ELECTRON BEAM FOCUS VOLTAGE CIRCUIT

Abstract: There is provided a circuit and method for generating a focus voltage having a predetermined waveform and magnitude for application to an electron beam focus means of a cathode ray tube having a display surface. A horizontal deflection current having a predetermined waveform and a frequency which is selectable over a predetermined range of frequencies is used to generate substantially horizontal scan lines on the display surface of the cathode ray tube by causing at least one electron beam to sweep across the display surface. The circuit has an input for receiving a signal having a current waveform representative of the waveform and frequency of the horizontal deflection current. In response to the received signal, the circuit produces a focus voltage output, the waveform and magnitude of which remain substantially constant over the range of selected frequencies of the horizontal deflection current.

Description: ELECTRON BEAM FOCUS VOLTAGE CIRCUIT The present invention relates to video monitors and, in particular, to circuits for generating an electron beam focus voltage. Cathode ray tubes (CRTs) typically utilize CRT focus means to focus the electron beam to minimize distortion and other artifacts. Such focus means adjust the focus of the electron beam with an electrical field, in accordance with a control voltage applied to the focus means by focus circuitry. The electron beam is typically focused by the focus means before being deflected by the horizontal and vertical yoke deflection coils. Distortion addressed by the focus means is caused, for example, by changes in the deflection angle of the electron beam of the CRT as horizontal scan lines are swept across the CRT face or screen. Thus, for example, the deflection angle in the four corners of a rectangular monitor are different from the deflection angles on horizontal scan lines displayed on the center of the monitor. Such changing deflection angles cause variations in the focus of the electron beam, in part because the CRT face is roughly planar and rectangular, rather than spherical. Various forms of focusing circuits have been developed to help change the focus of the electron beam at different horizontal and vertical points during the displaying of a frame or field, to reduce the effects of these and other variations on the focus of the electron beam. Such circuits are typically optimized for a given horizontal scanning frequency. For example, in the NTSC system, one focusing circuit may be optimized for the "1H" scan frequency of approximately 15,734 Hz. Another circuit may be optimized for the "2H" scan frequency of approximately 31 ,468 Hz. However, it may be desirable to utilize variable-frequency monitors, in which the horizontal scanning frequency of the monitor is dynamically switched to one of a plurality of optional horizontal scanning frequencies. Since prior art focusing circuits are typically optimized for use at a given horizontal scanning frequency, switching to other horizontal scanning frequencies will result in a degradation of electron beam focus. One possible approach to implementing a suitable focusing circuit for use with multiple horizontal scanning frequencies involves multiple capacitances that can be switched into and out of the focusing circuit as a function of the horizontal switching frequency. This approach is undesirable in practice because it adds complexity to the focusing circuit. Furthermore, capacitors in the focusing circuit typically may be charged to voltages that are equal to approximately 1 ,000 N. Switching voltages of this magnitude into and out of the focusing circuit is also undesirable. A horizontal deflection current having a predetermined waveform and a frequency which is selectable over a range of frequencies is used to generate substantially horizontal scan lines on the display surface of the cathode ray tube by causing at least one electron beam to sweep across the display surface. According to the inventive arrangements described herein, a circuit for generating a focus voltage for application to an electron beam focus means of a cathode ray tube has an input for receiving a signal having a current waveform representative of the waveform and frequency of the horizontal deflection current. The circuit conducts a portion of the signal current as a function of the frequency of the horizontal deflection current, thereby advantageously maintaining the waveform and magnitude of the focus voltage output substantially constant over the range of selected frequencies of the horizontal deflection current while simultaneously avoiding the complexity and disadvantages associated with switching capacitance into and out of the circuit. Such a circuit comprises: an input for receiving a signal having a current waveform representative of the waveform and frequency of the horizontal deflection current; and, means, responsive to the received signal, for producing a focus voltage output having a waveform and a magnitude which remain substantially constant over the range of frequencies of the horizontal deflection current. The means for producing a focus voltage output may comprise a variable impedance circuit having an impedance which is selectable in accordance with each selected frequency of the horizontal deflection current. The variable impedance circuit may comprise: series-connected first and second capacitors coupled to the input; and, means for selectively shunting at least a portion of current flowing through the first capacitor away from the second capacitor. The means for selectively shunting may be operative in response to an applied frequency select signal, and may comprise a switch, the switch operative in response to the frequency select signal. The means for selectively shunting additionally may comprise a resistor coupled between the junction of the first and second capacitors and a current return path through the switch. The means for selectively shunting also may comprise at least a second resistor coupled between the junction of the first and second capacitors and the current return path through at least a second switch, the switches operative in response to at least two frequency select signals. The variable impedance circuit may comprise: series- connected first and second capacitors coupled to the input; and, means for selectively shunting at least a portion of the signal current away from the series-connected first and second capacitors. The means for selectively shunting may comprise means for continuous variation of the magnitude of the current shunted away from the series-connected first and second capacitors in response to continuous variation of the frequency of the horizontal deflection current. According to a feature of the inventive arrangements described herein, the circuit comprises: an input for receiving a signal having a current waveform representative of the waveform and frequency of the horizontal deflection current waveform; a capacitance coupled to the input; and, means for selectively shunting at least a portion of current away from the capacitance; whereby the waveform and magnitude of the focus voltage waveform remain substantially constant over the range of selected frequencies of the horizontal deflection current. The capacitance may comprise series-connected first and second capacitors. The means for selectively shunting may be operative in response to an applied frequency select signal. The means for selectively shunting may comprise a switch, the switch operative in response to the frequency select signal. The shunting means additionally may comprise a resistor coupled between the junction of the first and second capacitors and a current return path through the switch. The means for selectively shunting may comprise means for continuous variation of the magnitude of the current shunted away from the capacitance in response to continuous variation of the frequency of the horizontal deflection current. The means for continuous variation may comprise: an amplifier circuit for providing a control voltage responsive to the focus voltage across the capacitance; and, an active device responsive to the control voltage for shunting the current away from the capacitance in accordance with the continuous variation of the frequency of the horizontal deflection current. The active device may comprise a transistor. FIGURE 1 is a schematic diagram of a dynamic focus circuit in accordance with the present invention. FIGURE 2 depicts selected scan lines on a CRT face. FIGURE 3 depicts parabolic focus voltage waveforms for focusing selected scan lines on the CRT face illustrated in FIGURE 2. FIGURE 4 depicts a parabolic DC component with the parabolic focus voltage waveforms of FIGURE 3 superimposed thereon in time sequence order. FIGURE 5 is a schematic diagram of an alternative switching portion of the dynamic focus circuit of FIGURE 1. FIGURE 6 is a schematic diagram of another alternative switching portion of the dynamic focus circuit of FIGURE 1. Referring now to FIGURE 1 , there is shown a schematic diagram of a dynamic focus circuit in accordance with the present invention, generally designated 100. Circuit 100 includes a current transformer 102 having primary winding or coil LI and secondary winding or coil L2. In one embodiment, coils LI and L2 have a turns ratio of 3T to 640T. Horizontal yoke 140 of a CRT (not shown) is coupled in series with coil LI . The other terminal of coil LI is coupled to ground through capacitor CS. Capacitors Cl and C2, which combine to form an equivalent capacitance CE, are coupled in series across coil L2. Coil L2 is coupled at one end through capacitor C3 to the wiper lead of potentiometer PI and to a dynamic element of a focus means (not shown) of the CRT. Potentiometer PI is connected in parallel with potentiometer P2, and each is further coupled between a 10 KN voltage source and ground. The wiper lead from potentiometer P2 is coupled to a static element of the focus means. The focus means (not shown) is a device that varies an electrical field applied to the electron beam in accordance with variations in focus voltages applied to the static and dynamic elements of the focus means. This varying electrical field adjusts the focus of the electron beam as the beam is being deflected by the yoke coils of the CRT. The voltage NCE across capacitance CE is the focus voltage applied to the dynamic element of the focus means. As the focus voltage NCE changes, the electric field generated by the dynamic element of the focus means changes, thereby changing the focus of the electron beam. Potentiometers PI and P2 are used to adjust the overall focus applied to the electron beam and the resulting picture displayed on the CRT face. A switch portion 120 of circuit 100 includes a switch 122. In the embodiment depicted in FIGURE 1, the switch is a field- effect transistor Ml having a source terminal S, a drain terminal D, and a gate terminal G. The junction of capacitors Cl and C2 is coupled to the drain terminal D of field-effect transistor Ml of switch portion 120 through resistor Rl . The drain terminal D of transistor Ml is further coupled to a 220N power supply through resistor R2. The source terminal S of transistor Ml is coupled to ground, and the gate terminal G of transistor Ml is coupled to ground through resistor R3. A frequency select signal FREQ_SELECT is coupled to the gate terminal G of transistor Ml through resistor R4. The frequency select signal FREQ_SELECT may be generated in a conventional manner which is not described herein. Transistor Ml of switch portion 120 functions as a switching element of the current path from the junction Jl of capacitors Cl and C2 through resistor Rl to ground, under control of the frequency select signal. Circuit 100 further comprises vertical component portion 130, which provides a DC voltage, the magnitude of which varies parabolically depending upon the vertical position of the current scan line. This DC voltage is added to the parabolic focus voltage waveforms used for each scan line displayed on the CRT face. This causes the magnitude of the parabolic focus voltage waveform for each scan line to vary in accordance with the scan line's vertical distance from center, as described below with reference to FIGURE 4. The vertical component portion 130 of circuit 100 comprises a resistor R5 connected in series with a capacitor C4. The other terminal of capacitor C4 is coupled to ground. The junction of resistor R5 and capacitor C4 at node 131 is coupled to the junction of coil L2 and capacitor C2 at node 172, as well as to the collector of a transistor Ql . The other terminal of resistor R5 is coupled to the 220N power supply and one terminal of a resistor R6. An input parabolic vertical signal NERT_SIG is coupled to the base of transistor Ql through capacitor C5 and resistor R7. The parabolic vertical signal NERT_SIG may be generated in a conventional manner which is not described herein. The emitter of transistor Ql is coupled to ground through resistor R9. The other terminal of resistor R6 is coupled to the base of transistor Ql . A resistor R8 is coupled between the base of transistor Ql and ground. The parabolically varying DC voltage provided at the NERT_SIG terminal of vertical component portion 130 has a period equal to the time required to display an entire field of scan lines, and has a peak-to-peak voltage of 5N. Vertical component portion 130 amplifies this voltage to provide a corresponding parabolically varying DC voltage at node 131 which has a peak-to-peak voltage of 200V. Vertical component portion 130 thus provides a means for adding a DC voltage to the parabolic voltage waveform for each scan line, wherein the magnitude of the DC voltage varies parabolically depending upon the vertical position on the display surface of each scan line. This means for adding a DC voltage includes (1) an input terminal at a terminal of capacitor C5 for receiving input vertical signal VERT_SIG, which has a parabolically varying voltage and a period which is substantially equal to the time required to display an entire field of scan lines; (2) an output terminal at node 131 coupled to the input of the focus voltage generating circuit at node 172; and (3) an amplifier, having an input coupled to the input terminal and an output coupled to the output terminal, for amplifying the input vertical signal VERT_SIG. In one embodiment, components of circuit 100 have the following values: LI = 4.18 μH (μHenries), L2 = 190 mH; Cl = 560pF; C2 = 560 pF; Rl = 2.4 KΩ; R2 = 100 KΩ; R3 = 22 KΩ; and R4 = 2.2 KΩ. Yoke 140 also has an effective inductance of approximately 250-300 μH. Transistor Ql is an npn transistor of type MPSW42, and transistor Ml is a FET of type IRF731. Potentiometers PI and P2, in one embodiment, each have a resistance of approximately 100 MegΩ. Yoke 140 comprises horizontal deflection coils that cause the electron beam to travel horizontally across the CRT face during the display of a scan line. Horizontal deflection coils operate by modulating an electrical field through which an electron beam passes to change the deflection angle Q of the beam. The horizontal deflection coils of yoke 140 receive a roughly linearly increasing, or ramping, current for each scan line, which causes the electron beam to sweep across the CRT face. Thus, the current waveform has a sawtooth shape. For a system in which there are 525 scan lines per field, for example, the current waveform applied to the horizontal deflection coils of yoke 140 includes one ramp per scan line, or 525 sawtooth wave peaks per field. Capacitor CS, sometimes referred to as an "S-shaping" capacitor, is designed to modify the shape of the current ramps of the current waveform so that they have a slight "S" shape rather than being purely linearly-increasing ramps. Horizontal deflection current IP1 , having a sawtooth shaped current waveform, is applied to the primary winding coil LI of transformer 102. In response, secondary winding coil L2 of transformer 102 generates current IP2, which is a signal having a current waveform representative of the waveform and frequency of horizontal deflection current IP1. The equivalent capacitance CE, coupled across the secondary winding coil L2, integrates the sawtooth wave shaped current ramps of current IP1 to provide a series of high-voltage parabolas, one for each ramp of current IP1 , in accordance with the equation V = 1/Cjl dt. Such a parabolic shaped voltage is useful in adjusting the focus of the electron beam to address the above-described focus problems caused by changes in deflection angle. In the present invention, the focus voltage VCE applied to the focus means of the CRT is varied parabolically within each scan line to keep the electron beam focused as the deflection angle changes. In one embodiment, the peak-to-peak voltage of the voltage parabola applied to the focus means for each scan line is the same, and the DC component of each voltage parabola varies for each scan line according to the scan line's vertical placement on the CRT face. This DC component itself varies parabolically along the vertical axis of the CRT face, and is supplied by vertical component portion 130, as previously described. The reason for changing the voltage parabola for each scan line is that scan lines off vertical center have different deflection angles. Referring now to FIGURE 2, there are depicted selected scan lines on CRT face 200, including scan lines 221, 222 and 223. Scan lines 221 and 223 are top and bottom scan lines, respectively, and are approximately equidistant from center scan line 222. Because the deflection angles Q are greater for scan lines 221 and 223 than for scan line 222, a greater amount of voltage VCE is required for focusing scan lines 221 and 223 than is required for scan line 222. Further, for each of scan lines 221 , 222 and 223, the left and right edges require a greater amount of focus voltage VCE than is required at the horizontal center of the respective scan line, since the deflection angle Q is greater the further that yoke 140 deflects the electron beam away from the horizontal center. 5 Referring now to FIGURES 3(a)-(c), there are respectively depicted parabolic focus voltage waveforms 321 , 322 and 323 having DC components VI , V2 and V3, respectively, for focusing scan lines 221, 222 and 223 on CRT face 200 of FIGURE 2. These voltage waveforms show changes in focus voltage VCE over a 0 period of time for a single scan line, from t = 0 to t = TP, where TP is the period of one scan line. As illustrated in focus voltage waveform 322, a greater amount of focus voltage is applied to the focus means at the beginning and end of scan line 222. Focus voltage waveforms 321 and 323 vary similarly, but have a larger 5 DC component (supplied by vertical component portion 130), since scan lines 221 and 223 require greater focus, due to the greater deflection angle Q associated with these scan lines. Thus, DC components VI and V3 of focus voltages 321 and 323, respectively, are approximately equal and are both greater than 0 the DC component V2 of focus voltage 322. Referring now to FIGURE 4, there is shown a parabolic waveform of DC component 430 with the parabolic focus voltage waveforms 321, 322 and 323 of FIGURE 3 superimposed thereon in time sequence order. One parabolic focus voltage waveform is 5 generated for each scan line per field displayed on the CRT face. The DC component of each voltage waveform itself varies parabolically, in accordance with parabolic envelope 430, which corresponds to the DC voltage generated by vertical component portion 130. This causes the parabolic voltage waveform for each 0 scan line to have a different magnitude of the DC component in accordance with the scan line's vertical distance from center. Different horizontal scanning frequencies can cause the focus voltage VCE waveforms to change. For example, suppose the horizontal scanning frequency changes from 2H, or approximately 3 5 31 ,468 Hz, to 2.4H, or approximately 37,762 Hz. In order to maintain the same scan line length, current IP1 is maintained at the same magnitude even when the horizontal scanning frequency changes. The peak-to-peak amplitude of the yoke current, IP1 , illustrated as flowing into coil LI in FIGURE 1 , causes an induced peak-to-peak current IP2 in coil L2. The instantaneous current in coil L2 corresponding to IP2 is equal to [IP2t/(TP/2)] - IP2. In accordance with the equation V = 1/Cjl dt, the focus voltage corresponding to the instantaneous current in coil L2 equals (IP2/CE) » (t2/TP - t), which has a parabolic waveform. Thus, if the horizontal scanning frequency changes from 2H to 2.4H, for example, the horizontal scanning period TP decreases and time parameter t decreases (since it takes less time to display a scan line at higher scan rates), having the net effect that focus voltage VCE decreases also. Similarly, if the scanning frequency decreases, focus voltage VCE increases. However, the waveform shape and peak-to-peak magnitude of focus voltage VCE applied to the focus means for a given scan line during the horizontal scanning period TP for that scan line ideally should not change just because the scanning frequency changes, since for a CRT face and scan lines of a given size, the same deflection angles exist and require focus correction. One way to maintain the desired peak-to-peak voltage of focus voltage VCE when the horizontal scanning frequency increases or decreases would be to switch in a capacitor having a lower or higher capacitance, respectively. However, high voltages on the order of 1000 volts may exist across the capacitor, making it undesirable to switch capacitors in such a situation. In accordance with the present invention, the waveform shape and peak-to-peak voltage of focus voltage VCE for a given scan line is advantageously maintained when the horizontal scanning frequency increases or decreases by increasing or decreasing, respectively, the effective current IP2 that flows through equivalent capacitance CE. The effective current IP2 that flows through equivalent capacitance CE is decreased by a predetermined amount by shunting a portion of the current IP2 away from equivalent capacitance CE by closing the transistor switch Ml at the lower scanning frequency, causing a portion 12 of the current IP2 to flow to ground through resistor Rl . At the higher horizontal scanning frequency, transistor switch Ml is opened so that none of current IP2 is shunted away from equivalent capacitance CE. Therefore, when transistor switch M l is opened, the effective current through equivalent capacitance CE is increased, causing an increase in focus voltage VCE. Thus, employing capacitors Cl and C2 to form equivalent capacitance CE, combined with resistor Rl and transistor switch Ml allows the amount of current that is being integrated by equivalent capacitance CE to be changed so as to prevent focus voltage VCE from changing due to changes in scanning frequency. According to the above description, the present invention provides a means, responsive to signal IP1, for producing a focus voltage output VCE at an output node 171, the waveform and peak-to-peak magnitude of which remain substantially constant over the range of selected frequencies of the horizontal deflection current IP1. The current IP2, which has a current waveform representative of the waveform and frequency of the horizontal deflection current IP1 , is applied to the input nodes 171 and 172 of the variable impedance circuit. The horizontal deflection current IP2 has a predetermined waveform and a frequency which is selectable over a predetermined range of frequencies. The means for producing a focus voltage includes capacitors Cl and C2, resistor Rl and transistor switch Ml of switch portion 120, which form a variable impedance circuit having an impedance which is selectable in accordance with each selected frequency of the horizontal deflection current. Capacitors Cl and C2, and resistor Rl and transistor switch Ml of switch portion 120 are used to effectively change the impedance seen by current IP2. In particular, the impedance is changed so that the waveform and peak-to-peak magnitude of the parabolic focus voltage waveform does not change when the scan line frequency changes. Capacitor Cl is coupled in series with capacitor C2 across the input nodes 171 and 172. Resistor Rl and transistor switch Ml of switch portion 120 form a means for selectively shunting at least a portion of current flowing through the first capacitor Cl away from the second capacitor C2. Transistor switch Ml of the means for selectively shunting is operative in response to an applied frequency select signal FREQ_SELECT. As will be appreciated, in alternative embodiments other switching and current shunting mechanisms are utilized in accordance with the present invention. Thus, in a two-capacitor embodiment such as illustrated in circuit 100, a switch such as that provided by transistor Ml may be utilized to selectively couple Rl to ground. Alternatively, two or more separate resistors RIA and RIB, each switchable to ground and having different values, could be utilized to support up to four basic horizontal scanning frequencies. Referring now to FIGURE 5, there is shown a schematic diagram of an alternative resistor and switch portion 500 of dynamic focus circuit 100 of FIGURE 1. Switch portion 500 includes resistors RIA and RIB, each coupled together at one end to the junction Jl of capacitors Cl and C2 of circuit 100. The other end of resistor RIA is coupled to ground through a switch 121, and the other end of resistor RIB is coupled to ground through a switch 122. Four scanning frequencies may be supported with resistors and switches of circuit 500 in the following configurations: (1) RIA open, RIB open (maximum current through equivalent capacitance CE for the lowest scanning frequency); (2) RIA open, RIB closed (for the second-lowest scanning frequency, assuming RIB > RIA); (3) RIA closed, RIB open (for the second-highest scanning frequency); and (4) RIA closed, RIB closed (for the highest scanning frequency). Each scanning frequency supported by a permutation of the opened or closed positions of the plurality of switches may be selected by a plurality of frequency select signals applied to the switches. As will be appreciated, different numbers and combinations of switchable resistors and series capacitors may be utilized to adapt circuit 100 to different numbers of variable scanning frequencies. In the embodiment of FIGURE 5, the means for selectively shunting described previously additionally includes a second resistor RIB coupled between the junction Jl of the first and second capacitors Cl and C2 and the current return path through a second switch 122, where both switches are operative in response to at least two frequency select signals. Referring now to FIGURE 6, there is shown a schematic diagram of an alternative switch portion 600 of dynamic focus circuit 100 of FIGURE 1 , which replaces switch portion 120 of FIGURE 1. Node Jl is coupled to ground through resistor Rl , transistor Q2 and resistor R25. Node Jl is also coupled to the non- inverting terminal of operational amplifier 610 through resistor R21. Node 172 is coupled to the inverting input of operational amplifier 610 through resistor R23. The non-inverting and inverting terminals of operational amplifier 610 are further coupled to ground through resistors R22 and R24, respectively. The collector of transistor Q2 is coupled to resistor Rl , and also to a 220V power supply through resistor R26. In this alternative embodiment, operational amplifier 610 is utilized to variably control the current II drawn through transistor Q2, to allow for continuous variations in scanning frequency and dynamic adjustment of focus in accordance therewith. For example, to provide for automatic dynamic correction, the voltage across capacitor C2 may be sensed via divided down dual inputs to an operational amplifier 610 to drive transistor Q2 in a linear mode, to help resist variations in the peak-to-peak voltage range of focus voltage VCE that would otherwise be caused by variations in horizontal scan frequency. Transistor Q2 operates in linear mode to draw a magnitude of current II in accordance with the error signal supplied by operational amplifier 610. The combination of resistor Rl and the variable resistance of transistor Q2 thus forms a variable resistive impedance, controlled by the error signal from operational amplifier 610. Operational amplifier 610 and resistors R21, R22, R23 and R24 form a means for supplying an error signal to transistor Q2 in accordance with variations in focus voltage VCE, and thus a means for varying the variable resistive impedance formed by resistor Rl and transistor Q2. For example, the bias point of operational amplifier 610 and transistor Q2 may be selected so that transistor Q2 begins to conduct more current II when the peak-to-peak focus voltage VCE begins to increase in response to a decrease in scan line frequency. As more current II is conducted through transistor Q2 and resistor R25, the lower will be the focus voltage VCE. In one embodiment, transistor Q2 is an npn transistor of type MPSW42, and operational amplifier 610 is of type LF356. In the embodiment, the means for selectively shunting previously described comprises a means for continuous variation of the magnitude of the current shunted away from the equivalent capacitance CE in response to continuous variation of the frequency of the horizontal deflection current IP1.

Claims: CLAIMS 1. A circuit for generating a focus voltage for application to an electron beam focus means of a cathode ray tube, wherein a horizontal deflection current waveform has a frequency which is selectable over a range of frequencies, the circuit comprising: means (102) for generating a signal (IP2) having a current waveform representative of the waveform and frequency of the horizontal deflection current (IP1); and, means (Cl , C2, 120) responsive to the generated signal for producing a focus voltage output having a waveform and a magnitude which remain substantially constant over the range of frequencies of the horizontal deflection current. 2. The circuit of claim 1 , wherein the means for producing a focus voltage output comprises a variable impedance circuit (Cl , C2, 120) having an impedance which is selectable in accordance with each selected frequency of the horizontal deflection current (IP 1 ) . 3. The circuit of claim 2, wherein the variable impedance circuit comprises: series-connected first (Cl) and second (C2) capacitors coupled to the input; and, means (120) for selectively shunting at least a portion of current flowing through the first capacitor away from the second capacitor. 4. The circuit of claim 3, wherein the means (120) for selectively shunting is operative in response to an applied frequency select signal (FREQ_SELECT). 5. The circuit of claim 4, wherein the means (120) for selectively shunting comprises a switch (122) , the switch operative in response to the frequency select signal (FREQ_SELECT). 6. The circuit of claim 5, wherein the means (120) for selectively shunting additionally comprises a resistor (Rl ) coupled between the junction of the first (Cl) and second (C2) capacitors and a current return path through the switch ( 122). 7. The circuit of claim 6, wherein the means ( 120) for selectively shunting additionally comprises at least a second resistor coupled between the junction of the first (Cl) and second (C2) capacitors and the current return path through at least a second switch, the switches operative in response to at least two frequency select signals. 8. The circuit of claim 2, wherein the variable impedance circuit . comprises: series-connected first (Cl) and second (C2) capacitors coupled to the input; and, means (120) for selectively shunting at least a portion of the signal current (IP2) away from the series-connected first and second capacitors. 9. The circuit of claim 8, wherein the means (120) for selectively shunting comprises means for continuous variation of the magnitude of the current shunted away from the series- connected first (Cl) and second (C2) capacitors in response to continuous variation of the frequency of the horizontal deflection current (IP1 ). 10. The circuit of claim 1, wherein the predetermined waveform of the focus voltage is a parabolic voltage waveform for each scan line. 11. The circuit of claim 10, further comprising means (130) for adding a DC voltage to the parabolic voltage waveform for each scan line, wherein the magnitude of the DC voltage varies parabolically depending upon the vertical position on the display surface of each scan line. 12. The circuit of claim 1 1 , wherein the means for adding a DC voltage comprises a vertical component circuit ( 130) comprising: an input terminal for receiving an input vertical signal (VERT_SIG) having a parabolically varying voltage and a period which is substantially equal to the time required to display an entire field of scan lines; an output terminal (131) coupled to the input (172) of the focus voltage generating circuit; and, an amplifier (Ql), having an input coupled to the input terminal and an output coupled to the output terminal, for amplifying the input vertical signal. 13. A circuit for generating a focus voltage waveform for a focus means of a cathode ray tube from a horizontal deflection current waveform, wherein the horizontal deflection current waveform has a frequency which is selectable over a range of frequencies, the circuit comprising: an input (171) for receiving a signal (IP2) having a current waveform representative of the waveform and frequency of the horizontal deflection current waveform (IP1); a capacitance (Cl, C2) coupled to the input; and, means (1200 for selectively shunting at least a portion of current away from the capacitance; whereby the waveform and magnitude of the focus voltage waveform remain substantially constant over the range of selected frequencies of the horizontal deflection current. 14. The circuit of claim 13, wherein the capacitance comprises series-connected first (Cl) and second (C2) capacitors. 15. The circuit of claim 14, wherein the means (120) for selectively shunting is operative in response to an applied frequency select signal (FREQ_SELECT). 16. The circuit of claim 15, wherein the means ( 120) for selectively shunting comprises a switch (122), the switch operative in response to the frequency select signal (FREQ_SELECT). 17. The circuit of claim 16, wherein the means (120) for selectively shunting additionally comprises a resistor (Rl ) coupled between the junction of the first (Cl) and second (C2) capacitors and a current return path through the switch (122). 18. The circuit of claim 13, wherein the means (120) for selectively shunting comprises means for continuous variation of the magnitude of the current shunted away from the capacitance in response to continuous variation of the frequency of the horizontal deflection current (IP1). 19. The circuit of claim 18, wherein the means for continuous variation comprises: an amplifier circuit for providing a control voltage responsive to the focus voltage across the capacitance; and, an active device responsive to the control voltage for shunting the current away from the capacitance in accordance with the continuous variation of the frequency of the horizontal deflection current. 20. The circuit of claim 19, wherein the active device comprises a transistor. AMENDED CLAIMS [received by the International Bureau on 17 April 1998 (17.04.98); original claims 1-20 amended; new claim 21 added; remaining claims unchanged (4 pages)] 1. A circuit for generating a focus voltage for application to an electron beam focus means of a cathode ray tube, wherein a deflection current waveform has a frequency which varies over a range of frequencies, the circuit comprising: a plurality of series-connected capacitances for producing the focus voltage output; and means coupled to a junction of said plurality of capacitances for shunting current from the plurality of capacitances such th at the focus voltage has a waveform and a magnitude which remain substantially constant over the range of frequencies of the deflection current. 2. The circuit of claim 1, wherein the means for shunting is operative in response to an applied frequency select signal. 3. The circuit of claim 2, wherein the means for shunting comprises a switch which is operative in response to th e frequency select signal. 4. The circuit of claim 3, wherein the means for shunting additionally comprises a resistor for coupling the switch to th e junction of the plurality of series-connected capacitances. 5. The circuit of claim 4, wherein the means for shunting additionally comprises at least a second resistor coupled between the junction of the plurality of series-connected capacitances and at least a second switch, the switches operative in response to a t least two frequency select signals. 6. The circuit of claim 1, wherein the plurality of capacitances comprises series-connected first and second capacitors. 7. The circuit of claim 6, wherein the means for shunting shunts at least a portion of current flowing through the first capacitor away from the second capacitor as a function of th e frequency of the horizontal deflection current waveform. 8. The circuit of claim 7, wherein the means for shunting comprises means for continuously varying of the magnitude of th e current shunted away from the series-connected first and second capacitors in response to continuous variation of the frequency of the horizontal deflection current. 9. The circuit of claim 8, wherein the means for continuously varying the current shunted away from the series- connected capacitors comprises: an amplifier circuit for providing a control voltage representative of the focus voltage; and, an active device responsive to the control voltage for shunting the current away from the capacitance. 10. The circuit of claim 9, wherein the focus voltage appears across the capacitance. 11. The circuit of claim 1, wherein the predetermined waveform of the focus voltage is a parabolic voltage waveform for each scan line. 12. The circuit of claim 11, further comprising means for adding a DC voltage to the parabolic voltage waveform for each scan line, wherein the magnitude of the DC voltage varies parabolically depending upon the vertical position on the display surface of each scan line. 13. The circuit of claim 12, wherein the means for adding a DC voltage comprises a vertical component circuit comprising: an input terminal for receiving an input vertical signal having a parabolically varying voltage and a period which is substantially equal to the time required to display an entire field of scan lines; an output terminal coupled to the focus voltage generating circuit; and, an amplifier, having an input coupled to the input terminal and an output coupled to the output terminal, for amplifying th e input vertical signal. 14. A circuit for generating a focus voltage waveform for a focus means of a cathode ray tube from a horizontal deflection current waveform, wherein the horizontal deflection current waveform has a frequency which is selectable over a range of frequencies, the circuit comprising: a transformer having a primary winding coupled to a horizontal deflection circuit and a secondary winding for providing a current waveform representative of the waveform and frequency of the horizontal deflection current waveform; a capacitance coupled to the secondary winding for generating the focus voltage waveform; and, means for shunting current away from the capacitance such that the waveform and magnitude of the focus voltage waveform remain substantially constant over the range of selected frequencies of the horizontal deflection current. 15. The circuit of claim 14, wherein the capacitance comprises series-connected first and second capacitors. 16. The circuit of claim 15, wherein the means for shunting is operative in response to an applied frequency select signal. 17. The circuit of claim 16, wherein the means for shunting comprises a switch which is operative in response to th e frequency select signal. 18. The circuit of claim 17, wherein the means for shunting additionally comprises a resistor for coupling the junction of th e first and second capacitors to the switch. 19. The circuit of claim 14, wherein the means for shunting comprises means for continuously varying the magnitude of th e current shunted away from the capacitance in response to continuous variation of the frequency of the horizontal deflection current. 20. The circuit of claim 19, wherein the means for continuously varying the current shunted away from the series- connected capacitors comprises: an amplifier circuit for providing a control voltage representative of the focus voltage; and, an active device responsive to the control voltage for shunting the current away from the capacitance. 21. The circuit of claim 20, wherein the focus voltage appears across the capacitance. AMENDED SHEET (ARTICLE 1$

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