High Definition Television (HDTV)

High Definition Television (HDTV) began broadcasting in the United States in 1998. It uses interlacing in its best mode (called 1080i), and non-interlacing, called progressive scanning, in its next best mode (called 740p). Progressive scanning means that all the lines are scanned in sequence rather than all the odd lines followed by all the even lines. (High quality computer monitors already do this.) 1080i means that there are 1080 horizontal scanning lines, split up into two sets of 540 alternating lines each, and the two sets are displayed one after the other ("interlaced"). Lines 1,3,5, etc. are in one set, and lines 2,4,6, etc. are in the other set.

HDTVs are shaped in the 16:9 aspect ratio ("Widescreen") and can scan at 24 or 30 frames per second, alleviating the problem of fitting 24 movie frames into 30 television frames, as just described. The US version of HDTV is all digital, while HDTV that already exists in Japan and Europe started out as analog and is converting to digital. Digital television transmission results in less "ghosts" and other interference that mars current analog transmission. All HDTVs are digital TVs (DTV), but not all digital televsions are HDTVs. There are many resolutions which can be used in digital TV transmissions, including 480 which is the same resolution as our current NTSC TVs. An additional feature of HDTVs is that they can line double, which means that our NTSC programs will look much better on an HDTV than on a standard TV. So, it is worthwhile to get an HDTV now, even though actual HDTV programming is sparse.
Many of the HDTVs available now can handle 1080i but not 720p. However, it is possible we may not see 720p programming for quite some time anyway. Projection HDTVs need 9" CRTs to display 1080i. Projection TVs with 7" CRTs or smaller cannot display the full 1080i resolution.