Look sharp: channels big and small make the turn to high-def.(What's on HDTV: the Spring...

Look what's finally here: high-definition television. Not only are cable- and broadcast-network executives increasingly interested in the quality of HDTV picture, but they're encouraged by the quantity of HD sets flowing into the marketplace.

This year alone, the Yankee Group estimates that Americans will buy 6 million or 7 million HD sets; that's on top of the 9 million that are already out there. It's now plausible that you know somebody with high-definition television. That wasn't so even 18 months ago.

As HD seems to be catching on, broadcasters and cable