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To study the specification and working of LCD TV

 

 

 

OBJECT: - To study the specification and working of LCD TV.

APPARATUS:-LCD Driving IC,LCD Panel,Rectifier,Inverter,Backlight lamp,source driven IC,Gate driven IC.

THEORY:- A liquid crystal display or LCD draws its definition from its name itself. It is combination of two states of matter, the solid and the liquid. LCD uses a liquid crystal to produce a visible image. Liquid crystal displays are super-thin technology display screen that are generally used in laptop computer screen, TVs, cell phones and portable video games. LCD’s technologies allow displays to be much thinner when compared to cathode ray tube (CRT) technology. Liquid crystal display is composed of several layers which include two polarized panel filters and electrodes. LCD technology is used for displaying the image in notebook or some other electronic devices like mini computers. Light is projected from a lens on a layer of liquid crystal. This combination of coloured light with the gray scale image of the crystal (formed as electric current flows through the crystal) forms the coloured image. This image is then displayed on the screen.

WORKING:-Liquid crystal display televisions (LCD TV) are television sets that use LCD display technology to produce images. LCD televisions are thinner and lighter than cathode ray tube (CRTs) of similar display size, and are available in much larger sizes. When manufacturing costs fell, this combination of features made LCDs practical for television receivers. LCD televisions produce a black and coloured image by selectively filtering a white light. The light was provided by a series of cold cathode fluorescent lamps (CCFLs) at the back of the screen. Today, most LCDTV displays use white or coloured LEDs as backlighting instead. Millions of individual LCD shutters arranged in a grid, open and close to allow a metered amount of the white light through. Each shutter is paired with a coloured filter to remove all but the red, green or blue (RGB) portion of the light from the original white source. Each shutter–filter pair forms a single sub pixel. The sub pixels are so small that when the display is viewed from even a short distance, the individual colours blend together to produce a single spot of colour, a pixel. The shade of colour is controlled by changing the relative intensity of the light passing through the sub pixels. Liquid crystals encompass a wide range of (typically) rod shaped polymers that naturally form into thin, ordered layers, as opposed to the more random alignment of a normal liquid. Some of these, the nematic liquid crystals, also show an alignment effect between the layers. The particular direction of the alignment of a nematic liquid crystal can be set by placing it in contact with an alignment layer or director, which is essentially a material with microscopic grooves in it, on the supporting substrates. When placed on a director, the layer in contact will align itself with the grooves, and the layers above will subsequently align themselves with the layers below, the bulk material taking on the director's alignment. In the case of a Twisted Nematic (TN) LCD, this effect is utilized by using two directors arranged at right angles and placed close together with the liquid crystal between them. This forces the layers to align themselves in two directions, creating a twisted structure with each layer aligned at a slightly different angle to the ones on either side. LCD shutters consist of a stack of three primary elements. On the bottom and top of the shutter are polarizer plates set at right angles. Normally light cannot travel through a pair of polarisers arranged in this fashion, and the display would be black. The polarisers also carry the directors to create the twisted structure aligned with the polarisers on either side. As the light flows out of the rear polarizer, it will naturally follow the liquid crystal's twist, exiting the front of the liquid crystal having been rotated through the correct angle that allows it to pass through the front polarizer. LCDs are normally transparent in this mode of operation. To turn a shutter off, a voltage is applied across it from front to back. The rod shaped molecules align themselves with the electric field instead of the directors, distorting the twisted structure. The light no longer changes polarization as it flows through the liquid crystal, and can no longer pass through the front polarizer. By controlling the voltage applied across the liquid crystal, the amount of remaining twist can be selected. This allows the transparency of the shutter to be controlled. To improve switching time, the cells are placed under pressure, which increases the force to realign themselves with the directors when the field is turned off. Several other variations and modifications have been used in order to improve performance in certain applications. In Plane Switching displays (IPS and SIPS) offer wider viewing angles and better colour reproduction, but are more difficult to construct and have slightly slower response times. Vertical Alignment (VA, SPVA and MVA) offer higher contrast ratios and good response times, but suffer from colour

Shifting when viewed from the side. In general, all of these displays work in a similar fashion by controlling the polarization of the light source. In order to address a single shutter on the display, a series of electrodes is deposited on the plates on either side of the liquid crystal. One side has horizontal stripes that form rows; the other has vertical stripes that form columns. By supplying voltage to one row and one column, a field will be generated at the point where they cross. Since a metal electrode would be opaque, LCDs use electrodes made of a transparent conductor, typically indium tin oxide. Since addressing a single shutter requires power to be supplied to an entire row and column, some of the field always leaks out into the surrounding shutters. Liquid crystals are quite sensitive, and even small amounts of leaked field will cause some level of switching to occur. This partial switching of the surrounding shutters blurs the resulting image. Another problem in early LCD systems was the voltages needed to set the shutters to a particular twist was very low, but that voltage was too low to make the crystals realign with reasonable performance. This resulted in slow response times and led to easily visible "ghosting" on these displays on fast moving images, like a mouse cursor on a computer screen. Even scrolling text often rendered as an unreadable blur, and the switching speed was far too slow to use as a useful television display.  In order to attack these problems, modern LCDs use an active matrix design. Instead of powering both electrodes, one set, typically the front, is attached to a common ground. On the rear, each shutter is paired with a thin film transistor that switches on in response to widely separated voltage levels, say 0 and +5 volts. A new addressing line, the gate line, is added as a separate switch for the transistors. The rows and columns are addressed as before, but the transistors ensure that only the single shutter at the crossing point is addressed; Any leaked field is too small to switch the surrounding transistors. When switched on, a constant and relatively high amount of charge flows from the source line through the transistor and into an associated capacitor. The capacitor is charged up until it holds the correct control voltage, slowly leaking this through the crystal to the common ground. The current is very fast and not suitable for fine control of the resulting store charge, so pulse code modulation is used to accurately control the overall flow. Not only does this allow for very accurate control over the shutters, since the capacitor can be filled or drained quickly, but the response time of the shutter is dramatically improved as well Building a display. A typical shutter assembly consists of a sandwich of several layers deposited on two thin glass sheets forming the front and back of the display. For smaller display sizes (under 30 inches), the glass sheets can be replaced with plastic. The rear sheet starts with a polarizing film, the glass sheet, the active matrix components and addressing electrodes, and then the director. The front sheet is similar, but lacks the active matrix components, replacing those with the patterned colour filters. Using a multistep construction process, both sheets can be produced on the same assembly line. The liquid crystal is placed between the two sheets in a patterned plastic sheet that divides the liquid into individual shutters and keeps the sheets at a precise distance from each other. The critical step in the manufacturing process is the deposition of the active matrix components. These have a relatively high failure rate, which renders those pixels on the screen "always on". If there are enough broken pixels, the screen has to be discarded. The number of discarded panels has a strong effect on the price of the resulting television sets, and the major downward fall in pricing between 2006 and 2008 was due mostly to improved processes. To produce a complete television, the shutter assembly is combined with control electronics and backlight. The backlight for small sets can be provided by a single lamp using a diffuser or frosted mirror to spread out the light, but for larger displays a single lamp is not bright enough and the rear surface is instead covered with a number of separate lamps. Achieving even lighting over the front of an entire display remains a challenge, and bright and dark spots are not uncommon.

CIRCUIT DIAGRAM:-

                                                        Fig:-13(a)     Fig:-13(b)   Fig:-13(c)

SPECIFICATIONS:-

1. Display: - 15”Diagonal Size, Flat panel display / TFT active matrix

    Max Resolution:-1280 x 1024

    Aspect ratio: - 4:3

2. Image

    Image Brightness: - 300 cd/m2

     Image Contrast Ratio: 350:1

     Image Max H-View Angle: 160

     Image Max V-View Angle: 160

3. Interface

    Analog Video Input: RGB VGA (HD-15)

    Analog Video Input: S-Video

    Composite Video Input: RCA Yellow,

    Audio Input: RCA- Left (White), Right (Red)

    Antenna RF Input: RF - SDTV/ PAL

4. Application: Used as Monitor (PC), DTV, SDTV, AV Player, S- Video Player

5. Computer Monitor Driver: Windows 98, XP, 2000

6. Tuner Channels: 2 to 69

7. On Screen display: Volume, Brightness, Contrast, Colour, Channel, Tuning

8. Remote Control functions: On screen display of Volume, Brightness, Contrast, and Channel

9. Audio Amplifier: 3 W PMPO

10. Faults: 5 No.

11. PCB Size: 15"x12” with Block diagram in multicolour section wise.

12. Power supply: 230V + 15% AC, 50 Hz, 60 watts.

13. Standard Accessories: 1. Trainer PCB with LCD display.

                                           2. Different input Cables

                                           3. A Manual having 10 practical

  Important factors to consider when evaluating an LCD:

  • Resolution versus range:-Fundamentally resolution is the granularity (or number of levels) with which a performance feature of the display is divided. Resolution is often confused with range or the total end to end output of the display. Each of the major features of a display has both are solution and a range that are tied to each other but very different. Frequently the range is an inherent limitation of the display while the resolution is a function of the electronics that make the display work.

 

  • Spatial performance:- LCDs come in only one size for a variety of applications and a variety of resolutions within each of those applications. LCD spatial performance is also sometimes described in terms of a "dot pitch". The size (or spatial range) of an LCD is always described in terms of the diagonal distance from one corner to its opposite. This is an historical remnant from the early days of CRT television when CRT screens were manufactured on the bottoms of glass bottles, a direct extension of cathode ray tubes used in oscilloscopes. The diameter of the bottle determined the size of the screen. Later, when televisions went to a squarer format, the squares screens were measured diagonally to compare with the older round screens. The spatial resolution of an LCD is expressed by the number of columns and rows of pixels (e.g., 1024×768). Each pixel is usually composed 3 sub pixels, a red, a green, and a blue one. This had been one of the few features of LCD performance that was easily understood and not subject to interpretation. However there are newer schemes that share sub pixels among pixels and to add additional colours of sub pixels. So going forward, spatial resolution may now be more subject to interpretation. One external factor to consider in evaluating display resolution is the resolution of the viewer's eyes. Assuming 20/20 vision, the resolution of the eyes is about one minute of arc. In practical terms that means for an older standard definition TV set the ideal viewing distance was about 8 times the height(not diagonal) of the screen away. At that distance the individual rows of pixels merge into a solid. If the viewer were closer to the screen than that, they would be able to see the individual rows of pixels. When observed from farther away, the image of the rows of pixels still merge, but the total image becomes smaller as the distance increases. For an HDTV set with slightly more than twice the number of rows of pixels, the ideal viewing distance is about half what it is for a standard definition set. The higher the resolution, the closer the viewer can sit or the larger the set can usefully be sitting at the same distance as an older standard definition display. For a computer monitor or some other LCD that is being viewed from a very close distance, resolution is often expressed in terms of dot pitch or pixels per inch. This is consistent with the printing industry (another form of a display). Magazines, and other premium printed media are often at 300 dots per inch. As with the distance discussion above, this provides a very solid looking and detailed image. LCDs, particularly on mobile devices, are frequently much less than this as the higher the dot pitch, the more optically inefficient the display and the more power it burns. Running the LCD is frequently half, or more, of the power consumed by a mobile device. An additional consideration in spatial performance is viewing cone and aspect ratio. The Aspect ratio is the ratio of the width to the height (for example, 4:3, 5:4, and 16:9 or 16:10). Older, standard definition TVs were 4:3. Newer High Definition televisions (HDTV) are 16:9, as are most new notebook computers. Movies are often filmed in much different (wider) aspect ratios, which is why there will frequently still be black bars at the top and bottom of an HDTV screen. The Viewing Angle of an LCD may be important depending on its use or location. The viewing angle is usually measured as the angle where the contrast of the LCD falls below 10:1. At this point, the colours usually start to change and can even invert, red becoming green and so forth. Viewing angles for LCDs used to be very restrictive however; improved optical films have been developed that give almost 180 degree viewing angles from left to right. Top to bottom viewing angles may still be restrictive, by design, as looking at an LCD from an extreme up or down angle is not a common usage model and these photons are wasted. Manufacturers commonly focus the light in a left to right plane to obtain a brighter image here.

 

  • Temporal/timing performance: - Contrary to spatial performance, temporal performance is a feature where smaller is better. Specifically, the range is the pixel response time of an LCD, or how quickly a sub pixel’s brightness changes from one level to another. For LCD monitors, this is measured in btb (black to black) or gtg (gray to gray). These different types of measurements make comparison difficult. Further, this number is almost never published in sales advertising Refresh rate or the temporal resolution of an LCD is the number of times per second in which the display draws the data it is being given. Since activated LCD pixels do not flash on/off between frames, LCD monitors exhibit no refresh induced flicker, no matter how low the refresh rate. High end LCD televisions now feature up to 240 Hz refresh rate, which requires advanced digital processing to insert additional interpolated frames between the real images to smooth the image motion. However, such high refresh rates may not be actually supported by pixel response times and the result can be visual artefacts that distort the image in unpleasant ways. Temporal performance can be further taxed if it is a 3D display. 3D displays work by showing a different series of images to each eye, alternating from eye to eye. Thus a 3D display must display twice as many images in the same period of time as a conventional display, and consequently the response time of the LCD is more important. 3D LCDs with marginal response times will exhibit image smearing. These artefacts are most noticeable in a person's black and white vision (rod cells) than in colour vision (cone cells). Thus they will be more likely to see flicker or any sort of temporal distortion in a display image by not looking directly at the display, because their eyes' rod cells are mostly grouped at the periphery of their vision.
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  • Colour performance: There are many terms to describe colour performance of an LCD. They include colour gamut which is the range of colours that can be displayed and colour depth which is the colour resolution or the resolution or fineness with which the colour range is divided. Although colour gamut can be expressed as three pairs of numbers, the XY coordinates within colour space of the reddest red, greenest green, and bluest blue, it is usually expressed as a ratio of the total area within colour space that a display can show relative to some standard such as saying that a display was "120% of NTSC". NTSC is the National Television Standards Committee, the old standard definition TV specification. Colour gamut is a relatively straight forward feature. However with clever optical techniques that are based on the way humans see colour, termed colour stretch, colours can be shown that are outside of the nominal range of the display. In any case, colour range is rarely discussed as a feature of the display as LCDs are designed to match the colour ranges of the content that they are intended to show. Having a colour range that exceeds the content is a useless feature. Colour depth or colour support is sometimes expressed in bits, either as the number of bits per sub pixel or the number of bits per pixel. This can be ambiguous as an 8bit colour LCD can be 8 total bits spread between red, green, and blue or 8 bits each for each colour in a different display. Further, LCDs sometimes use a technique called dithering which is time averaging colours to get intermediate colours such as alternating between two different colours to get a colour in between. This doubles the number of colours that can be displayed; However this is done at the expense of the temporal performance of the display. Dithering is commonly used on computer displays where the images are mostly static and the temporal performance is unimportant. When colour depth is reported as colour support, it is usually stated in terms of number of colours the LCD can show. The number of colours is the translation from the base 2 bit numbers into common base10.For example; 8 bit colour is 2 to the 8th power, which are 256 colours. 24 bit colour is 2 to the 24th power, or 256 x 256 x 256, a total of 16,777,216 colours. The colour resolution of the human eye depends on both the range of colours being sliced and the number of slices; But for most common displays the limit is about 28 bit colours. LCD TVs commonly display more than that as the digital processing can introduce colour  distortions and the additional levels of colour are needed to ensure true colours There are additional aspects to LCD colour and colour management, such as white point and gamma correction, which describe what colour white is and how the other colours are displayed relative to white.LCD televisions also frequently have facial recognition software, which recognizes that an image on the screen is a face and both adjust the colour and the focus differently from the rest of the image. These adjustments can have important effects on the consumer, but are not easily quantifiable; People like what they like and everyone does not like the same thing. There is no substitute for looking at the LCD one is going to buy before buying it. Portrait film, another form of display, has similar adjustments built into it. Kodak. Many years ago, Kodak had to overcome initial rejection of its portrait film in Japan because of these adjustments. In the U.S., people generally prefer a more colourful facial image than in reality (higher colour saturation). In Japan, consumers generally prefer a less saturated image. The film that initially sent to Japan was biased in the wrong direction for Japanese consumers. Television monitors have their built in biases as well.
  • Brightness and contrast ratio: Contrast ratio is the ratio of the brightness of a full on pixel to a full off pixel and, as such, would be directly tied to brightness if not for the invention of the blinking backlight (or burst dimming). The LCD itself is only a light valve and does not generate light; The light comes from a backlight that is either a fluorescent tube or a set of LEDs. The blinking backlight was developed to improve the motion performance of LCDs by turning the backlight off while the liquid crystals were in transition from one image to another. However, a side benefit of the blinking backlight was infinite contrast. The contrast reported on most LCDs is what the LCD is qualified at, not its actual performance. In any case, there are two large caveats to contrast ratio as a measure of LCD performance. The first caveat is that contrast ratios are measured in a completely dark room. In actual use, the room is never completely dark, as one will always have the light from the LCD itself. Beyond that, there may be sunlight coming in through a window or other room lights that reflect off of the surface of the LCD and degrades the contrast. As a practical matter, the contrast of an LCD, or any display, is governed by the amount of surface reflections, not by the performance of the display. The second caveat is that the human eye can only image a contrast ratio of a maximum of about 200:1. Black print on a white paper is about 15–20:1. That is why viewing angles are specified to the point where they fall below 10:1. A 10:1 image is not great, but is discernible. Brightness is usually stated as the maximum output of the LCD. In the CRT era, Trinitron CRTs had a brightness advantage over the competition, so brightness was commonly discussed in TV advertising. With current LCD technology, brightness, though important, is usually similar from maker to maker and consequently is not discussed much, except for laptop LCDs and other displays that will be viewed in bright sunlight. In general, brighter is better, but there is always a trade off between brightness and battery life in a mobile device.

PRECAUTIONS:-

1)      Connect the devices properly.

2)      Use an uncluttered and clean work area to do the experiment

RESULT:-We have studied the working and specification of an 15”Diagonal Size Flat panel display LCD TV.

QUESTIONS:-

  1. What is LCD TV?
  2. What is the difference between an LCD TV and LED TV?
  3. What is the difference between an LCD TV and Plasma TV?

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