Forensic Hum

[DF96]

See BBC report on mains hum as a forensic tool for authenticating sound recordings.

[bigwill]

That's very clever! Even a high-pass filtered signal could work due to the harmonics.

[DF96]

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.

[Pano]

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.

[bigwill]

One could also add a subtle time varying pitch / speed shift to swamp the known frequency variations, to wipe any embedded "timestamp".

[Pano]

But wouldn't that show up as tampering?

[bigwill]

Originally Posted by Pano

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

[Pano]

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!

[DF96]

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.

[Pano]

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.