Beginner's Home Theater Video - Part 3 Front Projection
Contents:
Introduction
For the closest thing to the real cinema experience, consider front projection, where the sky (or at least the ceiling) is the limit on screen size. The sky is also the limit on how much you can spend on a front projection set up, so take some time and learn the basics first. Doing a front projection system over is a very expensive endeavor.
Front projection is the most complicated way to display images in your home theater, but can be the most rewarding also. Front projection is much more than just the projector, since the projector sits out in your theater with you, the quality of the projected image is influenced by the room design and decoration, the screen selection, plus ambient and room lighting conditions.
System Components
System Source, Projector and Screen
The quality of the projected image is of course dependent on the type and quality of the projector, but not solely on the projector. The projector is part of a system that starts with the video signal source and ends at your eyeballs. A projection system requires balance. Balance between the brightness of the projector and the screen size and gain, balance between the resolution of the source and projector and the screen size and viewing distance.
Lets step through this projection system from beginning to end.
Source
What do you like to watch? What is available to you? Do you watch a lot of sports on regular TV? Are you a DVD junkie? Have you made the move to HDTV? All of the above? What you watch will determine the requirements put on the projection system. In general the higher the resolution (quality) of the source, the better the projection system has to be to reproduce it correctly.
- NTSC/PAL TV: This is "regular" TV, from broadcast, cable or satellite. This is the lowest resolution signal. The challenge here is not choosing a projection system that can handle the signal, all can, the challenge is to try to keep these sources from showing all the signal's faults when shown eight feet wide.
-DVD: Nearly everyone considering a projection system watches DVD's. DVD's have similar resolution to regular TV, but much lower noise and much higher picture quality. The minimum source requirement is a good quality player; a progressive scan player can be beneficial in some systems. This topic is discussed further this under scalers and processors.
-HDTV: Whether broadcast, D-VHS or satellite received this is the most demanding source. With fully three times the resolution of DVD and requiring five times the bandwidth. If you plan on watching much HD material, you will need a projector that can do it justice.
Scaling/processing
Scaling and signal processing is the mysterious, confusing and most misunderstood part of the complete projection system. The basic job of a scaler/processor is to take the video signal from the source and change it so that the projector gets a signal that is a perfect match for its capabilities.
Digital projectors present two challenges to video reproduction. First, they have a fixed display resolution determined by the resolution of the imaging device and second, the display is inherently progressive instead of interlaced like direct view televisions
(see Part 1). The progressive display challenge is met by de-interlacing the video signal by taking two interlaced fields and making a full frame out of them, then the full frame must be enlarged, or scaled to fit the projectors display resolution. Digital projectors that accept composite, S-video, and component video signals do this de-interlacing and scaling internally, but many digital projectors were designed for the business market and do not do this job well. There are exceptions; many home theater specific projectors are designed with top quality processors built right in.
CRT projectors do not have fixed display resolutions and are capable of displaying an interlaced signal. A standard resolution (480i) signal will show visible scan lines on a CRT projector. The challenge with CRT projectors is again to de-interlace and scale the image so that these scan lines are minimized and a smooth image is displayed.
Processing can be as simple as hooking up your DVD player to your digital projector and letting the internal processor do the job, or as complex as a multi-thousand dollar processor. If the projector has a good quality processor, you are set, but if you are using a business type digital projector or a CRT projector, you will need an outboard processor.
Commercial processors vary from relatively inexpensive "line doublers" which perform the de-interlacing function without any scaling, to increasingly complex processors with multiple inputs, and variable scaling rates and output modes. Expect to pay anywhere from $600 for a basic unit to $4,000 for an advanced unit and $10,000 plus for a cutting edge unit. Better quality projectors demand better quality processing for maximum performance.
Another option is the Home Theater Personal Computer, or HTPC. An HTPC equipped with a DVD-ROM drive, video input board, and high quality video and sound cards and the right software can do an excellent job of video processing. HTPC's allow maximum flexibility in setting output resolutions. You also get the side benefits of being able to use the PC to play games on your projector, store your music collection, or surf the web on the big screen. With proper hardware and software an HTPC can become the control center for your home theater. The downside is that HTPC's are complex, non-standard devices that can really eat up a lot of time getting set up and performing. If you like this sort of thing, check one out.
What role do progressive scan DVD players have? If your projector will accept a progressive signal (480p), then you can bypass the projectors de-interlacing stage and just let the projector scale the image. If you are sending the DVD player through an external processor, you will want to feed it an interlaced (480i) signal and let the processor do the work.
HDTV generally does not require any processing, although there are some projectors (9" CRT's) that can benefit from going from 1080i to 1080p.
Cable
Projectors are often located many feet away from the source and scaler/processor components, so getting the video signal to the projector intact is paramount. Most projectors are connected to the source or processor by an RGB connection. An RGB connection is the highest quality analog video connection, the luminance signal for each color (Red Green and Blue) is transmitted on its own cable, the horizontal and vertical synch information is transmitted on separate lines also. The RGB connection has several form factors:
-RGBHV: This is the most common connection today; it is essentially VGA, the connection your computer monitor uses. It uses separate lines for each signal: Red, Green, Blue, Horizontal Synch, and Vertical Sync. The cable can be five separate coaxial cables or a bundle of coaxial cables under a common jacket. The terminations can be a 15-pin high-density connector (VGA connector) or five BNC coaxial connectors.
-RGBS: Basically the same as RGBHV, but combines the Horizontal and Vertical Synch signals onto one line. Normally uses four coaxial cables or four coaxial cables under one jacket with four BNC connectors.
-RGsB: A three coaxial cable interface, both synch signals are combined with the green luminance signal. Three BNC connectors.
There are also emerging direct digital interfaces, the most prominent of which is DVI, or Digital Visual Interface. Using a DVI interface with a compatible source allows the video signal to remain in the digital domain until the projector, thus eliminating many of the issues associated with analog cables.
Component and S-Video connections discussed in Part 1 of this series may also be used.
Cabling do's and don'ts:
-Never use several VGA extension cables chained together, the impedance mismatch at each connector will cause nasty reflections that will show up as double or triple images or ghosts on the display.
-Keep runs as short as possible.
-Use high quality commercial cables made for the purpose or construct your own from high quality components.
-Don't run your video lines with power lines, avoid parallel runs where possible, where not possible maintain at least six inches of separation.
Projector
The projector is where the rubber meets the road. A projector is to a video system as speakers are to an audio system. If the projector is not up to the task there is no way you will get a good picture, on the other hand; garbage in = garbage out. Like speakers, projectors interact with the room they are in. They are affected by ambient light, reflected light, dust and temperature; they produce heat, noise, and a large colorful image. Projectors and projector technology are discussed in depth later in this article.
Proper mounting, set up and calibration are all essential for getting the most from a projector.
Screen
You are going to be spending a lot of time looking at this, so make sure it's right. A screen needs to be flat and uniform across the entire width and height (there is such a thing as a curved torus screen, but is outside the scope of this article). Screens can be as simple as a painted wall or as complex as a high gain grey screen with automatic electric masking. Screens come in several configurations:
-Manual pull down screens: There are many inexpensive pull down screens readily available. I do not recommend these units, while priced attractively; they will not remain flat over time. If you need a pull down screen, consider a tab tensioned model, which will remain flat.
-Fixed screens: Fabric screens stretched onto a frame, available from many manufacturers. These screens can be home made by painting a section of a wall or a specially prepared board, or stretching and fabric onto a frame and painting it.
-Electric screens: Essentially the same units as manual pull down screens, just with electric motors that extend and retract the screen. Again, get a tab tensioned model. An electric operator will add a minimum of $600 -$800 to the price of a screen.
Screens vary by surface characteristics. The base surface characteristics are:
-Gain: Gain is a measure of how a screen reflects light. A smooth white wall would have a gain of 1, or unity gain. Unity gain means that all the light that hits the screen reflects normally, it is not absorbed nor is it refracted in any way. If the paint used on the wall includes materials that refract the light so that more light reflects back towards the source, then it has a gain of greater than 1. Higher gain screens appear brighter when you sit in front of them and can help offset a low light output from the projector. The higher the gain, the more important it is that you sit directly in front of the screen. Many older screens used tiny glass beads on the surface to give a high gain finish. These glass beads cause too much refraction, to the point where they affect color balance, and should not be used for home theaters.
-Color: Most screens are white, however some digital projectors can benefit from a "grey" screen. A grey screen absorbs some of the projected light and can make blacks appear "blacker", giving the image more impact.
The Room
As in audio reproduction, in video reproduction the room matters. The two most important factors are ambient light control and reflected light control. For a front projected image to look best, ambient light must be eliminated, this includes outside light from windows and general illumination in the room. For casual television viewing or for watching bright programs like sporting events where image quality is not paramount, some ambient light is not detrimental. With today's high contrast digital and CRT projectors; light reflected from the screen to the walls, ceiling, floors and furnishings and then back to the screen will make the image look washed out and not as natural as it could.
The size and shape of the room is also important. Projectors require a minimum amount space between the projector and the screen. This distance is called the "throw distance", it is often expressed as a ratio of screen width to distance from the screen. For example, to project a 100" wide image with a projector that has a throw distance of 1.5, then the projector must be mounted 150" (1.5 x 100) back from the screen. Zoom lenses, found on many digital projectors but not CRT's, can give you some flexibility. Consider the required throw distance and the size of your room when choosing a projector.
Consider the projector mount. The vast majority of projectors are ceiling mounted. Will your ceiling support a projector? Do you rent and cannot mount a projector permanently. All these factors will drive your projector choice.
Projector Technologies
Basics
Front projectors divide into two main classifications: digital and analog or CRT. The basics of the two technologies are:
-Digital: Digital projectors have a single lamp that produces white light. This white light is controlled by a fixed resolution imaging device to produce an image. The imaging device is digitally controlled and the output is measured in discrete picture elements or pixels.
Digital projectors each have a native resolution, which is a measure of the number of pixels in the imaging device. Your budget and viewing habits will determine what the best resolution is. Projectors, like televisions, come in both standard 4:3 and widescreen 16:9 aspect ratios. For 4:3 projectors consider 800x600 as the minimum for DVD viewing, for HDTV and full resolution DVD viewing 1024 x 768 is the minimum. Widescreen projectors have a minimum resolution of 848 x 600, great for DVD's but suboptimal for HDTV. High-resolution widescreen projectors are 1366 x 768, 1280 x 720 and 1400 x 1050.
Lamps in digital projectors lead a hard life. The lamps are asked to provide copious amounts of pure white light while being jammed into a tiny little box. Older projectors used halogen, and then metal halide lamps, these lamps do not produce nearly as much light and have much shorter life spans than today's lamps, avoid them. Current projectors use predominantly UHP, SHP, and UHM lamps which are all variations of a very high pressure mercury vapor arc lamp. Some projectors, notably JVC D-ILA's, use Xenon arc lamps. Xenon lamps have better color output than Mercury lamps but generate more heat, don't last as long and are more expensive. Generally the rated lamp life for projection bulbs is 2000 hrs up to 5000 hours for some models in a reduced light output mode.
Entry level and business class LCD's and DLP's can benefit greatly from the addition of a processor or HTPC. LCD's and DLP's also benefit from a grey colored screen, either with a high gain finish or without.
Analog or CRT: CRT projectors use three cathode ray tubes (CRT), like direct view televisions, except that each CRT produces only one color (Red, Green, or Blue), has a very high light output, and does not have shadow mask. The light from each of these CRT's is focused onto the screen through a lens, the images are lined up and a full color image is produced.
Specific Technologies
Digital projectors can be classified by the technology that is used to produce the image. A discussion of the major types follows.
LCD
LCD stands for Liquid Crystal Display. LCD projectors work by splitting the white light from the lamp into red, green and blue components by the use of polarizers and dichoric filters. Each color then passes through an LCD panel that modulates the light by turning off and on LCD elements for each pixel. These modulated red, green and blue light streams are then recombined in a prism block and focused through the projection lens onto the screen.
LCD's are the elder statesmen of digital projection. They have been around longer than DLP or LCOS. The actual LCD panels are made by several companies, including Sony and Epson, these panels are then incorporated into many different manufacturers designs.
LCD Pluses include high lumen output per bulb watt, decent color reproduction, large range of models available, and the least expensive digital technology.
LCD Minuses include lower contrast specifications, higher absolute black levels, and the "screen door" effect.
Contrast on LCD projectors is determined by how tightly the LCD panels can block or "shut off" the light, then multiply that figure by three to account for each panel. This results in an image that has dark grey blacks rather than "black" blacks.
The "screen door" effect is caused by the spaces between the elements of the LCD panels. These spaces block the light and result in the projection of a "grid" that appears as if you are looking at the image through a screen door. This effect is minimized in newer projectors through the use of a micro lens array (MLA) that makes the pixels appear "bigger" on the screen and cover the grid. This also helps the contrast levels a bit. All digital projectors have this effect to some degree; the degree to which it is evident is determined by the "fill factor" which is how much of each pixel is filled with image versus grid.
LCD's are good choices for entry to mid-level projectors as many good models are available at low prices. Entry level units are typically 800 x 600 resolution with sub 1000 lumen light output, priced as low as $1500 new, while top units which boast 1366 x 768 resolution, greater than 1000 lumens of light output and premium scaler/processors are $5000-$6000. LCD's are also good when ambient light control is not perfect. The higher lumen output can help overcome some ambient light.
DLP
DLP stands for Digital Light Processing, and is a Texas Instruments trademark for their patented Digital Micromirror Device (DMD). DLP projectors come in both single chip and three chip designs. The most common and affordable designs are single chip. To understand how DLP projectors work, you must understand what a DMD is. A DMD is an array of microscopic mirrors mounted on electrically operated hinges. As each mirror is tilted it reflects light in a different direction, either towards or away from the projection lens. Single chip projectors take the light from the projection lamp and shine it onto the DMD through a rapidly spinning color wheel with red, green and blue color segments. As each color segment passes in front of the light stream the mirrors on the DMD are modulated to send the correct amount of that color light to the screen for each pixel. The net effect is that alternating red colored, green colored, and blue colored images are flashed on the screen at a high rate. Due to the persistence of vision your eye combines the images into one full color image. Three chip DLP's do not use a color wheel, but split the white light from the lamp into red, green and blue light streams, just like LCD projectors do. Each color has a dedicated DMD, after which the colored light streams are recombined with a prism and sent through the projection lens to the screen as a full color image. Commercial digital cinemas use three chip DLP projectors with super bright Xenon lamp houses providing the light.
Pluses include good color reproduction for single chip designs and world class color reproduction for three chip designs, best contrast of digital technologies, good fill factor which results in low screen door effect, and moderate prices.
Minuses include: Class leading projectors are more expensive than LCD, "rainbow" and dithering or mosquito noise artifacts that some people are susceptible to, generally lower lumen ratings per watt of lamp power.
Contrast on DLP projectors is determined by the type of DMD used, Texas Instrument's latest designs have a higher tilt angle (12 degrees vs. 10 degrees) which means that the light in the "off" state is diverted even further away from the projection lens. These latest DMD's are named the HD2 for the widescreen 1280 x 720 version, and are a replacement for the very successful HD1 DMD. The quality of the optics has a large impact on contrast ratio also, the best designs use precision optics from the likes of Zeiss and Minolta to keep internal reflections and light dispersion to a minimum. Three chip DLP projectors tend to have lower contrast ratios than single chip designs; with three DMD's there is three times the scattered light to add to the absolute black level and reduce contrast.
The "rainbow" effect is caused by rapid flashing of the different colored frames in single chip DLP designs; the rainbow effect is not an issue with three chip designs. The rainbow effect is not detectable by many people; it is most noticeable when you move your eyes away from a bright spot in a scene with motion. This effect is minimized in newer projectors through the use of higher speed color wheels that flash more images per second onto thee screen.
DLP's are good choices for entry to high-end projectors. Entry level units are typically 800 x 600 business class projectors priced as low as $2000 new, while top units which have HD2 DMD's, 1280 x 720 resolution, greater than 1500:1 contrast ratios, and premium scaler/processors are $10,000-$17,000. Three chip designs are the ultimate solution for large screens in high end home theaters, their excellent color reproduction, high resolution and high lumen output make them good choices for large theaters, costs for basic models are $30,000 - $60,000. Top models are $150,000+.
LCOS/D-ILA
LCOS stands for Liquid Crystal on Silicon. D-ILA stands for Digital Imaging Light Amplification, JVC's implementation of LCOS technology. LCOS is similar to LCD in that it uses a liquid crystal imaging device, but like DLP, LCOS reflects rather than transmitting light. The light from the lamp is split into separate colored light streams like LCD, then passed through and LCD imager and reflected from a mirror that forms the backing to the LCD. The light streams are then recombined and projected.
LCOS/D-ILA has the highest fill factor of the three main digital projection technologies, it also has the highest resolutions, with up to QXGA (2048 x 1536) resolution. This combination of high fill factor and high resolution combine to give LCOS produced images a very smooth, integrated appearance.
LCOS has good contrast, but again with three light paths and three imagers, contrast is not as good as single chip DLP. Screen door effect is a non issue due to the high fill factor associated with LCOS, and there is no rainbow effect either because a full color image is projected. D-ILA projectors are noted for really requiring a good calibration to look their best.
LCOS is a higher end to very high end projection technology. Hitachi and JVC are the main competitors in this field. LCOS/D-ILA prices start at $5500 - $10,000 for consumer models with a high end of consumer models selling for $15,000 - $17,000. Commercial models are used in large ultra-high end home theaters, costs run $200,000+.
CRT
Good old cathode ray tubes have been the focus of the first two articles; first as direct view sets, then as rear projection sets and now as front projection sets. Front projection CRT's use three tubes, one that produces each primary color. Often called three-eyed monsters or beamers, these gentle giants are large and often cantankerous to get set up and performing their best. CRT's are analog devices, which allow them to display any signal within their operating range. CRT capabilities range from smaller video only units that were built for use in sports bars and restaurants to high resolution 2500 x 2500 capable machines built for demanding applications like flight simulators.
CRT Positives: The above mentioned flexibility, excellent contrast, excellent black levels, very good color reproduction, no lamps to change (CRT's typically last 10,000+ hrs) and smooth film-like images.
CRT Negatives: CRT's are more complex and difficult to set up, large and heavy (most are at least 140lbs), lower average lumen output, analog devices that must be realigned periodically, and largely discontinued from production. Barco and VDC (formerly Electrohome) still produce projectors, and Sony still has stock of new units.
CRT projectors are produced in three main "sizes", where size refers to the diagonal measure of the CRT tube face. These sizes are 7", 8" and 9", some 12" units are manufactured, but are a niche projector and not considered suitable for home theater. As size increases, the light output and available resolution increases. Generally manufacturers include more advanced electronics that support higher scan rates and bandwidth in the larger size projectors. The maximum resolution that a CRT projector can resolve is determined by the point at which the scan lines "overlap", at this point parts of the video image are being superimposed and the picture looses detail and begins to look "soft".
Properly set up CRT projectors, especially 9" models, are the current pinnacle of home theater picture quality. To achieve this requires a top quality source and processor, good cables, a good screen and an excellent set up and calibration. A 9" CRT projector is one of the only display devices that can resolve full HDTV resolution of 1920 x 1080 pixels.
CRT projectors are often paired with a higher gain white screen. The Imaging Science Foundation (ISF) recommends a 1.3 gain as optimal, but higher gains can be used if viewing angles are controlled.
New CRT projectors were and continue to be expensive, with entry level 7" models in the $4,000-$14,000 range, and high end 9" units retailing from $20,000 to $60,000. There is a brisk used market in CRT projectors with several reliable technicians and companies that recondition and sell CRT projectors. Used 7" projectors range from a few hundred dollars for a video only unit to $3,000 for a like new unit with multisync capabilities. 8" projectors are $2,500 - $7,000 used and 9" units jump to $6,000-$12,000+
Conclusions:
Tech Comparisons
The following table lists each projector technology and a relative score in each category, 1 is best, 4 is worst. For this table I only considered projectors aimed at the home theater market with a current sales price of under $20,000.
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Projector Technology
Max ResolutionContrastBrightnessLamp LifePrices
Single DMD DLP 1280 x 720 23 2 2
LCOS/D-ILA 1400 x 105032 3 , a 3
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Notes: a. Based on Xenon lamps, newest models use UHP
Projected images are like art, some people like Impressionist oils, others like modern watercolors. Each projection technology has a "look" that must be seen in person to be appreciated. If you are serious about getting into front projection, I urge you to see as many projectors in as many different settings as you can before you make your decision.
Shopping for a Projector
Projectors are gaining in popularity and affordability every year, but you are still not likely to find many at the local mall. Some large chain electronics retailers carry them, as do many high-end audio shops and custom installers. In any case, if you want to see some projectors in action, you are going to have to get around.
Local home theater enthusiast groups often have meetings in member's home theaters, these sessions represent excellent opportunities to see projectors and find out what works for others in similar situations. Listings for these groups can be found at the Home Theater Forum (www.hometheaterforum.com), Home Theater Spot (www.hometheaterspot.com), and at AV Science Forum (www.avsforum.com). These sites are also a great place to do research and ask questions about any aspect of home theater design, equipment, or construction.
When auditioning projectors, the same rules outlined in articles 1 and 2 regarding direct view and rear projection sets apply. Bring material you are familiar with. Use the same input that you plan to use (S-video, component, RGB), be aware that some projectors require a special cable or adapter to input a component video signal. This accessory is not always included. Consider the remote and what mounting options are available. Is there a standard mounting bracket available? Remember to check the throw distance required for the projectors you are considering, will they fit your room?
When it comes time to buy, where you buy will largely be a product of what you've decided to purchase. Many business class projectors are available through the same channels as personal computer equipment. Pricing is competitive, so shop around. Higher end and home theater specific projectors are often sold through a dealer network, or direct from the manufacturer so you are not likely to get a much of a discount off list on these units. Search for a reputable dealer that you can trust since you will be paying for the dealer's expertise.
CRT projectors are largely second hand, buy from a reputable dealer, ask lots of questions and see the unit if at all possible. There is a great article on buying CRT's here: http://www.dvdverdict.com/columns/techtalk/crtbuying.shtml
What about those Homemade Projectors for $9.95?
I've seen plans on e-Bay and elsewhere that show you how to build your own projector for only $9.95, what gives? Do they work?
What these plans will show you how to do is build a box, cut a hole in the front, put a TV set in it upside down (yes, upside down) and cover the hole with a fresnel lens. The fresnel lens specified is a flat sheet of plastic often used on the rear windows of RV's and trucks to enable the driver to see behind him when.
This scheme does actually work, if you fiddle with it enough you will get a washed out, out of focus, low resolution backwards picture (corrected by turning the TV or monitor upside down). After the novelty wears off, like in about five minutes, you'll take it apart and likely never reassemble it. A fun project to experiment with, but you are likely to enjoy watching the TV used directly better.
Future Tech
Several companies are working on viable laser projectors; efforts to date have been absolutely huge, expensive and take as much power as a small city to operate. But they're coming. Digital projection gets better with each successive generation and will eventually overtake CRT at the extreme high end of picture quality. For larger theaters that require higher light output, CRT is already off the list.
Variations
Front projectors can be used in other arrangements than the classic projector pointing at the screen. These arrangements can be used to solve certain set up challenges or address specific needs.
Rear projection: Most front projectors can be configured to project onto a rear projection screen. Rear projection screens are similar to those found in rear projection televisions (see Part 2), but generally larger, and very expensive. Rear projection is good for high ambient light conditions. These set ups take more space due to the need to mount the projector behind the screen. This extra room can be reduced by using mirrors or special lenses.
Mirrors: Mirrors can be used in both front and rear projection set ups to reduce the throw distance required. This is useful in small rooms, or where minimizing the space required behind a rear projection set up is needed.
Stacks: Front projectors may be used in tandem. Normally the two images are superimposed to increase the overall brightness and "punch" of the image. This is most commonly done with CRT projectors, but can be accomplished with digitals also. Projectors can be stacked over and under or side by side.
Summary
Front projection is the most complex and demanding way to reproduce the cinema in your home, but it is also very rewarding. A great film, some popcorn, a 100" picture with great surround sound is a wonderful way to experience the movies.
