If you live in America and have an old radio ? a really old, boxy one, vintage 1935 or so, with big vacuum tubes inside ? you can still listen to it. You'll hear lots of static, and it won't be easy to home in on a clear signal, but it does work if all the parts still function.

The same cannot be said of early television sets as of February 17, 2009. Americans who still have an old Air King or DuMont or Majestic-brand set (the kind that requires a so-called "rabbit-ears" antenna or one you'd string up on your roof) have been put on notice. That set won't work 14 months from now. The screen will go blank, and there'll be no sound. Even nearly new television sets that operate with analog components won't work.

In what promises to be the most stunning TV technology revolution since the advent of color television in the 1950s, broadcasters will simply stop sending analog signals through the air. Everything will be digital ? and only digital.

Analog signals are sent as a continuous stream of information. In contrast, in a digital broadcast, information is chopped up into tiny slivers of information, that can be represented by a digit: 1 or 0. A digital TV set converts those digits back into pictures and sound. Today, analog broadcast signals are most often transmitted through the air. Digital signals are most often transmitted electronically through wires.

Americans in the estimated 20 million households that now get only over-the-air TV signals will be able to purchase inexpensive converter boxes. But the makers of television sets, as well as cable and satellite-television program providers, hope that customers will instead simply junk their old sets or turn them into end tables, and buy brand-new, digitally-compatible equipment.

Critics of this doomsday for analog television worry that the elderly, those who live in remote areas far from cable systems and good satellite signals, and people who don't speak much English may not get the word about this big change. They may be confused and angry to find themselves literally in the dark come February 17, 2009. Television advertisers won't like it one bit, either, if a few million viewers wake up that winter morning and can't see their commercials.