[HN Gopher] How to date a recording using background electrical ...
___________________________________________________________________
How to date a recording using background electrical noise
Author : mvac
Score : 68 points
Date : 2023-02-24 15:42 UTC (7 hours ago)
(HTM) web link (robertheaton.com)
(TXT) w3m dump (robertheaton.com)
| nickdothutton wrote:
| 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.
| cynwoody wrote:
| It's not useful for mag tape or wire recordings, but
| archaeologists have available a technique that can date
| artifacts like campfires, pottery kilns, and burned out adobe
| houses within the last 10,000 years.
|
| It's called archaeomagnetic dating[0]. It turns out that heated
| ferromagnetic materials, such as magnetite, capture the
| magnitude and direction of the earth's magnetic field as they
| cool down through the Curie temperature[1]. That allows an
| investigator to ascertain the direction of magnetic north the
| last time a likely sample was heated above the Curie point.
| Over time, magnetic north changes with respect to true north.
| [0]https://en.wikipedia.org/wiki/Paleomagnetism
| [1]https://en.wikipedia.org/wiki/Curie_temperature
| hyperific wrote:
| Also described in http://hummingbirdclock.info/
| blt wrote:
| Could we use a phase-locked loop instead of a sliding window FFT?
| I guess only if you're sure the recording contains no splices?
| CamperBob2 wrote:
| Yes, indirectly, by using the PLL as a demodulator to extract
| the sidebands from the 50/60 Hz "carrier" frequency. These
| types of problems usually boil down to reducing the signal
| bandwidth as far as you can in order to get rid of as much
| noise as possible, and the loop filter in a PLL can be good for
| that.
|
| Some terms to Google for more information on that would be
| "synchronous detection" and "lock-in amplifier."
|
| The method in the article talks about Fourier transform
| techniques, but in reality, this is a correlation problem that
| doesn't have to be handled in the frequency domain at all.
| Essentially you'd do a dot product of the contents of a sliding
| window from the recording against the utility's own recording
| of the AC power waveform. When the peak value is reached, the
| window offset corresponds to the best estimate of the signal's
| position with respect to the timeframe of the recording. This
| benefits tremendously from bandpass filtering, in terms of
| saving computation time, but doesn't strictly require it.
|
| In real life, you'd use the STFT or something like it as the
| author describes, but you'd use it as a convolution filter, not
| to locate the frequency peak. That's kind of a red herring in
| an otherwise-excellent article.
| subpar wrote:
| 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.
|
| [1] https://www.youtube.com/watch?v=e0elNU0iOMY
| ShakataGaNai wrote:
| As soon as I saw this topic I knew someone would bring the Tom
| Scott. Excellent work!
| czbond wrote:
| Thank you - I was very interested!
| kzrdude wrote:
| Spain to Turkey seems like quite a distance in a synchronous
| grid. Is it delayed and warped if we compare the hums in two
| different locations?
| Rebelgecko wrote:
| It doesn't answer your question at all but you might find this
| interesting: https://hackaday.com/2018/03/09/europe-loses-six-
| minutes-due...
| LazyMans wrote:
| I think technically speaking there could be a delay between two
| points. However, the "hum" being fingerprinted is the precise
| frequency which is tightly coupled through electromechanical
| action of all the generators on the grid spinning collectively
| at the same speed.
|
| Im sure the waveform wouldn't be perfectly matched from one
| generation area with another, but when you look at a longer
| period of time, say a few seconds, you wouldn't be able to
| refute the waveform match.
| drc500free wrote:
| 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.
|
| 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.
|
| 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).
|
| 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.
___________________________________________________________________
(page generated 2023-02-24 23:02 UTC)