Response time of LCD TV- explained
June 24, 2007
What is Response time?
Response time of a LCD TV is certainly an important specification to consider when you make a choice among various LCD TV models available in the television market. Generally, considered as a weakness of a LCD technology, the response time of LCD TV naturally needs lot more study and attention. Perhaps no other area related to the performance of LCD TV attracted such an amount of attention from the LCD manufacturers. LCD TV makers are continually bringing improvements in their high end LCD to overcome this weakness.
Response time means how fast the pixels are able to make transition between two states – inactive and active or vice versa. In other words, how fast they go from black to white or vice versa. LCD technology uses liquid crystals which allow and block light provided by backlight device. Response time is usually quoted in milliseconds or ms. TVs having faster shifts between active and inactive will have lower response time while higher response time means delay which can be vexing to the viewers.
Ghosting and flat-panel displays
The Plasma TVs or CRTs which use phosphors to produce light, are able to make the transitions much faster due to their technologies. For instance, the conventional CRT display gets the image on the screen refreshed when the electron beam travels over the screen. Plasma TVs also work in a more or less similar way. However, LCD takes much longer time to make these transitions resulting in smudges or blurs around the image. This is what is so called ‘ghosting’.
The negative aspect of displays with high response time is that they cannot produce satisfactory display when playing fast paced games or fast actions. This is also true when you use them for displaying CAD designs which require fast, precise operations.
Usually older LCD displays suffered ‘ghosting’ due to their higher response time although this is not true with latest LCD TVs as the manufacturers have come with numerous solutions.
Measuring Response time
Response time between 8 ms and 16 ms had been quite acceptable until recent time. The ISO standard usually considers the full black and white transitions as standard and the response time of display systems has to be quoted with TrTf (Time rising, Time falling). However the gray transitions which are mostly in practice are far behind the ISO standard thus misleading the customers with regard to response time. The recent Response Time Compensation or RTC technology has been adopted in many LCD TVs to make measurement of response time to fall in line with ISO standard. This is done by reducing the grey transitions in a LCD TV.
New LCD TVs come with improved response time
However, LCD manufacturers in the recent past found two ways to minimize the ghosting. One such way is to decrease the screen size as ghosting is not much visible smaller panels. Another way is to use a technique called overdriving which uses black-to-gray and gray-to-gray transitions.
Newer LCD TVs adopt two methods to reduce the response time to eliminate the ghosting.
According to first method, the backlight device is fired at a rapid rate which will be usually less than the screen refresh rate. However, this method is likely to result in flickering which can be perceptible by eyes.
The second method is to use ‘motion compensation’ which actually doubles the screen refresh rate and produces transitional frames. Most high end LCD TVs combine these two techniques in their LCD
Contrast ratio of LCD TV - explained
June 24, 2007
The contrast ratio measures the capacity of a display device to produce the brightest colour and the darkest colour and it is usually expressed as a ratio of their light intensities. There is a lot of disagreement prevails in measuring the contrast ratio so the contrast ratio measured using different methods show different figures.
While manufacturers of LCD TV follow their own methods to measure the contrast ratio, most of them are reluctant to take into account the effect of room light conditions. Majority of them have a view that the contrast ratio of a display device like LCD TVs should be measured in an ideal room where there is no place for the light from TV to get reflected and all the light that a TV emits is completely absorbed. They usually prefer the method of the complete on/off when they measure the contrast ratio as it would show the maximum contrast ratio.
There is another method, the Ansi contrast which provides the most realistic ratio as it includes the effects of room into measurement. Ansi contrast is often used to measure static contrast ratio while complete on/off method is employed to measure the dynamic ratio.
The methods used for measuring pure black and white vary from one manufacturer to another. However, all these methods are less likely to give the contrast ratio acceptable by everyone. There is always discrepancy between the contrast ratio specified by the maker and the actual contrast ratio that a viewer discovers after he puts the TV under a test.
Newer LCD TVs use PVA (Patterned Vertical Alignment) and S-PVA (Super Patterned Vertical Alignment) to produce deeper black levels and greater viewing angles. S-PVA also removes ghosting to certain extent by providing fast response times using RTC technologies.
Good black depth is prerequisite for viewing dark scenes in well-lit rooms. PVA and S-PVA technologies enhance the contrast ratio up to 3000:1
Another method a LCD TV uses to offer black depth is by illuminating the wall behind the screen thus making the dark scenes appear darker.
Why viewing angles of a LCD TV are so important?
June 24, 2007
In the beginning, LCD displays were generally considered suitable for computer monitors and television sets with small screen sizes. These systems did not require much off-axis viewing as the viewer usually sits closer to the screen. When LCDs started to become an integral part of home theatre unit, the narrow viewing angles of older LCD TVs needed to be improved by the manufactueres. Wide viewing angles make the LCDs suitable for a large room where a large number of viewers can watch the pictures with uniform picture quality. Narrow viewing angles means you have to sit at right in front of the screen and another disadvantage of narrow viewing angles is that they do not allow much audience to watch the TV.
Viewing angle is another important spec that marks the performance a display device. Viewing angle of a LCD tells us how far we can sit away from its off-axis and still can have acceptable picture quality. Beyond the viewing angle on either side, the image becomes distorted and lacks colour saturation and contrast. This is because the TV sets which have narrow viewing angles tend to reflect more light towards the viewer but less light on either side. As a result, the viewer gets poor picture quality from the sides. Sometimes, the viewing angles of LCD TVs are measured in correlation with contrast ratio. That is, the viewing angle is the angle at which the contrast ratio LCD TV becomes 10:1 or less.
The viewing angle of flat screen is measured across the screen from one direction to another. The maximum viewing angle can be 180 degrees in the case of flat panel displays. Nowadays, the response time is quoted, considering both the horizontal and vertical axis of the view although viewing angle on the basis of horizontal axis carries much importance.
Modern LCDs have come with a few solutions for narrow viewing angles. One of them is to use Optically Compensated Bend (OCB) liquid crystals to eliminate of problem of narrow viewing angles. OCB enables the LCDs to have viewings angles more than 170 degrees. Nowadays, some manufacturers boast that their LCD TVs have viewing angles up to 178 degrees.
Recent developments in LCD technology
June 24, 2007
The LCD manufactures had made lot of innovations in the manufacturing of a lCD TV. Currently different types liquid crystals like dual super twisted nematics (STN), dual scan twisted nematics (DSTN), ferroelectric liquid crystal (FLC) and surface stabilized ferroelectric liquid crystal (SSFLC) are used. In addition, latest LCD TVs started using special plastic in place of glass making the TV lighter and flexible.
Backlight system
As back light of LCD TV is one of the most important determiners of the picture quality of LCD TV, developments are constantly happening in this area with each manufacturing coming with new backlight technologies.
With regard to backlight source , the following techniques are employed by the LCD makers.
1. Manipulating backlight device
By controlling the brightness of florescent lamps that make up the backlight source.
This technique produces fast movements on the screen with smoothness and realism . The brightness of backlight is controlled in synchronization with the picture source to enhance black depth and picture details.
2. Using alternative back light sources:
FFL backlight
LCD TVs generally use cold cathode fluorescent tubular lights (CCFL). Some new LCD TVs, as manufactured by Samsung employ the use of a flat fluorescent lamp (FFL) which offers thin design combined with uniform scattering of light on the screen. This new technology also provides improved brightness and contrast of the image.
Some LCD manufacturers use their unique florescent backlight by improving the existing CCFL in their LCD panels. The conventionally used CCFL is usually made up of three colours – red, green and the blue. However, the leading LCD make Sharp has come with its new 4-colour florescent backlight which adds fourth colour. Adding the fourth colour also known as red emitter, 4-colour florescent backlight enhances the colour clarity showing different red shades. There is also a LED absed backlight technology which can give better colour quality than 4-color florescent backlight. However, cost wise 4-color florescent backlight is much cheaper
LED based backlight
The backlight technologies based on LED, though costlier, are employed in some newer LCD TVs as they have certain advantages over the CCFL backlight technology.
LED based backlight device presents colours with unrivalled naturalness and true-to-life quality that cannot be expected from LCDs that standard CCFL backlight. This is because, the colours in LED based TVs are created based on the frequency or wave length of light. As a result, colours look more precise and resemble the natural hues. It has following advantages:
- The overall picture quality is much better in LCDs which use LED based backlight technology.
- LCD TVs which are based on LED backlight sources usually consume less electricity.
- LED based backlight sources have better longevity and can last three times longer than normal CCFL backlight.
Color Field Sequential Technology
Colour filters are generally used to produce a gamut of colours in a LCD display and they weigh about one-fourths of a LCD TV. The newer LCD TVs have found a way to replace colour filters using a method called Colour Sequential. This new technology allows the backlight device to discharge only one colour at a time, so in a sequence of three colours (RGB) each colour is displayed one after another. The colour sequential component and the backlight source work together to produce improved colour spectrum thus the colours look more crisp and richer.
Colour filters also suck up nearly three-fourths of light provided by backlight source. So, the use of colour filiters not only involves higher power consumption but will also lead to the short life of backlight. As the colour sequential system does not require colour filters, the power consumption is reduced to minimum. The display is much brighter while using the same amount of electricity.When colour filters are removed, it also makes a LCD lighter by nearly 25 percent.
Since, only one colour is displayed at a quick succession, each sub-pixel almost functions like an individual pixel. While this makes the screen to have three times more resolution than the normal screen, this technology also keeps the number of bad pixels to a bare minimum. The Sequential colour system changes the colours so rapidly that all the three colours produce a full-colour image which looks richer and better colour detailed.
Optically Compensated Bend (OCB)
A few LCD makers like Toshiba are well known for their small LCD panels and they have been using Optically Compensated Bend (OCB) liquid crystals to eliminate of problem of narrow viewing angles. Narrow viewing angles make the viewer to sit within the supported field of view to get the acceptable picture quality and the image usually gets deteriorated when seen from off-axis. But OCB enables the LCDs to have viewings angles more than 170 degrees.
These liquid crystals twist and untwist at much faster speed, so the response time is also greatly improved up to 3 ms while the standard response times is 8 ms.
Another drawback of LCD is also overcome by using OCB. They can withstand extreme temperatures especially cold temperatures, so LCDs perform better in such conditions.(sub zero temperatures).
Advantages of LCD TVs
June 24, 2007
Advantages of LCD TVs
Slim profile
Like Plasma TVs, LCD TVs are slim, sleek and stylish. Even a larger 52†wide screen LCD is less than 4†in depth. They weigh less when compared with Plasma TVs of similar sizes. Because of being light, LCD TVs can be easily mounted on walls.
High resolution
LCD TVs have sidelined Plasma TVs in terms of resolution. They have better resolution than plasma TVs. Because they have better resolution, the images on LCD screen are sharper, smoother and more-detailed. A LCD of 45†model can display a full High Density resolution of 1080p (1920×1080 pixels).
Longer lifetime
LCD TVs generally have a better life span by contrast with Plasma TVs and CRTs. The lifetime of a display device is measured by its half-life, which refers to the time when its lighting device loses half of its original brightness. The half-life of newer LCD TVs crosses more than 60,000 hrs. There is also an option of replacing backlight resource when it is fully used out.
No screen burns
LCD TVs are free from screen burns as they don’t use phosphors, instead, they use liquid crystals and colour filters supported by a backlight source for displaying images.
No glare
Images on LCD TVs look brighter and their screens are not as glossy as plasma TV screens. Moreover, the feature of anti-glare technology prevents glare, even in bright rooms.
Economic power consumption
LCD TVs consume very little electric power. A wide screen LCD with 65†screen size will consume 30 percent less electricity than its counterpart Plasma TV. This is because more power is required by plasma TV to make the phosphors glow whereas LCD requires power only for producing its backlight, which undergoes filtering through the liquid crystals. LCD monitors can operate on batteries for a longer time, so they are ideal for laptops.
