Display Technology Shoot-Out

Comparison of CRT, LCD, Plasma, DLP and LCoS Technologies

 

Dr. Raymond M. Soneira

President, DisplayMate Technologies Corp.

Copyright © 1990-2006 by DisplayMate Technologies Corporation. All Rights Reserved.
This article, or any part thereof, may not be copied, reproduced, mirrored, distributed or incorporated
into any other work without the prior written permission of DisplayMate Technologies Corporation

 

Article Links:  Overview

Part I   Part II   Part III   Part IV

Part A   Part B   Part C   Part D

LCoS HDTV Manufacturers Sidebar

Shoot-Out Hardware and Software Sidebar

 

Fifteen years ago the CRT had a virtual monopoly for displays used in televisions and computers. Today we have half a dozen competing display technologies. In spite of all of this competition, the CRT has managed to hold onto its crown title as the Reference Standard against which all of the other display technologies are measured. There are two reasons for this: each new technology had to mimic the dominant CRT in order to be accepted in the marketplace; and second, the image and picture quality delivered by the best CRTs was simply outstanding and untouchable by any of the new technologies. Virtually all professionally produced content is still produced and optimized on CRT monitors. So, while all of the other technologies have been quite successful at chipping away at the CRT’s total market share, it has managed to hold on to the very top-end of the market and continue on as the Reference Standard for image and picture quality. But its lead has been steadily slipping and we’ll review its current status here.

Below we compare the state-of-the art for each of the display technologies, based upon the best consumer and commercial grade units available today, regardless of price. Most of the comparisons are based on the in-depth and comprehensive testing and analysis that we performed in the Display Technology Shoot-Out series. Because the state-of-the-art is always changing (although not as much as the manufacturers and published specification sheets would have you believe), we’ve also taken into account evaluations and observations of newer shipping units and product introductions at tradeshows.

 

In Table 1 we compare all of the technologies side-by-side: CRT, LCD, Plasma, DLP and LCoS. We’ve compared the technologies in 15 categories. No technology comes even close to wining in every category, but there are definite performance trends. DLP appears twice: in 1-Chip configurations, by far the most common, which currently use a spinning color wheel, and 3-Chip configurations, which are currently found only in high-end front projectors. LCD also appears twice: in Direct View and Projection. Each entry is an evaluation based on a letter grade system, with A for excellent, B for good and C for fair, with pluses and minuses added to indicate differences where needed. Some entries have two grades listed, together with a keyword suggesting the source of the duality. They are also visually color coded: A is green, B is yellow and C is red.

 

Each table entry is explained further below. They are all judgment calls based on my own measurements and detailed evaluations. Manufacturers are likely to question or challenge some of them, but I have no affiliation or vested interest in any of the technologies, so they are as objective a set of observations as you’re likely to find.

 

Table 1: Display Technology Assessments

 

Table Entry Explanations

We are going to cover a lot of material here, so we’ll only be able to briefly explain the meaning and interpretation of each of the entries in Table 1. For in-depth definitions, explanations and interpretations please refer to all eight of the Display Technology Shoot-Out articles, which are the foundation of virtually all of the material discussed here.

 

Peak Brightness is important only for high ambient light applications. It’s overrated since most displays are already too bright for their intended application. The excess brightness can actually be put to good use by trading it in for improved contrast, gray-scale accuracy, and viewing angle. Peak Brightness for Plasma displays is significantly reduced when there is a high Average Picture Level due to limitations in power dissipation.

 

Screen Brightness indicates how well a display can operate in low ambient light with lower brightness levels. An iris, backlight or lamp control is desirable to optimize screen brightness, otherwise the Contrast Ratio is reduced and Artifacts increase (when using the Contrast Control to reduce screen brightness).

 

Black Level is very important for low ambient light and not very important for high ambient light. Brightness and Black Level determine the Contrast Ratio. The additional enhancement from an adjustable or dynamic iris is not part of these ratings.

 

Contrast Ratio is very important for low ambient light and not very important for high ambient light. For projectors the indicated Contrast Ratio can generally be further increased by 100 percent or more by constricting the optical path with an iris, which reduces brightness and the overall optical efficiency (lumens per watt). The additional enhancement from an adjustable or dynamic iris is not part of these ratings. Plasmas are again affected by the Average Picture Level.

 

Checkerboard Display Contrast is highest for the direct view LCD and Plasma displays. Plasmas are again affected by the Average Picture Level. CRTs and projectors have lower values due to the amount of glass in the light path. Checkerboard Contrast has only a minor effect on perceived picture quality (see the discussion in Part B).

 

Color Primaries indicates how easy it is to adjust the primary colors for a given technology. It’s easiest for projection optics, but harder to change the sub-pixel filters in LCDs and the phosphors in CRT and Plasma displays.

 

Drive Electronics indicates how capable the electronics is in producing artifact free images with accurate gray-scales, and in compensating for irregularities and limitations in the display devices themselves. DLP and LCoS have very sophisticated electronics that contribute significantly to their performance. CRT analog electronics is excellent in professional units but is frequently lacking in consumer and commercial units. LCD and Plasma have the most to gain in enhancing their drive electronics.

 

Gamma Curve Transfer Function indicates how accurately displays follow the ideal power-law gamma of 2.20 via Drive Electronics and calibration. LCoS is the best because all 256 signal levels are measured and adjusted to match the ideal relation. Other technologies lack this detailed calibration or are pushed for peak brightness instead of gray-scale accuracy. Modern CRTs require some signal processing to deliver the ideal gamma.

 

Gray-Scale Contouring arises from irregularities in the Gamma Curve, which introduces false intensity steps and visible contours in an image.

 

Gray-Scale Dark Artifacts: the dark-end of the intensity scale is especially difficult to generate accurately, often leading to all sorts of irregularities. Temporal Dithering due to Pulse Width Modulation produces visible screen noise at low intensities for Plasma and DLP.

 

Motion Artifacts include any form of smear, flicker or breakup when the image is changing. This can be caused by limitations in the drive algorithms, processing or electronics in addition to device latency.

 

Viewing Angle artifacts arise from undesirable variations in intensity, contrast or color with angle. LCDs have improved significantly. Low gain projection screens perform much better than high gain screens that are used to boost screen brightness.

 

Aging refers to a change in performance over time. Replaceable lamps in projectors are not considered aging here. The backlight in many Direct View LCDs is difficult to replace. The phosphors in CRT and Plasma units may age non-uniformly, but this is currently not a serious problem (See Part IV). LCoS and DLP devices do not deteriorate noticeably with age (based on lab testing as well as actual field use over 60,000 hours).

 

Peak Resolution is highest for LCoS. CRT peak resolution is affected by Other Artifacts.

 

Other Artifacts: although CRTs have the longest list of Other Artifacts, their primary effect is a softer image, which is sometimes desirable. The Color Wheel in 1-Chip DLPs can cause rainbows to appear occasionally but it depends on the image and sensitivity varies from person to person. Many DLP projectors double the screen resolution by using a dithering process called Wobulation.

 

The New Reference Standard

Now that we’ve discussed all 15 parameters and categories, it’s time to consider which display technology currently offers the best overall image and picture quality. The winner should be crowned the Reference Standard. To come up with an Overall Grade and Overall Rank we need a scheme to appropriately weight all of the entries. Because there are so many diverse applications for displays, even for HDTVs, no single weighting scheme is appropriate for everyone. Readers are encouraged to apply their own weightings based on their particular applications, interests and personal biases. For example, in high ambient lighting conditions, the C grades received by LCDs for Black Level and Contrast Ratio are irrelevant, and should receive a low weight, possibly even zero. On the other hand, in a high-end Home Theater, those parameters are very important and might receive the highest weights of all.

 

A generic approach for the Overall Grade is to take a straight unweighted average of all of the individual grades. For general applications, it turns out that the end result is relatively independent of the weighting scheme because there is a fair degree of consistency in the grades for each technology. LCoS received almost straight As, the CRT got mostly As with some Bs and Cs, DLP received an even mix of As and Bs, both LCDs got mostly Bs with some As and Cs, and Plasma got a relatively even A to C distribution. With that point of view I came up with the grades and ranks shown in the table (which are relatively close to the unweighted averages): LCoS got an A and Rank 1, close behind was the CRT with an A- and Rank 2, DLP got a B+, and Rank 3, with a slight advantage for the 3-Chip over the 1-Chip. Plasma and both LCDs received a B, and tied for Rank 4. They are simply too close to call for general applications, but any specific application will most likely favor one significantly over the others.

 

And the winner is… LCoS, the new Reference Standard for overall image and picture quality, dethroning the CRT after more than 75 years at the top. An impressive achievement for a technology that has only recently started shipping in quantity. The new display technologies have now moved out from under the shadow of the CRT to stand on their own accomplishments.

 

What’s Coming Next

The Display Technology Shoot-Out series will be continuing in the near future with articles on a number of exciting new display technologies. They will continue to be in-depth display technology assessments rather than product reviews.

 

Potentially the most interesting is the Canon-Toshiba Surface-conduction Electron-emitter Display, SED, which is a very thin (under 1 centimeter) CRT-like phosphor based display technology. It has digitally addressed pixels, however, the brightness of each pixel is produced through an analog process, so it should be free of the digital artifacts that are present in Plasma and DLP displays, which have digital on-off intensity controls produced with Pulse Width Modulation  (see Part III). In this regard SED is very similar to LCoS. As an emissive technology, SED already produces CRT-like Black Levels, with Contrast Ratios in the 10,000 to 100,000 range (much better than Plasma because it doesn’t need to maintain a background level for priming the discharge). The response time is speced at 1 ms, which is very fast. SED is a type of Field Emission Display, FED, another difficult technology that has been under development for over 15 years. The SED products were initially announced to ship in 2005, but that date has been repeatedly pushed back. The most recent announcement as of March 2006 is for a product launch at the end of 2007, with volume shipments to begin in 2008. That’s disappointing but typical for a new display technology. The SED prototypes that have been periodically shown since 2005 are very impressive. Canon has so far been very skittish about allowing us to test any of their SED prototypes, but with the current schedule we’ll be able to evaluate an early production unit by the latter part 2007.

 

There are a whole series of very exciting display technology developments based on LED lighting for (front and rear) projectors and direct-view LCD panels. The extended color gamut that is possible with LEDs seems to be getting most of the attention. But as we have pointed out in Part II and Part B, an extended color gamut is undesirable for HDTVs because the primary colors need to match the standard Rec.709 primaries, which are used for color balancing all professionally produced content. If you have an HDTV with an extended color gamut you’ll see inaccurate gaudy colors.

 

Two important advantages of LED lighting are their very long lifetimes and their long-term color stability. The UHP lamps used in most projectors have a spectrum and an arc profile that change with age, so brightness, color accuracy, screen uniformity and Contrast Ratio all vary over the life of the lamp. LEDs are immune from all of these effects. You’ll also save money by not buying the relatively expensive replacement lamps.

 

But the most amazing advantage of LEDs is that they can be pulsed at incredibly fast rates. This strobing can be used to cut down on visible motion smear in LCDs (Part III) and improve the Contrast Ratio. For DLPs it can dramatically reduce dithering artifacts (Part III), dispense with a color wheel, and best of all eliminate visible rainbow artifacts (Part IV).  Both of these developing technologies will be fantastic candidates for future Shoot-Out articles. We will be evaluating them at the appropriate time in the near future.

 

LEDs can also behave like a super-fast dynamic iris, so they can dramatically improve Black Levels and Contrast Ratios (but with constraints that generally introduce some gray-scale artifacts, Part B). There’s still more… by using a very large number of addressable LEDs for backlighting a direct-view LCD panel, it’s possible to produce a display with an incredibly large Dynamic Range with reduced artifacts. For example, with a 1000:1 Contrast Ratio LCD panel illuminated by LEDs with a duty cycle range of a 1000:1, it’s possible to produce a display with a 1 million:1 Dynamic Range. BrightSide Technologies and Sharp are each developing displays with values of 200,000 and 1 million, respectively. We plan on evaluating them at the appropriate time in the near future.

 

To take proper advantage of all of the opportunities provided by pulsed LEDs without introducing significant artifacts will require the development of new and very sophisticated image data processing and control algorithms and electronics. It will probably take years for optimum implementations to be achieved. Once again, it will be really interesting to watch all of these exciting new technologies develop, improve, jockey for position, and keep the renaissance in display technology going strong…

 

If you know of a technology or product that is appropriate for a future Display Technology Shoot-Out article please let us know at dtso.info@displaymate.com. The Shoot-Out articles appear in select publications worldwide.

 

Article Links

Series Overview

 

CRT, LCD, Plasma and DLP

Part I: The Primary Specs

Part II: Gray-Scale and Color Accuracy

Part III: Display Artifacts and Image Quality

Part IV: Display Technology Assessments

 

LCoS – a New Mainstream Display Technology

Part A: Introduction to LCoS Technology

Part B: LCoS Color and Gray-Scale Accuracy

Part C: Test Pattern and Jury Panel Evaluations

Part D: Comparison with CRT, LCD, Plasma and DLP

 

Sidebar: LCoS HDTV Manufacturers

Sidebar: Shoot-Out Hardware and Software

 

 

About the Author

Dr. Raymond Soneira is President of DisplayMate Technologies Corporation of Amherst, New Hampshire, which produces video calibration, evaluation, and diagnostic products for consumers, technicians, and manufacturers. See www.displaymate.com. He is a research scientist with a career that spans physics, computer science, and television system design. Dr. Soneira obtained his Ph.D. in Theoretical Physics from Princeton University, spent 5 years as a Long-Term Member of the world famous Institute for Advanced Study in Princeton, another 5 years as a Principal Investigator in the Computer Systems Research Laboratory at AT&T Bell Laboratories, and has also designed, tested, and installed color television broadcast equipment for the CBS Television Network Engineering and Development Department. He has authored over 35 research articles in scientific journals in physics and computer science, including Scientific American. If you have any comments or questions about the article, you can contact him at dtso.info@displaymate.com.

 

 

Copyright © 1990-2006 by DisplayMate Technologies Corporation. All Rights Reserved.
This article, or any part thereof, may not be copied, reproduced, mirrored, distributed or incorporated
into any other work without the prior written permission of DisplayMate Technologies Corporation