Magnetic System Images Audiotape

Hank Hogan

Physicist David P. Pappas, project leader of magnetic recording measurements at the National Institute of Standards and Technology's Electromagnetics Div. in Boulder, Colo., thinks that there's more than one way to spot a problem on an audio recording. He has the proof that he's right, thanks to a real-time magnetic imaging system from NIST that enables forensic experts to "see" tampering on audiotape. Potential applications of the technology include the nondestructive testing of circuit boards and semiconductors as well as materials analysis.

Using magnetic sensor technology similar in design and fabrication to what is found in the read heads of hard-disk drives, the system, developed by Pappas and others at NIST, employs 64 small magnetic elements to scan a 4-mm audiotape. These measure the magnetic field at a low frequency. The results are converted into a visual representation of electrical resistance measurements of the tape.

"I envision this as essentially a magnetic-field camera," he said.

A real-time magnetic imaging system developed at the National Institute of Standards and Technology can "see" evidence of erasure and overdubbing on audiotapes. Here, the system images the data on a tape of a Michael Jackson tune. Courtesy of David P. Pappas.

A pristine audiotape displays predictable and consistent patterns. One that's been altered by erasing or overdubbing has characteristic smudges and other anomalies.

The imaging approach, which is being evaluated for use by the FBI, is faster and less susceptible to error than current tape verification methods, which include listening to a tape for suspicious sounds and viewing suspect regions under a microscope.

A version of the forensic analysis tape scanner with 256 elements should be ready in 2005. The magnetic imaging technology could also find use in materials analysis or in spotting defects in electronics.

"In these applications, the magnetic field generated by shorts or eddy currents can be mapped and inverted to understand the cause of failures and give early warnings," Pappas said.