Projection Hdtv

Some of you may be familiar with The Perfect Vision magazine, which is ranked among the top A/V magazines in the country. Gary Merson is a senior editor at The Perfect Vision, in addition to publisher of the well-respected industry newsletter called the HDTV Insider Newsletter. Gary has been reviewing HDTV products and writing about the technology for years, and is a noted expert in the field. I thought I'd tap Gary for some basic insights and pointers on buying HDTVs, but I recommend you also check out The Perfect Vision website for some of Gary's stories that cover these areas in more detail (note that some stories are free for downloading in PDF format, others cost $3 each, an entire current issue is $10, and back issues are priced at $6 each).

Calculate Screen Size based on Viewing Distance Before getting into the specifics of various HDTV technologies, Gary said the first thing you must determine is your typical viewing distance from the HDTV set, as that will help establish the optimal screen size for viewing 1080i and 720p content. Further, the viewing distance helps determine whether you can consider a direct view CRT (which presently tops out at 34" diagonal for new models), or if you must move to larger projection or flat-screen technologies.

While various HDTV manufacturers provide optimal screen size and viewing distance charts (especially front projector vendors), Gary suggested a few simple rules of thumb to calculate optimal viewing distance based on screen height. You must measure the actual screen height of the HDTV in inches (active screen area within the bezel, top to bottom). If you don't have a ruler handy, simply divide the diagonal measurement in half to get an approximate screen height for 16:9 aspect ratio HDTVs. Next, multiply the height by 3.2 to determine optimal viewing distance for 1080i content, or multiply by 4.8 to get the optimal distance for 720p content.

With my 720p Samsung 43" diagonal HLN4365W, the screen height is roughly 21 inches, which when multiplied by 4.8 yields 100.8 inches, or 8.4 feet. I typically view the set from 8-12 ft in the family room, depending on the seat. Assuming my average viewing distance is 10ft, Merson says my screen is a bit too small for my eyes to discern the maximum resolution from the TV screen. Using quick math, 10ft is 120 inches, and dividing by 4.8, that would require a 25 inch vertical height, or approximately a 50 inch diagonal HDTV for optimal perception of the 720p screen resolution from 10ft. I still find the experience quite satisfying overall, and recall I have the constraint of having the TV fit inside a wife-specified corner entertainment unit, and the 43" diagonal DLP is about as large as I can fit at the moment. When I set up my basement next year, these equations will definitely be factored heavily into my decision, (and the wife's preferences are NOT part of the equation! OK, you're right, this is really a test to see if she reads my stuff...).

Given rear-projection HDTVs are quite popular, I asked Gary for his opinions on popular rear-projection technologies, focusing on highlights and lowlights, based on his years of lab testing experiences. I'll present excerpts from our discussion in abbreviated, summary format below.

Rear Projection CRTs: Inexpensive HDTVs, relatively speaking. Excellent performance for the least amount of money. Reliable, and have been around a long time. Multiple scan rates, which can help with up-conversion of poor signals. The have great black levels, but sharpness is limited given it's an analog technology. Projection CRTs must be properly focused at the factory, and often need convergence and other adjustments to get a good picture out-of-the-box.

When buying CRTs, look for fine screen pitch (.52mm is popular in higher-end sets and better than .72mm seen in many lower-cost units), and try to buy a set with a copy-protected interface (such as DVI/HDCP or FireWire/5C). If you can afford it, Gary prefers sets with built-in HDTV tuners and two-way FireWire/5C connectors. Within 2002 product lines, Mitsubishi and Hitachi HDTV sets met these criteria.

Scalers are improving, but even with good scalers, if you have a lousy analog signal, you'll still see smearing, ghosting, and other problems on an HDTV.

CRT rear projection HDTVs are still a very worthwhile technology, and if a set is properly calibrated and optimized, it can give you a fantastic picture with the blackest blacks of any technology. CRTs are generally rated at 15000 hours until half-brightness, assuming the user control for contrast is set properly (generally between 30%-40%), which is not the typical factory default setting.

Rear Projection LCDs: Gary has a big problem with these devices and doesn't have much positive to say about them. Per Gary, black levels are terrible, and he's never found a properly calibrated LCD unit out-of-the-box. The units also suffer poor contrast ratio, because you can't get a good, dark black. Gary has experienced degradation with LCD pixels dying over time. He's also observed a few LCD televisions on retail showroom floors that have been active for a year or longer and displaying 12 or more stuck or dead pixels. Bottom line is that Gary would never recommend transmissive LCD technology, and he thinks it's a bad technology for consumers given all the problems he's seen. On the plus side, unlike plasma displays, burn-in is not an issue with LCD rear projectors, and they have a slim form-factor.

Rear Projection DLP: I wanted Gary to provide expanded thoughts in this area, since I recently settled on a DLP rear projection set in the Samsung HLN4365W. Gary said DLP is a mixed bag, but the sets have a very crisp-looking picture, and black levels are deep enough (though not quite as dark as CRTs) to provide excellent contrast ratios. Scalers in DLP sets are optimized for a single scan rate, which is good, and DLPs have a very good angle of view from side to side, but viewing below or above center screen can be an issue for some people with certain DLP rear projectors at close viewing distances.

Persistence of vision (aka rainbow effect, see our main story for details) is also a problem for a small percentage of people. Dithering gray scales using the micromirrors creates a form of visible noise with certain video material. Essentially, artifacts will vary depending on source material, the person viewing, and the viewing distance. Unlike plasma displays, burn-in is never an issue with DLP technology.

Factory preset gamma level may be an issue in some DLP projection sets, and is not inherent in DLP technology, but more a factor of implementation. While gamma is often described in technical terms, Merson says it can be translated simply as follows: a proper display device should have a linear transition from black to white areas, and bad gamma settings in a display does not allow such a linear transition. Bad gamma settings can also cause white crush or white clipping, where you see no gradient tones of brightness (white levels) above a certain point. When a bright light is shining on a face, for example, there may be a lack of detail and tonality that is quite evident, and the faces don't look natural. Gary says the upcoming RCA DLP units (referring to Thomson's RCA Scenium units displayed at its launch event in NYC recently) appeared to handle gamma quite well, based on his initial visual inspection, and they may be one of the best executions of DLP rear-projection technology to date.

Gary believes DLP rear projection HDTVs are very nice devices, though certain limitations exist with the current state of the art. Black levels are at acceptable levels, not jet black, but a neutral dark black, plus you can now get a bright white. DLP sets do not have degradation problems of LCDs. And if a company implements DLPs properly in terms of scaling and gamma, you can have a compelling DLP picture.

Regarding DLP enhancements, we may see designs with more segments on the color wheels, faster spin rates, and brighter lights. We may even see Archimedes-style color wheels. Instead of today's color wheels, which look like six-segment ViewMaster discs, the Archimedes wheels are shaped more like spiral lollypops, where all three colors can be projected on the DMD device at once, similar to Philips' upcoming single-chip LCoS scrolling color system. This can increase brightness by 40% and get rid of the rainbow effect. Gary noted that Sim2 Seleco just demonstrated an Archimedes sequential wheel at Infocomm in Orlando, but it would not disclose when the technology would be incorporated in future products.

Gary is very high on LCoS designs, and has written two very insightful articles about the Toshiba 57HLX82 second-generation LCoS-based HDTV that you can obtain for $3 each. Loyd Case wrote about the set briefly, as well. Note that RCA delivered an LCoS Scenium unit to the market last year, but couldn't quite get it to work properly, so they went back to the LCoS drawing board for now, and instead opted to launch the new DLP lineup in the meantime. But Toshiba seems to have gotten things right with its three LCoS chip-based, 1080p-capable 1920x1080 57" HDTV monitor. Plus Toshiba is expected to come out with a new three-chip LCoS set in September at a list price of $5500 and a minimum advertised price (MAP) of $5000.

As mentioned in our main story, Gary also noted that Philips will be coming out with a single-chip 720p LCoS-based HDTV later this summer that looks very promising with its scrolling color technology.

Though we didn't discuss plasma TVs above, Gary did point out that they are now getting blacker blacks with higher contrast ratios, and they have excellent viewing angles side to side, and from above and below the center of the screen. In addition, they have very bright pictures. And prices are starting to come down significantly, with 42" HDTV models now starting around $3999.

Thanks for your insights Gary!

Copyright © 2004 Ziff Davis Media Inc. All Rights Reserved. Originally appearing in ExtremeTech.

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