In audio forensics, the Electrical Network Frequency (ENF) has been shown to be a very useful and reliable reference signal. Since the Total Harmonic Distortion (THD) is another relevant quality parameter, we studied the variation of both ENF and THD in different locations of the electric grid in Portugal. To this purpose, we describe the design and test of a complete ENF and THD monitoring system which includes autonomous sensors that are able to extract these parameters, and send them via web to a server. An interactive web interface is also integrated which allows real-time monitoring and access to stored data. ENF and THD data have been collected on different periods and locations of the Portuguese electric grid. Besides confirming the known features of the ENF parameter, results have shown that THD data is strongly affected by local factors and therefore exhibits little geographical consistency. In a forensic perspective, while the ENF is tailored to spot events in time, the THD has the potential to spot events in space.
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