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Old 12th December 2012, 03:04 PM   #1
DF96 is offline DF96  England
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Default Forensic Hum

See BBC report on mains hum as a forensic tool for authenticating sound recordings.
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Old 12th December 2012, 03:11 PM   #2
bigwill is offline bigwill  United Kingdom
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That's very clever! Even a high-pass filtered signal could work due to the harmonics.
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Old 12th December 2012, 03:45 PM   #3
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Now we know they are doing it, it might be possible to add some hum from a hum recording made at the time the main recording is supposed to have been made. To do this the 'native' hum would have to be very low so it was well hidden by the added hum. One of the snags with any forensic method is that Mr. Plod and his lawyers often end up believing that it is 100% reliable, and this makes them lazy and careless about doing proper police work.
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Old 12th December 2012, 05:03 PM   #4
Pano is offline Pano  United States
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Very clever! I wonder if there is a database for the US power grids?

I'll try to catch the Radio 4 program this evening.
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Old 12th December 2012, 05:06 PM   #5
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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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Old 12th December 2012, 05:08 PM   #6
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But wouldn't that show up as tampering?
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Old 12th December 2012, 05:41 PM   #7
bigwill is offline bigwill  United Kingdom
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But wouldn't that show up as tampering?
Well I mean, from the perspective of the criminal you might want an audio recording to be impervious to this technique altogether
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Old 12th December 2012, 09:26 PM   #8
Pano is offline Pano  United States
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Arrggggghh! Listening to the BBC 4 program about forensic voice work. Now all I get is:
"Due to rights restrictions this part of the program is unassailable. Due to rights restrictions this part of the program is unassailable. Due to rights restrictions this part of the program is unassailable. Due to rights restrictions this part of the program is unassailable."

WTF? Been listening all afternoon. Oh, there it's back again.
Anyway, I guessed the voice in the lineup. #4.

Show came back just in time for the mains hum story. Good stuff!
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Old 12th December 2012, 09:37 PM   #9
DF96 is offline DF96  England
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I thought it was just things like sports or music which are limited to certain markets. Some Radio 4 programmes get repeated on World Service, although the BBC is generally losing interest in that because the government has stopped funding it.
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Old 13th December 2012, 09:12 PM   #10
Pano is offline Pano  United States
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It was strange, as it was just one story in the whole block.
I wonder who they are recording the mains frequency. The radio story mentioned once every 1.5 seconds. Would be interesting to know how they do it, and what format it's stored in.

Also fun to listen to the mix and how and where the voices were placed. The Beeb does good work.
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