At midnight on Feb. 17, 2009, the rabbit ears and the rooftop antennas that still guide television signals into nearly 1 of every 5 American homes will be rendered useless — unless they are tethered to a new device, including two versions unveiled yesterday, that the government will spend as much as $80 a household to help families buy.

Among the advantages of digital television being promoted by the broadcasters is superior image quality and that the signal required is compressed, so a station may be able to send out four streams of programming where once it had only one, freeing up spectrum space.