The universal conversion from analog to digital TV is coming within the next 48 hours, and if you're not yet familiar with that, and depend on airborne broadcast signals, here's what you need to know.

As of this Thursday and Friday, TV broadcasters nationwide will begin broadcasting solely digital signals, consisting of a stream of ones and zeros (think of the sound you hear when you've mistakenly dialed a fax machine) instead of the old analog waves, which will go dead.

They're doing that because simple up-or-down, on-or-off digital signals survive interference, and thus provide far better clarity than analog waves do. Analogy: try to learn exercises by watching people through leafy bushes: you'll get the up-and-down pattern of jumping jacks quickly, but the subtle whooshes of tai chi? Not so much. Interrupted analog waves are like those bushes, obscuring the picture.

So here come the digital signals. Televisions made since 2007 will automatically read them, and so will TVs hooked up via cable or satellite. But older-TV-with-antenna people will find fuzz or darkness unless they get a converter box that hears the digital signals and converts them back to analog.

The feds are giving away $40 coupons toward the boxes' cost (they run $40 to $80), and most TVs will work just fine, and even look and sound a bit better, once they're hooked up. Some rural residents and very old TVs, however, may not work as well. So buying a converter box and testing is crucial if TV is important to your daily routine.

To learn about the federal government coupon program, call 888.DTV.2009 or visit www.DTV2009.gov.

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