In the field of forensic authentication of digital audio recordings, the ENF (electric network frequency) Criterion is one of the possible tools and has shown promising results. An important task for forensic authentication is to determine whether the recordings are tampered or not. Previous work performs tampering detection by looking for the discontinuity in either the extracted ENF or phase angle from digital recordings. However, using only frequency or phase angle to detect tampering may not be sufficient. In this paper both frequency and phase angle with a corresponding reference database are used to do tampering detection of digital recordings, which result in more reliable detection. This paper briefly introduces the Frequency Monitoring Network (FNET) at UTK and its frequency and phase angle reference database. A Short-Time Fourier transform (STFT) is employed to estimate the ENF and phase angle embedded in audio files. A procedure of using the ENF criterion to detect tampering, ranging from signal preprocessing, ENF and phase angle estimation, frequency database matching to tampering detection, is proposed. Results show that utilizing frequency and phase angle jointly can improve the reliability of tampering detection in authentication of digital recordings.
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