Examination of Firearms Review: 2007 to 2010 - Interpol

differences between analog and digital media. To address the need to authenticate
digital audio, Grigoras (16) presented statistical tools to detect traces of
(re)compression data. The tool is used to assess compression generation and to
discriminate between different compression algorithms. In a technique specific to MP3
compression, Yang, Qu and Huang (17) examined the frame offsets to detect
manipulation. MP3 audio samples are divided into frames to encode with each frame
having its own offset after encoding. Manipulation can lead to broken frame grids.
Detection rates using this technique were reported to be 94%. In order to detect buttsliced
edits, Cooper (18) presented a simple signal processing method having
significant benefits over traditional approaches. This method is based on high pass
filtering the audio and modeling a discontinuity at higher frequencies; it is used to
complement other techniques since no single method can determine authenticity.
While not as frequent as with digital recordings, authentication of analog recordings is
still necessary on occasion. Visualization of magnetic features is the most effective
method for authentication of analog recording. Boss (19) described methods of
visualization including using crystals with polarizing microscopes, a MOSeS (Magneto-
Optical Sensor System) device, and a MOA-KOF (Magneto-Optical Analysis) device.
Images of visualized recordings including start and stop marks; an example is given of
how visualization can explain certain recording peculiarities (in this case likely due to a
tape fold). Another discussion of magnetic visualization methods was presented by
Broyles (20). The advantages and disadvantages of ferrofluids compared to magnetooptical
film imaging methods are discussed. Supporting test images are included.
Koenig, Lacey and Killion (21) described the components of a macrophotography
system to capture images of Bitter (ferrofluid development) patterns commonly used on
audio duplication, enhancement, authentication and voice comparison examination of
audio tapes.
In a technique valid to both analog and digital audio recordings, Malik and Farid (22)
described a technique to estimate the amount of reverberation in a recording.
Differences in estimated reverberation in a recording purported to be from a single
environment can be used as evidence of manipulation.
3. Speaker Identification (Matthew Graves)
This review will focus on speaker identification and the developments and research
conducted in this field between 2007 and 2010. While the overview provided here
contains many sources for information regarding speaker identification, members of the
Digital Evidence Branch at the United States Army Criminal Investigation Laboratory
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