Definitions

lcd-television

Liquid crystal display television


Liquid-crystal display televisions (LCD TV) are television sets that use LCD technology to produce images. Benefits of LCD technology include lower weight and reduced power requirements when compared to other display types. Often, LCD television screens can also be used as computer monitors.

LCD Technology

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smoothly; thus the TV must display the moving object in "correct" places for as long as possible, and erase it from outdated places as quickly as possible. LCD televisions are also a good component for video games.

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. LCD technology is based on manipulation of polarized light. Two thin polarizing sheets are laminated to two glass substrates containing a thin layer of liquid-crystal. A regular 2-dimensional grid of electrodes allows each pixel in the array to be selected and activated individually. Several LCD technologies are used for the realization of large format television screens (e.g. TN, IPS, PVA, FFS), all in combination with active-matrix addressing.

It had been widely believed that LCD technology was suited only to smaller sized flat-panel televisions at sizes of 40" or smaller. Early LCDs could not compete with plasma technology for screens larger than this because plasma held the edge in cost and performance. However, LCD TVs can now offer essentially the same performance.

Developments in LCD televisions

Current sixth-generation panels by major manufacturers such as Samsung, Sony, LG Display, and the Sharp Corporation have announced larger sized models:

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

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.

Improvements in LCD technology have narrowed the technological gap, allowing producers to offer lower weight and higher available resolution (crucial for HDTV), and lower power consumption. LCD TVs are now more competitive against plasma displays in the television set market. It is noted that LCDs are now overtaking plasmas, particularly in the important 40" and above segment where plasma had enjoyed strong dominance. 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 colors, however in new televisions this is compensated to some degree using a technique called overdriving.

Recent research

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

See also

References

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