Tuesday, October 16, 2007

Forensics: Power Line Signatures and other Audio Forensics in the news

A frequent contributor to this blog (a.a.) found this Wired article about audio forensics that came out of the annual AES  (Audio Engineering Society) convention in New York City (USA) last week.  Aural steganography (hiding data in audio files, such as MP3s), power line signature recognition, and digital editing/tampering all get mentioned.

Aside: a.a. and I have discussed the technique of power line signature recognition on several occasions.  The basic idea of this technique is that the the fundamental frequency of each local power grid varies over time. In other words, if you measured the frequency at your wall outlet, it would not read 50 Hz or 60 Hz, depending on what country you are in, exactly and constantly.  Instead, the frequency would constantly move around a little bit, perhaps as much as a few Hz up and down, all day and night, in a somewhat random fashion.  This power line "hum" leaks out too - into devices that are plugged into the power grid and even out through the air into nearby devices such as handheld audio recorders.  If one can then extract this frequency or its harmonics (i.e. multiples) from a recording and compare this "signature" to a database of the frequency variations of the same power grid, one could theoretically determine what time the recording was made.

Returning to the article, one has to wonder exactly how power line signature recognition could be used to determine the date of a recording made in a cave - do they typically wire up caves to the power grid?  I could believe that a cave would have a portable power generator, but how would the examiner get a record of its frequency variations?  Perhaps I am missing something here or the author misunderstood something from the presentation and/or interview.

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