Voice identification not a precise science

November 14, 2002|By Jeremy Manier, Tribune staff reporter.

Making sure that the voice on a newly released audiotape is really Osama bin Laden will take exhaustive scrutiny by computer voice identification programs and human analysts--and even then officials may never be certain, experts say.

Voice identification is far from an exact science, and some experts say they still trust people who are familiar with someone's voice more than a computer program. But the technology has advanced in the last few decades, fueled by computer software that can make a voice profile based on hundreds of audio samples on file.

Each person's voice makes a distinctive pattern of audio waves based on how the sound comes through the throat, nose and mouth, and on the shape and tension of the vocal cords. Although the pattern is not different enough to allow identification as reliable as a fingerprint, it does make imitating someone else's voice precisely almost impossible.

Experts said the sheer volume of recorded statements that bin Laden has released should make good raw material for voice identification software, giving technicians at the Department of Defense an edge in making out the voice on the new tape.

The analysis still may not show whether bin Laden is alive--for example, the task could be stymied if the recording consists of patched-together words taken from bin Laden's recordings. But the computer programs should be able to tell whether the voice is bin Laden's or just a skilled mimic's, said Bradley Horowitz, chief technology officer at Virage Inc., a California company that supplies audio and video analysis software to federal intelligence and law-enforcement agencies.

"This is the best-case scenario, where we're matching a long audio snippet against a well-established library of other utterances," Horowitz said. He said the reliability of such an analysis could be "in the high 90s."

Lawmakers and administration officials said Wednesday they are going on the assumption that bin Laden made the tape, though they said they can't be 100 percent certain.

Voice identification experts said the task is easiest in cases like this, when technicians have reason to believe the voice belongs to a specific person or someone in a known group of people. It's hardest when analysts have no idea who is speaking.

"You can't do something like walk into a gate at the Super Bowl and the program knows your name," said Alex "Sandy" Pentland, a professor of media arts at the Massachusetts Institute of Technology. "You need to know it's one of a small group of people before you can get really reliable."

Although voice mimics may do an uncannily good job by human standards, computer programs usually can rule out such trickery by looking at the voiceprint pattern.

"Even if Rich Little is doing an imitation of Nixon, he might get the phrasing and intonations right, but the actual spectrum of someone's voice is very hard to replicate," Horowitz said.

Someone determined to fake an audiotape could use digital techniques to stitch together words and sentences the person had uttered, experts said. But that would require sophisticated computer work that is tedious, time-consuming and expensive--comparable to the effort that goes into a computer-generated movie, Horowitz said.

Even if the tape were stitched together, human analysts have ways of telling whether speech is natural. They can look for distinctive intonation, phrasing and how often the speaker uses certain words to conclude whether the voice was processed or came from a natural conversation.

Arabic speakers trained in linguistics also can detect the regional variations in accent characteristic of bin Laden's speech.

"I'm sure the intelligence agencies have people who have spent large parts of their lives listening to everything bin Laden ever said," said Tito Poza, an expert in audio forensics and former researcher at the Stanford Research Institute. "To me that's more powerful than anything we can do technically."

New Jersey-based voice identification expert Tom Owen said he hopes to complete an independent analysis of the new tape Thursday, working with three Arabic-speaking colleagues. He said past tapes show that bin Laden tends to talk in a calm monotone, and so far he hears the same general pattern in the new recording.

"That's one in the `for' column," said Owen, president of Owl Investigations Inc.

The most accurate analysis comes from comparing the same words spoken on different tapes, experts said.

"In the best scenario, you'd get someone to say the whole Pledge of Allegiance both times," Horowitz said.