Scientists may be asked to investigate recordings for a number of reasons. For example, authorities might want to know if a tape has been copied or taped over a number of times. The heart of the new detection system is an array of 64 customized magnetic sensors, or read heads, which the NIST team placed in a regular audiotape deck. As a tape is played, these sensors detect changes in the magnetic data, which is then converted to an image by software on a connected computer. Playing a new tape that has been used just once results in a predictable, uninterrupted pattern. A tape that has been erased or had portions rerecorded, however, displays characteristic marks that let investigators know it has been tampered with. We are the first to implement real-time magnetic imaging of audiotapes, NIST project leader David Pappas says, and now, users can listen to the tape at the same time [that it's being analyzed].
The software can also produce a 3-D image of any smudges it finds. In the picture above, for example, the right side shows an uninterrupted audio test pattern, whereas the left side depicts where a recording was stopped. Although stringent testing of the new technique is necessary before it will be validated for use as a forensics tool to analyze evidence at the FBI, the approach has the benefit of being noninvasive, unlike methods that involve applying magnetically sensitive fluids onto the tape, Pappas points out. The team is also working on a second system with 256 sensors, which will produce images with a 400 percent increase in resolution.