[time-nuts] Building a mains frequency monitor

[Mark Sims]

And that can be very interesting...  a while back I read some stories on how the NSA, police, etc could find out where an audio recording was made by correlating AC hum in the recording with logs that they had of the power grids.  Apparently logging AC mains is rather popular among the spooky / law enforcement types around the world (Britain appears to be rather fond of it).
I did an experiment where I logged the AC mains at a few places around town along with some audio recordings made in the same areas.  I used some simple correlation software and could easily determine where and when the recordings were made.
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> Phase noise? The line frequency shifts phase every time a major
electrical load is added or dropped from the power line.