Under the circumstance of alternate current (AC) electric field or around AC electrical equipment, multimedia recording equipment can capture the frequency fluctuation of electric networks simultaneously as it records video, image or audio programs. Electric network frequency (ENF) has the characteristics of real-time, continuity and consistency with surrounding electric networks, thus, the ENF information estimated from these multimedia files can be used for time stamping, information source detection, operation forensics, and media synchronization. In spite of different signal formats of ENF in audio, image and video programs, the detection and estimation methods of ENF are similar, and some processing methods are universal. Up to now, plenty of research works have been done on ENF in audio, but the works on ENF in image and video are far more behind. This paper presents a comprehensive review on the relevant researches, including characteristics, estimation and applications of ENF in images and videos. Firstly, the characteristics of ENF signal are explored by analyzing the processes of capturing images and videos. Then, the problems existing in the detection and estimation of ENF signal and corresponding solving approaches are summarized and analyzed. Finally, the applications of ENF in information forensics and the possible research directions in the future are discussed.
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