This paper presents a study on forensic analysis of lossy compression artefacts on Apple Voice Memos recordings. Advanced Audio Coding (AAC) is one of the most common digital audio formats that can be found in real forensic casework, such as in Apple and Samsung audio files. Different decoders can be used to convert an original AAC content into a WAV PCM file or audio CD track for further forensic analysis of the signal. The scientific framework and best practice guidelines for forensic authentication of digital audio recordings include lossy compression level analysis of the evidence signal. Reference recordings from the suspect recorder or mobile phone, or a database of reference recordings are necessary for a detailed forensic analysis, including the assessment of previous traces of lossy compression. This article presents the analysis of audio files collected from over 50 different Apple iOS versions, and the lossy compression artefacts left by different digital signal processing. It also proposes a framework and methods to collect original reference AAC files, configure a database for further forensic comparisons, assess the lossy compression traces, report the results as likelihood ratios and error rates, and provides guidelines for AAC lossy compression verifications or identification in real cases when the suspect mobile phone is not available.
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