May 26, 2003 

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Black Box Recovery
July 25, 2000

Investigators of airplane crashes and criminal cases often depend on audio recordings to solve those mysteries. But if the tapes are damaged or tampered with, they’re often out of luck.

Now, using the same technology found in home computers, scientists at two government laboratories have developed a new device which may be just the high-tech tool needed to recover evidence.

Seeing is believing

Researchers at the National Institute of Standards and Technology (NIST) and the National Telecommunications and Information Administration’s Institute for Telecommunication Sciences have come up with a special kind of microscope that may be able to help authenticate audio tapes and decipher information from them even if they have been damaged in an accident, or deliberately tampered with.

The scanning magnetoresistive microscope

The device uses the same tiny sensors that read hard-disk drives in home computers. These sensors can "see" the sounds hidden on damaged audio and video tapes and other magnetic media, such as floppy disks.

The technology was originally developed by physicists at the University of California, San Diego in 1996 as a tool for the hard-disk industry. When government researchers were asked by the FBI and the National Transportation Safety Board for a way to recover information from audio tape, they adapted the scanning magnetoresistive microscope for forensic use.

The instrument works by painstakingly passing a piece of magnetic material, such as tape, under a tiny sensor from a computer hard drive. As the sensor moves around on top of the magnetic pattern embedded in the material, it maps the magnetic field. This information can then be reconstructed in a computer to get an image, or interpret the data directly into sound.

The new device creates visual maps of magnetic tape which can then be recreated and heard.

If you prefer to view it with Real Player, click here.

clip courtesy NIST

The result is highly detailed. "The size of the tracks right now are actually on the order of one or two microns, that’s about one-hundredth the width of a human hair," says NIST scientist David Pappas. "So we’re able to get that kind of resolution and transfer that to make a very high-resolution image from a cassette tape."

According to Pappas, the main advantage of the technique is that it gives the actual size and strength of the magnetic field. The current technique, which uses magnetic fluids, gives no indication of how strong the field is and can also ruin digital tapes and disks. "In this technique we can actually go in and see how strong and what the direction is and when we do that we can actually decode information that’s on a tape," says Pappas.

Sound advice

But it may not be just what’s on the tape that can now be deciphered. The FBI is testing the technique for analyzing evidence such as erase marks. One of the most famous erasures of all time—the 18 1/2-minute gap from the Watergate tapes—may now come under scrutiny. Officials at the National Archives and Records Administration are considering using the device to examine the gap and hopefully fill it in.

But Pappas isn’t overly optimistic. "If a cassette recorder completely erases a signal we cannot recover that signal," he says. "It’ll be comparable to if you wrote something on a piece of paper and then erased it. If you don’t do a thorough job of erasing it, somebody may still be able to decipher what you’ve written."

Damaged black box
Damaged "black box" flight recordings can now be saved.

In the case where tapes are damaged, however, such as from flight recorders recovered from plane crashes, the device could be quite useful. "This technology being kind of myopic—in other words it’s somewhat nearsighted—it’s actually very nice for small samples," explains Pappas. "So in case a tape was damaged and you only had small scraps of tape that you wouldn’t be able to play in a recorder, you’d be able to get whatever magnetic information is left there."

But tapes aren’t the only magnetic sources of information. As Pappas points out, the list of magnetic media is vast. For example, this type of data is on boarding passes for airplanes, answering machine tapes, cassette recorders, digital audio tapes, VHS tapes, and perhaps most ubiquitous of all—credit cards. With this technology, even cutting up credit cards into small pieces won’t destroy their information.

Right now, though, the technology is too time-consuming and costly to be used on a large scale or for evil purposes, according to Pappas. But he also says it makes good sense to cut up your credit cards into several pieces and dispose of them in separate trash cans.

Elsewhere on the web:

Magnifier May Crack Crimes, Crashes - Science News Online

American College of Forensic Examiners

Audio Engineering Society

Tape Enhancement from Applied Forensic Technologies Intl., Inc.

Recent major aviation accident investigations



produced by Jill Max


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