HDTV technologies: the power and the light (1992)

To hdtv visionaries, the minimal HDTV display will measure about 50 in. (1.5 m) on the diagonal and contain about 1.5M full-color pixels. These displays, regardless of technology, will be part of television receivers that plug into a standard power outlet. So, unlike displays intended for battery operation, power consumption does not seem on the surface to be a critical issue. (See “Plasma and EL Displays: The Efficiency War,” Information Display, February, 1990.) But let's look again. With over 200-million TV receivers in North America, perhaps 150 million in Europe, and roughly 100 million in Japan, finding a serious answer to a tongue-in-cheek display-industry riddle begins to seem worthwhile. The riddle? What emits light with a system efficacy of less than 0.1 lm/W? Answer: A space heater. Nobody wants to back a technology that will produce space heaters, so answering the following questions is of serious interest: At this relatively early stage in the development of large-screen HDTV displays, is it possible to compare the various technologies and obtain projections that can provide useful answers to these questions? We believe it is. The most important figure of merit in comparing these systems is the luminous efficacy of the display head—the total luminous flux emitted by the display head divided by the electrical power required to produce it—under typical operating conditions. (The display head is the display device itself along with its associated drive and control electronics.) If we can project the average luminous efficacies of comparable HDTV display heads based on the various technologies, we can also project the average power consumptions. We will look at the three technologies widely identified as the three leading ones for HDTV: cathode-ray tubes (CRTs), thin-film-transistor active-matrix liquid-crystal displays (TFT-LCDs), and plasma display panels (PDPs). We will largely ignore the fact that these technologies each have many variations, but we won't ignore the fact that LCD and CRT displays are available in both projection and direct-view versions. To make our projections useful but not too speculative, we will compare the three technologies on the basis of a 40-in.-diagonal monitor, even though 60 in. is more likely as a typical size for HDTV once the technology has passed its introductory commercial stage. (Sixty inches is a target because at a viewing distance of 8 ft. a display of this size presents a horizontal viewing angle of 30°—similar to the viewing angle in a typical movie theater.) For practical comparisons of HDTV display systems, the values for power consumption and luminous efficacy must be obtained under typical panel load conditions. The average picture level (APL) for a television video image is a function of picture content. In the U.S., typical APL values for TV images run from 20 to 30% according to Sol Sherr of Westland Electronics, so we will use an APL of 25% as the basis for comparing the different display technologies. (For “white-on-black” alphanumeric applications on dot-matrix displays, a typical APL is 5%.) So much for the generalities. Now let's look at the specific technologies themselves, with a bit of perspective on each of them before we focus on power-consumption issues. CRTs still set the standard for picture quality. Any flat-panel alternative, whatever its attractive packaging possibilities, must match the CRT's high luminance, excellent color chromaticity, 8-bit (256-level) gray-scale capability, and video speed (at least 60 frames/sec, progressive scan). However it is not picture quality, but the physical characteristics of large-screen CRT HDTV receivers that have spurred the search for flat-panel alternatives. Large-screen CRTs are necessarily heavy and deep. (It may not even be possible to make a practical 40-in. HDTV CRT receiver that fits through a standard North-American 28-in. interior doorway. The Sony HDM-4130, 38-in. high-definition monitor cannot.) More to the point of this article is that the excellent luminous efficiency of conventional CRT television receivers decreases dramatically for high-resolution 40-in. CRTs. Such a display requires a small electron-beam spot size with minimal aberrations from the electron lens. The electron-beam spot size is also increased by space-charge forces—the mutual repulsion of the electrons in the beam because they all carry a negative electrical charge, which becomes significant in high-brightness high-resolution CRTs. Lens aberrations can be reduced and space-charge repulsion counteracted with a wider electron lens, but this requires a wider-neck tube. While deflection power increases by the square of the neck diameter, the power consumption of the focusing optics increases as well. CRT designers are also faced with another set of power-related problems as they scale up their displays. If the beam current and tube design are held constant, spot size will increase linearly with tube dimensions, and luminance will decrease as the square of the screen diagonal. To counter this, both beam current and anode voltage are usually increased in large-area high-resolution displays, and both changes increase power consumption. Simultaneously, product designers may be pressuring the CRT designer to increase the beam-deflection angle so a tube of a given screen size will have less depth. (Remember that 28-in. doorway.) This will increase the CRT's inherent deflection defocusing. Remedies are available, but they also consume power. HDTV also requires a higher deflection frequency, which increases power consumption, and deflection power also increases linearly with the number of scanned lines. (HDTV will have, very roughly, twice the scan lines of NTSC TV, so deflection power will double from this alone.) The higher resolution of HDTV displays also requires increasing the number of slots or slits in the shadow mask for a given picture area and decreasing the slit aperture. This reduces the beam transmission per pixel, which tends to produce a tube with less brightness. As an example of the combined effect of these factors, conventional CRTs up to 27 in. in diagonal typically have a maximum usable luminance of at least 100 fL. Sony's 32-in. Trinitron monitor (Model 32HSR10), which has a horizontal resolution of 560 lines and a vertical resolution of 600 lines, has a maximum usable luminance of 62 fL and a power consumption of 160 W. (See Video Review Magazine, p. 118, December 1989.) Larger yet is Sony's high-definition Sony's HDM-4130 high-definition 38-in. CRT monitor is an HDTV monitor you can buy today. Trinitron monitor Model HDM-4130 with a horizontal resolution of 1000 lines and a vertical resolution of 750 lines (Fig. 1). Its peak luminance is 29 fL and its power consumption is 350 W. (Data courtesy of NHK Labs and Shinjiro Umeya of Sony.) This monitor has a depth of 34 in. and weighs 405 lbs. The big Sony monitor's electron gun consumes approximately 45 W of power at an APL of 0.4, with the heater consuming about 3 W, according to Mr. Umeya. We estimate the deflector power at 90 W. (The horizontal deflection frequency is 33.75 kHz, interlaced.) The beam-convergence correction system with digital memory for the high-resolution monitor probably consumes another 40 W, according to an estimate by Dr. Iwao Ohishi of Asahi Glass. (The rest of the circuitry consumes the remaining 170 W.) This is enough information to calculate the monitor's luminous efficacy. Japanese manufacturers specify the maximum power requirements of their television sets at an APL of 40%. We need to adjust Sony's figures to the more typical standard of 25% that we've set for our comparisons. In a CRT, the only subsystem whose power consumption varies significantly with APL is the electron gun. By using the Sony figures, but correcting for the slightly larger 40-in. size and slightly higher 35-fL luminance, we obtain an average luminous efficacy of 0,23 Im/W for a 40-in. high-resolution CRT display head (Table I). (Sony has kept the power consumption of the HDM-4130 down by using a 90° horizontal deflection angle, and our figure reflects this. The trend toward more compact TVs will increase this angle and will thus increase power consumption.) The difficulties in designing large-area direct-view high-resolution CRTs have spurred the development of projection CRT systems. Because ambient home lighting degrades the contrast on highly reflective front-projection screens, more than 90% of the consumer market now goes to rear-projection sets, which can use screens having a low reflective coefficient. Direct comparison of the luminous efficacies of direct-view and rear-projection CRT displays is difficult because of differences in screen size, contrast, and angle of view. Nonetheless, the rear projection display clearly has an advantage because of the (1) large gain associated with the superposition of RGB cells and elimination of the shadow mask, (2) the use of higher-efficiency small CRTs, and (3) the use of a front-screen multielement high-contrast lenticular lens that distributes the projected image within a more restricted angle of view, especially in the vertical direction. These design features more than compensate for losses in the refractive optics and give the new generation of HDTV projection displays incorporating 7- or 9-in. CRTs luminous efficacies at least twice that of our comparison 40-in. direct-view CRT monitor. The development of a more efficient blue CRT phosphor could permit a return to 5-in. projection CRTs, which would further increase luminous efficacy and reduce monitor depth, weight, and cost. All of this would result in a product with greater consumer acceptance. State-of-the-art flat-panel portable TVs incorporate backlit TFT-LCDs, with virtually all using amorphous-silicon technology for the thin-film transistors. The largest LCD televisions currently being sold in commercial quantities in Japan are the 5-in.-diagonal size such as Hitachi's LCM-501 (5.06-in. diagonal and 114,720 pixels), the only one for which we have complete technical data. We will therefore use the Hitachi data to estimate the performance of a 40 in. HDTV TFT-LCD. The LCM-501 uses a 5-in. high-efficiency Hitachi-made U-shaped fluorescent lamp. The backlight produces an average area luminance of 730 fL from an input power of 2.6 W at 5 V. Of the 730 fL, 3.5% gets through the color filters and diffusion plate (state of the art for color video TFT-LCD monitors) to deliver an average area luminance of 26 fL for white. The overall power consumption of the monitor is 5.5 W, independent of APL—the backlight is always on. Of this, the lamp consumes 2.1 W, the lamp inverter circuit consumes 0.5 W, the display panel with drive circuitry consumes 0.7 W (estimated), and the remaining TV circuitry consumes 2.2 W. The active screen area of the LCM-501 is 0.00793 m2, so the 26-fL area luminance—which is equivalent to 280 lm/m2—corresponds to a total luminous flux of 0.55 lm at an APL of 25%. The total power consumption of the display head is 3.3 W, so the average luminous efficacy of the LCM-501's display head is therefore 0.17 lm/W. This efficacy can be extrapolated to a 40-in. monitor. Lengthening the fluorescent tube will improve its efficiency. A reflective coating behind the lamp's bottom phosphor section and a more efficient inverter will help even more. This will have to be balanced by swapping Hitachi's very luminous but not-very-long-lived hot-cathode lamp for a longer-life cold-cathode lamp. Let's assume there will be no net change in lamp efficiency. As TFT-LCDs grow larger, the front-panel transparent electrodes must be made more conductive—and somewhat less transmissive—to compensate for their increased length. But increased size also means larger pixels and an increased aperture ratio with respect to the opaque TFT pixel circuitry, and this translates into more transmissivity. We think these opposing factors will essentially cancel each other's effect and that a 40-in. TFT-LCD will have about the same luminous efficacy as a 5-in. TFT-LCD. However, using optical-interference color filters instead of today's dye type could raise the efficacy by as much as 50%. Even greater improvements could be had if more trans-missive LCD configurations—such as polymer-dispersed LCDs—prove practical. All of this looks very attractive, but we must acknowledge practical realities. The world's best manufacturers are currently struggling to get manufacturing yields of 10-in. TIT-LCD color computer displays over 20%, and the largest fully integrated laboratory TFT-LCD color TV reported to date (by NTT) is only 15 in. on the diagonal. In the TFT-LCD world, increased size comes only with large amounts of ingenuity, perspiration, and financial investment. (More than $3.0 billion has been invested over the last decade to bring the major manufacturers to where they are today.) Given these enormous difficulties, the near-term HDTV solution for the LCD fraternity is projection TFT-LCDs. Small TFT-LCDs suitable for use in projection displays are in production today. Projection TFT-LCD systems enjoy several advantages over projection CRT systems: smaller size, lighter weight, greater ruggedness, and—potentially—lower cost. The efficacy improvements described for the scaling up of direct-view TFT-LCD displays work in reverse for smaller projection panels, but there are improvements associated with the superposition of RGB cells (as opposed to the spatially adjacent arrangement used in direct-view systems), the possibility of using metal-halide projection lamps, and the elimination of a diffuser plate in the backlight. These advantages are partially countered by optical losses in the projection system. On balance, a 40-in. TFT-LCD projection HDTV using current technology could double the efficacy of a 40-in. direct-view TFT-LCD. The development of more transmissive optical-interference color filters could produce additional efficiency improvements in the future. At least 10 major programs are currently under way in Japan to develop plasma-display panels for large-size HDTV. In fact, this past April the Plasma Display Technical Forum was founded in Tokyo with 50 Japanese corporate members. However, there are currently no commercial plasma TVs from which to extrapolate data for our comparison—although Photonics Imaging will begin shipment in late 1992 of a prototype 19-in. AC-PDP improved-definition-lelevision (IDTV) video monitor that was shown this past May at SID '92 in Boston (Fig. 2). Video image on Photonics Imaging's prototype 19-in. color plasma-display video monitor suggests the possibility of a large-screen HDTV you can hang on the wall. Developmental memory-type color video monitors exhibit high luminosity and the same scalability to large sizes as do monochrome plasma displays. Both ac and dc color PDPs are primarily thick-film devices made from window float glass, which gives them a potential for low cost—in comparison to TFT-LCDs-that has spurred continued development. Early problems with device lifetime have been largely solved, with groups in Europe, Japan, and the U.S. reporting lifetimes on the order of 10,000 hours. With this development, color plasma displays—although not video displays—became commercially available late in 1991 from several sources. The range of measured luminous efficacies reported for color PDPs varies from 0.2 lm/W to the 4.0 lm/W reported by Hitachi. To obtain conservative values for a demonstrated low-cost large-size device, we will base our efficacy and power-consumption values on those reported in 1991 by NEC at the IDRC in San Diego. This panel has demonstrated an average luminous efficacy of 0.50 lm/W, a figure that is nearly constant over APL values ranging from 20 to 100%. However, this figure does not appear to include power losses associated with the drive circuitry. To be conservative we assume a 15% parasitic loss, reducing the luminous efficacy from 0.50 to 0.42 lm/W. Color PDPs break the mold of other display technologies in that color versions can have higher luminous efficacies than monochrome versions. This is the case because color PDPs use a potentially more efficient mechanism for light production: phosphor photoluminescence from an ultraviolet gas discharge—the same mechanism used in fluorescent lamps. (Monochrome PDPs emit light via direct visible emission from a neon gas discharge.) The display-head average luminous efficacies and typical display-head power consumptions of the leading color HDTV displays based on CRTs, TFT-LCDs, PDPs, and projection systems are projected to fall within the range of 0.2–0.5 lm/W—which we believe is within the limits of consumer acceptability (Table I), Commercial success will thus depend on factors other than brightness and power consumption: cost, bulk, weight, and manufacturability. That should favor CRT and TFT-LCD projection systems at the beginning, with increasing interest in PDPs as the technology is commercialized. ▪