Having spent a decade in biometrics technology consulting, which is a similar "identify whether this thing is unique, and also you might use this in court" set of technologies... I wonder what the error rates are here.<p>Oddly enough, court testimony for e.g. fingerprint analysis hangs on the testimony of a human expert claiming 100% certainty, rather than the error characteristics of an automated algorithm. But we DO have recorded and proven False Match and False Non-Match rates from the manufacturers, independent companies, and NIST when it comes to algorithmic techniques. This seems similar to voice comparisons, where error rates are a function of how long the sample is.<p>I can see fairly easily showing that there are no clear discontinuities in the hum compared to what would be expected from random splicing (though as a defense attorney I would challenge that the very people who are presenting the clip are the ones introducing a spoofable signal; biometric error rates are against RANDOM and non-adversarial presentation, spoofing detection is an entirely different beast).<p>However, showing uniqueness of a hum sample that is n seconds long compared to the entire continuous history of background hum would be a more rigorous analysis. I wonder if the defense team requested that given that this was a new and unproven technique.
If this is interesting to you or your kids, here's a less detailed but pretty accessible video [1] explaining how mains hum forensics work, complete with British accent.<p>[1] <a href="https://www.youtube.com/watch?v=e0elNU0iOMY">https://www.youtube.com/watch?v=e0elNU0iOMY</a>
There was a IEEE Signal Processing Society student competition to do this in 2016.[1] I've been trying to find the results of the competition, but the relevant links are dead. The Internet Archive may be able to help.<p>It takes hundreds of seconds of data to see power line frequency changes on a major grid. All that synchronous rotating machinery attached to the grid has to physically change speed slightly, and there's huge inertia. Here's data for the UK national grid.[2] (This is supposed to be live, but is 2 days old.) It looks like the control systems are set up to take action at 0.1Hz error, because the frequency wanders around in that range, but as soon as it gets outside, there's a speedup or slowdown to get it back in bounds.<p>Claims made about short audio samples are probably bogus.<p>[1] <a href="https://sigport.org/sites/default/files/Information%20on%20the%20MAST%20ENF%20Power%20Signature%20Dataset_1.pdf" rel="nofollow">https://sigport.org/sites/default/files/Information%20on%20t...</a><p>[2] <a href="https://gridwatch.co.uk/frequency" rel="nofollow">https://gridwatch.co.uk/frequency</a>
I have occasionally wondered if a tape recording (audio, VHS, etc) would capture something of the earths magnetic field at the time and position of the information was committed to the media, and if this could be discerned.
While reading it I was wondering if it was possible to do it with videos too and the lights containing the mains "flickering", since videos has lower sampling rates than audio it must be harder. It looks like it's harder but possible and there is an improved method for it in this paper:<p><a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8538866/" rel="nofollow">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8538866/</a>