Liquid crystal display television

From Free net encyclopedia

Liquid crystal display television (LCD TV) is a television using LCD technology for its visual output. This technology is generally TFT. It is contrasted to alternate visual output technologies, such as cathode ray or plasma.

Early LCD television had drawbacks relative to traditional visual display technologies. It displayed fast-moving action with "ghosting" and could be viewed best only looking straight at the screen or from a slight angle. These problems have largely been overcome in recent years, and LCD televisions, along with plasma displays, have taken over the dominant market position worldwide from cathode ray displays.

For a long time it was widely believed that LCD technology was suited only to smaller sized televisions, and could not compete with plasma technology at larger sizes. This belief has been undermined by the announcements of ever-larger panels by companies such as Sharp Corporation, Samsung and LG.Philips LCD:

  • 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" panel.[1]

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.)

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Universal capabilities

Modern LCD TV sets are geographically universal because they have a multisystem tuner, to display PAL, NTSC and SECAM norms. And they include an electronic (step-down & step-up) transformer that automatically can use 110/200 V AC indifferently and universal grounded adapter plugs.

Also, the vast majority are no longer just for TV and HDTV (DVB). They can also be used as a computer monitor with a VGA/DVI signal, although resolution support can vary widely.

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Wireless AV kit (SmartLink)

Some newer LCD TV sets can connect to a host computer via a bluetooth or WiFi wireless link. See SmartLink, Streamium.

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Teletext and Electronic Programme Guide

They generally include teletext and NexTView for EPG.

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Developments in LCD televisions

TVs based on PVA and S-PVA LCD panels deliver quite good angle of view, up to 140 degrees. They also deliver an adequate contrast ratio for viewing bright scenes; and also dark scenes in bright rooms. 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. However, PVA and S-PVA panels generally have difficulty with ghosting when going between different shades of dark colors.

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 41.(6) 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, tries to actually 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 often as possible, and erase it from outdated places as fast as possible.

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 much hated 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.

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See also

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External links

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