The first approach involves eliminating the analog feed for cable channels. Cable companies that take this approach--Cablevision and Comcast--are doing so with channels that they are also broadcasting in a digital format.

Approach number two is termed . In this situation, channels are on a system that includes a switch, so they only get piped into your house if your cable box requests them. The arrangement works similarly to the way video-on-demand does. Say you want channel 132: You request it using your remote, after which the cable company sends the signal to your cable box. Before companies instituted this switch system, all signals were continuously broadcast through the pipe, and all you had to do was tune in to the station you wanted.

Switched digital works only with a set-top box, however. If you use a --which some cable companies supply in place of a box, and which users can slip into newer TVs or digital video recorders (like TiVo's Series3 boxes)--you'll find that the card can't communicate back to the cable provider. As a result, you won't be able to record channels on the switch. The next generation of CableCard, , will correct this deficiency. Tru2way allows interactive communication between the card and the cable company; unfortunately, as yet it remalns a rarity.

Time Warner Cable has deployed switched digital in some markets; only two Time Warner markets--New York and Los Angeles--have gone all-digital (and even there, only in some areas). Cox Communications is using switched digital technology in Arizona, Northern Virginia, and Orange County, California, and it has plans to roll out the technology in other markets this year.