Choosing a big screen

If you go into any medium to large electrical or computer store, you are bound to see some big screens on offer. There are a whole range of different types and prices making it difficult to immediately point out the perfect set. The three main types of large TVs are flat panel, rear projection and front projection. CRT’s generally do not go any larger than 40” due to their sheer weight and depth.

As no two large screen lovers have the same environment where they intend setting up their large TV, there are many things to consider before choosing a set. For example while a front projection system is relatively cheap and can produce a huge image, it would not be suitable in a brightly lit environment unless it is only used at night.

Both rear projection and flat panel TV sets do well for all sorts of viewing whether checking up the news and weather, watching full movies or playing games on. If considering a front projection system it may be worth getting a small television also. The light bulb in a projector and some rear projection sets gets the most wear & tear during its warming up and cooling of stage, but in this case it would be better to watch short events such as the weather forecast or doing a quick channel check on the small TV set before powering up the projector.

Here is a general guide to each type of TV and what each of the different specifications stands for. I would certainly recommend reading online reviews such as across forums before making the purchase as sometimes manufacturer’s will ‘tweak’ something to exaggerate a specification.

Flat Panel

The two most popular flat panel sets widely available are Plasma and LCD. Like TFT monitors, the main advantage with these sets is their narrow depth which allows them to be placed closer to a wall or even hung like a picture! This is useful for smaller rooms where a bulky CRT or rear projection set would take up too much space.

However the main drawback with flat panel sets is the high price. These generally cost over double that of an equivalent size rear-projection set.

Note that some flat panel sets do not incorporate a TV tuner and thus may rely on an external AV source. Generally any AV source containing a TV tuner will do just fine such as a VCR, however these will lack some useful TV features such as Teletext (Europe) or Closed Captions (US).


Plasma TV

Plasma TV’s were the original large flat panel TV’s that were launched. The screen shares similar characteristics with a CRT, but uses UV light instead of electrons to illuminate the phosphors on the screen. When current flows through the plasma behind a particular phosphor (pixel), it emits invisible UV light. When this strikes the phosphor, it illuminates either with a red, green or blue glow depending on the phosphor similar to how a phosphor illuminates on a CRT when struck by electrons.

The picture sharpness is generally as good as the source since each phosphor is lit by a separate circuit unlike a TV where the same electron beam scans the entire screen. However just like CRT TV’s, plasma sets can also suffer from screen-burn if a fixed image is shown over a long period of time. The other drawback is their shorter life time of around 20,000 hours compared with around 70,000 for a CRT.

In my opinion, the colour is not as pure as other TVs. For example the red on plasma looks more like red-orange and green looks more like green-yellow (compared with an LCD TV at a pub). Finally the shades of colour did not look as good as other TVs. I noticed this with several plasma sets at different places.


LCD TV

With size restrictions on LCD being overcome, this looks to overtake the plasma once its prices drop. LCD has several advantages and a few drawbacks over plasma sets. The main features over plasma are improved contrast ratio in brightly lit up areas, longer life, lower power consumption and lighter weight. The only part that may end up wearing out is the back light for illuminating the screen. However with the advancements in white LED technology, this may no longer be an issue if white LED technology is used in newer sets. However unlike front & rear projection sets, the user may never end up replacing a bulb in its life

The major advantage with LCD is its way of subtraction method of displaying pixels. An LCD pixel either blocks light or becomes transparent. A red, green or blue colour filter changes the transparent state to match the filter colour. Finally illuminating the rear lights up the transparent pixels. Unlike other types of sets, a pixel blocking light does not reflect light either which means that the visible actual contrast ratio does not significantly fall the brighter the room gets unlike CRT and rear projection sets and particularly front projection systems.

LCD TV’s do carry two drawbacks like PC TFT monitors however. LCD is the only technology has a limited viewing angle. Most high end sets have around a viewing angle of 120 degrees left to right, although only up to around 60 degrees up & down. The hue and contrast starts changing once viewed beyond its optimal position. ‘Stuck’ pixels can be another issue. A stuck pixel is generally a permanently lit red, blue or green pixel. Some users find this quite irritating, especially if one or more appears near the middle of the screen.

Front Projection

While many think about projectors as just a presentation device, these also often work very well as a large TV low host home cinema solution. They also take up very little space and in fact none at all if mounted from the ceiling. Like with any other projector system such as cine-film and slide projectors, these need a suitable display for showing the picture. A matt white to light-blue wall will generally do fine, however a projection screen will generally give a sharper, smoother and brighter picture.

A front projection system is the equivalent to a ‘separates’ Hi-Fi system. A complete front projection display consists of a projector, screen, speaker system and one or more AV sources. Very few if any projectors incorporate a TV Tuner and those that include speakers generally have very small speakers more suited for those sound effects in slide presentations rather than watching TV.

There are several things to look out for when comparing or deciding on a projector. The main specifications to look out for are native resolution, maximum resolution, aspect ratio, image forming technology, lumens, contrast ratio and lamp life.


Native Resolution

Most projectors come in either SVGA or XGA resolutions. SVGA is generally 800x600 and XGA is generally 1024x768. The higher the native resolution, the more physical pixels the projector is capable of producing. Native HDTV projectors usually have a resolution of 1280x720.

If the projector will be primarily used for watching TV, DVD and other standard definition content, an SVGA would be ideal here to save on costs. However the SVGA resolution may be too low for general PC usage since many software applications and websites are designed for use with a 1024x768 resolution display.

An XGA capable projector would also be useful should the user consider viewing high definition content at a later stage. In this case, the display would show greater detail than with an SVGA projector.

Finally a native HDTV resolution projector with an aspect ratio of 16:9 would suit all needs as well as show 720i/p HDTV content without resorting to lossy compression on the resolution.

A few manufacturers may rate the resolution in mega-pixel similar to digital cameras. The mega pixel rating for LCD projectors is worked out by multiplying the number of pixels horizontally by the number of pixels vertically by three (one for red, green and blue). The most common ratings for LCD projectors are:

1.44M = SVGA (800 x 600)
1.55M = 960 x 540
2.36M = XGA (1024 x 768)
2.76M = 720p HDTV (1280 x 720)
3.93M = SXGA (1280 x 768)
6.22M = 1080p HDTV (1920 x 1080)


Maximum Resolution

Pretty much all projectors are capable of taking in a higher resolution than its native resolution. When a higher resolution is provided as a source, the projector will resample the image to its native resolution before displaying. This may result in distorted detail or make small writing difficult to read.

Many widescreen projectors will accept both HDTV 720i/p and 1080i/p resolutions however these are resampled to its native resolution and will result in less detail shown than with a higher resolution display.

When comparing projectors, be sure to carefully look at the specifications when determining the native resolution. Some manufacturers place their maximum resolution in a large font, but its native resolution hidden in the specifications. I have seen many projectors advertised as ‘XGA’ which only had a native resolution of 800x600.


Image Forming Technology

There are several technologies used for developing the image. The main two are LCD and DLP.


LCD Projectors

The LCD panel in a projector works similar to a regular TFT monitor; however the LCD panel in a projector is generally around 1” in size with a powerful light source to project its image (similar to a slide projector). Some LCD projectors have three LCD panels using one for blue, one for red and one for green. A set of mirrors usually combine the three primary colour images to produce a full colour image.

LCD projectors are known for producing excellent colour rendering; however the maximum contrast ratio is generally no better than 800:1. LCD technology use to have an annoying ‘update’ latency where a pixel would take some time to change to a new state; however this is generally not an issue with most modern projectors.

Some of the lower end and particularly older projector models suffer from a ‘screen door’ effect. This is where a noticeable gap is present between each pixel, thus giving the appearance of a ‘grid’ present over the picture. Most of the newer LCD models, particularly the Panasonic models no longer have a noticeable grid effect when viewed beyond one to two metres from the screen.


DLP Projectors

DLP projectors use an array of miniature mirrors to produce the image. Each mirror represents a single pixel and can either be in an on or off state. When the mirror moves to its on position, the light is reflected onto the next stage, however if the mirror is in its off position, the light is reflected towards a black surface to absorb the unwanted pixel. The final stage usually consists of a rotating colour wheel to make up a full colour image. To display a colour for a given pixel, the mirror for that pixel is only set to the on position while the light is passing through the appropriate colour part of the wheel. To display different shades of colour, the mirror for a pixel is rapidly switched on & off (several thousand times per second) with on-time being varied to produce the intended shade.

The main advantage with DLP technology over LCD is the ability for the panel to handle greater heat. This allows the size of the DLP and weight projector to be reduced as well as allow for brighter bulbs to produce a brighter image. This is the main reason my most large venue projectors now mainly use DLP technology.

As DLP projectors reflect light using mirrors, they have virtually no update latency issues and also generally have a much higher contrast ratio than LCD projectors. Good DLP projectors generally have a rating of up to 3000:1 contrast ratio.

The one small downside to DLP projectors is that colour rendering is generally not as good as with LCD projectors. Some makes also suffer with displaying faint ‘Rainbow’ effects which some picky users have reported quite annoying.

Due to the higher lumen output and contrast rating of DLP projectors compared with LCD projectors, these would be better suited for use in lit-up areas.


Lumens

The lumen rating of a projector is the maximum light it can output. The lumen rating is the equivalent to the rating of a lamp where 750 lumens is the light output of a 60 watt tungsten filament light bulb.

The rating becomes important when deciding on the intended use. If the projector will be used in a dark room with a picture size up to 100”, then any lumen rating will generally do fine. However for images over 100” or when considering operating the projector in a lit up room, it is well worth going for a higher lumen output. In a lit up area, the projected image must be bright enough to significantly overcome the ambient light being reflected from the screen to show better contrast and colour rendering.

Note that as the lumen rating is the maximum output, this may be the rating with all contrast, brightness, gamma controls, aperture (if present), bulb brightness and other controls set to maximum as well as a new bulb. The actual lumen output with the controls properly adjusted is often less than half of the advertised rating and also gradually drops as its light bulb ages.


Contrast Ratio

In the ideal environment where the room is pitch black, the contrast ratio is the number of times the brightest possible shade of white is brighter than the darkest shade of black in the projected image. Again like the lumen rating, the advertised contrast ratio is usually with the contrast, brightness and RGB controls (if present) tweaked.

The contrast ratio becomes important with images that have dark areas or the whole image is dark. With a low contrast ratio, dark areas can look rather ‘hazy’ and have poor colour rendering. High contrast ratio gives more realistic and darker night time scene images.

Note that the actual contrast ratio also depends on other characteristics such as if the room is lit up as well as any picture adjustments made on the projector. The more ambient light hitting the screen, the lower the contrast ratio gets. However in a lit up room, a higher the lumen rating means a higher actual contrast ratio.

Although LCD technology generally only has a contrast ratio of up to 800:1, some manufacturers tweak this by varying the bulb brightness depending on the image being displayed. For example if the bulb brightness is halved on a dark scene compared with a bright scene, this effectively doubles the contrast ratio. However the contrast ratio with images containing combination bright and dark areas is only as good as what the image forming technology can do.


Use in a lit up area

Just note that no matter how good the lumen or contrast ratio rating on the projector is, the darkest shade of black than can be present is the ambient light being reflected from the display. This means that a pitch black picture shown in a sunny room will always appear white on the screen.

For frequent viewing in a lit up area, I would recommend considering a rear projection or flat panel TV. For occasional viewing during the day such as checking the news or whating short programmes, a 2nd small TV would be ideal here.


Lamp Life

Like cine-film and slide projectors, LCD and DLP projectors use a light source to project the image. These use a specially designed bulb with a short arc and high colour rendering properties. However just like a regular household light bulb, these have a limited lifetime of between 1,000 and 5,000 hours depending on the make of the bulb as well as how often the projector is operated in the ‘low lamp’ mode. Replacement bulbs are not cheap however which range between €200 ($240) and € 600 ($ 720) per bulb.

Rear Projection

Rear projection televisions share similar characteristics to a front projection system, but in this case the projector, screen, TV Tuner and speakers are generally built into a single fixed unit like a regular CRT television.

Like the projector in a front projection setup, a rear projection TV either contains a DLP, LCD or CRT image projection system. The early rear projection TV’s commonly used a CRT. In this case the CRT was the equivalent of a miniature extremely bright colour television screen or three primary colour television screens. A series of lens and mirrors are then used to project the image onto the large translucent screen on the TV.

The main advantage of a rear projection TV set over a large CRT TV is the huge difference in weight. A large CRT TV would require a lot of glass to develop as well as something to lift and move it.

There are a few advantages with rear projection TV over front projection. The main one is that the unit is complete - just place it in a suitable spot, hook up the peripherals and tune the stations in. Like high end TV sets, front projection sets often come with a wide range of features such as built in speakers, TV Tuner, Teletext (EU models), complete control with one remote and optionally an Internet browser. Projectors generally require an external source to tune TV channels, a remote for the picture, a remote for the audio and a 3rd remote to operate an external TV tuner such as the VCR. Finally the translucent screen looks much darker than the screen for a front projection system. This results in much better contrast and colour rendering than front projection systems when compared in a lit up environment.


Choosing a suitable front projection TV

When comparing different front projection TV’s, it is recommended to look at the specifications more closely than its advertised features. The main features to look out for on DLP & LCD sets are its native resolution, input sources, Lumens, Contrast ratio and lamp life. For more information on these specifications as well as the differences between LCD & DLP technology, see the front projection section above.

With rear projection TVs, DLP sets often have a higher contrast ratio and smoother picture than with LCD sets. As DLP uses miniature mirrors to produce the pixels in the image, it does not suffer from the heat issues that LCD has and thus allows the use of a higher lumen light source, thus resulting in a bright high contrast image.


Other resources

For more information on the technology and workings of front and rear projection sets as well as images, have a look at the following link:
http://electronics.howstuffworks.com/projection-tv.htm

As LCD televisions share the same characteristics of TFT displays, the following link will give detail on how these screens work:
http://electronics.howstuffworks.com/lcd.htm

For more information on how plasma displays work, have a look at the following link:
http://electronics.howstuffworks.com/plasma-display.htm