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Old 13th December 2012, 09:21 PM   #11
DF96 is offline DF96  England
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My guess is some sort of data logger. 1.5 secs is interesting, because how do you determine the frequency of 50Hz to the required accuracy in such a short time? They must use some form of rolling average or low pass filter. Possibly a frequency multiplier - maybe using digital techniques but then it might be subject to noise.

It is actually a non-trivial problem!
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Old 13th December 2012, 09:23 PM   #12
Pano is offline Pano  United States
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yeah, that's what I was thinkin' just how do you set about doing this right?
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Old 13th December 2012, 09:35 PM   #13
benb is offline benb  United States
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I recall a story about the US grid(s) being somewhat "deregulated" - the long-term stability (as used in many electric clocks, including electronic digital ones, to keep time) would go away, causing mains-based clocks to show the wrong time:

Power-grid experiment could confuse clocks - Technology & science - Innovation | NBC News

I don't know if they did that, or for how long, but I have no doubt there are millions of digital clocks still based on the 60 Hz power frequency for their timebase, and they're probably STILL being sold in Walmarts and other retailers around the US.
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One could also add a subtle time varying pitch / speed shift to swamp the known frequency variations, to wipe any embedded "timestamp".
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Well I mean, from the perspective of the criminal you might want an audio recording to be impervious to this technique altogether
It's a lot simpler than this - use an old-fashioned analog tape recorder, especially a cheap cassette recorder. Even the better studio analog recorders have playback variation of a substantial fraction of a second over a few minutes.

What makes this technology (as described in the story) possible (and what the reporter completelly missed, because he most likely doesn't have a clue about this - likely the police and engineers do, but forgot to say, or they intentionally did not say) isn't just the variation in the power line frequency, but also that over the last two decades or so digital recording has become ubiquitous and has replaced analog recording. It is usually crystal controlled, and so time and frequency reproduction are substantially more accurate than the power line itself.

Last edited by benb; 13th December 2012 at 09:46 PM.
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Old 13th December 2012, 09:42 PM   #14
DF96 is offline DF96  England
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Let's assume you need to know frequency to about 0.01Hz - it won't change much beyond about 0.5Hz except under fault conditions. Naively that means you need 100 secs. A frequency doubler gets that down to 50s. My guess is some sort of digital filtering to remove noise, combined with digital pattern matching to determine the 'instantaneous' frequency. Then low pass filter. Finally sample every 1.5s and write the result to a log file.

Whatever method is used would have to be used for the recording too, but there the hum will be buried under the signal. Fortunately it is a narrow-band signal. The mains frequency can't change suddenly because of the inertia of all the generators.

Finally, you have to do correlation analysis to determine the actual time - a bit like tree-ring matching.

benb:
Yes, the recording needs to be crystal locked. Can you imagine lawyers in court arguing about jitter on the measurement?

Last edited by DF96; 13th December 2012 at 09:44 PM. Reason: extend
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Old 14th December 2012, 12:51 AM   #15
Pano is offline Pano  United States
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It's a lot simpler than this - use an old-fashioned analog tape recorder, especially a cheap cassette recorder.
That was mentioned briefly, IIRC. That the ubiquitous digital recorders in cell phones and the like now make this possible.

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(and what the reporter completelly missed, because he most likely doesn't have a clue about this -
"She", actually. And it was alluded to. The story wasn't in depth enough to go into that sort of detail. Interesting too, was that they said it is still impossible to get a certain match from voice analysis. Human voice just change too much.
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