The
Department of Commerce’s 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 (FBI) 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 are just a few examples. At the
heart of the NIST technology is a cassette player modified
with
an array of 64 customized magnetic sensors
that 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.
Additionally, copies of tapes lack the original markings
specific to different types of tape players. Therefore, an
examiner can use the new system to help determine the authenticity
of a tape or if that tape is a copy.
|
Investigators
can then go back and take high resolution images to
produce a 3D computerized rendering of smudges for more
intensive analysis. This image shows audio test tone
patterns on the upper right, and a write-head stop event
on the left. Click
here to open a higher resolution version of this image.
|
“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, Colo., 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 1,600 dpi. That system will use 256 microscale sensors
designed by NIST.
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 approach
by recovering data from scraps of aircraft “black-box”
tape that were too short to be played in a conventional tape
deck.
For several
years, the FBI has been using a prototype of the NIST magnetic
imaging system that scans samples with a single sensor many
times to build up an image. Because the system is slow, it
has been used only for testing and special cases. For example,
the FBI used the technique on test tapes in a National Archives
and Records Administration study on the feasibility of recovering
audio from the famous 18-and-one-half minute gap in the Nixon
White House “Watergate” tapes. Although intelligible
speech could not be recovered from the test tapes, it was
possible to distinguish between two types of erased tapes:
one that had previously been recorded with audio and one that
had not.
NIST and the FBI funded development of the latest, real-time
audiotape imaging system.
As a
non-regulatory agency of the U.S. Department of Commerce’s
Technology Administration, NIST develops and promotes measurement,
standards and technology to enhance productivity, facilitate
trade and improve the quality of life.
Go
back to NIST News Page
|