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TV Reviews HDTVs

ContentsIntroductionWhat is an LCD TV?What is a plasma TV?What is an OLED TV?3D TVHD broadcastsHD moviesBuying an HDTV

Introduction

The last ten years have seen the TV market turn away from bulky cathode-ray-tube (CRT) screens and towards the world of the flat-panel. The hype surrounding recent developments such as Blu-ray, an optical disc format offering movies in high definition, has accelerated the consumer market's uptake of HDTVs. Recent studies suggest, for example, that over half of UK and US households now have an HDTV. For those in the apparent minority, therefore, this article looks in detail at all aspects of HDTV - from the technical background, to the suitability of the various technologies, to new developments like 3D TV. There's also a section on the availability of HD content. As a recent BBC study discovered, many users fail to track down this crucial piece of the puzzle. Finally, the article offers some advice on deciding which TV is right for the reader's circumstances, based on the size of their room, what they want to watch, and their budget.

Plasma and LCD screens are today's main contenders, but LCD's massive sales figures cement its position as the market's technology of choice. Although plasma-TV developers have managed to improve on many of their technology's disadvantages, plasma is still costlier, heavier, and less efficient than LCD. Nevertheless, many consumers prefer plasma because of its better home-cinema performance; LCD TVs, however, take the upper hand in bright rooms. New display technology OLED is beginning to make inroads, earning kudos for its accurate colours, deep blacks, and versatile manufacturing possibilities. But it's likely to take the new contender many years to knock LCD off the top spot. As yet, OLED hasn't even entered the HDTV mainstream.

Although many broadcasters are only just introducing HD content to their schedules, some countries have already begun 3D TV broadcasts. The initial test transmissions are expected to give way to full 3D channels within 2010, but customers will have to invest in a 3D-ready TV before this can become a reality. Observers fear many users will be reluctant to do so, having only recently upgraded from SD to HD - most likely at considerable expense. For those considering throwing caution to the wind, this article introduces the most important technologies in 3D TV and gives a round-up of current developments in the 3D sector - such as the recent announcement by Sony and Fifa that 25 matches of the 2010 World Cup will be recorded in 3D.

As the Buying an HDTV' section will emphasise, the prices of HDTVs have fallen dramatically in recent years, meaning HD is now accessible to all consumers. And if you haven't yet watched a Blu-ray on a Full HD screen, we highly recommend that you read on - it's truly breathtaking.

What is an LCD TV?

Liquid-crystal displays, or LCDs, are by far the most popular panel type in flat-screen TVs. Liquid crystals are a class of chemicals whose properties lie between those of a liquid and a crystalline solid. Varying the voltage applied to the liquid crystals changes their structure, and this is the basis of their use in displays. In a liquid-crystal display, plane-polarised light - light in which all of the rays vibrate in a single plane - shines through a layer of liquid crystals held on a sheet of glass. This in turn is coated with control circuitry that applies the necessary voltages to the crystals. Generally, the liquid-crystal layer lies between two polarisation filters whose axes of polarisation sit at 90 degrees to one another. Because LCD TVs require a lamp that shines from behind the panel (the "backlight"), the liquid crystals block light unless told otherwise - this is what we'd call a black screen. When a pixel needs to light up, the control circuitry opens' the cell by changing the voltage so that the crystals twist' the polarisation of the light. This is known as the "Twisted Nematic" principle, and means that the light can pass through both the first filter and - once repolarised - the second filter too.

The problem with having a backlight shining constantly is that the panel cannot block the passage of light altogether - in other words, some light still shows when the screen is displaying black. Typically, blacks will therefore appear as a dark blue on LCD TVs. Recent technologies have combated this problem by reducing the intensity of the backlight as a whole during darker scenes or by varying the brightness of the backlight in separate zones of the screen - the latter is known as "local dimming". Whereas backlights in LCD TVs traditionally use fluorescent lamps (CCFLs), many now use LEDs - but don't let this fool you: So-called "LED TVs" are actually just LCDs with an LED backlight.

LCD TVs suffer a number of other problems as a result of their display technology. Many exhibit poor motion depiction, for example, due to the time taken for the pixels to react to changes in the signal. Colours are also typically less accurate on an LCD than on a plasma or OLED screen, but this problem is slowly getting better as backlight technology improves. Viewing angles are also narrow on LCD TVs compared to those of plasma TVs - that is, viewers sitting to one side of the screen see weaker contrast and inaccurate colours.

One major benefit of LCDs, however, is their brightness: The backlights are entirely separate from the pixels, meaning they benefit from the efficiency and brightness of modern lighting technology. Other display types - so-called "emissive" technologies, such as plasma - produce light within the pixels themselves, limiting their overall light output. LCDs are therefore ideal for brighter rooms. The current popularity of LCD TVs and, therefore, the sheer number of them that are produced each year also bring price advantages over other display types.

What is a plasma TV?

Unlike LCD TVs, plasma TVs have no backlight. Instead, each pixel behaves like three tiny fluorescent tubes, one for each of the primary colours: red, green, and blue. These tubes, or cells, contain a gas that turns into a plasma when a voltage is applied. From this excited state, the gas then discharges' as it returns to the gaseous state, emitting a burst of energy in the form of photons. These photons collide with coatings, known as phosphors, on the cell walls and are the part of the screen that actually emits light. Each of the three cells in a pixel uses a slightly different phosphor in order to produce the relevant primary colour.

This process means that plasma pixels require excitation before they will produce light, and that they then do so for only an instant. The TV has to excite the pixels many tens of times per second in order to produce an image, and must vary the number of times per second in order to produce different brightness levels.

There are a number of benefits to producing the picture in this way. Since, for example, the light sources are (almost) on the surface of the screen, they emit light in all directions. This provides a wide viewing angle: viewers can watch the picture from one side without seeing a drop in contrast or colour accuracy. The rapid strobing of the pixels also results in sharper motion depiction than with LCDs - that is, there's no blur during fast motion, so news tickers or fast serves in a tennis game resolve clearly.

One major benefit of plasma screens is that they only produce light when they need to. Where LCDs have to struggle to block the light from their backlight, plasmas simply don't excite the pixels that are meant to be dark. In a dark room, this means plasmas can display very dark blacks. In bright rooms, however, a plasma screen shows up ambient illumination more strongly than an LCD - and this is essentially plasma's biggest shortcoming.

When used in ideal conditions, therefore, plasma screens produce excellent pictures. LCDs, on the other hand, perform better than plasmas in day-lit rooms. Plasmas also use a lot of power - or, at least, they did traditionally: Recent developments have reduced plasma TVs' power consumption, but they still use more power than LCDs.

What is an OLED TV?

OLED stands for Organic Light Emitting Diode. An OLED is a carbon-based (organic) material that emits light under electrical excitation. The light results from a type of luminescence known as fluorescence. We're all familiar with phosphorescence, a type of luminescence used to produce glow in the dark' plastics that continue emitting light after being charged up' from another light source. Fluorescence works in a similar way, but stops producing light almost immediately after the energy source is removed. In the case of OLED displays, this energy source is electricity. Each pixel of an OLED screen consists of three separate light sources - one for each primary colour - and produces light itself, rather than relying on a backlight as an LCD does. What OLED shares with LCD, however, is the thin layer of control circuitry applied to the panel. Colour OLED displays, such as those used in OLED TVs, use an active matrix circuit (AM-OLED) similar to that used in TFT-LCDs.

The beauty of OLED screens is that, like plasmas, they only produce light where it's needed, so they can display highly pure blacks and, as a result, almost infinite contrast ratios. Activating the pixels is also extremely fast, therefore ruling out the motion-blur problems seen on LCD screens. The colours from OLED sub-pixels are highly pure, and the pixels' uniform emission in all directions results in wide viewing angles. What's more, since OLED pixels can be made as small as the device requires, the screens can have very high resolutions. Last but not least, the highly efficient conversion of electricity into light means that the screens use relatively little power.

OLED has yet to make it to the HDTV mainstream. Sony, for example, has produced an 11-inch TV, the XEL-1, for the consumer market, but this tiny model costs 2, 500 GBP and is therefore simply too expensive. Manufacturers have shown off various prototypes of larger models, but none have yet introduced realistic sizes for the everyday consumer. OLED clearly has the potential to overtake both plasma and LCD, but it looks like it will take the technology a few years to become established.

3D TV

Once a novelty technology reserved for a handful of cinema releases, 3D now looks set to conquer the living room. Market researchers have predicted huge growth in sales of 3D-ready TVs over the next decade, and technologies such as 200-hertz panels are making it increasingly easy for manufacturers to deliver the 3D effect as well as high-definition imagery.

In order to see in 3D, the viewers' eyes need to receive images showing slightly different perspectives. These two images can be transmitted in a number of ways. One option is to display the images alternately, and to then give viewers a pair of glasses - "shutter glasses" - that open and close their lenses in time so that each eye only sees the picture it's meant to receive. When done at high speed, this switching is imperceptible to the viewers. The brain then uses the two images in the same way it would in normal binocular vision - and produces an effect of depth.

Alternatively, you can use filter glasses to separate the images for the viewers' eyes, which might be coloured differently, for example - red and green are the classic example. Projection systems allow users to filter the images using more effective techniques such as by polarising the two images differently, so that the viewers can then wear inexpensive filter glasses, instead of the costlier shutter glasses described above. Incidentally, the recent box-office hit Avatar used circularly polarised images and technology supplied by RealD 3D, a company that's been involved in 3D cinema for many years.

Nevertheless, the home market seems likely to favour active shutter glasses, and many of the 3D-ready TVs already announced by several major manufacturers will ship with one pair of shutter glasses. Further sets of glasses will be available from the manufacturers, but - initially at least - they won't come cheap. Almost all of the biggest names - Sony, Toshiba, LG, Panasonic - have announced plans to bring out 3D-ready TVs within 2010, and several broadcasters, including BSkyB in the UK, have begun test transmissions in 3D. Plans are already in motion around the world to introduce 3D channels in the near future.

HD Broadcasts

It's one thing buying an HDTV, but you have to make sure you have access to HD content first. This depends partly on your location and how much you're willing to pay. Terrestrial HD service Freeview HD is currently rolling out region-by-region in the UK, although it currently provides just a handful of HD channels - what's more, the required set-top boxes haven't yet gone on sale, so customers cannot currently view the broadcasts. On-demand HD content and some HD channels are available via digital-cable operator Virgin Media, which charges a monthly subscription. The free-to-air satellite service freesat also broadcasts some HD channels, but the only sizeable selection of HD broadcasts comes via BSkyB's subscription service, Sky. Sky currently provides customers with access to up to 37 HD channels, depending on their chosen package.

HD movies

As well as high-definition TV content, viewers can now enjoy high-definition movies, thanks to the arrival of the Blu-ray disc. Falling prices of Blu-ray discs and Blu-ray disc players reflect the growing popularity of the new format in the living room.

Blu-ray discs can hold five times as much data as a DVD, allowing a single disc to hold a film in HD resolution along with high-quality surround sound. The name "Blu-ray" refers to the blue laser that BD players use to read the discs - this laser's wavelength is shorter than that of the red laser used in DVD players. The lower wavelength allows the player to focus the laser onto a smaller "spot size" and, thus, allows the discs to store more data in the same area. Blu-rays carry video in one of several different HD resolutions: 1080/50i and 1080/60i are common for material recorded on a video camera, such as documentaries and TV series. 1080/24p is the format used for films and preserves the original 24-hertz frame rate recorded by a film camera. In the names of these formats, "1080" is the number of horizontal lines that make up the picture. In widescreen (16:9) format, this corresponds to a resolution of 1, 920 x 1, 080 pixels.

Another benefit of Blu-ray discs is their higher-quality audio; users can enjoy up to eight channels (in a 7.1 arrangement) of audio in one of the new lossless audio formats, such as DTS-HD Master Audio or Dolby TrueHD. Since these are not supported on all Blu-ray players, however, a second lower-definition soundtrack always accompanies the HD versions. Blu-rays also have a larger capacity for storing bonus materials - in some cases the discs contain hours of additional footage, as well as games and information relating to the movie. BD-Live is a system introduced in the more-recent BD Profile 2.0 and allows players with an internet connection to access yet more bonus materials - including, for example, quizzes, downloadable games, and live chats.

Buying an HDTV

There are a number of factors to consider when buying an HDTV: Viewing distance, resolution, display technology, budget, multimedia capabilities, and design are just a few - the list is long, so decide which factors are most important to you. If, for example, you're putting the TV in the middle of your living room, you might want to try and match the room's design. Similarly, if you're planning to watch TV in bright daylight, you'll need an LCD - so the chosen display technology then becomes one of your top criteria.

Let's run through a few of the main factors, starting with viewing distance. Actually, you'll need to consider this from both perspectives: If you want to sit far away from the screen, you'll need a big screen. Similarly, if your budget limits you to a smaller screen diagonal, make sure you'll be able to sit quite close. The rule of thumb: The screen's width should equal about half of the viewing distance. This means that the picture's perceived size fills the active area of the viewer's vision, and will make it easier for them to become absorbed in the action. So if you know you're going to sit two metres from the screen, buy a TV whose screen is one metre wide - this corresponds to a screen diagonal of around 46 inches.

Also bear the video's resolution in mind: HD pictures show extra detail, so you can sit a little nearer, but you should view standard-definition (SD) content from a little farther away. Decide, therefore, whether you're going to spend most of your time watching HD or SD. In the example above - sitting two metres from the screen - you could probably get away with 37 inches if you're only planning to watch in SD.

One of the biggest questions buyers ask themselves is: LCD or plasma? The two display technologies each suit different situations, so there's no straightforward answer. LCD is a great all-rounder and performs well in bright rooms. Plasma, on the other hand, offers better contrast and motion depiction, but it's dimmer and therefore needs low ambient light in order to deliver its best pictures. So it all really depends on what you're going to watch and when you're going to watch it. Afternoon TV or sports fans should probably opt for an LCD, but evening-time movie fans can safely choose plasma - once the lights are dimmed, the plasma will give you crisper motion and deeper blacks.

Additional features such as photo or network playback and internet access are becoming increasingly common, but they're not necessary for what many users want from their TVs. Don't be afraid, therefore, to choose a cheaper screen that lacks an internet connection or a motorised swivel stand, for example - often, you'll get the same top-quality screen as in the manufacturer's pricier models, just without all the fancy extras. Refer to online reviews to find out about the quality of the screens at various price levels. Ideally, you'll find a TV that does everything you need and that comes at a fair price.