Even in the 21st century, old-fashioned audiotape can be critical evidence in criminal investigations. But determining a tape's authenticity has required painstaking manual work and a sharp pair of ears—until now. The National Institute of Standards and Technology has developed a real-time magnetic imaging system that lets investigators "see" signs of tampering while tapes are playing.
NIST modified a cassette player with 64 customized magnetic sensors that detect and map the microscopic magnetic fields on audiotapes as they're played. A desktop computer connected to the sensor array converts the magnetic data into displayable images.
Authentic, original tapes produce images with non-interrupted, predictable patterns. Erase and record functions, however, produce characteristic "smudges" that correlate to "pops" and "thumps" in the audio signal. Copies of recordings also lack telltale markings specific to different types of tape players.
These images have a resolution of about 400 dpi. NIST already is at work on a next-generation system that could provide 1600-dpi resolution, courtesy of 256 microscale sensors. The technology can be applied to tapes from all kinds of devices, such as answering machines, cassette recorders, and even digital audiotape (DAT) recorders.
For more information, go to www.nist.gov.