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, but it is not used for broadcast TV (yet).
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.
