2) and second (Q3) active devices are arranged such that said first and second active devices provide conduction in opposite directions.
7. The deflection circuit of claim 6, characterized in that each of said first (Q2) and second (Q3) active devices are coupled in parallel with an impedance.
8. The deflection circuit of claim 7, characterized in that one of said active devices is biased to drive completely during a period when the other of said active devices modulates said deflection current (IH).
9. 9. The deflection circuit of claim 8, characterized in that the modulation of said deflection current (IH) is achieved by linearly varying a conductivity of said other active devices during a portion of a vertical scan interval. The deflection circuit of claim 9, characterized in that those of said plurality of scan lines in an upper portion of said box are changed to a first side edge of said box and those of a plurality of scan lines in a portion of said box. The lower part of said frame is shifted towards a second lateral edge of said box opposite said first lateral edge. RESU M IN A beam of electrons tends to tilt downward as it deviates horizontally to form a box in a video display apparatus. The inclination of the beam can cause geometric errors in the box, for example, orthogonality and parallelogram errors. A box correction circuit compensates for orthogonality and parallelogram errors in a box by modulating a horizontal deflection current in a vertical sweep regime. The correction current of the box is phased in relation to a deflection current of horizontal regime so that the scan lines in a portion of the upper half of the box are changed to the right and the scan lines in a portion of the lower half of the box are changed to the left.