Basic ideas for HDTV

The concept behind high-definition television is actually not to increase the definition per unit area ... but rather to increase the percentage of the visual field contained by the image.

The majority of proposed analogue and digital HDTV systems are working toward approximately a 100% increase in the number of horizontal and vertical pixels. (Proposals are roughly 1 MB per frame with roughly 1000 lines by 1000 horizontal points). This typically results in a factor of 2-3 improvement in the angle of the vertical and horizontal fields. Most of HDTV proposals also change the aspect ratio to 16/9 from 4/3 -- making the image more "movie-like".

NOTE: The aspect ratio of the picture is defined to be the ratio of the picture width W to its height H. The optimal viewing distance (expressed in picture heights, H) is the distance at which the eye can just perceive the detail elements in the picture.

MPEG-2

Broadcasters are having to squeeze the increased picture detail and higher quality surround sound into the same 6-megahertz (MHz) bandwidth used by analog television. Compression software, very similar to what is used in personal computing, allows this to happen.

Digital TV relies on a compression and encoding scheme known as MPEG-2 to fit its stunning images into a reasonable amount of bandwidth. In each image, the MPEG-2 software records just enough of the picture without making it look like something is missing. In subsequent frames, the software only records changes to the image and leaves the rest of the image as-is from the previous frame. MPEG-2 reduces the amount of data by about 55 to 1.

MPEG-2 already is the industry standard for DVD videos and some of the satellite TV broadcast systems. Compression reduces image quality from what is seen by the digital camera at the studio. However, MPEG-2 is very good at throwing away image detail that the human eye ignores anyway. The quality of the image is very good, and significantly better than traditional analog TV.

The use of MPEG-2 permits an HDTV receiver to interact with computer multimedia applications directly. For example, an HDTV show could be recorded on a multimedia computer, and CD-ROM applications could be played on HDTV systems. A digital TV decodes the MPEG-2 signal and displays it just as a computer monitor does, giving it high resolution and stability.

HDTV Stations

There are HDTV stations "on the air" in many large cities. The first HDTV station was WRAL-HD in Raleigh, NC.