Investigators take high resolution images to produce a 3D computerized rendering of smudges for more intensive analysis.

The result is a real-time magnetic imaging system that enables criminal investigators to “see” signs of tampering in audiotapes—erasing, overdubbing and other alterations—while listening to the tapes has been developed by researchers at the National Institute of Standards and Technology (NIST).

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The new system, which permits faster screening and more accurate audiotape analysis than currently possible, was recently delivered to the Federal Bureau of Investigation (FBI) and will be evaluated for its possible routine use in criminal investigations.

The FBI’s Forensic Audio Analysis Unit receives hundreds of audiotapes annually for analysis. Representing evidence from crimes such as terrorism, homicide and fraud, these tapes come from a wide variety of devices, including answering machines, cassette recorders and digital audiotape recorders.

At the heart of the NIST technology is a cassette player modified with an array of 64 customized magnetic sensors that detects and maps the microscopic magnetic fields on audiotapes as they are played. The array is connected to a desktop computer programmed to convert the magnetic data into a displayable image.

Authentic, original tapes produce images with non-interrupted, predictable patterns, while erase and record functions produce characteristic “smudges” in an image that correlate to “pops” and “thumps” in the audio signal.

An examiner can also use the new system to help determine the authenticity of a

tape or if that tape is a copy.

“We are the first to implement real-time magnetic imaging of audiotapes, and now, users can listen to the tape at the same time,” says project leader David Pappas of NIST’s <?xml:namespace prefix = st1 ns = “urn:schemas-microsoft-com:office:smarttags” />Boulder, Colo., laboratories.

A second-generation audiotape imaging system is under development, which is expected to provide ultrahigh image resolution of 1,600 dots per inch. That system will use 256 microscale sensors designed by NIST.