All About HDTV: Answers To Your Questions
Buying a TV used to mean deciding what size screen you wanted, then going out and finding the set with the best-looking picture you could afford--with not much more to it than that. In the end, such basic choices are still mostly what it's all about. Yet somehow the process manages to be, or at least to seem, much more complicated.
One reason is that, increasingly, most of the TVs available today are high-definition models. And along with dramatically improved picture and sound, HDTV brings a boatload of new terminology, new features, and new connection options. To help make sense of it all, here are the answers to the most common and most important questions posed by today's HDTV shoppers.
Q. What is HDTV?
A. HDTV (high-definition television) comprises a family of digital broadcast formats that offer extremely clear wide-screen pictures and Dolby Digital sound. An HDTV broadcast contains about six times as much picture information as a conventional analog TV broadcast.
Q. What does "wide screen" mean?
A. In television, a wide-screen picture is one with a 16:9 aspect ratio--that is, 16 units of screen width for every 9 units of screen height. This is very close to the aspect ratio most commonly used in the film industry and a better match to the human visual field than the traditional, almost square 4:3 aspect ratio of conventional television (and of movies made before the early 1950s).
Q. What are 1080i and 720p?
A. These are the picture formats used for HDTV broadcasts. Some networks and stations use 1080i (CBS and NBC, for example), whereas others (including ABC and Fox) use 720p. TV creates an illusion of motion by displaying a succession of still images. In a 1080i signal, each complete video frame is 1920 pixels (picture elements) wide by 1080 pixels high, transmitted in interlaced format. A 720p signal, on the other hand, contains 1280 by 720-pixel frames transmitted in progressive format.
Q. What is 1080p?
A. Basically, it's a progressive version of 1080i (see above). HD DVDs and Blu-ray Discs carry video in 1080p format; it is not used for broadcast.
Q. What's the difference between interlaced and progressive video?
A. Progressive video presents a complete frame at a time. Interlaced video, on the other hand, builds each frame out of two fields--one comprising the odd-numbered pixel rows, or scan lines, the other the even-numbered pixel rows--which are presented alternately. Whereas the frame rate for 720p (progressive) HDTV is 60 per second (60 fps), the frame rate for 1080i (interlaced) is just 30 per second, but with a field rate of 60 per second.
If you're thinking, "Well, that seems like a bit of a cheat," you're right. It's a pretty effective one most of the time, however, which is why interlacing was adopted for the analog television systems of the twentieth century. It allowed greater effective picture resolution within the transmission bandwidth allotted for TV channels, yet the screen was refreshed frequently enough to avoid obvious flicker on the cathode-ray tube (CRT) displays of the era. Its advantages outweighed its drawbacks.
Q. Which is better, 720p or 1080i?
A. That depends on the amount of motion in the program. For still images or scenes containing relatively little motion, 1080i can deliver greater detail. But 720p excels on scenes containing fast motion. That's why the networks with the heaviest commitments to sports programming have adopted 720p. ABC owns ESPN, for example, and Fox owns many regional sports networks.