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Liquid crystal display television



Liquid Crystal Display Televisions (LCD TV) are televisions that use LCD technology for visual output (producing an image). The technology used is generally TFT, because this allows for size, especially depth reduction, lower weight and reduced energy consumpton. These LCD screens are also used as computer monitors.

Early generation LCD televisions had drawbacks relative to the traditional visual display technologies for televisions and computer monitors. Fast-moving action was displayed with "ghosting" and because light emitted focused directly ahead, viewing was best only when looking directly at the screen or from a slight angle. Developments in recent years have solved most of these problems, and LCD televisions, along with Plasma displays, have become more popular worldwide than Cathode Ray display televisions.

In the early 2000s, LCD flat-panels captured a large part of the computer monitor market from the traditional CRTs type, with aspects such as its energy efficiency being favourable over the CRT design. Continuing advances in LCD TV technology enable it to compete against its rival technologies,Plasma flat panels and rear-projection televisions (DLP and also two variants of the LCD technology LCD and LCoS) for large-screen HDTV.

For much of its continuing time on the television market, it was widely believed that LCD technology was suited only to smaller sized flat-panel televisions at sizes of 40" or smaller, as it could not compete with plasma technology for screens larger than this because plasma held the edge in cost and performance. However , LCD TV's can now offer the same performance.

This is shown by the announcements of seventh-generation panels by major manufacturers such as Samsung, Sony, LG.Philips LCD, and the Sharp Corporation:

* In October 2004, 40" to 45" televisions were widely available, and Sharp had announced the successful manufacture of a 65" panel.
* In March 2005, Samsung announced an 82" LCD panel.[1]
* In August 2006, LG.Philips Consumer Electronics announced a 100" LCD television [2]
* In January 2007, Sharp displayed a 108" LCD panel branded under the AQUOS brand name at CES in Las Vegas.[3]

Manufacturers have announced plans to invest billions of dollars in LCD production over the next few years, with televisions expected to be a key market. (The other main market for LCD displays is in computer monitors.)

Improvements in LCD technology have narrowed the technological gap, allowing producers to offer the lower weight, falling prices, higher available resolution (crucial for HDTV), and lower electrical power consumption which previously made a gap with plasmas. This means they are now more competitive against plasma displays in the television set market.

Analysts of late 2006, noted that LCDs were overtaking plasmas, particularly in the important 40" and above segment where plasma had enjoyed strong dominance a couple of years before.






LCD Technology

LCD technology is based on the properties of polarized light. Two thin, polarized panels sandwich a thin liquid-crystal gel that is divided into individual pixels. An X/Y grid of wires allows each pixel in the array to be activated individually. When an LCD pixel darkens, it polarizes at 90 degrees to the polarizing screens.

This pixel has darkened. The pixel darkens in proportion to the voltage applied to it: for a bright detail, a low voltage is applied to the pixel; for a dark shadow area, a higher voltage is applied. LCDs are not completely opaque to light, however; some light will always go through even the blackest LCD pixels.




Developments in LCD televisions

TVs based on PVA and S-PVA LCD panels deliver a broad viewing angle, up to 178 degrees. They also deliver an adequate contrast ratio for viewing bright scenes, as well as dark scenes in bright rooms. The dynamic contrast technique improves contrast when viewing dark scenes in a dark room. Alternatively, some manufacturers produce LCD TVs that throw light on the wall behind it to help make dark scenes look darker. PVA and S-PVA panels generally have difficulty with ghosting when going between different shades of dark colours, however in new televisions this is compensated to some degree using a technique called overdriving.

Moving pictures on a CRT TV do not exhibit any sort of "ghosting" because the CRT's phosphor, charged by the strike of electrons, emits most of the light in a very short time, under 1 ms, compared with the refresh period of e.g. 20 ms (for 50 fps video). In LCDs, each pixel emits light of set intensity for a full period of 20 ms (in this example), plus the time it takes for it to switch to the next state, typically 12 to 25 ms.

The second time (called the "response time") can be shortened by the panel design (for black-to-white transitions), and by using the technique called overdriving (for black-to-gray and gray-to-gray transitions); however this only can go down to as short as the refresh period.

This is usually enough for watching film-based material, where the refresh period is so long (1/24 s, or nearly 42 ms), and jitter is so strong on moving objects that film producers actually almost always try to keep object of interest immobile in the film's frame.

Video material, shot at 50 or 60 frames a second, actually tries to capture the motion. When the eye of a viewer tracks a moving object in video, it doesn't jump to its next predicted position on the screen with every refresh cycle, but it moves smoothly; thus the TV must display the moving object in "correct" places for as long as possible, and erase it from outdated places as quickly as possible.

Although ghosting was a problem when LCD TVs were newer, the manufacturers have been able to shorten response time to 2ms on many computer monitors and around an average of 8 ms for TVs.

There are two emerging techniques to solve this problem. First, the backlight of the LCD panel may be fired during a shorter period of time than the refresh period, preferably as short as possible, and preferably when the pixel has already settled to the intended brightness. This technique resurrects the flicker problem of the CRTs, because the eye is able to sense flicker at the typical 50 or 60 Hz refresh rates.

Another approach is to double the refresh rate of the LCD panel, and reconstruct the intermediate frames using various motion compensation techniques, extensively tested on high-end "100 Hz" CRT televisions in Europe.

The best approach may be a combination of two, possibly allowing the viewer to switch them on or off when viewing video- or film-based material.

Some manufacturers are also experimenting with extending colour reproduction of LCD televisions. Although current LCD panels are able to deliver all sRGB colours using an appropriate combination of backlight's spectrum and optical filters, manufacturers want to display even more colours. One of the approaches is to use a fourth, or even fifth and sixth colour in the optical colour filter array. Another approach is to use two sets of suitably narrowband backlights (e.g. LEDs), with slightly differing colours, in combination with broadband optical filters in the panel, and alternating backlights each consecutive frame.

Fully using the extended colour gamut will naturally require an appropriately captured material and some modifications to the distribution channel. Otherwise, the only use of the extra colours would be to let the viewer boost the colour saturation of the TV picture beyond what was intended by the producer, but avoiding the otherwise unavoidable loss of detail ("burnout") in saturated areas.