NIST System "Sees" Crimes on Audiotape

Investigators made this 3D computerized rendering of smudges that show audio test tone patterns and a write-head stop event.

The National Institute of Standards and Technology (NIST) has developed 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. The new system, which permits faster screening and more accurate audiotape analysis than currently possible, recently was delivered to the Federal Bureau of Investigation 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 (DAT) recorders. The need for quick and accurate tape analysis is just as diverse: determining authenticity, comparing voices and identifying duplication.

A cassette player modified with an array of 64 customized magnetic sensors 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 also can use the new system to help determine if the tape is authentic or 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 Boulder, Colorado, laboratories.

The benefits of the NIST system are its speed in correlating sounds with magnetic marks on tape and the fact that it makes an image without damaging the tape. Currently, the most common technique involves listening to a tape and stopping it when a suspicious sound is heard. The cassette then must be removed from the player, the tape extracted from its housing and a solution of magnetically sensitive fluid applied to the tape surface. Finally, the image of the audio track containing the suspicious sound is viewed under a microscope to determine whether or not an actual tampering event occurred. This is time consuming and subject to errors caused by particle contamination when applied to digital tapes.

For the new system, NIST scientists fabricated a 64-sensor linear array read head and placed it next to the standard read head in a commercial audiotape deck. The customized sensor array can scan a 4-millimeter-wide tape. Each sensor in the array changes its electrical resistance in response to magnetic fields detected from the tape. Software converts the sensor resistance measurements to visual images, which have a resolution of about 400 dots per square inch (dpi).

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