In addition to a program that was created in HD and transmitted in that format, you need a TV that's capable of displaying it. The TV screen must display enough lines or pixels--a spec called native resolution--to depict all the detail in an HD signal.

You also need a digital tuner that can receive HD and other digital signals. The tuner can either be built in to the TV or provided by an external device. An ATSC tuner can receive off-air signals from an antenna. Digital cable and satellite use a different type of digital tuner, which is contained in cable box or satellite receiver.

HD-ready sets, sometimes called HD monitors, contain only an NTSC tuner, the same as the TVs we grew up with. Right out of the box, these sets can receive the standard-definition programming you get on regular TV. However, these TVs require an external digital tuner to decode HD signals. If you get your TV service via cable or satellite, you would need the tuner contained in an HD-capable cable box or satellite receiver (which also requires a special dish setup). To get free broadcast digital signals via antenna, you would connect the TV to a set-top box containing a digital ATSC tuner. (Note that an antenna pulls in only the channels available on network TV, not the expanded lineup offered by cable and satellite.) Set-top boxes generally cost a few hundred dollars. However, HD-ready sets are usually a few hundred dollars cheaper than a comparable integrated set with a built-in tuner.

TVs with built-in digital tuners are called integrated HDTVs. Their internal ATSC tuner can receive free broadcast digital signals, including HD, when the set is attached to a VHF/UHF antenna. To get HD programs and premium channels via cable or satellite, integrated TVs require a cable or satellite box.

Some integrated HDTVs also have QAM tuners. Besides getting digital signals via antenna, they can receive unscrambled digital-cable signals--including the local HD channels in basic cable packages--via a cable into the TV, without a cable box.

Digital-cable-ready (DCR) or plug-and-play TVs are the next step up. Besides receiving off-air digital signals and unscrambled digital-cable signals, they can get HD and premium channels without using a cable box; instead, you insert a CableCard (provided by the cable company, usually for a fee) into a slot on the TV. The downside is that current DCR TVs are one-way, so you still need a cable box to get an interactive program guide, video on demand, and other interactive services. Two-way capability isn’t expected to be available for a while.

You may be able to watch HDTV on some ED sets, which usually cost less than an HDTV. EDTVs, mostly plasma sets, have a lower native resolution than true HDTVs, but many can display HD signals at a lower resolution (provided they’re used with a digital tuner). The picture quality won’t match that of true HD, but it can be quite impressive on the best sets.

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