Forensic linguistics: the contribution of humanities computing

Forensic linguistics: the contribution of humanities
computing

László
Hunyadi

University of Debrecen
hunyadi@llab2.arts.klte.hu

Enikő
Tóth

University of Debrecen
teniko@pmail.arts.klte.hu

Kálmán
Abari

University of Debrecen
abarik@pmail.arts.klte.hu

2002

University of Tübingen

Tübingen

ALLC/ACH 2002

editor

Harald
Fuchs

encoder

Sara
A.
Schmidt

Abstract:
The aim of the talk is to demonstrate how useful humanities computing can
prove to be in solving issues of a seemingly as distant field as forensic
science. We will present a case study of an actual forensic linguistic
assignment whose aim it was to determine if a digitalized recording of a
conversation had been tempered. The task, highly challenging due to its
novelty both in applied linguistics and forensic practice, was carried out
by investigating three independent aspects of the issue: those of
experimental phonetics, situational semantics and computation. The results
of the three approaches were synthesized to give a comprehensive basis for
an answer to the initial question.

Introduction:
To find a proof for or against the assumption that a given document has been
tempered with is one of the important tasks of forensic science. With the
introduction of various voice-recording techniques it became especially
important to decide whether or not such a recording can authentically
represent a certain event of reference. The case of magnetic tape recordings
is relatively simple, since any (either electronic or mechanical)
modification of the tape leaves a trace behind, characteristic even of the
kind of manipulation (cf. Gruber et al. 1995, Gruber et al. 1993, Poza
1979). However, with digital recordings gaining more and more popularity in
our days, on the one hand, and digital manipulation becoming technically
possible and easy, on the other, one might believe that the discovery of
such a manipulation is highly unlikely. The novelty of the issue in
scientific literature just adds to this challenge. In order to carry out the
task, we had two assumptions: a. due to human voice having a highly complex
structure, even digital tempering by a person might leave a significant
trace, and b. due to conversations also having their strict internal
structure, the removal of a segment of the given conversation might also be
noticeable.

Discussion:
The findings of the three approaches were as follows:

1. A detailed spectrographic analysis of digitalized voice recordings
found a characteristic pattern with a duration of 8-10 milliseconds at
the place where a segment had been digitally removed. Although this
pattern varied according to the actual immediate environment, its
characteristic features could be established (a symmetrical increase of
intensity as well as a 500 Hz increase of frequency). This
spectrographic pattern was only manifested in the exact location of
recordings with a digital cut. Applying the methodology to the given
task, we found that no spectrographic trace of this kind of manipulation
could be identified.
2. The aim of the situational-semantic study was to find out if an
eventual cutting of a portion of the text could have possibly left a
trace that can be identified as semantically significant. Our attention
was directed to the analysis of the appropriateness of pieces of
linguistic material with a referential function, including pronouns,
determiners and names (cf. Brown et al. 1983, Kamp 1981). The analysis
pointed at some places where the reference was not unambiguously
computable from the immediate linguistic environment, but since in
running conversation such immediate turns often happen, they could not
be decided on the basis of linguistic content alone. These locations
became thus subject to spectrographic analysis, and the latter concluded
that no tempering could be identified there.
3. In our work we included a separate computational task to find out
if segments of an eventual cut in the voice recording were still
recoverable from the hard disk originally used for digitizing. Since it
turned out that the hard disk had been completely reformatted, we could
not complete this task. However, we elaborated a methodology for similar
tasks and applied it in model situations. The applied statistical method
of zero crossing proved to yield significant interpretable results in
the differentiation of headerless segments of certain types of files.
This method showed a significant difference between .bmp and .txt files,
and .wav files also had a characteristic value for zero-crossing. Thus,
we suggest that this methodology may be useful for the possible
differentiation of at least a few types of files in future work.

Summary:
This case study showed us that humanities computing can have a significant
contribution to forensic science, especially in the form of a combination of
theoretical and applied linguistics as well as statistics and computing.
This assignment was a real challenge for us, and it resulted in the
elaboration of a new methodology both in experimental phonetics and
computation. Thus proving the inspiring force of humanities computing across
various fields of science.

References:

G.
Brown

G.
Yule

Discourse Analysis

CUP
1983

H.
Kamp

A theory of truth and semantic representation

J.
Groenendijk

T.
Janssen

M.
Stokhof

Formal Methods in the Study of Language

Amsterdam
Mathematical Centre
1981

J.
S.
Gruber

F.
Poza

Voicegram Identification Evidence

American Jurisprudence Trials

Lawyers Cooperative Publishing
54

1995

J.
S.
Gruber

F.
Poza

A.
J.
Pellicano

Audio Recordings: Evidence, Experts and
Technology

American Jurisprudence Trials

Lawyers Cooperative Publishing
48

1993

On the Theory and Practice of Voice Identification

F.
Poza

(Technical Consultant)

National Academy of Sciences
1979