The Miller Channel
By Kevin Miller
March 30, 2001 - When it comes to video display devices (TVs) size matters more than you might think. My experience as a custom installer taught me that customers always want the biggest picture they can possibly get when purchasing an expensive home theater system. While the overall size of the picture in a home theater definitely adds to the impact of the home theater experience, bigger is not always better. The optimum size of the screen depends on several factors.
What kind of display device will be used?
For a big screen home theater you need to go either front or rear projection. There are basically three types of display devices available today, and each one has its limitations. The oldest and still the reigning champ in picture quality is the CRT (Cathode Ray Tube) based projection TV. While CRT based RPTVs and FPTVs produce the best picture, especially in terms of resolution, black level and contrast ratio, they are wanting in the all-important area of light output. For this reason screen size is a very important factor when dealing with CRT based projection TVs. Obviously RPTVs come in fixed picture sizes so they don't apply to this discussion.
CRT projector guidelines
Front projectors, however, are another story. For optimum picture quality on CRT based front projectors follow these guidelines. For 7-inch CRT front projectors the optimum screen size is a 72-inch wide 16:9 aspect ratio screen. For 9-inch CRT projectors the optimum screen size would be an 84-inch wide 16:9 aspect ratio screen. Can you go larger? Yeah sure. Most projectors are capable of producing 15-foot wide 16:9 aspect ratio screen sizes. However, once you begin to exceed the optimum screen size you have to drive the projector harder and harder, which has two real nasty side effects. First the tubes will burn out much faster than with more reasonably sized screens, and secondly picture quality begins to suffer in a number of areas. When you crank contrast beyond a projector's linear operating range you introduce geometric distortion, and color fidelity goes all to hell. Not to mention that retubing a CRT projector is very expensive. Finally, the larger the picture you create with a CRT projector the more visible the scan lines that make up the picture become. Properly setup on a reasonably sized screen, CRTs can last up to about 10,000 hours. If you severely overdrive the projector to accommodate a larger screen, tube life is cut by 60% or more.
Fixed pixel display device guidelines
The two main categories of fixed pixel projectors are LCD (Liquid Crystal Display) and DLP (Digital Light Processing). Neither of these technologies suffer from the same problems as CRT based projectors when projecting too large an image, but they have their own set of problems. Both LCD and DLP based projectors have plenty of light output to drive big screens. In fact, in the case of DLP projectors, there is plenty of light to spare.
However, both LCD and DLP projectors, being fixed pixel displays, have picture elements that make up the structure of the picture. When you blow up the size of the picture these elements become increasingly visible. In the case of LCD tiny little pixels make up the picture. With DLP it is micro-mirrors that make up the structure of the picture. In both cases, the higher the resolution of the panels the smaller the picture elements, and therefore the better the picture quality. But even the highest resolution LCD and DLP projectors will show the structure of the picture if producing too large an image.
The most common resolutions of both LCD and DLP projectors are 800 x 600 and 1024 x 768. There are only two manufacturers that currently offer true 16:9 widescreen performance with either of these technologies. Sony has the only 16:9 LCD panel projector on the market right now. The VPLVW10HT with a resolution of 1366 x 768, has the highest resolution of any LCD projector designed for home theater use. Runco currently makes two DLP projectors coupled with controllers that act as both video processors (scalars) and aspect ratio controllers. The VX-101c and the VX-1c are 800 x 600 and 1024 x 768 resolution devices respectively. The controllers crop some of the resolution in order to do aspect ratio control the way a CRT projector does; creating a 4:3 aspect ratio inside of a widescreen.
The largest size screen I would recommend for either of these devices would be an 84-inch wide 16:9 aspect ratio. For other 4:3 LCD and DLP projectors stick with an 84-inch diagonal screen for the best results. If you go beyond these recommendations depending on how far you are sitting from the screen you will get what is commonly referred to as the "screen door" effect. It's literally like looking through a screen door. You'll see the little square pixels or mirrors that make up the structure of the picture, which is very distracting.
Seating distance from the screen is also key to the best possible picture
The last thing to take into consideration when deciding on the size of the screen you want, for any of these types of projectors, is how far from the screen your main seating position will be. Generally speaking the farther you are away from the screen the better the picture will be, but you don't want to be so far away that you loose the cinematic impact of the home theater experience. There are some basic guidelines that you should consider. Assuming there is video processing (line doubling, tripling, scaling etc.) the ideal seating distance would be 3 to 4 times the picture height from screen surface to seating position. For example, with a 7 or an 8-inch CRT projector using the ideal screen size of 72-inches wide, 121.5 to 162 inches (10 to 13.5 feet) from screen surface to seating position would be ideal. Provided you have a high quality projector with good video processing this should yield excellent results. If you're seating position is going to be any closer than that you should seriously consider down sizing the screen.
So do yourself and your projector a favor and resist the temptation to go with too big a screen size, just for the sake of size. Take into serious consideration the type of projector you will be using and the room constraints. For the best home theater experience, bigger is not always better.
3/20/01
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