Forensic Investigation of MP3 Audio Recordings

Special aspects of MP3-recordings technical investigation are addressed. The following features of formation and research of MP3 phonograms are explained: traces of MP3 coding in time and spectral domain, special aspects of MP3-files structure analysis, detection methods of re-coding of MP3-recordings, methods of group identification of MP3-recorders and MP3-codecs.

MP3 coding leaves certain traces of its usage. Due to the psychoacoustic model inaudible spectral components are deleted from the signal spectrum. Traces of psychoacoustic codecs usage are also clearly seen via dynamic spectrogram as rectangular areas of zero spectral amplitude. The methods discussed in this paper enable the investigating expert to detect the exact position of the MP3 frame in the signal by its properties even without any information from the file header. This method reveals the coding itself, multiple coding and also audio editing by the investigation of the periodicity of the extracted frames’ positions.

MP3 file format specifies the structure of the frame header providing a perfect instrument to detect any periodicity of any peculiarities of MP3 frames. The tool based on this approach reveals MP3 frames disorder caused by editing in the “digital” domain – manual deletion of audio information using HEX editor.

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