CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent application Serial No. 60/640,949 filed on December 31, 2004, the entire contents of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to cathode ray tubes (CRTs) for displays such as, for example, High Definition Television (HDTV). More particularly, it relates to an apparatus for controlling heater voltages in CRTs operating in a vertical scan mode and a method of operating the CRT in the vertical scan mode.

BACKGROUND OF THE INVENTION

The popularity of HDTV has prompted an increased demand for television sets capable of displaying HDTV images. Such demand has prompted an increase in demand for larger aspect ratio, true flat screen displays having a shallower depth, increased deflection angle and improved visual resolution performance.

The demand for shallow, flat screen displays has led to efforts to improve spot performance so that spot size and shape exhibit greater uniformity across the entire screen for improved visual resolution performance. To this end, most displays now make use of dynamic focus. Increasing the deflection angle also yields an improvement in spot performance in the central area of the screen because increasing the deflection angle results in a decreased gun-to-screen distance, hereinafter referred to as the 'throw distance' . Figure 1

illustrates the basic geometrical relationship between throw distance and deflection angle for a typical CRT. Increasing the deflection angle (A) reduces the throw distance, thus allowing for production of a shorter CRT and ultimately, a slimmer television set.

As the deflection angle increases, the throw distance decreases and spot size decreases in a non-linear relationship. The following formula mathematically approximates relationship between spot size and throw distance:

Spot Size ~ B * Throw ^λA (Equation 1) where the exponent 1.4 represents an approximation taking into consideration the effects of magnification and space charge effects over a useful range of beam current. The term B represents a system-related proportionality constant. Considering this relationship, for a tube having a diagonal dimension of 760 mm, increasing the corner to corner deflection angle from 100 degrees to 120 degrees while decreasing the center throw distance, for example, from 413mm to 313mm yields a 32% reduction in spot size at the center of the screen.

Increasing the deflection angle in these displays gives rise to increases in obliquity, which is defined as the effect of a beam intercepting the screen at an oblique angle, thereby causing an elongation of the spot. The problem of obliquity becomes especially apparent in CRTs having a standard horizontal gun orientation, i.e., a CRT whose guns have a horizontal alignment along the major axis of the screen. As obliquity increases, a spot having a generally circular shape at the center of the screen becomes oblong or elongated as the spot moves toward edges of the screen. Based on this geometrical relationship, in a large aspect ratio screen, such as a 16 x 9 screen, the spot appears most elongated at the edges of the major axis and at the screen corners. Thus it becomes apparent that the obliquity effect causes the spot size to grow. The following equation defines the spot size radius SS _ra(ϋ ai:

SSra _d i _a i = SS _nO πnai/cos(A) (Equation 2) where A represents deflection angle, as measured from Dc to De as shown in Figure 1 and

nominal spot size SS _n0πna i represents the spot size without obliquity.

In addition to the obliquity effect, yoke deflection effects in self-converging CRTs having a horizontal gun orientation can compromises spot shape uniformity. To achieve self convergence, CRT's typically include a horizontal yoke that generates a pincushion shaped field and a vertical yoke that generates a barrel shaped field. These yoke fields cause the spot shape to become elongated. This elongation adds to the obliquity effect by further increasing spot distortion at the three-o'clock and nine o'clock positions (referred to as the "3/9" positions) and at corner positions on the screen.

Various attempts have been made to address spot distortion and obliquity. For example, U.S. Patent No. 5, 170,102 describes a CRT with a vertical electron gun orientation whose un-deflected beams appear parallel to the short axis of the display screen. The deflection system described in this patent includes a signal generator for causing scanning of the display screen in a raster-scan fashion, thereby yielding a plurality of lines oriented along the short axis of the display screen. The deflection system also comprises a first set of coils for generating a substantially pincushion-shaped deflection field for deflecting the beams in the direction of the short axis of the display screen. A second set of coils generates a substantially barrel shaped deflection field for deflecting the beams in the direction in the long axis of the display screen. The deflection system's coils generally distort spots by elongating them vertically. This vertical elongation compensates for obliquity effects, thereby reducing spot distortion at the 3/9 and corner positions on the screen. The barrel shaped field required to achieve self convergence at 3/9 screen locations overcompensates for obliquity and vertically elongates the spot at the 3/9 and corner locations as shown in Figure 10 of the U.S. Patent No. 5, 170,102. (In effect, the barrel shaped field overcompensates, thus making the spot shape at the 3/9 position and the screen corners a vertically oriented ellipse). Orienting the electron guns along the vertical or minor axis will yield improvements in a self-

converging system, but spot distortion remains problematic at the 3/9 positions and at the corner screen locations.

Thus, a need exists for a CRT system that overcomes the aforementioned disadvantages when increasing the deflection angle A and thereby reducing throw distance and thus the overall depth of the CRT. More specifically, the reduction in throw arm distance reduces the beam spread, thereby resulting in a smaller center spot.

Furthermore wide angle deflection increases the center-to-center spot growth due to increase inclination angles of the beam in the corners of the display screen. However, the shortened throw arm provided compensation, and the absolute spot size at 3/9 and the corners of the standard scan tubes (CRTs) match that of the transposed scan display device of the present principles.

Those of skill in the art will recognize that when there is a loss of scan in a CRT, the CRT is subject to screen burn if the anode voltage and gun voltages are still present. This risk can be minimized by removing the heater voltage when scan is lost. As such, the present principles propose to base the heater voltage control on the presence of scan in the CRT: In this manner, screen burn can be prevented when the scan is lost.

SUMMARY OF THE INVENTION

Briefly, in accordance with a preferred embodiment of the present principles, there is provided a video display system that comprises a cathode ray tube having a picture display area. The display system includes a deflection system for the cathode ray tube to provide line rate scanning in a vertical direction.

A video signal processing system serves to transpose video signals supplied to the deflection system. The display system includes a circuit for controlling the heater voltages to the CRT by sensing the presence of a scan.

In accordance with one embodiment, the transposed scan cathode ray tube (CRT) includes a tubular envelope with a screen for displaying images, an electronic gun positioned within the envelope and having a cathode for generating electrons which strike the screen, a heater for heating the cathode, and a heater control circuit having an input connected ito a transformer, said heater control circuit for providing voltage to the heater when pulses are present at said input. The transformer can be the flyback transformer and the pulses represent retrace pulses from the, flyback transformer. The heater control circuit also includes means for activating the heater control circuit in response to the presence of the pulses. The activating means may be, for example, the AC coupling of the pulses.

A fast scan circuit having an energy storage inductor can be included where the aforementioned "transformer" is the energy storage inductor, and the pulses represent the presence of a fast scan output from the fast scan circuit. The pulses sensed by the heater control circuit may be either negative or positive in polarity.

According to a further embodiment, the transposed scan cathode ray tube (CRT) includes a tubular envelope with a screen for displaying images, an electronic gun positioned within the envelope and having a cathode for generating electrons which strike the screen, a

heater for heating the cathode, and a heater control circuit having an input connected to an energy storage device. The heater control circuit provides voltage to the heater of the CRT.

The heater control circuit includes means for detecting the retrace pulses of a CRT anode supply or scan circuitry. The energy storage device can be, for example, flyback transformer outputting voltage pulses during retrace intervals, or may be an inductor in a fast scan circuit, where the inductor outputs voltage pulses during fast scan. The voltage pulses may be either of negative or positive polarity.

In accordance with yet further embodiment of the present principles, the heater control circuit further includes a heater control transistor and the energy storage device provides a DC level to the base of the heater control transistor. The heater control circuit provides stable voltage to the CRT heater when the DC level is at least equal to 6 volts and less than 12 volts, and the heater control circuit provides insufficient voltage to the heater of the CRT when the DC level falls below 1 volt.

In another further embodiment, the method for controlling heater voltages in a transposed scan cathode ray tube includes the steps of providing a heater control circuit having an input connected to an energy storage device within an anode power supply circuit, sensing the presence of a control signal at the input, and outputting a voltage to the cathode heater for turning on the cathode heater and heating the cathode when the control signal is present at the input. The control signal is indicative of pulses from the energy storage device during a retrace interval. These pulses may have either positive or negative polarity.

In a further modified embodiment, the a high voltage transformer is provided within the anode power supply circuit such that said high voltage transformer operates as the energy storage device.

In yet another embodiment of the present principles, the method for controlling heater voltages in a transposed scan cathode ray tube (CRT), includes providing a heater control

circuit having an input connected to an energy storage device within a fast scan circuit of the transposed scan CRT, sensing the presence of fast scan at the input, and outputting _. a voltage to the cathode heater for turning on the cathode heater and heating the cathode when the fast scan is present at the input. The fast scan can include pulse signals having positive or negative polarity.

The energy storage device can be an energy storage inductor is provided in the fast scan circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference to the accompanying figures, wherein like reference numerals depict similar elements throughout the views:

Figure 1 is a diagram depicting the basic geometrical relationship between the throw distance and deflection angle in a typical CRT;

Figure 2a is a diagrammatic cross-sectional view of a CRT according to a preferred embodiment of the present principles; Figure 2b is a diagram representing the lines and pixels of a standard scan CRT;

Figure 2c is a diagram representing the lines and pixels of the transposed scan display according to an embodiment of the present principles;

Figure 3 is a diagram of the screen of the CRT of FIG. 2 illustrating a mis- convergence pattern in accordance with the present principles; Figure 4 is a diagram depicting optimization of spot shape in accordance with the present principles;

Figure 5 is a block diagram of the transposed scan display system incoiporating the heater voltage control of the present principles;

Figures 6a-6c are illustrative schematic diagrams of the video/deflection system having the heater voltage control of the present principles;

Figure 6d is an illustrative schematic diagram of the anode power supply and heater voltage control circuits according to an embodiment of the present principles;

Figure 7a is an illustrative schematic diagram of an oscillator circuit within the anode power supply according to an embodiment of the present principles; Figure 7b is an illustrative schematic diagram of a high voltage regulator circuit within

the anode power supply according to an embodiment of the present principles;

Figure 7c is an illustrative schematic diagram of a drive circuit within the anode power supply according to an embodiment of the present principles;

Figure 7d is an illustrative schematic diagram of the high voltage output circuit of the anode power supply and the heater, voltage control circuit according to an embodiment of the present principles;

Figure 8 is an illustrative schematic diagram of the heater control circuit according to an embodiment of the present principles; and

Figure 9 is an illustrative schematic diagram of the output stage of the fast scan circuit according to an embodiment of the present principles.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Prior to discussing the CRT display system of the present principles, a brief discussion of the facets of a typical cathode ray tube will prove helpful. Figure 2a illustrates a cathode ray tube (CRT) 1, for example a W76 wide screen tube, having a glass envelope 2 having a rectangular faceplate panel 3 and a tubular neck 4 connected by a funnel 5. The funnel 5 has an internal conductive coating (not shown) that extends from an anode button 6 toward the faceplate panel 3 and to the neck 4. The faceplate panel 3 comprises a viewing faceplate 8 and a peripheral flange or sidewall 9, which is sealed to the funnel 5 by a glass frit 7. The inner surface of the faceplate panel 3 carries a three-color phosphor screen 12. The screen 12 comprises a line screen with the phosphor lines arranged in triads. Each triad includes a phosphor line of three primary colors, typically Red, Green and Blue, and extends generally parallel to the major axis of the screen 12.

A mask assembly 10 lies in a predetermined spaced relation with the screen 12. The mask assembly 10 has a multiplicity of elongated slits extending generally parallel to the major axis of the screen 12. An electron gun assembly 13, shown schematically by dashed lines in Figure 2a, is centrally mounted within the neck 4 to generate three inline electron beams, a center beam and two side or outer beams, directed along convergent paths through the mask frame assembly 10 to strike the screen 12. The electron gun assembly 13 has three vertically oriented guns, each generating an electron beam for a separate one of the three colors, Red, Green and Blue. The three guns lie in a linear array extending parallel to a minor axis of the screen 12.

The CRT 1 employs an external magnetic deflection system comprised of a yoke 14 situated in the neighborhood of the funnel-to-neck junction. When activated with deflection currents, the yoke 14 generates magnetic fields that cause the beams to scan over the screen 12 vertically and horizontally in a rectangular raster.

Conventional video signal transmission assumes a pixel-by-pixel time sequence such that transmission of Red, Green and Blue images effectively occurs as a series of scan lines proceeding from the left edge to the right edge of the image along a scan line and then moving down to the next scan line where again the signal sequence proceeds from left to right. This process continues from top to bottom, in either a progressive scan mode or an interlaced scan mode, as is known in the art. Figure 2b shows an example of a standard orientation (scan) CRT having 720 horizontally scanned lines each having a pixel width of 1280.

To achieve a vertical scan (or transposed scan) display, the image must undergo a translation into a vertical scan pattern such that the signal sequence starts at the upper left hand corner of the image. The subsequent signal elements then follow along a vertical line from top to bottom along the left edge. After an appropriate retrace interval, generation of a signal element at the top edge of the image at the second scan line occurs, followed by the signal elements corresponding to a sequence from top to bottom along the second scan line. ' Similarly the third scan line starts at the top and proceeds to the bottom of the image, and thus the corresponding top to bottom signal element must be provided. This process continues through the last scan line at right vertical edge of the image.

To effect vertical scanning, the horizontal scan sequence must undergo a change from a conventional left-to-right and stepwise top-to-bottom regimen to a top-to- bottom and stepwise left to right transposed sequence. Figure 2c shows an example of the vertical scanning of the transposed scan display according to an embodiment of the present principles. As shown in this example, there are 1280 vertically scanned lines, each having a length of 720 pixels.

For the purposes of the following discussion, the terms "Digital Orthogonal Scan" and/or DOS refer to the above-described transposition operation and are used herein interchangeably with the term "transposed scan display".

In accordance with one aspect of the present principles, the electron beam undergoes spot shaping. To understand spot shaping, a discussion of the yoke 14 and the effect of the yoke fields will prove helpful. As discussed, the yoke 14 lies in the neighborhood of the funnel-to-neck junction on the CRT 1 as shown in Figure 2a. In the illustrated embodiment, the yoke 14 has first deflection coil system (not shown) that generates a horizontal deflection yoke field that is substantially barrel-shaped. The yoke 14 has a second deflection coil system (not shown) electrically insulated from the first deflection coil system for generating a vertical yoke field that is substantially pincushion-shaped. These yoke fields affect beam convergence and spot shape. Rather than adjust for self-convergence, the horizontal barrel field shape associated with the first deflection system undergoes an adjustment (e.g., a reduction), to yield an optimized spot shape at the sides of the screen. The barrel shape of the yoke field attributable to the second deflection coil system undergoes a reduction. The combined effects of the barrel-shaped field and the dynamic astigmatism correction provided by the dynamic focus associated with the electron guns yields an optimized, nearly round spot shape at the 3/9 position and at the corner screen locations. The use of pincushion vertical field and a barrel horizontal field, where the barrel horizontal field is adjusted to improve spot shapes and allow some mis-convergence of the electron beams along the screen edges is characterized as quasi- self-convergent deflection fields.

The field reduction that results in improved spot shape from self-convergence actually causes mis-convergence at certain locations on the screen. Figure 3 illustrates a transposed scan display screen showing the resulting mis-convergence from such a reduced barrel-shaped field. For example, when the barrel field undergoes a reduction to achieve an optimized spot at the 3/9 positions and at the corner locations of the screen, the beams over-converge at the sides of the screen. Over-convergence as used here refers to a condition that results from the

red and blue beams crossing over each other prior to striking the screen. The amount of over- convergence varies as a function beam deflection. Thus, the resultant pattern appears converged at the center of the screen while appearing mis-converged at the sides of the screen. Assuming the electron gun assembly 13 of Figure 2a has its red, green, and guns orientated from top to bottom, the over-convergence causes the electron beams to generate a blue, green, red convergence pattern at the sides of the screen as seen in FIG. 3. The resultant over- convergence at the screen sides in this example was measured at 15 millimeters. Other CRT designs having different geometries or different yoke field distributions will result in more or less over-convergence, for example, in the range of 5 to 35 millimeters. The addition of multipole coils, such as the quadrupole coils 16 shown in Figure 2a, can correct for mis-convergence, or over-convergence that results from the yoke effect described above. In particular, locating the quadrupole coils 16 on the gun side of the yoke 14 will dynamically correct for the yoke effect. The quadrupole coils 16 are fixed to the yoke 14 or alternatively, can be applied to the neck and have their four poles oriented at approximately 90° angles relative to each other as is known in the art. The adjacent poles of the coils 16

have alternating polarity and the orientation of their poles lies at 45° from the tube axes so that

the resultant magnetic field displaces the outer (red and blue) beams in a vertical direction to provide correction for the mis-convergence pattern shown in FIG. 3. Alternatively, the quadrupole coils 16 can lie behind the yoke 14 approximately at or near the dynamic astigmatism correction point of the guns of the electron gun assembly 13.

Operating under dynamic control, the quadrupole coils 16 create a correction field for adjusting mis-convergence on the screen. The quadrupole coils 16 in this embodiment are driven in synchronism with the horizontal deflection. The signal driving the quadrupole coils 16 has a magnitude selected to correct the over-convergence described above. In an illustrated embodiment, the quadrupole coil signal has a waveform whose shape approximates

a parabola.

The electron gun assembly 13 of the CRT 1 has electrostatic dynamic focus. astigmatism correction to achieve optimum focus in both the horizontal and vertical directions of each of the three beams. This electrostatic dynamic astigmatism correction occurs separately for each beam, thereby allowing for correction of the horizontal-to-vertical focus voltage differences without affecting convergence. Although the quadrupole coils 16 affect beam focus, their location near the dynamic astigmatism point of the guns of the electron gun assembly 13 allows for correction of this effect by adjusting the electrostatic dynamic astigmatism voltage so that there is a minimal effect on the spot. This enables correction of mis-convergence at selected locations on the screen without affecting the spot shape. Advantageously, modification of the yoke field design can optimize spot shape and the dynamically driven quadrupole coils 16 can correct for any resultant mis-convergence.

Figure 4 illustrates one quadrant of the screen of a W76 CRT with an aspect ratio of

16:9 and a 120° deflection angle and shows the improvement in spot shape and size obtained

by the design of the yoke 14 and the use of the quadrupole coils 16 as discussed above. The spots illustrated by the dotted lines represent the effects of throw distance and obliquity referenced to a round center spot. Optimized spots obtained in accordance with the present principles appear with solid lines. Significant improvements in spot size and shape appear at the sides and corners of the screen. Table 1 lists experimental results for an illustrative embodiment in accordance with the present principles, with H representing the horizontal dimension of each spot, and V representing the vertical dimension of each spot normalized to the center spot. Table 1 compares the cumulative effect of gun orientation, yoke field effects and dynamically controlled

quadrupole coils with dynamic astigmatism correction applied to traditional horizontal inline gun CRTs.

TABLE l

The center column of Table 1 lists the spot dimensions for a prior art standard horizontal gun orientation CRT with self -convergent beams, whereas the right-hand column represents the result ^' s for a CRT with vertical gun alignment in accordance with the present principles wherein the beams undergo dynamically controlled convergence. Although spot shape suffers a slight compromise at the 6 O'clock and 12 O'clock screen positions (6/12 or otherwise as the top and bottom), spot size uniformity shows great improvement at the 3 O'clock and 9 O'clock positions (3/9 or otherwise as the side) and at the corner locations. The present technique advantageously provides more uniform spot shape across the screen, thus enhancing visual resolution. Although the invention is applicable to CRTs having deflection angles at 100 or above, the invention has particular applicability to much larger deflection angles such as systems exceeding 120 degrees. In general, CRT displays exhibit raster distortions. The most common raster distortions pertain to geometric errors and to convergence errors. A geometric error results from non-linearities in the scanned positions of the beams as the raster traverses the screen. Convergence errors occur in a CRT display when the Red, Green and Blue rasters do not align

perfectly such that over some portion of the image, a Red sub-image appears offset with respect to the Green sub-image and the Blue sub-image appears offset to the right of the Green sub-image. Convergence errors of this type can occur in any direction and can appear anywhere in the displayed image. With known color CRT displays, both convergence and geometric errors can occur despite perfect alignment of the center region during the original manufacture of the CRT display, assuming that the deflection signals applied to the deflection coils ramp linearly. Traditional analog circuit techniques compensate for such distortions by modifying the deflection signals from linear ramps to more complex wave shapes. Also, adjustment in the details of the yoke design can reduce convergence errors and geometry errors. As the deflection angle increases beyond 100°, however, the traditional methods of geometry and convergence corrections become more difficult to implement.

It becomes clear from the above, that when the deflection angle is increased in a transposed scan display, thereby decreasing the throw distance of the electron gun, many considerations and corrections are required in order to compensate for the negative effects on the displayed image resulting from such changes in design.

The display system of the present invention includes a video deflection system for the CRT to provide line rate scanning in a transposed or the vertical direction. This digital orthogonal scanning (DOS) provides a fast scan in the short direction of a 16:9 format screen. Figure 5 shows a block diagram of the transposed scan display system 100 according to an embodiment of the present principles. As shown, an input from a high definition (HD) video source 102, such as, for example from a cable, satellite, network or other service provider is provided to the display system. The high resolution source input is fed to an FPGA 110 where it is processed and then input into the video processor 116. In some instances, an RGB to YPrPb converter 104 may be required to input the Y, Pr and Pb signals

to the video processor 116. In addition, content source 102 provides horizontal and vertical sync signals (H & V) which are processed by the FPGA 110 and sent to the sync processor 118.

The video processor 116 outputs the RGB video signals to the video drivers 133 which 5 drive the electron gun of the slim transposed scan (DOS) CRT 200.

The sync processor 118 outputs several signals including synchronization signals to a waveform generator 120 embodied within the microprocessor 112 in order to generate the appropriate waveform for the quad coil drivers 130, and for N-S Pincushion Modulator 124. In other contemplated embodiments, the waveform generator can be incorporated into the 10 FPGA 110 and thus be eliminated from the microprocessor the circuit shown in Figure 3.

The sync processor 118 is responsible for handling the synchronization of the output signals to the transposed scan (DOS) CRT 200. As such, it is responsible for the fast scan (V Drive) and slow scan (H Drive) signals input to the V scan 128 and H scan 126 circuits, respectively. Sync processor 118 also provides control signals to the focus modulation 15 generator 120, which controls the dynamic focus output 121 connected to the anode power

supply 134.

In accordance with one embodiment, the video processor 116 may include OSD insertion 117 capabilities. In other contemplated embodiments, the OSD may be integrated into video processor 116, or the microprocessor 112, or the FPGA 110 without departing from 20 the spirit of the present disclosure. Those of skill in the art will recognize that many of the components shown in the block diagram of Figure 5 can be embodied in an application specific integrated circuit (ASIC), or other specialized integrated circuit without departing from the spirit of the present principles. Examples of such circuits that could be embodied in one or more ASICS would be, FPGA 110, Microprocessor 112, RGB to YPrPb converter 104, 25 sync processor 118, video processor 116 and/or focus modulation generator 120.

Microprocessor 112 functions to control the video processor 116, the OSD 117, the FPGA 110 and the SW mode power supply 113. An IR pickup 114/ keyboard or other user interface device may be connected to the microprocessor for providing remote control capability to the system 100. The Anode power supply outputs the G2, G3 and G5 voltages to the appropriate pins

(not shown) of the electron gun 13. In addition, it provides a 3OkV anode voltage to the transposed scan CRT 200. The quad drivers 130 drive the quad coils 16 of the CRT, and the V scan 128 and H scan 126 circuits drive the yoke 14. The video drivers 133 provide the video signals to the electron gun for display on the CRT 200. The fast scan sync waveform generated by the V scan circuit 128, is used by: the sync processor 118 for phase correction; the video processor 116 to generate blanking; the SW mode power supply 113 for synchronization and the anode power supply 134 for synchronization.

The present principles provide a method and corresponding circuitry for controlling ' the heater voltages to the heaters in CRTs and transposed scan CRTs. Figures 6a-6d show exemplary schematic circuit diagrams blocked according to the block diagram of Figure 5. The details of the inner-workings of these circuits are described below with reference to the schematic diagrams shown in Figures 7-9.

Figure 6d shows the anode power supply 134 and the dynamic focus output circuit 121. According to one aspect of the present principles, anode power supply 134 includes several circuits 650, 660, 670 and 680, which are now described with reference to Figures 7a-

7d.

Figure 7a shows an exemplary circuit diagram of an oscillator circuit 650 according to an embodiment of the present principles. This oscillator circuit 650 functions to maintain the phase control of the anode power supply by allowing the retrace pulse to be set so as to phase it with respect to the scan. The oscillator 650 also functions to: 1) provides a reference +5

volt supply at output 71OA to the regulator circuit 660 at input 71OB (Figure 7b); and 2) provides an output signal through output 720A to input 720D of the high voltage circuit 680 _. (Figure 7d) to prevent failures of the circuit, and more particularly to prevent failure in the operating transistor LQ610 in the high voltage circuit 680. Figure 7b shows an exemplary circuit diagram of the high voltage regulator circuit 660 according to an embodiment of the present principles. The regulator circuit 660 outputs a High voltage supply voltage HVB+ at output 730B to the input 730D of the high voltage circuit 680 (Figure 7d).

Figure 7c shows an exemplary circuit diagram of the drive circuit 670 within the anode power supply according to an embodiment of the present principles. Drive circuit 670 provides the drive voltage and waveform to the drive transformed in the high voltage circuit 680. The drive circuit 670 receives the high voltage drive signal (HVDRV) from the oscillator circuit 650. The HVDRV signal is applied at input 750D for the drive circuit 670. The driver circuit outputs the HVDRV signal at output 740C, where it is input to the high voltage circuit at input 740D (Figure 7d).

Figure 7d shows an exemplary circuit diagram of a high voltage output circuit 680 within the anode power supply according to an embodiment of the present principles. The high voltage circuit 680 includes a heater voltage control circuit 800 that receives a voltage signal input from the energy storage inductor 702 in the anode supply, according to one embodiment, and from energy storage inductor 960 in the fast scan circuit, according to a second embodiment (See Figure 9). The voltage signal from energy storage inductor 702 is representative of the presence of retrace pulses in the flyback transformer. The voltage signal from energy storage inductor 960 is representative of the presence of fast scan. The input signal can be a pulse of either polarity or potentially be based on a DC level. Referring to Figure 8, during operation, Transistor LQ602 is turned on by resistor

LR618 when there is no signal sensed at the input 802 of the circuit 800. This causes the output voltage to drop to about 2V which is insufficient to keep the cathodes emitting electrons. When the retrace pulses from the anode supply are present (according to one embodiment), the input to circuit 800 receives the pulse signals (in the form of voltage signals from the energy storage inductor 702. When the fast scan output is present (in the second embodiment), voltage pulses from 914 ar inductor 960 (Figure 6) provide the input signal pulses to circuit 800. When the voltage pulses (e.g., retrace pulses) are present at input 802, the circuit responds by shutting off transistor LQ602 and providing a stable 6.3 volts to the CRT heaters via output 806. Resistor LR614 drops the 7 volts generated by the circuit 800 to the stable output voltage of 6.3 volts.

In operation, the incoming retrace pulse signal is AC coupled and DC restored, and the amplitude limited (e.g., components LC605, LR617 and LD609 function for this aspect of the circuit). Once DC restored, the input signal is rectified (LD606) and filtered (LC606) so as to ¹ provide a DC level signal to the transistor LQ602. The voltage level of the signal at the base of LQ602 functions to turn it on or off as described above.

In accordance with another embodiment, the control signals provided at the base (805) of transistor LQ602 can be in the form of a DC level having predetermined thresholds. For example, when the DC level at input 802 falls below 1 volt, the circuit 800 outputs a lowered regulated voltage to the heaters of the CRT, that is insufficient for normal operation. When the DC level at the base of transistor LQ602 is equal to or greater than 6 volts, but not more than 12 volts, the circuit 800 responds and turns on the heater voltage to the stable 6.3 volts required.

Those of skill in the art will recognize that the circuit element values shown in the figures are for exemplary purposes and other values may be substituted without departing from the spirit of the present principles. Those of skill in the art will also recognize that in

other implementations of the concept, the input signal may be a pulse of either polarity or could be a voltage of changing amplitude or polarity and that the source can be from a circuit other than a transformer.

While there have been shown, described and pointed out fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions, substitutions and changes in the form and details of the methods described and devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed, described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.