HORIZONTAL PARALLELOGRAM CORRECTION COMBINED WITH HORIZONTAL CENTERING

Abstract: An electron beam tends to slope downwardly as it is deflected horizontally to form a raster in a video display apparatus. The sloping of the beam can cause geometric errors in the raster, for example orthogonality and parallelogram errors. A raster correction circuit substantially offsets the downward slop of the electron beam by modulating a vertical deflection current (IV) with an induced horizontal-rate raster correction current, thereby substantially eliminating orthogonality and parallelogram errors in the raster. A raster correction transformer (41) utilizes a raster centering inductor (LC) for a primary winding, and a horizontal-rate centering current (IC) is magnetically coupled into the vertical deflection coils (LV1, LV2) to modulate the vertical deflection current.

Description: HORIZONTAL PARALLELOGRAM CORRECTION COMBINED WITH HORIZONTAL CENTERING BACKGROUND This invention relates generally to the field of raster correction circuits, and, in particular, to correction of orthogonality and parallelogram errors in a raster of a cathode-ray tube of a video display apparatus. A deflection system utilized in a video display apparatus, such as a television receiver or a video display monitor, typically includes circuitry that allows for the adjustment of a raster on the viewing screen of the apparatus's cathode-ray tube. Such circuitry is required because of, among other things, the nature of the scanning process and the geometry of the cathode-ray tube. For example, such circuitry may include a raster correction circuit for eliminating orthogonality and parallelogram errors in the scanned raster. The nature of the orthogonality and parallelogram errors and an approach to eliminating both of them is described in a U.S. patent application entitled "VERTICAL DEFLECTION CIRCUIT WITH RASTER CORRECTION", which was filed on May 17, 1996, in the name of Walter Truskalo et al., and was assigned Serial No. 08/649,409. That application discloses an arrangement for modulating a vertical deflection current at a horizontal rate for substantially offsetting a downhill scan effect caused by vertical deflection of the electron beam, thereby correcting orthogonality and parallelogram errors in the raster. A raster subject to orthogonality and parallelogram errors is illustrated in FIGURE 1. Such circuitry may also include a centering circuit for, illustratively, horizontally centering the raster on the viewing screen of the tube. Centering the raster is necessary to ensure the most efficient use of the tube, which occurs when the size of the scanned raster is substantially the same size as the tube's viewing screen. The need for horizontal centering is most pronounced when the amount of horizontal overscan is reduced or, in other words, when the size of the scanned raster is reduced to the size of the tube's viewing screen. Centering the raster is typically accomplished by causing a direct current of selected polarity and amplitude to flow through the appropriate deflection coils, either horizontal or vertical. In the manufacture of a video display apparatus, it is desirable to consolidate circuitry to the greatest possible extent. Such consolidation provides several advantages, among them: decreased parts count, decreased cost, increased reliability, and an increase in the amount of space available within the apparatus's chassis. Accordingly, it is desirable to consolidate the circuits that perform the raster correction and centering functions. SUMMARY The present invention is directed to a deflection system that satisfies the need to consolidate circuitry in a video display apparatus to the greatest possible extent. A deflection system according to the inventive arrangements taught herein comprises a vertical deflection coil for deflecting the scanning electron beam between upper and lower edges of the raster; a raster centering circuit, which has a centering inductor, for centering the raster on the screen; and a raster correction transformer. The raster correction transformer uses the centering inductor for a primary winding and has a secondary winding coupled to the vertical deflection coil. The centering inductor and the secondary winding are advantageously wound around the same core. The vertical deflection coil may comprise first and second vertical deflection windings coupled in either a series or a shunt arrangement. It is advantageous to use the centering inductor as the primary winding of the raster correction transformer because then the vertical deflection circuit and the raster centering circuit can both be mounted with the deflection yoke assembly on a neck portion of the cathode-ray tube of the video display apparatus. This simplifies assembly of the video display apparatus because it obviates the need to run wires from the chassis of the video display apparatus to the vertical deflection circuit and the raster centering circuit. The above, and other features, aspects, and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIGURE 1 is useful for explaining orthogonality and parallelogram errors in a raster; FIGURES 2 and 3 show diagrams, in block and schematic form, of deflection systems for video display apparatus according to inventive arrangements described herein; FIGURES 4a-4f shows voltage and current waveforms useful for explaining the operation of the deflection systems of FIGURES 2 and 3; FIGURE 5 is a diagram, in block and schematic form, of a first equivalent horizontal centering circuit for the deflection systems of FIGURES 2 and 3; FIGURE 6 shows a voltage waveform characteristic of the equivalent centering circuit of FIGURE 5; FIGURE 7 is a diagram, in block and schematic form, of a second equivalent horizontal centering circuit for the deflection systems of FIGURES 2 and 3; FIGURE 8 shows a voltage waveform characteristic of the equivalent horizontal centering circuit of FIGURE 7; and FIGURES 9 and 10 show current waveforms characteristic of the deflection system of FIGURE 3. DESCRIPTION OF THE PREFERRED EMBODIMENTS An inventive embodiment of a deflection system 400 for a video display apparatus, such as a television receiver or a video display monitor, is shown in FIGURE 2. A horizontal deflection circuit 100 and a vertical deflection circuit 200 cooperatively deflect a scanning electron beam to form a raster on a screen of the video display apparatus. The horizontal deflection circuit 100 deflects the scanning electron beam across the screen at a horizontal scanning rate. Simultaneously, the vertical deflection circuit 200 deflects the electron beam downwardly at a slower, vertical scanning rate. A raster centering circuit 300 derives energy from the horizontal deflection circuit 100 in order to horizontally center the scanned raster on the screen of the video display apparatus. In order to consolidate circuitry within the video display apparatus, the vertical deflection circuit 200 advantageously uses a horizontal centering inductor Lc of the raster centering circuit 300 as a primary winding of a raster correction transformer 41. The embodiments described herein employ horizontal deflection, or scanning, rates equal to approximately 31 ,468 Hz, commonly referred to as the "2H" scanning frequency. It will become apparent to those skilled in the art that the inventive arrangements described herein are not limited to any particular horizontal or vertical deflection frequencies, but can be utilized throughout the entire range of useful horizontal and vertical deflection frequencies. Voltage and current waveforms associated with the horizontal deflection circuit 100 are shown in FIGURES 4a-4f; current flow is defined as positive in the directions indicated in FIGURE 2. Referring to FIGURE 2, a B+ voltage of approximately 140 Vdc is impressed across an S-correction capacitor Cs through a primary winding LPRI of a high- voltage transformer IHVT. As an electron beam is deflected to an upper left-hand corner of the raster, a horizontal output transistor Ql does not conduct a current. Energy previously stored in a horizontal deflection coil L H causes a current to flow through a forward-biased damper diode Dl and the horizontal deflection coil LH and into the S- correction capacitor Cs . At this point both a damper current ID and a horizontal deflection current IH attain their peak negative values. When the scanning electron beam reaches the center of the raster, the energy stored in the horizontal deflection coil LH has decayed to zero and the horizontal deflection current IH and the damper current ID are equal to approximately zero. The damper diode Dl becomes reverse biased and a horizontal deflection oscillator 10 causes the horizontal output transistor Ql to conduct a current IH O T - The horizontal deflection current IH reverses direction, and energy supplied to the horizontal deflection coil LH by the S-correction capacitor Cs allows the horizontal deflection current IH to increase linearly. When the scanning electron beam reaches the right edge of the raster, the horizontal deflection oscillator 10 causes the horizontal output transistor Ql to discontinue conducting the current IH O T and the damper diode Dl remains reverse biased. During this retrace interval, the decaying horizontal deflection current IH flows rapidly into the retrace capacitor CR . When horizontal deflection current IH decays to approximately zero, it reverses direction and is then supplied by retrace capacitor CR . Once the retrace capacitor CR has discharged its stored energy through the horizontal deflection coil LH , the electron beam has been returned to the upper left-hand corner of the raster, and the process repeats. In the vertical deflection circuit 200 shown in FIGURE 2, a vertical-rate sawtooth generator 61 provides a vertical-rate sawtooth waveform to a non-inverting input of a vertical output amplifier 62. The vertical output amplifier 62 is coupled to a positive supply voltage, for example +24 V, and a negative supply voltage, for example a ground potential, and may comprise a complementary or quasi-complementary push-pull transistor output stage. The vertical output amplifier 62 drives first and second vertical deflection windings Ly i and Lγ 2 of a vertical deflection coil with a vertical-rate sawtooth current Iγ . The vertical deflection windings Ly i and Lγ2 are coupled in a series arrangement; the current flowing through these windings may have a peak-to-peak amplitude equal to approximately 2 A. A voltage divider formed by resistors R3 and R4 generates a feedback voltage, which is coupled to the inverting input of the vertical output amplifier 62 through a resistor R5. A capacitor C3 provides S correction for the vertical deflection current Iγ . A series arrangement of resistors R l and R2 and a potentiometer P I is coupled in parallel with the two vertical deflection windings Lγ ι and Lγ2- The resistors Rl and R2 and the potentiometer PI are selected during the design of a deflection yoke for the cathode-ray tube, and these resistances are included as part of a deflection yoke assembly. The three resistances are used to adjust the convergence of the electron beams within the cathode-ray tube. The potentiometer PI is adjusted to achieve a desired crossover of the electron beams from the outer electron guns, typically red and blue, at a vertical center line of the cathode-ray tube. In a presently preferred embodiment of an inventive arrangement described herein the horizontal deflection circuit 100 combines with a vertical deflection circuit 200' to form a deflection system 400', which is shown in FIGURE 3. In the vertical deflection circuit 200', the vertical deflection windings Lv i and Lγ 2 are coupled in a shunt arrangement; the shunt arrangement is advantageously used in order to obtain a shorter vertical retrace time and to enable a lower inductance for the vertical deflection coil for the same applied voltage. The coupling of the secondary winding of transformer 41 to the first and second vertical deflection windings Lv i and Lγ2 does not disturb the shunt nature of the arrangement of the vertical deflection windings Lγ ι and Lγ2 - The peak-to-peak amplitude of currents I'LV I and I'LV2 flowing through each of the vertical deflection windings may have a peak-to-peak amplitude equal to approximately 2 A. A feedback voltage is generated across a resistor R8 and is coupled to the inverting input of the vertical output amplifier 62 by a resistor R9. Resistors R6 and R7 and a capacitor C4 provide a damping network for the deflection windings Lγι and Lγ2- The raster centering circuit 300 of FIGURES 2 and 3 comprises a horizontal centering inductor Lc, a centering capacitor Cc, diodes D2 and D3, a switch device SI, and a variable resistance P2, which may comprise a potentiometer. The horizontal centering inductor Lc has, for example, Nl turns and typically has a greater inductance, and hence conducts a lower peak-to-peak current, than does the horizontal deflection coil LH - The switch device SI may comprise, for example, a slide switch or a single- pole, double-throw rotary switch of the type disclosed in U.S. Patent 4,703,233, issued on October 27, 1987, to E. Rodriguez- Cavazos. As shown in FIGURES 2 and 3, the centering circuit 300 derives energy from the horizontal deflection circuit 100. For purposes of the present description, the switch device SI makes a connection with the anode of the diode D3 to provide an equivalent centering circuit 300', which is shown in FIGURE 5. Referring to FIGURE 5, during a negative portion of the horizontal deflection current IH , which corresponds to the flow of the damper current ID through the horizontal deflection coil LH , and, thus, to deflection of the electron beam from the left edge to the center of the raster, the diode D2 is reverse biased, the diode D3 is forward biased, and the horizontal-rate centering current lc charges the S- correction capacitor Cs through the diode D3. A small, positive centering voltage V'c , clamped to approximately the sum of the forward voltage drop of the diode D3, is established across the centering capacitor Cc , as shown in FIGURE 6, and a negative portion of the horizontal-rate centering current lc flows through the horizontal centering inductor Lc. As the electron beam reaches the center of the raster, the horizontal deflection current IH reverses direction and becomes positive, which corresponds to the flow of the current IH o T through the horizontal deflection coil LH and, thus, to deflection of the electron beam from the center to the right edge of the raster. The horizontal-rate centering current lc also becomes positive. The diode D2 is now forward biased, the diode D3 is now reverse biased, and a horizontal-rate current flows through the diode D2 and the variable resistance P2. The centering voltage V'c becomes negative, as shown in FIGURE 6, and is equal to approximately the voltage Vp2 generated across the variable resistance P2. The successive magnitudes of the negative peaks of the centering voltage V'c produce an average voltage V' aV g» as shown in FIGURE 6. The voltage V' avg generates a positive component of the horizontal-rate centering current lc flowing through the horizontal centering inductor Lc- Setting the switch device S I to make its connection to the anode of the diode D3 may prove to be inadequate to center the raster properly on the face of the cathode-ray tube. In that event, the switch device S I is adjusted to make its connection to the cathode of the diode Dl to provide an equivalent centering circuit 300", which is shown in FIGURE 7. The circuit of FIGURE 7 operates similarly to the circuit of FIGURE 5, with the exception that the voltages provided across the centering capacitor Cc in the two circuits have opposite polarities. Referring to FIGURE 7, during a negative portion of the horizontal deflection current IH , which corresponds to the flow of the damper current ID through the horizontal deflection coil LH and, thus, to deflection of the electron beam from the left edge to the center of the raster, a negative portion of the horizontal-rate centering current lc flows through the horizontal centering inductor Lc • The diode D2 is reverse biased, the diode D3 is forward biased, and the horizontal-rate centering current lc charges the S-correction capacitor Cs through the variable resistance P2 and the diode D3. A positive centering voltage V"c is established across centering capacitor Cc, as shown in FIGURE 8, and is equal to approximately the voltage Vp2 generated across the variable resistance P2. The successive magnitudes of the positive peaks of the centering voltage V"c produce an average voltage V" avg , which is shown in FIGURE 8. The voltage V" aV g generates the horizontal- rate centering current lc through the horizontal deflection coil LH in the same direction as the damper current ID - As the electron beam reaches the center of the raster, the horizontal deflection current IH reverses direction and becomes positive, which corresponds to the flow of the current IH O T through the horizontal deflection coil LH and, thus, to deflection of the electron beam from the center to the right edge of the raster. The horizontal-rate centering current lc also becomes positive. The diode D2 is now forward biased, the diode D3 is now reverse biased, and a horizontal-rate current flows through the diode D2. The centering voltage V'c becomes negative, as shown in FIGURE 8, and is clamped to approximately the sum of the forward voltage drop of diode D2. In accordance with an inventive arrangement described herein, the deflection systems 400 and 400' shown in FIGURES 2 and 3, respectively, advantageously use the horizontal centering inductor Lc of the raster centering circuit 300 as the primary winding of the raster correction transformer 41. In the presently preferred embodiment shown in FIGURE 3, the horizontal centering inductor Lc has 380 turns. The secondary winding of the transformer 41 is coupled in series with the first and second vertical deflection windings Lγ ι and Lv2 and has 16 turns. A center-tap 47 divides the secondary winding into a first winding 43a and a second winding 43b, each of which has 8 turns. The particular number of primary and secondary turns of the raster correction transformer 41, and hence its turns ratio, is dependent upon the requirements of a particular deflection system and is left to the judgment of one skilled in the art. Both the horizontal centering inductor Lc and the first and second windings 43a and 43b are advantageously wound around the same core, for example a ferrite rod core which, in a presently preferred embodiment, has a diameter of approximately 0.399 inches and a length of approximately 1 inch. The use of a rod core is illustrative, and is not intended to suggest that a core configuration which has a closed-loop magnetic path length, for example a toroid, cannot be used. A significant factor for one skilled in the art to take into account when selecting a particular core is the need to avoid saturating the core with the horizontal- rate centering current lc flowing through the horizontal centering inductor Lc and with vertical currents ILV I and ILV2 (in a series arrangement) and I'L V I and I'L V 2 (in a shunt arrangement) flowing through the first and second vertical deflection windings Ly i and Lv2ϊ such saturation can cause undesirable distortions in the parallelogram correction currents. It is advantageous to use the horizontal centering inductor Lc as the primary winding of the raster correction transformer 41 because then the vertical deflection circuit 200 or 200' and the raster centering circuit 300 can both be mounted with the deflection yoke assembly on a neck portion of the cathode-ray tube of the video display apparatus. This simplifies assembly of the video display apparatus because it obviates the need to run wires from the chassis of the video display apparatus to the vertical deflection circuit 200 or 200' and the raster centering circuit 300. In the embodiments shown in FIGURES 2 and 3, the horizontal deflection circuit 100 generates a horizontal deflection voltage VQ I , which is shown as FIGURE 4b and typically has a peak-to-peak voltage which is approximately equal to 1200 V. The horizontal deflection voltage VQ i is stepped down in accordance with the turns ratio of raster correction transformer N 2 41, which is equal to TTT. The resulting stepped-down, horizontal- rate pulse waveform is divided substantially equally between the first and second windings 43a and 43b. For example, in the presently preferred embodiment of FIGURE 3, the stepped-down horizontal-rate pulse waveform has a peak-to-peak voltage of approximately 28 V and is divided substantially equally across first and second windings 43a and 43b of secondary winding 43. Thus, first and second windings 43a and 43b are each provided with a horizontal-rate pulse waveform which has a peak-to-peak voltage of approximately 14 V. Returning to the deflection system 400 of FIGURE 2, the stepped-down horizontal-rate pulse waveforms across the first and second windings 43a and 43b induce the horizontal-rate raster correction currents ILV I and ILV2, respectively, for the first and second vertical deflection windings Lvi and Lv2- The raster correction currents ILV I and ILV 2 are not constrained to have equal peak-to-peak amplitudes by virtue of the center- tap 47. In addition, the peak-to-peak amplitudes of the raster correction currents ILV I and ILV 2 may vary as the coupling between the horizontal centering inductor Lc and the secondary winding of transformer 41 changes for different choices of the ferrite core. In the deflection system 400' of FIGURE 3, the stepped-down horizontal-rate pulse waveforms across the first and second windings 43a and 43b induce the horizontal-rate raster correction currents I'LV I and I'LV 2 , as shown in FIGURES 9 and 10. The raster correction currents I'LV I and I'LV 2 are not constrained to have equal peak-to-peak amplitudes by virtue of the shunt arrangement of windings Ly i and Lγ2- In addition, the peak-to- peak amplitudes of the raster correction currents I'LV I and I'L V 2 may vary as the coupling between the horizontal centering inductor Lc and the secondary winding 43 changes for different choices of the ferrite core. The raster correction currents IL V I and ILV 2 (in a series arrangement) and I'LV I and I'LV2 (in a shunt arrangement) flow through the first and second vertical deflection windings Ly i and L γ 2 , respectively, in a direction such that a magnetic field is created which opposes the downhill scan effect. In this way the vertical deflection current is modulated at a horizontal rate and the downhill scan effect is substantially offset for each horizontal scanning line of the raster. Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.

Claims: CLAIMS: 1. A deflection system for forming a raster on a screen of a video display apparatus, said deflection system comprising: a vertical deflection coil (Lγ ι , Lγ 2 ) for deflecting said scanning electron beam between upper and lower edges of said raster; a raster centering circuit (300) for centering said raster on said screen, said centering circuit comprising a centering inductance (Lc); and a raster correction transformer (41) having said centering inductance for a primary winding and a secondary winding ((43a, 43b) coupled to said vertical deflection coil. 2. The deflection system of claim 1 , wherein said vertical deflection coil comprises first (Lγ ι ) and second (Lγ2) vertical deflection windings. 3. The deflection system of claim 2, wherein said first and second vertical deflection windings (Lγ ι , Lγ2) are coupled in a series arrangement. 4. The deflection system of claim 2, wherein said first and second vertical deflection windings (Lγ ι , Lγ2) are coupled in a shunt arrangement. 5. The deflection system of claim 2, wherein said secondary winding (43a, 43b) is coupled in series with said first and second vertical deflection windings (Lγι , Lγ2). 6. The deflection system of claim 5, further comprising a series interconnection of a plurality of resistances (Rl , PI , R2) wherein one of said plurality has a terminal coupled to one of said vertical deflection windings (Lγ ι ) and another of said plurality has a terminal coupled to the other of said vertical deflection windings (Lγ2). 7. The deflection system of claim 6, wherein one of said plurality of resistances comprises a potentiometer (PI). 8. The deflection system of claim 7, wherein: said secondary winding (43a, 43b) comprises a center-tap (47); and said center-tap is coupled to a wiper (W) of said potentiometer.

Inventors: FERNSLER RONALD EUGENE, TRUSKALO WALTER, FERNSLER, RONALD, EUGENE, TRUSKALO, WALTER
Applicants: FERNSLER RONALD EUGENE, THOMSON CONSUMER ELECTRONICS, TRUSKALO WALTER, FERNSLER, RONALD, EUGENE, THOMSON CONSUMER ELECTRONICS, INC., TRUSKALO, WALTER
Application year: 1997
Main classification: H04N3
IPC codes: G09G1, H04N3