A CIRCUIT AND METHOD FOR TIME MULTIPLEXING VOLTAGE SIGNALS

Abstract: A circuit for time multiplexing a voltage signal for controlling the color balance of a flat panel display (200). Within an FED screen, a matrix of rows (230) and columns (250) is provided and emitters are situated within each row-column intersection (100). Rows are sequentially activated during "row on-time windows" by row drivers (220) and corresponding individual gray scale information (voltages) are driven over the columns by column drivers (240). Within each column driver, the present invention provides selection circuitry for driving a first voltage signal during a first part of the row on-time window and a second voltage during a second part of the row on-time window. The lengths of the first part and second part of the row on-time window can be adjusted for a given color, to adjust the color balance with respect to color.

Description: -1- A CIRCUIT AND METHOD FOR TIME MULTIPLEXING VOLTAGE SIGNALS BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to the field of flat panel display screens. More specifically, the present invention relates to the field of flat panel field emission display (FED) screens. In one embodiment the invention discloses a circuit and method for time multiplexing voltage signals for controlling the color balance of a flat panel display unit. RELATED ART In the field of flat panel display devices, much like conventional cathode ray tube (CRT) displays, a white pixel is composed of a red, a green and a blue color point or "spot." When each color point of the pixel is excited simultaneously, the pixel appears white. To produce different colors at the pixel, the intensity to which the red, green and blue points are driven is altered using well known techniques. The separate red, green and blue data that correspond to the color intensities of a particular pixel are called the pixel's color data. Color data is often called gray scale data. The degree to which different colors can be achieved within a pixel is referred to as gray scale resolution and is directly related to the amount of different intensities to which each red, green and blue point can be driven. Field emission display (FED) screens, like CRT displays, utilize phosphor spots to generate the red, green and blue color points of a pixel. Often, during manufacturing, the characteristics of the phosphor of the display screen for a particular color can vary from screen to screen. If the phosphor has different characteristics, then its color intensity will vary from screen to screen thereby producing screens with different color balance. Therefore, it is important that a display screen have a mechanism for altering the relative color intensities of the color points so that manufacturing variations in the phosphor can be compensated for in the display screen. The method of altering the relative color intensities of the color points across a display screen is called white balance adjustment (also referred to as color balance adjustment or color temperature adjustment). Another reason for providing color balance adjustment, in addition to correcting for manufacturing variations in the phosphor, is to correct for phosphor aging through prolonged display use. It is typical for the light emitting characteristics of the phosphor of an FED screen to change over time as it is used. Therefore, it is important that a display screen have a mechanism for altering its color balance to correct for phosphor aging to maintain image quality throughout the life of the FED screen. A further reason for providing color balance adjustment within a display screen is to allow the viewer to manually adjust the color balance. Using a manual adjustment, users can adjust the white balance of the display screen to their particular viewing taste. One method for correcting or altering the color balance within a display screen is to alter, on the fly, the color data used to render a screen. Instead of sending a particular color point a color value of X, the color value of X is first passed through a function that has complex gain and offset adjustments. The output of the function, Y, is then sent to the color point. The function compensates for any variations in the color temperature caused by phosphor variations. The gain and offset factors of the above function can be altered as the color temperature needs to be increased or decreased. Although offering dynamic color balance adjustment, this prior art mechanism for altering the color balance is disadvantageous because it requires relatively complex circuitry for -3- altering a relatively large volume of color data. For instance, in order to represent the color balance function, a look-up table (LUT) is used for each column. The additional circuitry (e.g., a LUT) that this prior art mechanism requires adds significantly to the overall size of the driver circuits and negatively impacts performance speed. Assuming a horizontal screen resolution of 1024 white pixels, there can be as many as 3072 column drivers per FED screen and a complex LUT circuit replicated over 3072 column drivers may require too much substrate area for practical fabrication. Secondly, this prior art mechanism may degrade the quality of the image by reducing the gray-scale resolution of the flat panel display. It is desirable to provide a color balance adjustment mechanism for a flat panel display screen that does not alter the image data nor compromise the gray-scale resolution of the image. Another method of correcting for color balance within a flat panel display screen is used in active matrix flat panel display screens (AMLCD). This method pertains to altering the physical color filters used to generate the red : green and blue color points. By altering the color the filters, the color temperature of the AMLCD screen can be adjusted. However, this adjustment is not dynamic because the color filters need to be physically (e.g., manually) replaced each time adjustment is required. It would be advantageous to provide a color balancing mechanism for a flat panel display screen that can respond, dynamically, to required changes in the color temperature of the display. Figure 1 illustrates a graph 6 of a typical data-in voltage-out curve that is embedded within a digital to analog converter circuit of an AMLCD flat panel display. The digital to analog converter is responsible for transforming the digital color data to voltages that are used to generate the actual color intensity. When presented with color data from 0 to 63, the voltages corresponding to curve portion 2 are supplied as output to drive the color points. When presented with color data from 64 to 127, the voltages corresponding to curve portion 4 are supplied as output to drive the color points. Curve portion 4 may be the same as curve portion 2 except with a DC voltage offset. Curve portion 4 and curve portion 2 are used in alternating refresh cycles so that no net DC voltage is applied to the cells of the AMLCD display. Prolonged exposure to DC voltage can destroy the AMLCD display. Therefore, the gray scale resolution of the AMLCD device using curves 2 and 4 is only from 0 to 63, although 127 data positions exist. This is the case because positions 64 to 127 are only duplicates, respectively, of positions 0 to 63. Although used in the manner described above, the data-in voltage-out function of Figure 1 has never been applied to perform any type of color balancing operations. Accordingly, the present invention provides a mechanism and method for dynamically adjusting the color balance of a flat panel display. The present invention provides a mechanism and method for adjusting the color balance of a flat panel display screen that does not significantly compromise the gray-scale resolution of the pixels of the display screen. Further, the present invention provides a mechanism and method for adjusting the color balance of a flat panel display screen without significantly increasing the size of the column driver circuits. Further, the present invention provides a mechanism and method for controlling the color balance of a flat panel FED screen while providing a power savings operational mode. These and other advantages of the present invention not specifically mentioned above will become clear within discussions of the present invention presented herein. -5- SUMMARY OF THE INVENTION A circuit and method are described for time multiplexing a voltage signal for controlling the color balance of a flat panel display. Adjustment of color balancing can be done in response to tube aging, viewer taste and/or manufacturing variations in the phosphor. Within an FED screen, a matrix of rows and columns is provided and emitters are situated within each row-column intersection. Rows are sequentially activated during "row on-time windows" by row drivers and corresponding individual gray scale information (voltages) are driven over the columns by column drivers. When the proper voltage is applied across the cathode and anode of the emitters, electrons are released toward a phosphor spot, e.g., red, green, blue, causing illumination. Within each column driver, the present invention provides selection circuitry for driving a first voltage signal during a first ("full") part of the row on-time window and a second voltage signal during a second ("half) part of the row on-time window. The total or effective voltage applied to a given column is therefore a weighted average of the two voltages applied during the first part and the second part of the row on-time window. The weights of the weighted average is represented by the respective lengths of the first and second parts, respectively. The lengths of the first part and second part of the row on-time window can be adjusted, individually for a given color, to adjust the total voltage applied. This effectively adjusts the color balance with respect to that color, e.g., red, green or blue. In one embodiment of the present invention, a shift register is used to divide a digital representation of the first voltage value in half for application during the second part of the row on-time window. The first voltage value being applied during the first part of the row on-time window. In a second embodiment, a multiplexer is used to divide the first voltage value in half for -6- application during the second part. Again, the first voltage value being applied during the first part of the row on-time window. In a third embodiment, the order of the first and second parts of the row on-time window are swapped with respect to every other consecutive row on-time window such that two first parts occur consecutively and two second parts occur consecutively over a period of two row on-time windows. The third embodiment reduces the frequency of voltage changes and thereby saves power. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a data-in voltage-out function used by an active matrix liquid crystal display (AMLCD) of the prior art. Figure 2 is a cross-section structural view of part of a flat panel FED screen that utilizes a gated field emitter situated at the intersection of a row line and a column line. Figure 3 illustrates a plan view of an flat panel FED screen in accordance with the present invention illustrating row and column drivers and numerous intersecting rows and columns. Figure 4 is a plan view of internal portions of the flat panel FED screen of the present invention and illustrates several intersecting row lines and column lines of the display, including at least one pixel. Figure 5 is an illustration of three exemplary column drivers (red/green/blue) of the flat panel FED screen of the present invention. Figure 6 is an overall block diagram of a circuit of the present invention for the time multiplexed application of column voltages for color balancing. Figure 7 illustrates the red, green and blue column driver amplifier circuits of an exemplary ith white pixel group in accordance with the present invention. Figure 8A is a circuit diagram of color balance adjustment circuitry used by a first embodiment the present invention in an exemplary ith red column driver for driving the ith red column line. Figure 8B is a circuit diagram of color balance adjustment circuitry used by the first embodiment of the present invention in an exemplary ith green column driver for driving the ith green column line. Figure 8C is a circuit diagram of color balance adjustment circuitry used by the first embodiment of the present invention in an exemplary ith blue column driver for driving the ith blue column line. -8- Figure 9A is a circuit diagram of color balance adjustment circuitry used by a second embodiment the present invention in an exemplary ith red column driver for driving the ith red column line. Figure 9B is a circuit diagram of color balance adjustment circuitry used by the second embodiment of the present invention in an exemplary ith green column driver for driving the ith green column line. Figure 9C is a circuit diagram of color balance adjustment circuitry used by the second embodiment of the present invention in an exemplary ith blue column driver for driving the ith blue column line. Figure 10 illustrates a multiplexing circuit used by the second embodiment of the present invention to perform color balancing. Figure 11 illustrates circuitry for generating red, green and blue selection signals used by the first and second embodiments of the present invention for performing color balancing. Figure 12A illustrates timing diagrams of the relevant signals used by the first and second color balancing embodiments of the present invention for an exemplary color, e.g., red. Figure 12B illustrates timing diagrams of the relevant signals used by the first and second color balancing embodiments of the present invention for an exemplary color, e.g., green. Figure 13 illustrates a ramp generator circuit used by a third embodiment of the present invention for generating timing signals for time multiplexing voltage signals for one color. Figure 14 illustrates a ramp generator circuit used by a third embodiment of the present invention for generating timing signals for time multiplexing voltage signals for red, green and blue colors. -9- Figure 15 illustrates timing diagrams of the relevant signals used by the third color balancing embodiment of the present invention for an exemplary color, e.g., red. Figure 16 illustrates timing diagrams of the relevant signals used by the third color balancing embodiment of the present invention for an exemplary color, e.g., green. -10- DETAILED DESCRIPTION OF THE INVENTION In the following detailed description of the present invention, a method and mechanism for using time multiplexing of voltage signals for dynamically altering the color balance within a flat panel FED screen without significantly compromising gray-scale resolution, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one skilled in the art that the present invention may be practiced without these specific details or with equivalents thereof. In other instances, well known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. FLAT PANEL FED SCREEN ORGANIZATION OF THE PRESENT INVENTION Embodiments of the present invention are drawn to mechanisms and methods for providing color balance adjustments within an FED display screen. Preceding a discussion of the color balance adjustment circuitry of the present invention, a discussion of certain elements of an FED display screen is discussed. Specifically, a discussion of an emitter of a field emission display (FED) is now presented. Figure 2 illustrates a cross-sectional diagram of a multi-layer structure 75 which is. a portion of an FED flat panel display. The multi-layer structure 75 contains a field-emission backplate structure 45, also called a baseplate structure, and an electron-receiving faceplate structure 70. An image is generated by faceplate structure 70. Backplate structure 45 commonly consists of an electrically insulating backplate 65, an emitter (or cathode) electrode 60, an electrically insulating layer 55, a patterned gate electrode 50, -11- and a conical electron-emissive element 40 situated in an aperture through insulating layer 55. One type of electron-emissive element 40 is described in United States Patent Number 5,608,283, issued on March 4, 1997 to Twichell et al. and another type is described in United States Patent Number 5,607,335, issued on March 4, 1997 to Spindt et al., which are both incorporated herein by reference. The tip of the electron-emissive element 40 is exposed through a corresponding opening in gate electrode 50. Emitter electrode 60 and electron- emissive element 40 together constitute a cathode of the illustrated portion 75 of the. FED flat panel display 75. Faceplate structure 70 is formed with an electrically insulating faceplate 15, an anode 20, and a coating of phosphors 25. Electrons emitted from element 40 are received by phosphors portion 30. Anode 20 of Figure 2 is maintained at a positive voltage relative to cathode 60/40. In one embodiment, the anode voltage is 100-300 volts for spacing of 100-200 urn between structures 45 and 70 but in other embodiments with greater spacing the anode voltage is in the kilovolt range. Because anode 20 is in contact with phosphors 25, the anode voltage is also impressed on phosphors 25. When a suitable gate voltage is applied to gate electrode 50, electrons are emitted from electron-emissive element 40 at various values of off- normal emission angle theta 42. The emitted electrons follow non-linear (e.g., parabolic) trajectories indicated by lines 35 in Figure 2 and impact on a target portion 30 of the phosphors 25. The phosphors struck by the emitted electrons produce light of a selected color and represent a phosphor spot or point. A single phosphor spot can be illuminated by thousands of emitters. Phosphors 25 of Figure 2 are part of a picture element ("pixel") that contains other phosphors (not shown) which emit light of different color than that produced by phosphors 25. Typically a pixel contains three phosphor or "color" spots, a red spot, a green spot and a blue spot. Also, the pixel containing -12- phosphors 25 adjoins one or more other pixels (not shown) in the FED flat panel display. If some of the electrons intended for phosphors 25 consistently strike other phosphors (in the same or another pixels), the image resolution and color purity can become degraded. As discussed in more detail below, the pixels of an FED flat panel screen are arranged in a matrix form including n columns and x rows. In one implementation, a pixel is composed of three phosphor spots aligned in the same row, but having three separate columns. Therefore, a single pixel is uniquely identified by one row and three separate columns (a red column, a green column and a blue column). As described more fully below, each column of the three columns that constitute a pixel is associated with its own column driver circuit. The size of target phosphor portion 30 depends on the applied voltages and geometric and dimensional characteristics of the FED flat panel display 75. Increasing the anode/phosphor voltage to 1,500 to 10,000 volts in the FED flat panel display 75 of Figure 2 requires that the spacing between the backplate structure 45 and the faceplate structure 70 be much greater than 100-200 urn. Increasing the interstructure spacing to the value needed for a phosphor potential of 1,500 to 10,000 causes a larger phosphor portion 30, unless electron focusing elements are added to the FED flat panel display of Figure 2. Such focusing elements can be included within FED flat panel display structure 75 and are described in United States Patent Number 5,528,103 issued on June 18, 1996 to Spindt, et al., which is incorporated herein by reference. Importantly, the intensity of the target phosphor portion 30 of Figure 2 depends on the magnitude of the incident current which is itself dependent upon the voltage potential applied across the cathode 60/40 and the gate 50. Thus, the intensity of a color spot is related to the voltage differential applied between the row and column at whose intersection the color spot is located. The larger -13- the voltage potential, the larger the intensity of the target phosphor portion 30. Secondly, the intensity of the target phosphor portion 30 depends on the amount of time a voltage is applied across the cathode 40/60 and the gate 50 (e.g., on- time window). The larger the on-time window, the larger the intensity of the target phosphor portion 30. Therefore, within the present invention, the intensity of FED flat panel structure 75 is dependent on the voltage and the amount of time (e.g., "on-time") the voltage is applied across cathode 60/40 and the gate 50. The effective voltage (EV) is obtained by taking both voltage amplitude and voltage on-time into consideration. As shown in Figure 3, the FED flat panel display 200 is subdivided into an array of x horizontally aligned row lines 230 ("rows") and n vertically aligned column lines 250 ("columns"). The pixels of the FED flat panel display 200 are also aligned vertically and horizontally. Color points (also called "phosphor spots") are formed at each intersection of row and a column. Three adjacent color points of a same row, a red, a green and a blue, form a pixel. For n pixels horizontally, there are 3n columns. For x pixels vertically, there are x rows. The FED flat panel display 200 of Figure 3 is described in more detail further below. A portion 100 of this FED flat panel display 200 is shown in more detail in Figure 4 and includes at least one full pixel. Specifically, Figure 4 illustrates a respective pixel 125 (also called "white group"). The respective pixel 125 of Figure 4 contains a red phosphor spot 125a, a green phosphor spot 125b and a blue phosphor spot 125c of a same emitter line (also called "row electrode" or "row") 230. In one embodiment, each phosphor spot of a pixel is controlled by a different column driver, but all phosphor spots of a pixel are controlled by the same row driver because all phosphor spots of a same pixel reside within the same row 230. The exemplary ith pixel 125 is therefore located at the ith red column line, ith green column line, the ith blue column line and the jth row line. -14- The boundaries of the respective pixel 125 of Figure 4 are indicated by dashed lines. Three separate emitter lines 230 (row lines) are also shown. Each emitter line 230 is a row electrode for one of the rows of pixels in the array. The middle row electrode 230 is coupled to the emitter cathodes 60/40 (Figure 2) of each emitter of the particular row associated with the electrode. A portion of one pixel row is indicated in Figure 4 and is situated between a pair of adjacent spacer walls 135. A pixel row is comprised of all of the pixels along one row line 250. Two or more pixel rows (and as much as 24-100 pixel rows), are generally located between each pair of adjacent spacer walls 135. Each column of pixels has three gate lines (also called "columns") 250: (1) one for red; (2) a second for green; and (3) a third for blue. Likewise, each pixel column includes one of each phosphor stripes (red, green, blue), three stripes total. Each of the gate lines 250 is coupled to the gate 50 (Figure 2) of each emitter structure of the associated column. This structure 100 is described in more detail in United States Patent Number 5,477,105 issued on December 19, 1995 to Curtin, et al., which is incorporated herein by reference. In one embodiment, the red, green and blue phosphor stripes 25 (Figure 2) are maintained at a positive voltage of 1 ,500 to 10,000 volts relative to the voltage of the emitter-electrode 60/40. When one of the sets of electron- emission elements 40 is suitably excited by adjusting the voltage of the corresponding row (cathode) lines 230 and column (gate) lines 250, elements 40 in that set emit electrons which are accelerated toward a target portion 30 of the phosphors in the corresponding color. The excited phosphors then emit light. During a screen frame refresh cycle (performed at a rate of approximately 60 Hz in one embodiment), only one row is active at a time and the column lines are energized to illuminate the one row of pixels for the row on-time period. This is performed sequentially in time, row by row, until all pixel rows have been -15- illuminated to display the frame. Frames are presented at 60 Hz. Assuming n rows of the display array, each row is energized during the row on-time window at a rate of 16.7/n ms. The above FED 100 is described in more detail in the following United States Patents: US Patent No. 5,541 ,473 issued on July 30, 1996 to Duboc, Jr. et al.; US Patent No. 5,559,389 issued on September 24, 1996 to Spindt et al.; US Patent No. 5,564,959 issued on October 15, 1996 to Spindt et al.; and US Patent No. 5,578,899 issued November 26, 1996 to Haven et al., which are incorporated herein by reference. Row and Column Array. As discussed above, Figure 3 illustrates an FED flat panel display screen 200 organized as an array of rows and columns in accordance with the present invention. Specifically, the screen contains x rows and n columns of "pixels". Region 100, as described with respect to Figure 4, is also shown in its relative position in Figure 3. The FED flat panel display screen 200 consists of x number of row lines (horizontal) and 3n number of column lines (vertical) to achieve (xn) total pixels, e.g., three column lines per pixel are required. For clarity, a row line is called a "row" and a column line is called a "column." Row lines are driven by x row driver circuits 220a-220c which in one embodiment are integrated circuits. Shown in Figure 3 are exemplary row groups 230a, 230b and 230c. Each row group contains an arbitrary number of rows (e.g., y) that are all associated with a particular row driver circuit; three respective row driver circuits are shown 220a-220c. In one embodiment of the present invention, there are over 400 rows (x=400) and therefore 400/y number of individual row groups 230 and associated row drivers 220. However, it is appreciated that the present invention is equally well suited to an FED flat panel display screen 200 having any number of rows. Also shown in Figure 3 are column groups 250a, 250b, 250c and 250d which in one embodiment are integrated circuits. In one embodiment of the -16- present invention there are over 1920 columns to allow n=640 pixels (1920/3 = 640). A pixel requires three columns (red, green, blue), therefore, 1920 columns provides at least 640 pixel resolution horizontally. However, it is appreciated that the present invention is equally well suited to an FED flat panel display screen having any number of columns. Like row drivers 220, the column drivers 240 can be separated into multiple independent column drivers each responsible for driving a group of columns. The Row Driver Circuits 220. Row driver circuits 220a-220c of Figure 3 are preferably placed along the periphery of the substrate area FED flat panel display screen 200. In Figure 3, only three row drivers are shown for clarity. As discussed, each row driver 220a-220c is responsible for driving a group of rows. For instance, row driver 220a drives rows 230a, row driver 220b drives rows 230b and row driver 220c drives rows 230c. Although an individual row driver is responsible for driving a group of rows, only one row is active (e.g., driven) at a time across the entire FED flat panel display screen 200. Therefore, any individual row driver circuit drives at most one row line at a time, and when the active row line is not in its group during a refresh cycle it is not driving any row line. A supply voltage line 212 is coupled in parallel to all row drivers 220a- 220c and supplies the row drivers with a driving voltage for application to the cathode 60/40 of the emitters. In one embodiment, the row driving voltage is negative in polarity, but could be positive in other embodiments. An enable signal is also supplied to each row driver 220a-220c in parallel over enable line 216 of Figure 3. When the enable line 216 is low, all row drivers 220a-220c of FED screen 200 are disabled and no row is energized. When the enable line 216 is high, the row drivers 220a-220c are enabled. -17- A horizontal clock signal ("H SYNCH") is also supplied to each row driver 220a-220c of Figure 3 in parallel over clock line 214 of Figure 3. The horizontal clock signal 214 (or synchronization signal) pulses each time a new row is to be energized and marks the start of a row on-time window. The horizontal clock signal 214 also synchronizes the loading of new column color data into the column driver circuits 240. Therefore, the x rows of a display frame are energized, one at a time, with the columns receiving the respective data. When all rows have been energized, a frame of data is displayed. Assuming an exemplary frame update rate of 60 Hz, all rows are updated once every 16.67 milliseconds. Assuming x rows per frame update. the horizontal clock signal 214 pulses once every 16.67/x milliseconds. In other words a new row is energized every 16.67/n milliseconds. If x is 400, the horizontal clock signal 214 pulses once every 41.67 microseconds. All row drivers of FED 200 are configured to implement one large serial shift register having x bits of storage, one bit per row. Row data is shifted through these row drivers using a row data line 212 that is coupled to the row drivers 220a-220c in serial fashion. During sequential frame update mode, all but one of the bits of the n bits within the row drivers contain a "0" and the other one contains a "1". Therefore, the "1" is shifted serially through all n rows, one at a time, from the upper most row to the bottom most row. Upon a given horizontal clock signal pulse, the row corresponding to the "1" is then driven for the on-time window. The bits of the shift registers are shifted through the row drivers 220a- 220c once every pulse of the horizontal clock as provided by line 214. In interlace mode, the odd rows are updated in series followed by the even rows. A different bit pattern and clocking scheme is therefore used. The row corresponding to the shifted "1 " becomes driven responsive to the horizontal clock pulse over line 214. The row remains on during a particular -18- "on-time" window. During this on-time window, the corresponding row is driven with the voltage value as seen over voltage supply line 212 provided the row drivers are also enabled. During the on-time window, the other rows are not driven with any voltage. In one embodiment, the rows are energized with a negative voltage, which could be a positive voltage in other embodiments. The Column Driver Circuits 240. As shown by Figure 4, there are three columns per pixel (or "white group") within the FED flat panel display screen 200 of the present invention. Column lines 250a of Figure 3 control one column of pixels, column lines 250b control another column of pixels, etc. Figure 3 also illustrates the column drivers 240 that control the gray-scale information for each pixel. In an analogous fashion to the row driver circuits, the column drivers 240 can be broken into separate circuits that each drive groups of column lines. In accordance with the present invention, the column drivers 240 drive time multiplexed, amplitude modulated, voltage signals over the column lines 250. The amplitude modulated voltage signals driven over the column lines 250a- 250e represent gray-scale data for a respective row of pixels. The larger the effective voltage (EV) of the column voltage, the larger the light intensity of the corresponding color point. The lower the effective voltage (EV) of the column voltage, the lower the light intensity for the corresponding color point. Once every pulse of the horizontal clock signal at line 214, the column drivers 240 receive gray-scale digital color data (clocked by line 205) to independently control all of the column lines 250a-250e of a pixel row of the FED flat panel display screen 200. Therefore, while only one row is energized per horizontal clock, all columns 250a-250e are energized during the row on-time window. The horizontal clock signal over line 214 synchronizes the loading of a pixel row of gray-scale data into the column drivers 240. Column drivers 240 receive column data over column data line 520 and column drivers 240 are also -19- coupled in common to a number of voltage tap lines which are included within column voltage supply line 515. Different voltages are applied to the column lines by the column drivers 240 to realize different gray-scale colors. In operation, all column lines are driven with gray-scale data (over column data line 520) and simultaneously one row is activated. This causes a row of pixels of illuminate with the proper grayscale data. This is then repeated for another row, etc., once per pulse of the horizontal clock signal of line 214, until the entire frame is filled. To increase speed, while one row is being energized, the gray-scale data for the next pixel row is simultaneously loaded into the column drivers 240. Like the row drivers, 220a-220c the column drivers assert their voltages within the on-time window. Further, like the row drivers 220a-220c, the column drivers 240 have an enable line. In one embodiment, the columns are energized with a positive voltage. Multiplexing Column Voltages. As discussed more fully below, the present invention time multiplexes certain column voltages during the row on- time window to alter the color balance of the FED flat panel display screen 200 of Figure 3. Specifically, to increase the color intensity for a particular color, the effective column voltages for that color (e.g., applied to all n columns of that color) are increased during the row on-time window. To decrease the color intensity for a particular color, the effective column voltages for that color (e.g., applied to all n columns of that color) are decreased during the row on-time window. Since the color data of the column drivers are not altered during color balancing, the present invention does not significantly degrade gray-scale resolution by altering color balancing in the above fashion. The following describes the mechanisms used by embodiments of the present invention for providing dynamic color balance adjustment within the framework of an FED screen 200 as described above. -20- COLOR BALANCE CONTROL CIRCUITRY OF THE PRESENT INVENTION As described more fully below, the present invention provides a mechanism for uniformly increasing or decreasing the effective voltages applied from the column drivers, of a particular color, in order to perform color balancing on that color. Each color can be adjusted separately and simultaneously. More specifically, the present invention provides a mechanism for uniformly increasing or decreasing the effective voltage applied during the row on-time window by all red (or green or blue) column drivers by a particular percentage to increase or decrease, respectively, the intensity of the red (or green or blue) spots uniformly over the FED screen 200. In accordance with the present invention, the effective voltage applied is adjusted by time multiplexing two different column voltages over the row on-time window. In one embodiment, a full column voltage is applied during a first part of the row on-time window and a second or "half column voltage is then applied over a second part of the row on-time window. The effective voltage then applied over the row-time window is the weighted average of the two voltages (full and half) weighted in accordance with the lengths of the first and second parts, respectively. The lengths of the first and second parts of the row on-time window are the same for a given color but can vary from color to color. In this way, color balancing is applied uniformly with respect to a given color. Figure 5 illustrates three separate and exemplary column drivers 240a- 240c of FED flat panel display screen 200 that drive exemplary column lines 250f-250h, respectively. These three column lines 250f-250h correspond to the red, green and blue lines of a vertically aligned column of pixels (also called a column of white groups). Gray-scale information is supplied over data bus 520 -21- as digital color data to the column drivers 240a-240c and is clocked in by clock 205. The gray-scale information causes the column drivers to assert different voltage amplitudes to realize the different gray-scale contents of the pixel. Different gray-scale data for a row of pixels are presented to the column drivers 240a-240c for each pulse of the horizontal clock signal 214. As discussed more fully below, the present invention provides a mechanism for adjusting the color balance of a pixel by controlling circuitry within each column driver, e.g., 240a, 240b and 250c. In one embodiment, the digital color data is presented to each column driver in a seven bit word but could alternatively be presented using only six bits, or any number of bits. Each column driver 240a-240c of Figure 5 also has an enable input that is coupled to enable line 510 which is supplied in parallel to each column driver 240a-240c. Each column driver 240a-240c is coupled to a column voltage line 515 which includes voltage tap lines that originate from a resistor chain. These voltage tap lines are coupled to digital to analog converter circuits located within each column driver, e.g., 240a, 240b and 250c. The column drivers 240a-240c also receive a column clock signal 205 for clocking in the gray-scale data for a particular row of pixels. A timing bus 345 includes a red timing signal 345a, a green timing signal 345b and a blue timing signal 345c used by the present invention. Bus 345 [s generated by timing circuit 550 (Figure 11) in the first and second embodiments of the present invention and generated by timing circuit 750 (Figure 14) in the third embodiment. In accordance with the present invention, the color intensity of ail color spots of the FED screen 200 of a particular color can be adjusted to perform color balancing. Adjustments to the color balance can be performed in response to FED screen aging or to manufacturing variations of the phosphors within the FED screen 200. Alternatively, adjustments to the color balance can be -22- performed by the viewer based on individual viewing taste. The following describes the circuitry used by the first, second and third embodiments of the present invention for altering the color intensity of each color spot of a particular color within the frame work of the FED screen 200. CIRCUIT OVERVIEW Figure 6 illustrates a block diagram of a circuit 300 in accordance with the present invention for performing dynamic adjustments to the color balance of an FED screen 200. Within circuit 300, digital color data, over bus 520, representing a complete row of image data, including red data, green data and blue data, is serially clocked into multiple (e.g., 3n) shift registers 310. The process of loading the above data is initiated by the horizontal synchronization clock 214. Clock signal 205 is the column clock signal and operates at a frequency sufficient to load all digital color data for a row of pixels within the period of successive horizontal clock signal pulses of line 214. Assuming FED screen 200 contains n pixels along the vertical, there are 3n column drivers in the FED screen 200. More specifically, there are n number of blue column drivers and, for a given row of image data, each blue column driver receives an individual digital blue data. There are n number of red column drivers and, for a given row of image data, each red column driver receives an individual digital red data. Likewise, there are n number of green column drivers and, for a given row of image data, each green column driver receives an individual digital green data. Each color data, in one embodiment, is seven bits wide. Therefore, shift register 310 of Figure 6 actually represents 3n individual shift registers with each shift register (within each column driver) receiving seven bits of digital color data. Since a pixel requires one red, one green and one blue color, a pixel of color data requires 7X3 color bits. -23- Blocks 320a-370a of Figure 6 represent the circuitry required to drive red color data over the red column lines and also to perform color balancing for the n number of red column drivers 240a to uniformly alter the red color across the FED 200 according to a signal, RSEL 345a. Blocks 320b-370b represent the circuitry required to drive green color data over the green column lines and also to perform color balancing for the n number of green column drivers 240b to uniformly alter the green color across the FED 200 according to a signal, GSEL 345b. Lastly, Blocks 330c-370c represent the circuitry required to drive blue color data over the blue column lines and also to perform color balancing for the n number of blue column drivers 240c to uniformly alter the blue color across the FED 200 according to a signal,. BSEL 345c. The horizontal synchronization signal 214 latches in a row of image data from bus 315 into 3n output registers 320a-320c that also contain divide by two circuitry in accordance with the present invention. Bus 315a represents all of the red color data of the row of image data and, in one embodrment, this comprises n number of 7-bit data which are input to n circuits 320a for red. Bus 315b represents all of the green color data of the row of image data and, in one embodiment, this comprises n number of 7-bit data which are input to n circuits 320b for green. Bus 315c represents all of the blue color data of the row of image data and, in one embodiment, this comprises n number of 7-bit data which are input to n circuits 320c for blue. Circuits 320a of Figure 6 are responsible for presenting n separate digital values representing n first column voltages over n separate red buses 317a during a first part of the row on-time window and is also responsible for then presenting n separate digital values representing n second column voltages (e.g., half of the first column voltages) over the n separate red buses 317a during a second part of the row on-time window. The relative lengths of the first and -24- second parts being defined by the RSEL signal over line 340a. The RSEL signal 345a is applied uniformly to all n red circuits 320a. In this fashion, the red timing signal 345a is used for all red column drivers to control the intervals over which analog voltages are time multiplexed over the individual red column lines 250(red). Circuits 320b perform analogous functions for the n green column buses 317b and the relative lengths of the first and second parts for these circuits 320b are defined by the GSEL signal of line 345b which is applied uniformly to all n green circuits 320b. Circuits 320c perform an analogous function for the n blue column buses 317c and the relative lengths of the first and second parts for these circuits 320c are defined by the BSEL signal of line 345c which is applied uniformly to all n blue circuits 320c. Block 330a of Figure 6 represents n decoders, one for each red column driver. Each decoder receives a different digital red color data from buses 317a. In one embodiment, six of the 7 bits of color data are used by the decoders 330a to determine one of 64 different red color values for each red column driver. In another embodiment, 7 bits of color data produce 128 different red color values. Block 340a of Figure 6 represents n digital to analog converters, one for each red column driver. In accordance with the present invention, each digital to analog converter of each red column driver contains an analog switch circuit that receives its corresponding red color data value. The analog switch circuit is coupled to the above referenced tap lines and maintains a data-in voltage-out function and thereby generates an analog voltage output. The data-in voltage- out function determines a particular column voltage based on the input color data. The column voltage in turn translates to a particular color intensity for red. Block 370a of Figure 6 represents n channel amplifiers 370a, one for each of the n red column drivers. Each channel amplifier receives an analog voltage from its corresponding digital to analog converter circuit of 340a and asserts this -25- signal over its corresponding red column line. In the aggregate, n column outputs 250(red) are individually generated simultaneously by block 370a. As discussed above, block 320a, block 330a, block 340a and block 370a represent circuitry that is duplicated and therefore distributed within each red column driver 240a of FED screen 200. Circuit blocks 320b, 330b, 340b and 370b of Figure 6 are analogous to blocks 320a, 330a, 340a and 370a, but cover the n circuits that apply to the n green column drivers and alter the green color to affect color balancing. A green timing signal (GSEL) 345b is used for all green column drivers to control the time multiplexing of the column voltage signals over the individual green column lines 250(green). Therefore, block 320b, block 330b, block 340b and block 370b represent circuitry that is duplicated and distributed within each green column driver 240b of FED screen 200. Likewise, circuit blocks 320c, 330c, 340c and 370c of Figure 6 are analogous to blocks 320a, 330a, 340a and 370a, but cover the n circuits that apply to the n blue column drivers and alter the blue color to affect color balancing. A blue timing signal (BSEL) 345c is used for all blue column drivers to control the time multiplexing of the column voltage signals over the individual blue column lines 250(blue). Therefore, block 320c, block 330c, block 340c and block 370c represent circuitry that is duplicated and distributed within each blue column driver 240c of FED screen 200. Figure 7 partially illustrates the circuitry within three exemplary column drivers 240a(i), 240b(i) and 240c(i) that control the ith pixel column of FED screen 200. Specifically, only the driver amplifier circuits 370a(i), 370b(i) and 370c(i) are illustrated. The remainder of the column driver circuitry for these column drivers 240a(i), 240b(i) and 240c(i) is shown in Figure 8A, Figure 8B and Figure 8C, respectively. -26- Figure 7 illustrates that the amplifier circuits 370a(i), 370b(i) and 370c(i) are directly coupled to receive the outputs from lines 365a(i), 365b(i) and 365c(i), respectively, and drive their respective column lines with these voltage levels. When row 230j (e.g., the jth row) is active, column driver 240a(i) drives a column voltage over ith red column line 250f to illuminate the ith red spot 460a; column driver 240b(i) drives a column voltage over ith green column line 250g to illuminate the ith green spot 460b; and column driver 240c(i) drives a column voltage over ith blue column line 250h to illuminate the ith blue spot 460c. It is appreciated that the red spot 460a, the green spot 460b and the blue spot 460c comprise the ith pixel for a given row, e.g., row 230j. OUTPUT REGISTER HAVING DIVIDE BY TWO FUNCTION FOR TIME MULTIPLEXING COLUMN VOLTAGES OVER ROW-ON TIME Figure 8A, Figure 8B and Figure 8C illustrate the circuitry used by a first embodiment of the present invention for adjusting color balance within an FED screen 200 for three exemplary column drivers: the ith red column driver 240a(i) of the n red column drivers 240a, the ith green column driver 240b(i) of the n green column drivers 240b and the ith blue column driver 240c(i) of the n blue column drivers 240c. These three exemplary ith column drivers provide the column voltage signals for the ith pixel along a given row of pixels during a first part and a second part of the row on-time window. The first embodiment uses an output shift right register to perform a divide by two function, described below, to generate the voltages applied during the first and second parts. Components with Figures 8A, 8B and 8C that have the "(i)" designation are replicated for each of the n column drivers of the same color as the exemplary column driver, (i), to which they are described. Components without the "(i)" designation are not replicated within each column driver but rather are -27- shared by all column drivers, or all column drivers of a similar color, as described more particularly below. Figure 8A illustrates circuitry within the exemplary red column driver 240a(i) that drives the ith red column (250f of Figure 7) within the ith pixel (of the n horizontal pixels) of the FED screen 200. Prior to the next pulse of the horizontal synchronization signal 214, the input shift register 310a(i) serially receives (over bus 520) one seven bit color data value for the red intensity of the ith pixel of a row (e.g., row j). This data is clocked in based on signal 205. On the next pulse of horizontal synchronization signal 214, a new row on-time window starts. When a new row on-time window starts, the "first voltage" data from the input register 310a(i) is then loaded in parallel to the output shift register 320a(i) over the lines of bus 315a(i). The first voltage data is held in shift register 320a(i), and output over lines of bus 317a(i), until a pulse is received from the shift right generator circuit 321a. One circuit 321a is coupled to and used by all of the n red column drivers 240a. Circuit 321 a is coupled to receive the RSEL signal 345a and according to the present invention generates a pulse to the output shift register 320a(i) when the RSEL signal 345a transitions. When the pulse is received from circuit 321a of Figure 8A, the output shift register 320a(i) of the present invention serially shifts its bit contents by one bit position to the right, effectively performing a divide by two operation on the first voltage data. During the right shift operation, a zero bit is inserted into the left most bit position (e.g., the MSB). The resulting digital value, a six bit "second voltage" data, represents half of the "first voltage" data and is held on lines 317a(i) until the start of the next row on-time window (e.g., until the next pulse of line 214). The data bits (either of the first or the second voltage data) are forwarded over bus 317a(i) in parallel to decoder circuit 330a(i) which in response re¬ generates a signal over a single output line of bus 319a(i). If seven bits of color data are used, then decoder circuit 330a(i) is a 0 to 127 decoder (as shown). Alternatively, if six bits of color data are used, then decoder circuit 330a(i) is a 0 to 63 decoder. For a given input over bus 317a(i), the decoder circuit 330a(i) generates a single active signal over one of the lines of bus 319a(i) to the digital to analog ("DA") voltage converter circuit 340a(i). Since the first and second voltage data are presented, time multiplexed, within a given row on-time window, decoder circuit 330a(i) generates two separate time multiplexed outputs to the DA voltage circuit 340a(i) during the row on-time window. The DA voltage circuit 340a(i) of Figure 8A contains a function of switches that can provide any transformation function (e.g., linear or non-linear) depending on the programmed configuration of certain internal switches coupled to a resistor chain which is coupled to the previously described voltage taps. This is described in more detail in co-pending US Patent Application entitled, "A Circuit and Method for Controlling the Color Balance of a Flat Panel Display Without Reducing Gray Scale Resolution," filed September 25, 1997, serial number 08/938,194, by Hansen, et. al., and incorporated herein by reference. Using its transformation function, the DA voltage circuit 340a(i) generates, over line 365a(i), a first analog voltage corresponding to the first voltage data. Subsequently, DA voltage circuit 340a(i) generates a second analog voltage corresponding to the second voltage data. The channel amplifier circuit 370a(i) receives these time multiplexed analog voltage signals over line 365a(i) and drives these values over the ith red column line 250f as appropriate. It is appreciated that circuit 321a, signal 345a, the horizontal synchronization signal 214, the clock signal 205 and column data bus 520 are used by all of the n red column driver circuits 240a of the present invention. The -29- mechanism for generating the RSEL signal 345a in accordance with the present invention is described further below (Figure 11). Figure 8B illustrates circuitry with an exemplary green column driver 240b(i) that drives the ith green column line 250g (Figure 7) for the ith pixel (of the n horizontal pixels) of the FED screen 200. The circuitry of Figure 8B, although replicated for and pertinent to the ith green column driver 240b(i), is analogous to the circuitry of Figure 8A except a green color data value is received over bus 520 for the ith pixel and the row on-time window is time multiplexed according to a GSEL line 345b, not the RSEL line 345a. Also, a different shift right generator circuit 321 b is used for the green columns. It is appreciated that circuit 321 b, signal 345b, the horizontal synchronization signal 214, the clock signal 205 and column data bus 520 are used by all of the n green column driver circuits 240b of the present invention. The mechanism for generating the GSEL signal 345b in accordance with the present invention is described further below. As discussed with reference to Figure 8A, the output shift register 320b(i) generates two different green voltage data values, a first and a second, which are time multiplexed and fed to decoder 330b(i). The channel amplifier 370b(i) therefore generates two different time multiplexed green analog voltage signals over column line 250g. The time multiplexing for green is controlled by the GSEL line 345b. Figure 8C illustrates circuitry with an exemplary blue column driver 240c(i) that drives the ith blue column line 250h (Figure 7) for the ith pixel (of the n horizontal pixels) of the FED screen 200. The circuitry of Figure 8C, although replicated for and pertinent to the ith blue column driver 240c(i), is analogous to the circuitry of Figure 8A except a blue color data value is received over bus 520 for the ith pixel and the row on-time window is time multiplexed according to a -30- BSEL line 345c, not the RSEL line 345a. Also, a different shift right generator circuit 321c is used for the blue columns. It is appreciated that circuit 321c, signal 345c, the horizontal synchronization signal 214, the clock signal 205 and column data bus 520 are used by all of the n blue column driver circuits 240c of the present invention. The mechanism for generating the BSEL signal 345c in accordance with the present invention is described further below. As discussed with reference to Figure 8A, the output shift register 320c(i) generates two different blue voltage data values, a first and a second, which are time multiplexed and fed to decoder 330c(i). The channel amplifier 370c(i) therefore generates two different time multiplexed blue analog voltage signals over column line 250h. The time multiplexing for blue is controlled by the BSEL line 345c. Figure 9A, Figure 9B and Figure 9C illustrate the circuitry used by a second embodiment of the present invention for adjusting color balance within an FED screen 200 for three exemplary column drivers: the ith red column driver 240a(i)' of the n red column drivers 240a, the itn green column driver 240b(i)' of the n green column drivers 240b and the ith blue column driver 240c(i)' of the n blue column drivers 240c. These three exemplary ith column drivers represent the ith pixel along a given row of pixels. The second embodiment uses a multiplexer configuration, rather than a shift register, to perform the divide by two function, described below. Components with Figures 9A, 9B and 9C that have the "(i)" designation are replicated for each column driver of the same color as the exemplary column driver to which they are described. Components without the "(i)" designation are not replicated within each column driver but rather are shared by all column drivers, or all column drivers of a similar color, as described more particularly below. -31- Figure 9A illustrates circuitry within the exemplary red column driver 240a(i)' that drives the ith red column (250f of Figure 7) within the ith pixel (of the n horizontal pixels) of the FED screen 200. Prior to the next pulse of the horizontal synchronization signal 214, the input shift register 310a(i) serially receives (over bus 520) one seven bit color data value for the red intensity of the ith pixel of a row (e.g., row j). This data is clocked in based on signal 205. On the next pulse of horizontal synchronization signal 214, a new row on-time window starts. When a new row on-time window starts, the "first voltage" data from the input register 310a(i) is then loaded in parallel onto lines 0 to 6 of bus 315a(i). Lines 0 to 6 of bus 315a(i) are coupled to one input 542a(i) of multiplexer 544a(i). Lines 1 to 6 are coupled to a second input 540a(i) of multiplexer 544a(i) starting from the LSB(0) position. This digitally provides that the value represented by input 540a(i) is half of the value represented by input 542a(i). In accordance with the second embodiment of the present invention, the first input 542a(i) contains the first red voltage data and the second input 540a(i) contains the second red voltage data. The RSEL line 345a is used as a selection control on mux 544a(i) such that mux input one 542a(i) is first provided to the output register 320a(i) and latched in according to signal 214. Then, when RSEL 345a transitions, mux input two 540a(i) is then provided to the output register 320a(i) and latched in according to signal 345a. The OR gate 522a, used for all of the n red driver circuits, receives both signals 214 and 345a to provide the latching function for output register 320a(i). Circuits 330a(i), 340a(i) and 370a(i) operate in a fashion analogous to Figure 8A to drive time multiplexed voltage signals over the ith red column 250f. As seen, column driver 240a(i)' is analogous to column driver 240a(i) of Figure 8A except a multiplexing circuit is used to provide the divide by two function rather than a shift register. -32- It is appreciated that circuit 522a, signal 345a, the horizontal synchronization signal 214, the clock signal 205 and column data bus 520 are used by all of the n red column driver circuits of the second embodiment of the present invention. Figure 9B illustrates circuitry with an exemplary green column driver 240b(i)' that drives the ith green column line 250g (Figure 7) for the ith pixel (of the n horizontal pixels) of the FED screen 200. The circuitry of Figure 9B, although replicated for and pertinent to the ith green column driver 240b(i)\ is analogous to the circuitry of Figure 9A except a green color data value is received over bus 520 for the ith pixel and the row on-time window is time multiplexed according to a GSEL line 345b, not the RSEL line 345a. Also, a different OR gate circuit 522b is used. It is appreciated that circuit 522b, signal 345b, the horizontal synchronization signal 214, the clock signal 205 and column data bus 520 are used by all of the n green column driver circuits of the second embodiment of the present invention. The channel amplifier 370b(i) generates two different time multiplexed green voltage signals over column line 250g. The time multiplexing for green is controlled by the GSEL line 345b. Figure 9C illustrates circuitry with an exemplary blue column driver 240b(i)' that drives the ith blue column line 250h (Figure 7) for the ith pixel (of the n horizontal pixels) of the FED screen 200. The circuitry of Figure 9C, although replicated for and pertinent to the ith blue column driver 240c(i)\ is analogous to the circuitry of Figure 9A except a blue color data value is received over bus 520 for the ith pixel and the row on-time window is time multiplexed according to a BSEL line 345c, not the RSEL line 345a. Also, a different OR gate circuit 522c is used. It is appreciated that circuit 522c, signal 345c, the horizontal synchronization signal 214, the clock signal 205 and column data bus 520 are used by all of the n blue column driver circuits of the second embodiment of the -33- present invention. The channel amplifier 370c(i) therefore generates two different time multiplexed blue voltage signals over column line 250h. The time multiplexing for blue is controlled by the BSEL line 345c. Figure 10 illustrates an exemplary configuration for realizing the multiplexer 544a(i), first input 542a(i) and second input 540a(i) of Figure 9A. In this configuration, the lines of bus 315a(i) are coupled to the inputs of seven two- input multiplexers 528 having select inputs which are all controlled by line 345a. The inputs to these two-input multiplexers 528 are configured as shown in Figure 10 to provide for the first voltage and its divided-by-two second voltage value. The outputs 530 are then provided to the output shift register 320a(i). Figure 11 illustrates one timing circuit 550 for generating the signals of the RSEL line 345a, the GSEL line 345b and the BSEL line 345c. Circuit 550 can be used in the first and second embodiments of the present invention described above. In circuit 550, three separate one-shot circuits 570a-570c are provided. Each one-shot circuit 570 contains its own separate user-adjustable resistor- capacitor network 572a-572c to vary the period of each output signal. The one- shot circuits 570a-570c are all clocked by the horizontal synchronization signal 214. Circuit 550 provides separate and programmable signals for RSEL 345a, GSEL 345b and BSEL 345c so that the red, green and blue components of the pixels of FED screen 200 can be adjusted independently for color balance. Figure 12A illustrates timing diagrams of the pertinent signals used by the first and second embodiments of the present invention for the exemplary red column driver 240a(i) of Figure 8A and for the exemplary column driver 240a(i)' of Figure 9A. The horizontal synchronization clock 214 is shown divided into four exemplary consecutive row on-time windows 580a-580d. Row on-time windows 580a-580d correspond to the sequential activation of four adjacent rows of FED 200. At the start of a row-on time window 580a, a designated row -34- receives an enabling voltage level while the other rows are disabled. Before the start of the row on-time window 580a, the digital color data for all columns of this row have been loaded into each respective column driver. The RSEL signal 345a of Figure 12A divides each row on-time window 580 into two parts, a first part which presents the first or "full" voltage data and a second part which presents the second or "half voltage data. (In one alternate embodiment, the half voltage data is gauged such that half current is drawn.) Also shown in Figure 12A is the analog voltage signal driven on the ith column line 250f for producing light intensity at red color spot 460a (Figure 7). For example, during row on-time window 580a of Figure 12A, first voltage v1 is driven during the first part 585a and second, or half, voltage (v1 / 2) is driven during the second part 585b of row on-time window 580a. The relative lengths of first part 585a and second part 585b can be adjusted by adjusting the resistor- capacitor network 572a (Figure 11). The effective voltage amplitude, VE, for window 580a is therefore the weighted average of v1 and (v1 / 2) over their respective on-time parts 585a-585b according to: VE = [ (V1 * L585a) + ((V1 / 2) * L585b) ] / [L585a + L585b] where L585a is the length of row on-time first part 585a and L585b is the length of row on-time second part 585b. Likewise, for row on-time 580b, voltages v2 and (v2 / 2) are driven as shown. For row on-time 580c, voltages v3 and (v3 / 2) are driven as shown and for row on-time 580d, voltages v4 and (v4 / 2) are driven as shown. Figure 12B illustrates timing diagrams of the pertinent signals used by the first and second embodiments of the present invention for the exemplary green column driver 240b(i) of Figure 8B and for the exemplary column driver 240b(i)' -35- of Figure 9B. The horizontal synchronization clock 214 is shown divided into the four exemplary consecutive row on-time windows 580a-580d of Figure 12A. The GSEL signal 345b divides each row on-time window 580 into two parts, a first part which presents the first or "full" voltage data and a second part which presents the second or "half voltage data. Also shown in Figure 12B is the analog voltage signal driven on the ith column line 250g for producing light intensity at green color spot 460b (Figure 7). For example, during row on-time window 580a of Figure 12B, voltage v1 is driven during the first part 585c and half voltage (v1 / 2) is driven during the second part 585d of row on-time window 580a. The relative lengths of first part 585c and second part 585d can be adjusted by adjusting the resistor-capacitor network 572b (Figure 11). Likewise, for row on-time 580b, voltages v2 and (v2 / 2) are driven as shown. For row on- time 580c, voltages v3 and (v3 / 2) are driven as shown and for row on-time 580d, voltages v4 and (v4 / 2) are driven as shown. It is appreciated that V1-V4 of Figure 12A are not the same voltage values as V1 -V4 of Figure 12B. According to the teachings above, the color balance of the first and second embodiments of the present invention can be adjusted by varying the RSEL signal 345a, the GSEL signal 345b and the BSEL signal 345c according to the circuit 550 of Figure 11. The red component of the current color balance can be increased by altering RSEL signal 345a such that the first part of the row on-time window that corresponds to the red color is increased. This increases the period in which the first or "full" voltage is applied. Since the red timing pulse RSEL 345A is applied to all red column drivers 240a, they will uniformly adjust up the respective effective column voltages which are used to generate the red color intensities. Although each red column driver receives different red color data, all red color intensities will be uniformly increased by the same amount. Likewise, the red component of the current color balance can be decreased by -36- altering RSEL signal 345a such that the second part of the row on-time window that corresponds to the red color is increased. This increases the period in which the second or "half voltage is applied. The same is true with respect to the green and blue color components which can be altered by similarly altering the GSEL 345b and the BSEL 345c, respectively. POWER SAVINGS THIRD EMBODIMENT OF THE PRESENT INVENTION As shown in Figure 12A and Figure 12B, the first and second parts of the row on-time windows 580a-580d occur in sequential and alternating order, e.g., the first or "full" part always following the second or "half part which follows a first part, etc. Although effective to provide color balancing, this alternating scheme of the first and second embodiments of the present invention generates some frequency of voltage change with respect to the voltage signals driven on the columns (e.g., columns 250f and 250g). For instance, every full analog voltage level is followed by its half voltage level which is followed again by a full voltage of a next row-on time window, and so on. The third embodiment of the present invention provides a mechanism for altering the order of the first and second parts of a row on-time window to decrease the overall frequency of voltage change on the columns while still providing for the same level of color balance functionality provided by the first and second embodiments of the present invention. Specifically, the third embodiment of the present invention provides a mechanisms whereby, for the period of two consecutive row on-time windows, two consecutive full parts are followed by two consecutive half parts. In other words, the order of the first ("FULL") and second ("HALF") parts of the row on-time window, compared to the -37- first and second embodiments, are swapped for every other row on-time window. The result produces the following ordering within the third embodiment: . . . FULL1 HALF1 HALF2 FULL2 FULL3 HALF3 HALF4 FULL4 . . . rather than: . . . FULL1 HALF1 FULL2 HALF2 FULL3 HALF3 FULL4 HALF4 . . . which is produced by the first and second embodiments. Figure 13 illustrates a circuit 700 used by the third embodiment of the present invention for providing the proper color select signals to realize the above ordering of full and half parts. Specifically, circuit 700 can be used to generate either signal 345a, 345b or 345c, any one of which is represented by the reference "345x" and "XSEL." Circuit 700 includes a divide-by-two circuit 710 which receives the horizontal synchronization signal 214 and divides its frequency by two to produce a "HALF H SYNCH" signal at node 715. Any of a number of well known divide-by-two circuits can be used and the configured D flip-flop 710 shown in Figure 13 is exemplary only. The HALF H SYNCH signal of node 715 controls a ramp generator circuit 720. Specifically, the signal at node 715 controls the enable line of a charging constant current source 722 and the inverse of the signal at node 715 (via inverter 726) controls the enable of a discharging constant current source 724. The charging constant current source 722 is coupled to a voltage source Vcc, and coupled to node 730. Node 730 is coupled to the discharging constant current source 724 which is coupled to ground or a negative voltage supply Vpp. -38- Node 730 of Figure 13 is also coupled to a resistor 732 which is coupled to Vcc. Node 730 is coupled to a resistor 734 which is coupled to Vpp. Node 730 is also provided as the positive input of a comparator 740x. The negative input of comparator 740x is coupled to receive a threshold voltage VTX which is coupled to a resistor 742x which is coupled to Vpp. When the voltage at 730 is greater than the threshold voltage VTX, a signal is asserted over line 345x, otherwise, the signal line 345x is not asserted. By altering the threshold voltage VTX, the signal 345x is altered and therefore the relative lengths of the first and second parts of the row on-time window are also altered. Figure 14 illustrates a timing circuit 750 that can be used to generate each of the RSEL 345a, the GSEL 345b and BSEL 345c signals based on three separate input threshold voltages, VTR, VTG and VTB, respectively, for red, green and blue. These signals, VTR, VTG and VTB, are user programmable based on desired a color balance and can be generated using a number of well known methods and components. The horizontal synchronization signal 214 is provided to a single divide-by-two circuit 710. The divided frequency signal is provided at 715 to a single ramp generator circuit 720. The ramp signal 730 generated by the ramp signal generator 720 is provided to the positive input of three comparator circuits 740a, 740b and 740c. Each comparator circuit of 740a-740c also, at its negative input, receives a separate threshold voltage VTR for red, VTG for green and VTB for blue. Comparator circuit 740a then generates RSEL 345a, comparator circuit 740b generates GSEL 345b and comparator circuit 740c generates BSEL 345c. In accordance with the third embodiment of the present invention, the signals 345a- 345c are then respectively coupled to the column driver circuits 240a-240c as shown in Figure 6, Figures 8A-8C and Figures 9A-9C. -39- Figure 15 illustrates timing diagrams of the pertinent signals used by the third embodiment of the present invention for the exemplary red column driver 240a(i)' of Figure 9A. (In order for the exemplary red column driver 240a(i) to operate with the third embodiment, the driver would need to be modified such the output shift register 320a(i) was able to simultaneously supply both the first or "full" voltage data and the second or "half voltage data.) The horizontal synchronization clock 214 is shown divided into four exemplary consecutive row on-time windows 580a-580d. The HALF H SYNCH signal 715 is also shown. During the first row-on time window 580a, the ramp signal 730 is charging, during the second row-on time window 580b, the ramp signal 730 is discharging. This sequence continues over windows 580c and 580d. Although shown as analog, the ramp generator circuit 750 could also be implemented using digital circuits. In this digital implementation, the charging of node 730 can be simulated by upcounting a counter circuit and the discharging of node 730 can be simulated by downcounting the counter circuit wherein signal 715 controls the count direction. In this implementation, a digital comparator is used for circuit 740x and the threshold value VTX would be a digital number. Figure 15 also illustrates the constant threshold voltage VTR. As shown by the RSEL signal 345a, for those periods when the ramp signal 730 exceeds the threshold voltage VTR, then RSEL signal 345a is asserted and deasserted otherwise. These signals create the following ordering. During the first window 580a, the first or "FULL" part is asserted followed by its second or "HALF' part. However, during the second window 580b, the HALF part is asserted followed by its FULL part. During the third window 580c, the FULL part is asserted followed by its HALF part and during the fourth window 580d, the HALF part is asserted followed by its FULL part. Although the order of the FULL and HALF parts have 9/50816 -40- beeή altered, compared to the ordering of the first and second embodiments, the lengths of each FULL part of Figure 15 are the same and the lengths of each HALF part of Figure 15 are the same. It is appreciated that by varying the level of the threshold voltage VTR, the relative lengths of the FULL and HALF parts can be adjusted. The resulting analog voltage signal driven over the ith red column line 250f is also shown in Figure 15. By ordering the assertion of the FULL and HALF parts of the row on-time windows 580a-580d as shown in Figure 15, the frequency of voltage change (and therefore integrated circuit power dissipation) is significantly reduced. For instance, V1 is asserted followed by (V1 12) followed by (V212) followed by V2 followed by V3 followed by (V412) followed by V4. Essentially by placing as many FULL voltage levels consecutive as possible and placing as many HALF voltage levels consecutive as possible, the present invention reduces the incidents of wide voltage level changes in the column driving voltages, thereby saving power. Figure 16 illustrates timing diagrams of the pertinent signals used by the third embodiment of the present invention for the exemplary green column driver 240b(i)' of Figure 9B. (In order for the exemplary green column driver 240b(i) to operate with the third embodiment, the driver would need to be modified such that the output shift register 320b(i) was able to simultaneously supply both the first or "full" voltage data and the second or "half voltage data.) The horizontal synchronization clock 214 is shown divided into the four exemplary consecutive row on-time windows 580a-580d. The- HALF H SYNCH signal 715 is also shown. The same ramp generation signal 730 is shown in Figure 16 as is shown in Figure 15. Figure 16 also illustrates the constant threshold voltage VTG which is lower in value than VTR of Figure 15. As a result, the HALF parts of Figure 16 sOT sτrr«τ£ SHEET am** ) -41- are larger in duration than the HALF parts of Figure 15. As shown by the GSEL signal 345b, for those periods when the ramp signal 730 exceeds the threshold voltage VTG, then GSEL signal 345b is asserted and deasserted otherwise. These signals create the following ordering. During the first window 580a, the first or "FULL" part is asserted followed by its second or "HALF part. However, during the second window 580b, the HALF part is asserted followed by its FULL part. During the third window 580c, the FULL part is asserted followed by its HALF part and during the fourth window 580d, the HALF part is asserted followed by its FULL part. It is appreciated that by varying the level of the threshold voltage VTG, the relative lengths of the FULL and HALF parts can be adjusted. The resulting analog voltage signal driven over the ith green column line 250g is also shown in Figure 16. By ordering the assertion of the FULL and HALF parts of the row on-time windows 580a-580d as shown in Figure 16, the frequency of voltage change (and therefore integrated circuit power dissipation) is significantly reduced as described with respect to Figure 15. The preferred embodiment of the present invention, a method and mechanism for using time multiplexing of voltage signals for dynamically altering the color balance within a flat panel FED screen without significantly compromising gray-scale resolution, is thus described. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the below claims.

Claims: -42- CLA1MS What is claimed is: 1. A field emission display device comprising: a plurality of row drivers, each coupled to a respective row line, for driving a row voltage signal over one row line at a time during a row on-time window, wherein a pixel includes intersections of one row line and multiple column lines; a horizontal synchronization clock signal for synchronizing said plurality of row drivers by initiating row on-time windows; a plurality of column drivers of first, second and third colors, each column driver coupled to a respective column line and for time multiplexing thereon a first analog voltage and a second analog voltage, respectively, during a first part and a second part of each row on-time window; and each column driver comprising a color balancing circuit responsive to a color select signal and for generating said first analog voltage based on a first voltage data and for generating said second analog voltage based on a second voltage data. 2. A field emission display device as described in Claim 1 wherein said color balancing circuit comprises: a shift register for receiving said first voltage data representing said first analog voltage and for generating said second voltage data therefrom responsive to said color select signal, said second voltage data representing said second analog voltage; a decoder coupled to said shift register for decoding said first and second voltage data; and 99/50816 -43- an digital to analog converter coupled to said decoder for converting said first and second voltage data to said first and second analog voltage signals. 3. A field emission display device as described in Claim 2 further comprising a timing circuit coupled to said horizontal synchronization clock signal for generating a first color select line that is coupled to said shift register of each column driver of said first color, said first color select line for causing said shift register of each column driver of said first color to generate said second voltage data. 4. A field emission display device as described in Claim 3 wherein said timing circuit is also for generating second and third color select lines, said second color select line for causing said shift register of each column driver of said second color to generate said second voltage data and said third color select line for causing said shift register of each column driver of said third color to generate said second voltage data. 5. A field emission display device as described in Claim 2 wherein said second voltage data is half of said first voltage data. 6. A field emission display device as described in Claim 5 wherein said first voltage data is 7-bits and said second voltage data is 6-bits. 7. A field emission display device as described in Claim 2 wherein for each pair of consecutive row on-time windows, said first and second part are ordered as follows: first; second; first; second. -44- 8. A field emission display device as described in Claim 4 wherein for each pair of consecutive row on-time windows, said first and second part are ordered as follows: first; second; first; second. 9. A field emission display device comprising: a plurality of row drivers, each coupled to a respective row line, for driving a row voltage signal over one row line at a time during a row on-time window, wherein a pixel includes intersections of one row line and multiple column lines; a horizontal synchronization clock signal for synchronizing said plurality of row drivers by initiating row on-time windows; and a plurality of column drivers of first, second and third colors, each column driver coupled to a respective column line and for time multiplexing thereon a first analog voltage and a second analog voltage, respectively, during a first part and a second part of each row on-time window.veach column driver comprising: a multiplexer circuit for selecting between a first voltage data representing said first analog voltage and a second voltage data representing said second analog voltage; a decoder coupled to an output of said multiplexer circuit for decoding said first and second voltage data; and an digital to analog converter coupled to said decoder for converting said first and second voltage data to said first and second analog voltage signals. 10. A field emission display device as described in Claim 9 further comprising a timing circuit coupled to said horizontal synchronization clock signal for generating a first color select line causing said multiplexer circuit of -45- each column driver of said first color to select said first voltage data during said first part and to select said second voltage data during said second part. 11. A field emission display device as described in Claim 10 wherein said timing circuit is also for generating second and third color select lines coupled, respectively, to each multiplexer circuit of said column drivers of said second and third colors, wherein said second color select line is for causing said multiplexer circuit of each column driver of said second color to select said first voltage data during said first part and to select said second voltage data during said second part and, wherein said third color select line is for causing each multiplexer of each column driver of said third color to select said first voltage data during said first part and to select said second voltage data during said second part. 12. A field emission display device as described in Claim 9 wherein said second voltage data is half of said first voltage data. 13. A field emission display device as described in Claim 12 wherein said first voltage data is 7-bits and said second voltage data is 6-bits. 14. A field emission display device as described in Claim 9 further comprising a timing circuit for controlling said multiplexer of each column driver of said first color wherein, for each pair of consecutive row on-time windows, said multiplexer is for ordering said first and second parts as follows: first; second; first; second. 9/50816 -46- 15. A field emission display device as described in Claim 9 further comprising a timing circuit for controlling said multiplexer of each column driver of said first color wherein, for each pair of consecutive row on-time windows, said multiplexer is for ordering said first and second parts as follows: first; second; second; first. 16. A field emission display device comprising: a plurality of row drivers, each coupled to a respective row line, for driving a row voltage signal over one row line at a time during a row on-time window, wherein a pixel includes intersections of one row line and a red, a green and a blue column line; a horizontal synchronization clock signal for synchronizing said plurality of row drivers by initiating row on-time windows; and a plurality of column drivers of red, green and blue colors, each column driver coupled to a respective column line and for time multiplexing thereon a first analog voltage and a second analog voltage, respectively, during a first part and a second part of each row on-time window, each column driver comprising: a divide circuit for receiving a first voltage data representing said first analog voltage and for supplying said first voltage data and for generating and supplying a second voltage data representing said second analog voltage; a decoder coupled to said divide circuit for decoding said first and second voltage data; and an digital to analog converter coupled to said decoder for converting said first and second voltage data to said first and second analog voltage signals. -47- 17. A field emission display device as described in Claim 16 further comprising a timing circuit coupled to said horizontal synchronization clock signal for generating a blue color select line coupled to each column driver of said blue color and for causing said divide circuit of each column driver of said blue color to supply said first voltage data during said first part and to supply said second voltage data during said second part. 18. A field emission display device as described in Claim 17 wherein said timing circuit is also for generating separate green and blue color select lines coupled, respectively, to each divide circuit of said column drivers of said green and blue colors, wherein said green color select line is for causing said divide circuit of each column driver of said green color to supply said first voltage data during said first part and to supply said second voltage data during said second part and, wherein said blue color select line is for causing said divide circuit of each column driver of said blue color to supply said first voltage data during said first part and to supply said second voltage data during said second part. 19. A field emission display device as described in Claim 16 wherein said second voltage data is half of said first voltage data. 20. A field emission display device as described in Claim 16 wherein for each pair of consecutive row on-time windows, said first and second part are ordered as follows: first; second; first; second. -48- 21. A field emission display device as described in Claim 16 wherein for each pair of consecutive row on-time windows, said first and second part are ordered as follows: first; second; second; first.

Inventors: FRIEDMAN JAY, HANSEN RONALD L, STOIAN LEE, FRIEDMAN, JAY, HANSEN, RONALD, L., STOIAN, LEE
Applicants: CANDESCENT TECH CORP, CANDESCENT TECHNOLOGIES CORPORATION
Application year: 1998
Main classification: G09G3
IPC codes: G09G3