Standard definition (SD) vs. HD: HD always looks better, but some HDTV are better than others. Moreover, the term SD is seldom used now. If it isn't labeled HD, it's SD.
Analog vs. digital: Television broadcasting uses analog signals whose frequencies vary smoothly. Analog will disappear in February 2009 and be replaced by more efficient digital television (DTV), which can fit more channels and signals into a smaller segment of broadcast spectrum. To view DTV on an older set, you'll need a special device or nonbroadcast service -- cable or satellite -- that converts digital broadcast signals to analog. Digital provides better pictures than analog, but it is not necessarily HD. However, HD is always digital.
Definition
High-definition (HD) television refers to better-than-standard-quality, sharper pictures. How much better requires more terminology to decipher.
Widescreen vs. standard: Older TV sets mimicked then-current movie screens, with a picture 1.33 times as wide as it was high. But most movies now use a wider format, often 2.35 times as wide as they are high. To present such movies on TV requires cropping or editing the picture or showing it with black bands on the top and bottom. This "letterboxing" preserves the intended format of the original movie but can make it look tiny on smaller TV sets. With HD and DTV, the industry has a new standard screen almost twice as wide as it is high (the actual ratio is 16:9). The new format means less compromise for movies, making editing or letterboxing less objectionable. In most situations, it also provides more image content. This is especially noticeable with televised sports.
Plasma vs. LCD vs. projection vs. DLP vs. CRT: These terms refer to how the TV set physically creates the image you see on the screen. Any of these technologies might also be HD, but without more information, you just don't know.
Broadcast vs. DVD: DVDs show standard-definition pictures but generally make them look better than they would on an SD broadcast. Newer DVD players use a conversion process to make an SD picture look almost as good as HD. Until early 2008, two incompatible formats competed for true HD on DVDs, but 's Blu-ray won the war while lost. Blu-ray players are than standard DVD players, and standard DVD players can't play .
What About Those Numbers and Letters?
An HDTV spec sheet may boast 720i or 1,080p, but what do those numbers and letters mean?
The numbers: They refer to the theoretically best possible picture quality -- the total number of pixel lines from top to bottom that the set can display.
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.
The p stands for progressive scanning. Here, the set paints the entire picture in sequence, starting at the top and going to the bottom without skipping anything. In the newest sets, this happens twice as fast, as in 1/120 second. The picture looks smoother, and action scenes look better than with interlaced scanning.
To compare the picture quality of TV screens with that of computer monitors, 720p is equivalent to a computer display of 1280 by 720 pixels, while 1080p is 1920 by 1080.
For the same number, p generally looks better than i, but you can't always tell the difference unless everything else in the viewing chain is optimized.
For example, depending on your equipment, even a 1080p set may show only 1080i unless you use a special, more expensive type of interconnecting cable called high-definition multimedia interface, or HDMI.
Currently, 1080p is the highest-quality picture broadcast or viewable. (Newer, higher standards will replace it, but not for several years.) You may not be able to see a difference between 1080i and 1080p, but if you're shopping for a new set, go for the p. Still, 1080 will always look better than 720, and both will look make SD look bad. But remember, your old VHS tapes will look pretty fuzzy on the new set. And on a smaller set (15 inches or less), you may never see the difference between 720 and 1080.
Kay is a Computerworld contributing writer in Worcester, Mass. You can contact him at .