Helping the Forensic Research Institute of the French Gendarmerie to Identify a Suspect in the Presence of Voice Disguise or Voice Forgery

Abstract

In the field of forensic speaker recognition, the question of voice disguise presents a specific interest. Most criminals try to disguise their voice before making a malefic call or a terrorist threat. Their aim is to change the register of their voice quality in order to falsify their identity (voice disguise) or to mimic the voice of another person (voice forgery). This chapter proposes to analyse two different kinds of disguise: The first is the transformation of the voice by non-electronic and deliberate means; the second is the conversion of the voice by electronic and deliberate means. By considering both kinds of disguise (electronic and non-electronic) our analyses of voice transformation are based on an acoustic approach, which we use to measure specific changes in speech, and on an automatic approach to detect voice disguise. Four kinds of disguises which are considered the most common are studied: high pitched voice, low pitched voice, a hand over the mouth and pinched nostrils. A constraint of audibility and intelligibility has been imposed on the speakers who have recorded the database. The acoustic analysis of specific features reveals some differences according to the form of disguise, while in the automatic experiment we found the best way to detect a voice disguise is to use Support Vector Machines (SVM) technique. The level of performance is an AUC (area under curve) at 0.79. Voice conversion techniques are also proposed and applied in two forensic scenarios: first, the imitation of a politician from an Internet recording; and second, the application of voice disguise reversibility. Different kinds of tests are proposed to evaluate the relevance of the results, which are based on objective and subjective measurements. The best conversion is obtained from a GMM-ALISP voice conversion.