Newsday

If the credibility of the latest Osama bin Laden tape is attacked by his supporters in the Arab world, experts say, advanced digital-imaging forensics could be used to authenticate the sound and graphics.

"Editing can appear seamless to the layman, but it can be detected by the pro," says former Secret Service Special Agent Steve Cain of Applied Forensics Technology in Lake Geneva, Wis., who expects the government already has done such verification and will probably release a report on it.

Indeed, in an era when actors like Humphrey Bogart can become "synthespians" -digitized into a pitchman for beer -separating virtual reality from reality is becoming increasingly important in the courtroom and elsewhere.

Most video cameras and players offer at least some analog editing functions, and sophisticated "nonlinear" digital editing, once the province of only the biggest Hollywood studios, can be performed on ordinary personal computers equipped with a $200 video editing card.

Such manipulation can be hard to detect when viewed casually on TV or at the movies, according to Steve Aftergood of the Federation of American Scientists. But there are many telltale signs that show up when a videotape or disk is examined directly.

Analog video editing is the easiest to detect. Every tape has a track that incorporates a timing code. If video frames are inserted or removed, this disrupts the code. Audio editing can be somewhat more difficult to detect, but experts know how to look for telltale changes in pitch and background noise.

Similar issues appear on digital recordings, which are increasingly common. In many cases, though, it takes instrumentation to notice anomalies. By using such exotica as waveform analyzers and vectorscopes, a technician can trace minute cyclical variations in the color signal or unique flaws in the drive motor of the camera. If these are out of sequence, it's a good bet the signal has been tampered with.

In theory, it's possible to manipulate a digital signal to fool the experts, Cain said. "But I've never seen it, and by and large, people who do these things are clumsy and leave lots of telltale signs."

Aftergood added, "There is a tremendous disincentive to modify these tapes because if such a modification were exposed, the loss of credibility would be enormous for the government. At the micro level, there are too many ways to detect if it's been changed."