What is a LCD TV?
June 24, 2007
LCD TV – An Introduction
A LCD TV uses LCD technology which can provide slim design, a flat viewing surface, larger screens and higher-quality images. A liquid crystal display TV is an extremely thin TV set with a profile of 4†or less and consumes much less electricity in comparison with conventional CRT TVs or modern Plasma TVs. This is one of the reasons why LCD devices are ideal for electronic devices like laptops which can work for a longer time using batteries.
How does a LCD TV work?
June 24, 2007
As the name suggests, the display in a Liquid Crystal Display (LCD) TV is based on the inter-active properties of liquid crystals and light. The technology used in making a LCD is called Thin Film Transistor (TFT), which has made tremendous advancements during the last ten years.
What are liquid crystals?
Liquid crystals are cylindrical shaped molecules which allow light to pass through them in their natural, twisted phase. However, they start to untwist or relax, when they are placed in an electric field. The amount of electric current decides the degree of untwisting of crystalline molecules. The more the untwisting of molecules the more the blocking of light rays will be. In their extreme relaxed state, they completely block the light and become opaque. By varying the amount of current, the intensity of light can vary between extreme brightness and blackness.
How LCD TV works?
In a LCD TV, liquid crystals are sandwiched between two transparent electrodes and two polarizing filters. In other words, liquid crystals (most often in nematic phase) are packed between two glass plates. Two grids of electrodes (one is horizontally aligned and another is vertically aligned) contact either side of the crystals. The job of this network of electrodes is to access each pixel of the crystal layer and control the electric flow.
Liquid crystals are not luminious by themselves and it is the backlight device made of fluorescent lamps which give off the light behind them. The light of the backlight is then diffused by a white panel before passing through the liquid crystals. This white panel ensures that the entire surface of the screen gets uniform brightness. The front panel of LCD is made up of a grid of wires which are in contact with each pixel of the screen and activate it separately.
With in network of crystals, each molecule functions like a gate to open or close the passage of light through them depending on the voltage of the current. To produce bright display, the voltage is kept low while for dark details the voltage is increased. By nature, liquid crystals always allow a smaller amount of light to pass through them making the pure black levels somewhat impossible.
Passive Matrix structure and Active Matrix structure
There are two options for controlling voltage – using passive matrix structure or active matrix structure. In the passive matrix structure of electrodes, one pixel can be accessed at a time. LCD displays with lesser resolution like simple mobiles, wristwatches use this option. However, the refresh rate will be usually slow and the voltage control would not be proper. Picture may not have outstanding clarity.
Large LCD displays like flat panel LCD television use the advanced active matrix structure. The electrodes will have an additional template of thin film transistors (TFT) which is capable of addressing the exact pixel and changing the colour of sub-pixels without involving the nearby pixels, as a result, better clarity of details and more colour preciseness are achieved. TFT also function like capacitors storing electric charge during the refreshing cycle of pixels.
How does a LCD TV produce colours?
In earlier mono-chrome LCD displays, only two basic colours were available, most of them had green as their back-ground colour. In a colour LCD display, as in Plasma TV, a pixel (picture element) has three sub-pixels – red, blue and green. In addition, there is a colour filter placed before the crystal layer to produce the overall colour of the pixel. As the name indicates, colour filters filter the colours from the colour spectrum and produce innumerable combinations of basic colours (red, blue and green) to form coloured display
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.
