Every year the FBI is inundated with audiocassettes that may contain subtle criminal evidence of tampering. Now physicists David Pappas of the National Institute of Standards and Technology and Stephen Voran of the Institute for Telecommunication Sciences have discovered a way to see the traces of erasing and rerecording.
In their analysis, Pappas and Voran slide a minute disk-drive head over the surface of an audiotape. The head is attached to a special microscope that picks up the blobs of magnetism left by a tape recorder's heads, which can indicate whether the tape has been erased, modified, or copied. This technique can recover incompletely erased signals on the tape and could help extract digital data from damaged black-box recorders. The FBI is trying to get the tape-scanning device certified for use in court.
Magnetic snapshot shows an erasure ridge on an audiotape.Image by David Stevenson/NIST