The Supreme Court decided by a single vote yesterday to put a safety net under Hugo Zacchini, who bills himself as a "human cannonball."

The court ruled, 5 to 4, that the constitutional guarantee of a free press did not immunize from a lawsuit a Cleveland, Ohio, television station that broadcast his "entire act" without his consent.

"The broadcast of a film of (Zacchini's) entire act poses a substantial threat to the economic value of that performance," Justice Byron R. White wrote in the opinion for the court.

As defined by White, the "entire act" in the film clip shown by the Scipps-Howard Broadcasting Co. station stated with the cannon explosion that launched Zacchini and ended with his safe landing. The elapsed time was 15 seconds.

The film clip showed none of the preliminaries. Justice Lewis F. Powell Jr. assumed these may have stretched over several minutes and were accompanied "by suitably ominous commentary from the master of ceremonies."

White had "no doubt that entertainment, as well as news, enjoys First Amendment protection," but wrote that neither the public nor Zacchini will be deprived of the benefits of his performance "as long as his commercial stake in his act is appropriately recognized."

Powell, in a dissenting opinion joined by Justices William J. Brennan Jr. and Thurgood Marshall, doubted that the court's opinion "is appropriately sensitive to the First Amendment values at stake."

The station "simply reported on what [Zacchini] concedes to be a news worthy event, in a way hardly surprising for a television station - by means of film coverage," Powell said. "The report was part of an ordinary daily news program, consuming 15 seconds. It is a routine example of the press fulfilling the reporting function so vital to our system.

"The court's holding that the station's ordinary news report may give rise to substantial liability has disturbing implications, for the decision could lead to a degree of media selfcensorship," Powell continued.

"Hereafter, whenever a television news editor is unsure whether certain film footage received from a camera crew might be held to portray an 'entire act,' he may decline coverage - even of celarly newsworthy events - or confine the broadcast to watered down verbal reporting, perhaps with an occasional still picture. The public is then the loser. This is hardly the kind of news reportage that the First Amendment is meant to foster."

Justice John Paul Stevens dissented on different ground to the decision, which reversed the Ohio Supreme Court.

Over Zacchini's objection, a cameraman videotaped his act in 1972 at a county fair. He sued for $25,000 for appropriation of his property.