High Definition Television or HDTV is a term that can be better understood when we make a comparison between different digital television standards available today. In fact, HDTV is a relative term and is often analogous with higher resolution. When we date back to the days when TV was in its formative years, there were standards like NTSC, PAL, SECAM which measured the video quality of the TV in terms of scan lines and number of frames per second lines. It was in mid forties of last century , NTSC or National Television System Committee which universalized a video standard which had  4:3 aspect ratio and 525 scan lines refreshing at a rate of 30 interlaced frames, or 60 fields per second. A few nations like the UK, did not stick to this standard and made a change in refresh rate, thus UK followed TV standard based on 50 Hz instead of 60 HZ which was set as standard in countries like the US and Canada. However, PAL and SECAM which came as alternatives for NTSC had more scan lines (625 instead of 525 lines) to provide more detailed pictures on the screen. Although the pictures were better on PAL but the refresh rate tend to be slower resulting in flicker which used to annoy the viewers.

The real revolution took place in 1982, when ATSC or Advanced Television Systems Committee was formed to develop a new television technology that can altogether replace the analogue video formats like NTSC, PAL or SECAM. First began as technological advancement in analog television, ATSC soon started to develop digital TVs that include standard TV, enhanced TV and finally High-definition TV. All these DTVs promised better picture quality in terms of resolution, refresh rate, colour quality etc. Another advantage that DTVs holds over previous analog televisions was improved sound quality that comes with their impressive display.

All the Digital TVs that include LCD television or plasma TV differ depending up on how many pixels per inch make up the image (this is called resolution), number of scan lines over the screen (both vertically and horizontally) and the aspect ratio. The aspect ratio shows how wide is the screen in relation to its height. In early televisions (analog) and standard digital televisions the 4:3 is the aspect ratio is still followed which means the width will be slightly longer than the height of the screen. This aspect ratio is much closer to 16 mm or 35 mm movie, and was popular until the wider format of movies (70 mm) started to be used by most movie makers during the last two decades. The 16:9 or wider aspect ratio started to be followed by television makers to present movie like viewing experience and soon became the standard for the HDTVs.

Resolution is one of the major characteristics that help us to distinguish one TV from another. Compared with old analog televisions, present HDTVs can show ten times higher resolution or even twenty times! A high end HDTV LCD TV (which is 1980p resolution), can show more than 4 million pixels on its screen presenting imagers so sharper and more detailed. When resolution is taken into account, any digital TV that can show minimum resolution of 720p is HDTV. There are HDTV resolutions like 720p, 1080i, 1080p etc. While there is not much to compare between 720p and 1080i as they look more or less alike, however, 1080p images always look superior to any other HDTV resolution. Again, 720p and 1080p indicate the method of scanning is progressive scan which tends to present better image quality against the same resolution scanned by interlace method ( which is denoted by ‘i’ and which follows the resolution). 

Digital TVs also come with improved audio quality as they adopt new audio systems like Dolby Digital which minimize the unwanted noises and also use much more audio data on their tracks. Dolby uses five channels to provide surround sound quality which is more or less similar to what one experiences in a cinema hall. However, most of the HDTVs usually feature only two channels and only a few can support 5.1 channel feature of full Dolby Digital.

Screen burns and Image persistence
LCD screens may sometimes be afflicted by a temporary problem known as image persistence. As the continuous, static image produces so called ‘screen burns’ on a plasma screen, in a similar way, they cause LCD screen to display the previous image or part of it on the background thus producing picture of poor quality.

Difference between screen burns and Image persistence
Screen burns of CRT monitors and plasma TVs are often permanent as the phosphors that produce display become pre-maturely old or damaged by the continuous display of some bright, static image on the screen. Screen burns are not reversible because the phosphors cannot be replaced and this leads to permanent imprint of the image on the screen. This is what is normally called ‘screen burns’.

On the other hand, image persistence or image retention on a LCD screen, though produced similarly, is hardly permanent and reversible if you take some remedial steps. LCDs use liquid crystals that produce images using a backlight source. When the voltage is passed through the crystals, they twist and relax depending on the varying voltage. When you let any image on the screen for longer period, the same voltage continues for same amount of time resulting in crystals to stay in one state. So, when the next image appears on the screen, the colours on the affected area will still have a faint imprint of the previous image.

Why Image persistence occurs on a LCD screen?
Any static images that are displayed for a quite longer time like tool bars, program icons or background images tend to produce image persistence on LCD screens.

Fortunately, image persistence does not produce permanent effect on LCD screen, as it is caused by the state of crystals which tend to change depending on the voltage used. When different images appear on the screen they usually prevent image persistence from occurring. However, if any static image is left for a longer duration to cause image persistence, it produces permanent imprint of image only in rare circumstances.

Preventive measures 
Image persistence on LCD screens can be prevented by observing certain methods which are given below:

• If you use LCD screens with your PC for much longer time, you can use appropriate power settings in your PC, so that LCD monitors will be off for a few minutes after prolonged use.
• When you are using your monitor, either make some screen saver to rotate over the screen. Generally some graphic images or some blank screen savers are generally preferred for this purpose.
• If you use any background images on your desktop, it is highly recommended that you change them periodically to prevent image persistence from occurring.

Corrective measures
Image persistence can not only be prevented but also can be rectified using following methods:

• If a image persists on your LCD screen, you may keep your LCD screen unused for hours together. If it still persists, turn it off for a few weeks. Often it enables your LCD screen to correct the problem itself.
• Any graphical image used as screen saver often helps to remove the image persistence if it is kept rotating over the screen for longer periods.

What is interlaced scan?

Standard Television sets use anyone of the interlaced scan systems - namely NTSC or PAL. Both these systems display analog video signals that are broadcast from TV stations or from some video sources. Although the display using these scanning systems produced acceptable results, they failed to create exact images on a PC or when DVD materials are used. It is because the video images produced by these scan system always had a few limitations in terms of resolution and picture quality. In an interlaced scan system, an image is scanned in an alternate order (1, 3, 5) skipping next line. As a result, the images lack smoothness and details and also become prone to constant flickering. Although certain improvements like ‘line doubling’ have been made in this system to produce images of higher resolution, they also produced some undesirable effects like ‘motion lag’ on the screen. Some of the video processors handle these problems quite efficiently, but the display needed an entirely different scanning system to completely remove flaws of the interlaced images.

What is progressive scan?

On the other hand, progressive scan system scans images in a sequential manner (1, 2, 3 etc), from top to bottom on the screen. This scanning method renders more details and smoothness to the images, especially when some text materials are shown on the screen. Presently, progressive scan is used in DVD, Digital TV and HDTV. Progressive scan is ideal for viewing DVDs and HDTV broadcast so high end PCs and flat panel television sets use this scan system to display images.

Progressive scan with 3:2 pull down and different progressive scans

There are two kinds of progressive scans that are being used. Progressive scan and progressive scan combined with 3:2 pull down. Whether your display device uses 3:2 pull down or not, the images are definitely better in terms of picture quality when compared with interlaced video images. However, the 3:2 pull down offers a film-like image quality and is often preferred if you wish to watch some HDTV sources or HD DVDs. Based on number of lines or pixel rows scanned, there are different progress scans like 480p, 720p, 1080i and 1080p. While a typical digital TV uses 480p (480 lines progressively scanned), the rest of the scans are applied in HDTV broadcast. Thus a HDTV broadcast or DVD will have resolution of 720p or higher. So if you plan to view a DVD or HDTV materials and to get enriched visual experience, you should go for TVs that come with progressive scan inputs.

What you should look for in your LCD TV?

If you want to access progressive scan pictures from DVD or TV broadcast, your TV should have anyone or all of the following connectivity options:

• Digital Video Interface (DVI)
• High Definition Multi-media Interface (HDMI)
• Component video input

Together with these inputs, either your TV or your DVD player should have 3:2 pull down detection, so that you can view progressive scan images with reverse 3:2 pulldown.

What impresses anyone most, is the slim profile of a LCD TV. A LCD TV is not thick more than 4 or 5 inches and this enables us to hang it on the wall like a portrait or you can even get a mount that can fix your LCD on the ceiling. If you prefer neither of this installation options, you can go for traditional table stand or an elegant column floor stand.

When you prefer wall mountable LCD TVs, it not only saves space but also becomes a decorating element on the wall. Table-stand option is sturdier even though it occupies some space in your room. Mounting your LCD TV on the ceiling in your bedroom, indeed, is not an easy option and it involves some professional help. Therefore, you need to choose the suitable mounting option after evaluating its advantages and disadvantages.

Before starting with mounting your LCD TV, you need to plan the following things:
1. Find the right place for your LCD TV so that viewing can be comfortable.
2. Choose the right kind of mount which is feasible for your room.
3. As many cables run from LCD TV, you should find a way to hide the running cables so that it does not look messy.

How to mount a LCD TV?
If you have some basic carpentry skills and are able to understand the instructions provided in the installation guide, the mounting of a LCD TV will not pose a difficult task to you. All you need is some instruments like screw driver, screws, a tape to take measurements etc. However, when you install a wall mount, you should carefully fix it with a precise horizontal level. As the larger LCD TVs, say 42” display, weigh heavier, you may require assistance of someone to handle the job. As a rule of thumb, you should plan well before you go for installing your LCD TV.

When you choose a place for your TV in a room, the ideal place will be where no direct sunlight falls or a place which is completely free from any kind of reflections. So, don’t keep your LCD screen in a place where it will be facing windows or doors.

The next important thing to consider is the distance between the LCD screen and your seat. If your LCD TV is High Definition TV, the viewing distance should be minimum 3 times the screen width from the LCD screen. It is recommended that you choose the screen size taking the room space available into consideration. If you have insufficient room space, place your LCD where the corners meet, so that you can get more viewing distance from your LCD TV.

If you decide to hang your LCD TV on the wall, make sure the wall mount is fixed at optimum height so that the centre of the screen is at the eye level when you view the TV from your seat. Similarly, when you stand, your eye level should be at the top of the screen. This setting is often considered ideal for comfortable viewing.����

Don’t forget to hide the connecting cables.
While you use table stand for your LCD TV, it will often have a provision to hide all connecting cables inside its central column. However, if you opt for wall mount, the task of hiding cables becomes a bit arduous, as you need to run these cables either through some grooves inside the wall or use some camouflaging techniques.

The camouflaging method hides the cables inside wiring channels which are painted with the same colour that match with the colour of your room.�

If you opt for running cables through the walls, you may require some professional to get it done. Although it is expensive, the end result is far better than just camouflaging cables in the room.

Composite signals are essential for displaying brilliant images or pictures on your TV screen. They most commonly used video signals which offer lower quality. It also has another name as “the lowest denominator of video connectivity”. As it has limited band width, there are chances for overlapping of brightness, contrast and colour. This problem of overlapping creates another problem of inability to differentiate the identity between brightness information and chrominance information.

Later this confusion leads to fuzziness and artifacts on the screen of your LCD TV. An element called “comb filter” has been designed and implemented to remove those fuzziness arising out of composite signals. These filters use different filtering methods to clean this fuzziness. These filters can be availed in two distinct forms namely feed forward and feedback.It has classified on the basis of direction of signals.

1. Feed Forward – It is a unique system, mainly used for showing the feed forward behaviour responds. It can be measured in a pre-defined way with the support of feedback system. It is possible to implement this comb filters in 2 ways namely discrete time or continuous time.
2. Feedback – It is a kind of video signal which is always looped back within a system for controlling. This type of loop has been called as feedback loop. This control system has come with input as well as output. If the output of the system is feed back into controlling system of its output, that status is said to be as feedback.

Thus the comb filter element has been embedded within LCD televisions to serve a vital process of removing the fuzziness on the TV screen. So when you plan to purchase a new television make sure that the television is supported by the comb filter feature to get clear and bright images.