The 4-1 vote by the Federal Communications Commission upheld a 2001 FCC ruling that said cable companies only must carry one digital channel per station.
Digital broadcasts offer sharper pictures than the traditional analog transmissions used in most TV sets in American homes. A digital signal also can carry more information without using any more space on the broadcast spectrum.
Some stations have chosen to use their digital signals for crystal-clear, high-definition broadcasts, while others have established multiple channels.
Cable operators have voluntarily agreed to carry multiple digital channels in some cities, but broadcasters say making it a requirement would benefit the public by giving them more programming choices. They contend that cable companies are afraid of the competition that more free, local "over-the-air" channels would give popular cable-only movie and sports networks.
But the cable industry has argued that the First Amendment gives the operators, and not the government, the power to decide what channels to carry. Requiring up to six digital channels would take up valuable space because there is room for only a limited number of channels, cable operators say.
They also fear that broadcasters might use the extra capacity to carry nothing but shopping shows or infomercials.
The four commissioners who sided with cable companies questioned whether they had the authority to force cable operators to carry the extra channels and were skeptical of the need.
"This is not a test of whether multicast content is beneficial," Republican Commissioner Kathleen Abernathy said. "But multicasting is not necessary to ensure the continued preservation of free, over-the-air TV."
The commission's only dissenter, Republican Kevin Martin, contended the ruling would hamper the ability of small, independent minority and religious broadcasters to get on the air.
"The public could have benefited from more free programming," he said.
Congress has told broadcasters to complete the digital shift by December 2006 or when digital TV reaches 85 percent in a market, whichever comes later.
Some local stations are offering an all-weather digital channel in addition to a channel showing regular programming.
The FCC on Thursday also rebuffed another request from broadcasters to require cable companies to carry both digital and analog signals of local broadcasters as they switch to the new technology.