The future of TV looks impossibly sharp on RCA's 27F634T — except when it comes up as a blank blue screen with a "signal unavailable" message. This unremarkable-looking cathode-ray-tube set is the cheapest digital television yet sold. That makes it a fulfillment of the part of digital TV that many people don't know about: not high-definition TV, but free, over-the-air broadcasts with far better reception than analog (except when they conk out) and multiple channels for each station.