So you’re dying to have a karaoke machine in your living room but thought you just couldn’t afford one? We have two words for you: closed-captioning. Designed for the hearing-impaired, this typewritten scroll of dialogue can now be found all over the television dial-even on some MTV videos. (With many TV sets less than three years old, the writing pops up with just a few clicks on your remote control.) Finally you can understand what Billie Joe of Green Day is yammering on about. Finally you can sing along to Tom Petty without going to a cheesy Paramus, N.J., bar on Friday night. But that’s not the only advantage closed-captioning offers the not- necessarily-hearing-impaired couch potato. *It’s a handy tool when you’re on the phone. If your best friend calls during Melrose Place to whine about dating woes, you don’t have to stop enjoying the far more interesting girl trouble that Billy is experiencing. Simply mute your TV and read away, as you toss out an occasional ”Mm-hmmm” to your friend. *It’s educational. After a particularly tense interview with leggy supermodel Claudia Schiffer, David Letterman viewers with closed-captioning got to read the following: ”Band plays the Rolling Stones’ ‘Bitch.’ ” On another show, we got to read, ”Band playing peppy conga music.” For the musically illiterate, it’s like having a personal pop-culture tutor in the privacy of your own home. *It clarifies sound effects. Was that an ”Oh” or an ”Ohhhh”? A ”Groan” or a ”Grunt”? A ”Click” or a ”Clack”? With closed-captioning, you’ll know for sure. But best of all for troubled PTA members, it gets kids to read while watching TV! The tube doesn’t have to make us all illiterate. And stuff.