WHILE people like to complain that there is nothing to watch on television, about 21 million American households may find that literally true in February 2009. On the 17th of that month, most TV stations will quit broadcasting analog TV signals over the air, and older sets will go blank. But remarkably, half of the country does not realize a changeover is coming, according to a survey by the Cable and Telecommunications Association for Marketing. And despite advertising that proposes cable TV as the remedy for a dark screen, cable viewers with older sets — 40 million of them, according...