Digital television isn't just an evolution of today's standard for television broadcasting - it's a revolutionary change.
It requires completely new transmitting and receiving equipment and will require changes to everything related to TV, including videocassette recorders and cable TV systems - even the shape and size of TVs themselves.
The first difference is in how the signal is broadcast.
The current standard for TV transmission, developed by the National Television Standards Committee, uses the same principles to transmit pictures and sound that radio uses: Separate radio signals within a band of radio frequencies (or channels) carry the video and audio signals, as well as alternative signals such as closed captioning and second audio programs.
Digital TV, on the other hand, uses pulses of radio waves to transmit information broken down into the equivalent of ones and zeros. The National Aeronautics and Space Administration uses the same system to exchange data with satellites and space probes such as the Hubble Space Telescope.
Because the information in the digital signal isn't delivered by changes in strength or frequency, as in traditional radio, digital // PTC transmissions are less vulnerable to interference. That means they can travel farther and still deliver a clear picture, without the "ghosts" or "snow" associated with the current broadcast format.
The digital signals are converted back into pictures and sound at the receiver.
By their nature, digital TV signals can be compressed and carry much more information than traditional, analog TV signals. Thanks to electronics and software technology, the format of that information can be changed virtually on the fly.
This means broadcasters can send out two types of programming with digital transmissions: high-definition TV (HDTV) and standard-definition TV (SDTV).
HDTV transmissions will transmit a single, very high-resolution program with five-channel, compact-disk-quality Dolby sound, providing a picture and audio quality equal to what's available in a movie theater.
SDTV uses a lower-resolution signal, although it still provides better pictures than current signals. What makes SDTV attractive to broadcasters is that they can squeeze up to four SDTV programs into the same amount of bandwidth as a single high-definition program.
HDTV and SDTV broadcasts leave room to transmit other types of information, including computer data.
While converter boxes will make it possible to view SDTV broadcasts on current televisions, HDTV will require completely new sets. That's because of the difference in the resolution and the proportion (or "aspect ratio") of the HDTV and conventional screens.
The current standard TV picture has an aspect ratio of 4-to-3, so a 25-inch-diagonal screen is about 20 inches wide and 15 inches tall. The picture on that screen is divided into 525 lines, which are refreshed 30 times a second.
The screen on a digital TV, on the other hand, is proportionally wider and shorter. It will have an aspect ratio of 16-to-9, which is closer to most movie theater screens.
This means that a digital TV with a 25-inch-diagonal screen will be almost 22 inches wide and only a little more than 12 inches tall - so the picture will seem a bit smaller than that of today's 25-inch sets. However, HDTV broadcasts will have a resolution of more than 1,000 scan lines - resulting in a picture quality close to that of 35-mm film.
Pub Date: 5/25/98