Reinventing the color wheel (1991)

The basics of color television have changed little since the medium was commercialized 37 years ago. Although the integrated-circuit revolution has greatly improved the electronics that control the CRT, the CRT itself continues to depend upon the nearly 40-year-old shadow mask. (See this issue's companion article, “The Evolution of the Shadow-Mask Tube.”) And, despite their longevity and continuing success, shadow-mask tubes do have inherent limitations. Alternatives have been demonstrated over the years, but none of them was practical. The earliest was the color-wheel system, which was a serious contender at the 1950 U.S. Federal Communications Commission (FCC) hearings that led to the adoption of the shadow mask. Within the last year, Tektronix has demonstrated an electronic analog of the color-wheel system that solves many of the old system's shortcomings. Tektronix believes that the new system will soon be an attractive alternative to shadow-mask color in a variety of applications. The main attraction of the new system is that it does away with the limitations that stem from the basic architecture of the shadow-mask CRT, in which each of the displayed color pixels is made up of three (or occasionally four) differently colored dots or stripes. These dots or stripes are placed close enough to each other so that the viewer's eye integrates them into a single large pixel conveying one of a large number of possible colors. This works, but at the cost of the tube's resolution — the number of pixels that can be displayed per inch (or millimeter) — being substantially less than it would be for a black-and-white tube of similar quality. A second difficulty is that the three electron beams that excite the three dots or stripes in each color pixel must track together precisely. This “convergence” of the three beams must be increasingly accurate as tubes are developed with flatter faceplates, squarer corners, and much higher resolutions. And third, with their shadow masks, three guns, and convergence requirements, shadow-mask CRTs tend to be relatively bulky, complicated, and sensitive to shock and vibration. There is an alternative way of producing color. Instead of having the eye average red, green, and blue dots over a region on the tube's face, we can average the colors over time at each position on the tube. This approach — field-sequential color — was what the old color-wheel system attempted. It used an electric motor to rotate a wheel with red, green, and blue filters in front of a black-and-white TV screen. The size of the mechanical system, technical deficiencies, and incompatibility with the NTSC monochrome system were the color wheel's downfall in 1950. But field-sequential color had demonstrated some benefits, and in the 1980s it became possible to replace the color wheel with stationary filters, liquid-crystal “pi-cells,” and polarizers. It was time to take another look. The mechanical color wheel produced color images by synchronizing each of the color fields with the red, green, and blue filter sections of the wheel. The liquid-crystal electronic color shutter (LCS) achieves a similar effect without mechanical motion [Fig. 1]. The first component of the shutter display system is a white monochrome CRT using P45 phosphor, which has relatively narrow emission peaks in the red, green, and blue portions of the spectrum. Next is a set of two color polarizers — cyan (blue, green) and red which polarize the CRT light and perform the first separation into color components. After this comes the first of two pi-cells — very fast liquid-crystal switches. Its function is either to rotate the polarized light by 90° or to leave its original polarization unchanged. The second set of two color polarizers — yellow (red, green) and blue — follows. This set divides the blue component from the green. Next is the second pi-cell, followed by a polarizer whose only function is to absorb the unwanted horizontal component of light. A neutral-density filter is the final element. Only the desired color in the vertical orientation is transmitted. By selecting the appropriate combination of pi-cell states — rotating (OFF) or nonrotating (ON) — the filter can be switched to transmit a red, green, or blue colored image. Suppose we wish to select green. Both pi-cells would be set to non-rotating. Since the light transmitted through the shutter is always vertically polarized, we need only concern ourselves with the vertical components. The blue and green components from the first set of color polarizers pass through the first cell unaffected. The vertically oriented yellow polarizer then absorbs the blue component and leaves only the green. This passes through the next pi-cell and the front polarizer. The transmitting state is green. Similarly, the red component is transmitted by selecting the first pi-cell to rotate light by 90°. This puts the red component into the vertical position so that it can pass through the yellow polarizer unaffected and out through the neutral polarizer. Blue is obtained by placing both pi-cells in their rotating states. This rotates the initially vertically polarized blue component so it can pass through the horizontally oriented blue polarizer, and then reorients it to a vertical position for final transmission. The full-color liquid-crystal-shutter system uses a monochrome CRT and a series of color polarizers and switchable pi-cells for rotating the polarized light. Color images are produced by sequentially displaying red, green, and blue field information on the monochrome CRT while the LCS filter is synchronously switched to transmit red, green, and blue light. The color components of the image are alternated so rapidly that the human visual system integrates them to create a full-color image. This is accomplished by using a color field rate of 180 Hz, which produces a complete color picture at a 60-Hz frame rate. A wide gamut of colors is obtained by varying the CRT's beam current in each field. White is produced by displaying the same image in each field, and a deep black is produced by not displaying an image during any color filter sequence. The electronic color-shutter filtering system overcomes several technical problems associated with the old mechanical color wheel or shadow-mask tube: color purity, convergence, and broadcast compatibility. In addition, brightness — a traditional difficulty with field-sequential systems — is rapidly being improved. In the old color-wheel system, the persistence time of the phosphor used in the CRT was greater than the field time (which was also the time each filter remained in front of the monochrome screen). Thus, some of the red signal, for instance, would pass through the green filter and degrade color purity. By contrast, the P45 phosphor used in the LCS system decays to 1% of full luminance in less than 3 msec which makes it adequate for the field times. Further help in overcoming this comes from dividing the shutters into segments that are parallel to the CRT's raster lines. Each segment can be switched independently, which allows a given transmitting (color) state to be maintained in front of each portion of the CRT until the phosphor has extinguished. The segmentation also permits the liquid-crystal pi-cells to switch in about 2 msec. The pi-cells are segmented with 5-μm lines that are photo-lithographically produced on the glass and are narrow enough to be invisible to the user. Fabricated in this way, the LCS produces uniform and saturated colors. Field times in the color-wheel system were short enough so that the phosphors available in the 1950s could not develop enough luminance for an adequately bright picture. Brightness remains the greatest technical challenge for field-sequential systems. The LCS system transmits a theoretical maximum of 16.7% of the incident light — one-third of the total emitted light is spent on any one color and one-half of the light is lost in the polarizers. But real-world nonideal polarizers reduce the transmissivity to 14%, and real-world color dyes coupled with the spectral output of P45 bring the transmissivity of our current shutter down to 6.5%. A tube luminance of 500 fL is needed to obtain a display luminance of 30 fL, which would require a beam current of 400 μA for the video area of a 10-in. CRT with P45 phosphor. Although high by monochrome standards, this beam current is fairly typical of 19-in. shadow-mask CRTs. As a result, CRT cathode materials now exist that can tolerate this load with acceptable longevity. But can the phosphor tolerate this high a beam current? Fortunately, P45 is a durable phosphor. It can withstand an accumulated charge deposition of 140 C/cm2 before losing 50% of its initial brightness at constant beam current. Thus, a 10-in. display running with 400-μA beam current would take about 24,000 hours — 8 years at 8 hours of operation per day at full intensity — to age the phosphor to one-half its original brightness. Off-the-shelf PC video boards able to display 1024 × 768 non-interlaced pixels can be used to drive the 640 × 350-pixel 180–Hz vertical field-sequential monitor. These boards use a 60–65-MHz clock that meets the field-sequential monitor's video bandwidth requirement. A typical off-the-shelf PC video board can be modified with custom ASICs to drive the field-sequential monitor while retaining compatibility with IBM PC video standards [Fig. A]. The host CPU interface, timing and control generator, and the graphics and CRT controller blocks can all be implemented in a single ASIC, or in a small ASIC set. Conversion is accomplished by changing the BIOS ROM table used to load the CRT controller registers when video-BIOS mode calls are involved. An alternative to changing the BIOS ROM is to run a terminate-and-stay-resident program that intercepts the return from a video-BIOS mode call, changes the CRT controller registers to the field-sequential rate, and then releases program control. If you are willing to give up close-to-off-the-shelf availability, you can design a more efficient video board [Fig. B]. Such a board would use the same ASIC we just described to handle the host CPU interface, timing control generator, and graphics and CRT controller, thus ensuring software compatibility. The difference is that it would contain only a single output DAC and a smaller frame buffer. Single-output DACs are cheaper than their triple-output counterparts, but they do not contain look-up tables organized in a usable way for field-sequential output. This necessitates a separate DAC and look-up table. The costs of the alternative designs are essentially equal. Parallel-video controller. Field-sequential (serial-video) controller. The second implementation has the advantage of using less memory. Manufacturers generally include 512 bytes of frame-buffer memory for the 1024 × 768 16-color boards, and 1 Mbyte of memory for the 256-color versions. The required amount of memory for those resolutions, however, is actually 394 and 787 kbytes, respectively. The memory requirements for the 640 × 350 field-sequential monitor are 112 and 224 kbytes, respectively. Therefore, a likely design for frame-buffer memory size is 256 kbytes. Since memory is an appreciable portion of the component cost of a video board, this represents a significant cost reduction. –John Purdy, electrical engineer Tektronix To produce images, a display system must be compatible with the signals supplied to it. The color wheel of the 1950s was incompatible with NTSC broadcast television signals. Digital electronics saves the LCS system from a similar fate. In fact, compatibility has been made relatively easy by the manufacturers of computer-monitor video controller cards, who are designing their controllers so they can run with a large variety of software and monitors. The electronics needed to operate the LCS system is thus available as an almost-off-the-shelf item. Tektronix' developmental 640 × 350 monitor units are being run with third-party cards designed for shadow-mask tubes with 1024 × 768 addressability. The vertical refresh rate is programmable to 180 Hz, with pixel clock rates in the 60-MHz region. Tektronix adds a small video multiplexing board to switch between the red, green, and blue image components when a vertical sync pulse is detected. This board also operates the liquid-crystal shutter. (See “Almost-Off-the-Shelf Electronics for Field-Sequential Color.”) Compatibility is becoming more common because computer applications are increasingly being written so that they run independently of the monitor or display controller being used. This has been true for some time in Macintosh software and is a feature of Windows 3.0 for the PC and some workstation environments. Temporal integration of color primaries offers several advantages over spatial integration. The basic one is improved resolution. The color LCS system produces a color pixel that is essentially the same size as the spot produced by the monochrome CRT that feeds it. This also means that the complete area of any addressable region can be filled with a single desired color. As a result, a solidly colored area looks essentially the same regardless of viewing distance or magnification and does not have the “structure” seen in shadow-mask tubes. (See cover photo.) A full-color liquid-crystal shutter (LCS) changes the design criteria normally associated with a monochrome CRT display. The color polarizers, pi-cells, and neutral-density filter in a liquid-crystal shutter absorb most of the ambient light, resulting in excellent antire-flection properties and a very high contrast ratio. But the absorption works both ways. Only 6.5% of the light from the CRT is transmitted through the shutter. High luminance is the key design requirement for a CRT intended to drive an LCS display. For the shutter to function properly, a single-crystal phosphor — P45 — is used. P45's energy spectrum provides a distinctive red, green, and blue source from a single point. It is, however, less efficient than many white phosphors that are blends of two or more components. At a current density of 1 μA/in.2 and with 12 kV on the CRT's anode, P45 yields a luminance of 24 fL with monochrome glass. (For comparison, P4 yields 35 fL.) Six and one-half percent of 24 fL yields 1.6 fL out of the display. The CRT designer clearly has a challenge on his hands. A small-neck (20-mm) electron gun running at a nominal 75 μA can deliver 1.6 μA/in.2 on a 10-in. CRT. This would generate a full white screen at only 1.95 fL. But help is at hand. Switching to a glass bulb designed for color-projection CRTs eliminates the lead in standard monochrome bulbs that turns brown with use, and it permits the use of a higher anode voltage. That adds approximately 70% to the glass transmittance and 33% to the beam acceleration. In addition, spot size can be traded against cathode loading — the beam current generated per unit area of cathode surface. A high-resolution monochrome display has, by nature, a smaller spot than a color CRT. By allowing a larger spot size, the cathode loading can be held within the same range specified for a standard monochrome product and still deliver color VGA performance. Given the high contrast ratio of the LCS display, 20 fL is an acceptable luminance that provides a substantial improvement in viewability over most shadow-mask displays. Initial results indicate this can be attained in a 10-in. or smaller display with acceptable cathode life even with a traditional barium-oxide cathode. By incorporating these modifications into 29-mm electron guns, even larger and/or brighter displays will be possible. –Ken Compton, marketing manager Clinton Electronics Quite apart from resolution and convergence considerations, the three guns of a shadow-mask CRT must have different voltage-brightness curves to accommodate the characteristics of the three different phosphors. As a result, colors do not remain constant with changes in brightness [Table 1]. Similarly, as components age, the voltage-brightness curves will change in relation to each other and colors will shift. These problems never arise with a single-gun system. The LCS shutter is made of rugged materials and it sits in front of a monochrome CRT, which is simpler, lighter, shorter, and more rugged than a shadow-mask CRT. These parameters suggest several attractive areas of application for color LCS displays: Situations in which a large amount of legible information must be displayed on a small monitor. Situations in which displayed color must compare accurately with the color of printed material. Situations in which a color monitor must be operated in rugged environments. The following books and articles are suggested for further reading. P. Bos, T. Buzak, and R. Vatne, “A Full-Color Field-Sequential Color Display,” Proceedings of the SID, Vol. 26, No. 2 (1985). Harry H. Poole, Fundamentals of Display Systems (Macmillan, Ltd., 1966). Richard A. Perez, Electronic Display Devices (Tab Books, 1988). Ron O. Peterson, P45 Phosphor and a Samarium Red Enhanced Version (unpublished). G. Murch and L. Virgin, “Resolution and Addressability: How Much is Enough?” SID '85 Digest, Vol. 16 (1985).