Digital TV (DTV)
Digital TV (DTV), also known as high-definition TV (HDTV), was designed to replace the National Television Standards Committee (NTSC) system, which was invented in the 1940s and 1950s.
The goal of HDTV is to greatly improve the picture and sound quality. After more than a decade of evaluating alternative HDTV systems, the FCC has finalized the standards and decreed that HDTV will eventually become the U.S. TV standard by April 2009.
The first HDTV stations began transmission in the 10 largest U.S. cities on September 1, 1998. HDTV sets can now be purchased by the consumer, but they are still expensive. As more HDTV stations come online and as more HDTV programming becomes available, more consumers will buy HDTV receivers and the cost will drop dramatically.
The HDTV system is an extremely complex collection of digital, communication and computer techniques.
A full discussion is beyond the scope of this book. However, this section is a brief introduction to the basic concepts and techniques used in HDTV. HDTV Standards HDTV for the United States was developed by the Advanced Television Systems Committee (ATSC) in the 1980s and 1990s.
HDTV uses the scanning concept to paint a picture on the CRT, so you can continue to think of the HDTV screen in terms of scan lines, as you would think of the standard NTSC analog screen. However, you should also view the HDTV screen as being made up of thousands of tiny dots of light, called pixels.
Each pixel can be any of 256 colors. These pixels can be used to create any image. The greater the number of pixels on the screen, the greater the resolution and the finer the detail that can be represented.
Each horizontal scan line is divided into hundreds of pixels. The format of a HDTV screen is described in terms of the numbers of pixels per horizontal line by the number of vertical pixels (which is the same as the number of horizontal scan lines).
One major difference between conventional NTSC analog TV and HDTV is that HDTV can use progressive line scanning rather than interlaced scanning. In progressive scanning each line is scanned one at a time from top to bottom.
Since this format is compatible with computer video monitors, it is possible to display HDTV on computer screens. Interlaced scanning can be used on one of the HDTV formats. Interlaced scanning minimizes flicker but complicates the video compression process. Progressive scanning is preferred and at a 60-Hz frame rate, flicker is not a problem.
The FCC has defined a total of 18 different formats for HDTV. Most are variations of the basic formats as given in Table. Most plasma, LCD and larger screens only display these formats.The 480p (the p stands for “progressive”) standard offers performance comparable to that of the NTSC system. It uses a 4:3 aspect ratio for the screen.
The scanning is progressive. The vertical scan rate is selectable to fit the type of video being transmitted. This format is fully compatible with modern VGA computer monitors.
The format can use either progressive or interlaced scanning with either aspect ratio at the three vertical scan rates shown in Table.
The 720p format uses a larger aspect ratio of 16:9 (a 4:3 format is optional at this resolution also). This format is better for showing movies. Figure shows the difference between the current and new HDTV aspect ratios.
The 1080i format uses the 16:9 aspect ratio but with more scan lines and more pixels per line. This format obviously gives the best resolution. The HDTV set should be able to detect and receive any available format. The 720p at 60 Hz and 1080i formats are those designated HDTV.