Small frequency variations in the electrical power network, which are the same through the area serviced by the network, can be used to assess the integrity of audio and video evidence. Small amounts of hum usually leak into a recording, and this provides a unique time signature. By archiving power line frequencies over many years, the time of a recording can be determined by comparing it to the information in the archive database. Similarly, discontinuities in hum frequency or multiple frequency components provide additional forensic evidence.
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