When people first used the term HD, the technology was pegged at 720 lines (thus 720i or 720p). HD displays a picture more clearly than SD, and marketers were quick to label their products HD. But true HD really starts at 1080 lines (1080i or 1080p); 720 might more honestly be called "HD Lite."

The letters:The i stands for "interlaced scanning," where only half of each video frame is displayed on the screen every 1/60 of a second. The set presents every other line of pixels from top to bottom, then in the next 1/60 second, it goes back and fills in the blanks. This works because the human visual system tends to remember what was there previously, so interlacing tricks the eye into thinking the picture is complete. This same persistence-of-vision phenomenon is what makes motion pictures, filmed at 24 frames per second, look continuous.

HD Radio

To further muddle the meaning of HD, there's also high-definition radio, in which stations broadcast digital audio signals that can be decoded only by special receivers. HD radio sounds better than AM or FM, and may even sound better than a typical CD. But at present, most HD stations also broadcast in normal analog mode, and there are no plans to replace AM or FM.

Not yet, anyway.