The Federal Communications Commission's plan caps 10 years of work to clear the way for the biggest industry advance since color in the 1950s.
The 24-month rule will apply to stations owned or affiliated with ABC, CBS, NBC and Fox in the nation's top 10 markets. Stations in smaller markets will have 30 months to begin digital broadcasting, FCC sources said, speaking on condition of anonymity.
The commission already has received written pledges from at least 25 stations in the top 10 markets to offer digital broadcasts within 18 months - in time for the 1998 holiday shopping season, the FCC sources said.
The next step up - high definition television with its even sharper pictures and sound than digital TV - is being left up to each station under the FCC plan, according to sources familiar with it. Stations will be free to broadcast as little or as much digital high-definition programs as they want.
With the new digital technology, TV stations could cram more services into their airwaves space. For example, they could offer sports scores to laptop computer users or even establish a separate pay-for-view sports channel.
Once the plan is formally approved, the FCC will immediately begin issuing new digital broadcast licenses to every TV station in the country.
FCC Chair Reed Hundt had supported a short timetable for pushing digital television onto the market, a move endorsed by the manufacturers of TV sets.
As they convert to digital, broadcasters will be transmitting programs over two channels: their existing analog; and a second digital channel that they'll get from the government for free. This way, existing analog TV sets will not be rendered immediately useless.
The FCC's plan shortens the time broadcasters have to switch entirely to digital - from 15 years to nine years, or by 2006. Broadcasters don't like the new timetable, which matches a Clinton administration proposal.
After 2006, broadcasters will no longer transmit programs in analog. That means people will either have to buy a new digital TV by then, or a device that would enable existing TV sets to receive the new signals.
New digital TV sets, which will be wider than existing sets and more like movie screens, are expected to cost at least $2,000. The set-top, box-like convertors would range from $150 to $300.
Even with the new sets, most of the nearly 68 million cable TV subscribers in the country will have to have an antenna and an a/b switch to get the clearer pictures.
Tele-Communications Inc.'s system in Hartford, Conn., is now the only cable operator in the country that has gone digital. More cable systems are preparing to go digital, but until they do, viewers will have to rely on antennas.
The abandoned analog channels will be auctioned by the government for non-broadcast uses such as mobile phone, two-way paging and wireless Internet access.
The nation's 1,600 TV stations are getting the second digital TV channel for free even though their collective worth has been estimated at up to $70 billion. Critics, who wanted Congress to force broadcasters to pay for them, call it the biggest federal giveaway of the 20th century.