Although both systems are based on digital technologies, HDTV generally offers viewers nearly six times the sharpness of SDTV based broadcasts. In addition to classifying digital TV signals into HDTVand SDTV, each of these technologies has the following subtype classifications.
Resolution
- Resolution is defined as the amount of detail or pixels contained in an image displayed on a television set. This characteristic is one of the main reasons why people experience an improved viewing experience while watching a HDTV in comparison to standard analog based TVs. A television display has display capabilities of 644 x 483 pixels, although most people refer to it as 640 x 480. This again is not a true description of the display capabilities of a standard TV. For technical reasons, most viewers are unable to see the margins that surround the perimeter of their television screens. Consequently, most industry experts calculate the actual viewing area by subtracting the marginal areas. Such calculations result in an actual viewing area on a standard TV of 620 x 440 pixels. It is also possible to measure the resolution of a television display by the number of video lines available on the screen. The video lines are developed with a type of electronic gun that operates inside the television tube. This gun emits a beam of light that moves back and forth across the surface of the screen. As the beam moves across the inner surface of the screen, it creates an image, line by line.
As a general rule, the more lines created by the gun, the higher the resolution. The resolution of an IP HDTV program is dependent on the size of the TV display and the number of video lines present on the screen.
For example, clearer pictures are available to owners of new HD televisions because they are able to display 1080 lines of resolution versus 525 lines displayed on an ordinary television sets.
Interlaced and Progressive Scanning
- The process of scanning defines how the lines are displayed across a TV screen. There are two different scanning methods, namely, interlaced and progressive. Interlacing scanning is a system that was designed by TVengineers to overcome the inability of early television tubes to draw a video display on the screen before the top of the display began to fade. With interlacing the image is created by instantaneously applying light to every second line on the screen. Once this is completed the remaining lines are illuminated almost immediately. This happens so fast that
the human eye is unable to detect. Interlace scanning is popular in analog televisions and low resolution computer monitors.
Progressive scanning, also known as noninterlaced video, refers to a process of a television screen refreshing itself line by line. This is done at the same time therefore the image appears smooth to the human eye and stays sharp during motion in the video pictures. Progressive scanning is popular in HDTVs and computer monitors.
Aspect Ratio
- Aspect ratio defines the relationship between the horizontal width and vertical height of a TV screen. The aspect ratio of a traditional TV screen is 4 : 3. In other words, for every four units of measurement across the length of the TV screen the height will consist of three units. HDTV uses an aspect ratio of 16 : 9.
These classifications are used by broadcasters and TV manufacturers to define a notation for identifying different types of HDTV products. The notation used in the United States is based on lines of resolution followed by a letter that represents the type of scanning used. An ‘‘i’’ represents interlaced and a ‘‘p’’ represents progressive. The notation also includes a figure for the number of frames or fields per second. Examples of some of the most popular HDTV standards.
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