Of all the spectacles to come in the trial of D.C. Mayor Marion Barry, none has roused the anticipation of the public or the disquiet of his lawyers more than the FBI surveillance tapes that caught him unawares in Room 727 of the Vista Hotel.

Sources close to the mayor report that Barry's lawyers are daunted by the prospect that their client's own voice and image will be offered in evidence against him. The history of such evidence suggests the lawyers are right to be concerned: Most videotaped "stings" result in convictions.

But experts say even the most incriminating tapes offer opportunities for both sides. As law enforcement technology has grown more sophisticated, so have the tools of the defense: An industry of specialists has sprung up to provide frame-by-frame forensic analysis of videotapes and linguistic interpretation of the events recorded on them.

"The key on the video is you study it over and over as if you were a Hollywood producer, looking for any sign of resistance by the target," said lawyer Kenneth M. Robinson, who tried the Abscam case of then-Rep. John W. Jenrette (D-S.C.).

The Abscam investigations, the best-known examples of videotaped stings, are also the most encouraging for the prosecution. All seven members of Congress charged with accepting bribes were convicted. The appellate law emerging from their convictions, particularly those of Jenrette and Florida Republican Richard Kelly, established formidable tests for defendants wishing to assert that the government entrapped them or denied their right to due process.

"The word in the legal community," said Georgetown criminal law professor Paul Rothstein, "is that with a good videotape . . . the chances of conviction are enormous."

There are notable exceptions. Barry's lawyers, R. Kenneth Mundy and Robert W. Mance, will be looking for inspiration from the cases in which defendants beat the odds:

Donald E. Robinson Jr., then an assistant U.S. attorney here, was twice videotaped in 1976 accepting bribes in exchange for promises to use his influence to help defendants. Robinson said he'd taken the money in part because he feared the disclosure of a liaison with a local prostitute. He was acquitted.

John Z. DeLorean was videotaped in 1982 reaching for plastic bags of cocaine and saying they were "good as gold." The automaker convinced a jury that the drug deal was the government's idea. He was acquitted.

More than 20 alleged members of the Luchese crime family in New Jersey were recorded on videotape in apparently incriminating conversations. Defense lawyers exploited technical problems and inconsistencies in the evidence, and not one defendant was convicted of racketeering.

In Barry's case, three FBI cameras and several microphones awaited the mayor when he arrived at Hazel Diane "Rasheeda" Moore's room at the Vista on Jan. 18. The tapes, according to accounts made available to The Washington Post, depict the mayor drinking cognac, lounging and talking with Moore, and twice inhaling on a crack pipe after giving Moore $30 to buy cocaine.

Mundy could not be reached for comment, but he has told U.S. District Judge Thomas Penfield Jackson that he intends to offer challenges, or corrections, to an FBI-prepared transcript of the tape.

According to officials in the U.S. Attorney's Office, the FBI transcript omits the word "chicken" from Moore's statements in the moments before Barry smoked the crack. The Washington Post reported earlier this month that Moore urged Barry to smoke the drug, that she asked him if he was going to "do it," and, when Barry appeared to hesitate, asked whether Barry was "chicken." Stephens's office has declined to comment on the report.

Barry, in an interview Tuesday with The Washington Post, said the tape "is more damaging to the government than to me" because it shows the lengths to which authorities went to lure him there.

Sources said the tape also is murky and difficult to see clearly in places.

The tape offers documentary evidence of only one of 14 charges against the mayor. But prosecutors hope images of the mayor's drug use will bolster witnesses against Barry in nine other charges of possession, one charge of conspiracy to possess, and three charges of lying about cocaine.

Even if a jury never sees them, the tapes already have served a vital purpose for prosecutors. Sources close to the investigation say public disclosure of the tapes dealt a critical blow to the solidarity of potential witnesses. Suddenly, the sources said, the mayor seemed mortal, and witnesses -- however reluctantly -- began to cooperate.

But videotape alone is not enough to convict Barry even on one count.

"You can't tell the jury that what they're seeing, they're not seeing," said David Povich, who won the Robinson case in 1976. "You have to say, 'Yes, what you're seeing is what the government did to my client, not what he did to himself.' "

Henry W. Asbill, who helped win the New Jersey racketeering case, said, "If you're stuck with having to admit the act, but you're challenging the mens rea {criminal intent}, the videotape may work immensely to Barry's advantage . . . . Something will have happened that even if it's not fishy can be made to appear fishy to a jury."

In some high-stakes cases, the defense has hired experts to analyze the tapes. The gambit sometimes pays off with revelations of discontinuity or recording anomalies, said Anthony J. Pellicano, a Beverly Hills forensic analyst, but the lawyers' fondest hopes go unrealized.

"I've had all kinds of allegations made toward videotapes," Pellicano said, "but there's never been an FBI case involving videotape where I have found that the government has altered the tape. Ever."

More common, said Roger W. Shuy, a Georgetown linguist, are courtroom battles over the meaning of ordinary exchanges.

Does "uh-huh" signal agreement, or is it merely a neutral "feedback marker" in conversation? Is a finger across the nose an innocent gesture or code for a criminal act?

"That's where the battle will probably be fought," Shuy said. "The government will say here's what it means, and the defense will say here's what it means . . . . What I find regularly is ambiguity."

Staff writer Michael York contributed to this report.