The courts request the forensic audio laboratory to determine whether the digital audio files are original or modified, where the altered files have been modified, and which parts can be used as evidence. It is important to identify manipulation zones in audio files that contain speech as well as to determine the authenticity of the audio file. For this reason, when we examine the theory and rules of digitizing or digital recording, it has been found that inevitable manipulation traces must be found at the manipulation points, but these traces can be removed or reduced by certain software although some manipulations can be re-revealed. In this paper, a supporting method that uncovers the wide band oscillations signals known as the Gibbs effect caused by tape tampering but may have been obscured by software means is given, and it can distinguish such manipulations from other deceptive but benign transient signals. The procedure is applied on digital files in a computer. Lower frequencies are de-emphasized compared to higher frequencies in a special way by software so that manipulation points are easier to detect. Examples are presented. The aim is to reduce forensic analyst investigation time and fatigue. It has been used as a supporting method in our institution since 1998.
Key words: Forensic audio, digital editing, authentication, manipulated audio files, audio signal analysis
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