Magnetic Imaging + Audio = Truth About Tapes

The question of whether an audiotape has been tampered with in some way—erased, overdubbed, or otherwise altered—has for years vexed forensic investigators as well as archivists. Remember the 18.5 min. gap in a certain tape made in the White House some years ago? NIST researcher David Pappas and colleagues have developed a real-time magnetic imaging system that clearly reveals evidence of monkeying while the tapes are being listened to. It works with a variety of recording devices including answering machines, cassette recorders, and digital audiotape recorders. The technology uses a cassette player modified with an array of 256 customized magnetic sensors that can detect and map the magnetic fields on 4mm-wide audiotapes as they are played. When the customized read head is placed next to the standard read head in a commercial audiotape deck, each sensor in the array changes its electrical resistance in response to the magnetic fields it detects from the tape.

The array is connected to a desktop computer with software that converts the sensor resistance measurements to visual images with a resolution of ~1600 dpi. Authentic, original tapes produce images with noninterrupted, predictable patterns; erase and record functions produce characteristic "smudges" that correlate to pops and thumps in the audio signal. In addition, duplicate tapes lack the original markings specific to different types of tape players, allowing an examiner to help determine whether the tape is the original or a copy.

Novel features of Pappas's system are its speed in correlating sounds with magnetic marks on tape and the fact that it makes an image without damaging the tape. The conventional investigational approach is to stop a tape when a suspicious sound is heard; remove the cassette from the player; extract the tape from its housing; apply a solution of magnetically sensitive fluid to the tape surface; and examine the questionable audio track under a microscope to detect signs of tampering. The process is both time consuming and subject to errors caused by particle contamination when applied to digital tapes.

The basic magnetic imaging technology was developed in the late 1990s in a NIST collaboration with the Commerce Department's National Telecommunications and Information Administration to retrieve data from damaged or altered magnetic tapes and computer disks. The researchers first demonstrated the technique by recovering data from aircraft black-box tapes too short to be played in a conventional tape deck. As for the Watergate tapes, no intelligible speech could be recovered but the device was nonetheless able to distinguish between two types of erased tapes: one that had previously been recorded with audio and one that had not. The research was supported by NIST and the FBI.

 Short Takes
Short Takes

Contact David P. Pappas, National Institute of Standards and Technology, Boulder, CO; 303-497-3374, ,