2 ms·

There wasn't a good explanation in the article. Anyone want to break it down for the layman? I get that the power company can record variances in load and time

There wasn't a good explanation in the article. Anyone want to break it down for the layman? I get that the power company can record variances in load and time stamp them, but how does that signature have anything related to a recording made on a battery powered device?

ars 16y ago
Not load. Frequency. Basically the entire US hums at 60 hz. (There are about 7 independent grids each humming separately.) The grid is kept very synchronized, so the 60 hz is the same at every house. But the 60 hz is not exact, it changes slightly up/down. But when it changes, it changes everywhere. So what they do is record the slight variation in the 60 hz hum, and keep it long term. (Like it's 59.9992 then 60.0001, etc, etc.) The effect of the 60 hz, is to have a slight wobble in the DC voltage made by a power supply. This wobble then translates into a background noise in recordings. Then compare the exact frequency of the hum to your long term recording and look for a match. Even on a battery operated device the hum could still be there, because the local power line makes a slight electric/magnetic field, which is picked up.
- Entlin 16y ago
  So, with a long enough record of the wobble at hand, could we tell the exact moment any recording ever made was made, much like Dendrochronology? And also locate the recording, due to the independent grids?
  - ars 16y ago
    Only if you have a record of the frequency changes. The article says that in London they have a record going back 5 years. Is there any such record in the US? I have no idea. I don't think there would be - till this idea came up there was no reason to make such a record.
- djb_hackernews 16y ago
  so basically anything ever recorded with or near something on the grid has detectable background noise? Looking at the math I don't see how they could make any sensible conclusions about a 4 minute sample of audio. That's only 160 data points and if the variations are in as small a range as you propose...
  - DrJokepu 16y ago
    Even if they can only differentiate two states (say, below and above 60 Hz) 2^160 is a pretty large number so I don't think that would be a problem, unless I'm missing something.
a-priori 16y ago
I too am a layman on this topic, but here goes: When you make a recording in most environments, because of power lines and equipment nearby there will be a mains hum in the recording at about 60Hz. This is true even if the recording device is battery-powered because it's in an environment with mains-powered devices. But the characteristics of this mains hum will change slightly over time depending on the load of the power grid (Edit: It appears this isn't correct; see ars' comment). So apparently what they're doing is recording the power usage over time in one place, and correlating it with changes in the hum in recordings. Since they're attached to the same power grid, the variations should be similar at the two locations. If the changes line up, then the recording was probably taken at that time. Similarly, if it lines up with time T=X for part of the recording, then to time T=X+10sec later in the recording, you know that about 10 seconds of audio were edited out. Again, this is my interpretation, and I welcome corrections.

ars 16y ago

Not load. Frequency. Basically the entire US hums at 60 hz. (There are about 7 independent grids each humming separately.) The grid is kept very synchronized, so the 60 hz is the same at every house. But the 60 hz is not exact, it changes slightly up/down. But when it changes, it changes everywhere. So what they do is record the slight variation in the 60 hz hum, and keep it long term. (Like it's 59.9992 then 60.0001, etc, etc.) The effect of the 60 hz, is to have a slight wobble in the DC voltage made by a power supply. This wobble then translates into a background noise in recordings. Then compare the exact frequency of the hum to your long term recording and look for a match. Even on a battery operated device the hum could still be there, because the local power line makes a slight electric/magnetic field, which is picked up.

Entlin 16y ago
So, with a long enough record of the wobble at hand, could we tell the exact moment any recording ever made was made, much like Dendrochronology? And also locate the recording, due to the independent grids?
- ars 16y ago
  Only if you have a record of the frequency changes. The article says that in London they have a record going back 5 years. Is there any such record in the US? I have no idea. I don't think there would be - till this idea came up there was no reason to make such a record.
djb_hackernews 16y ago
so basically anything ever recorded with or near something on the grid has detectable background noise? Looking at the math I don't see how they could make any sensible conclusions about a 4 minute sample of audio. That's only 160 data points and if the variations are in as small a range as you propose...
- DrJokepu 16y ago
  Even if they can only differentiate two states (say, below and above 60 Hz) 2^160 is a pretty large number so I don't think that would be a problem, unless I'm missing something.

a-priori 16y ago

I too am a layman on this topic, but here goes: When you make a recording in most environments, because of power lines and equipment nearby there will be a mains hum in the recording at about 60Hz. This is true even if the recording device is battery-powered because it's in an environment with mains-powered devices. But the characteristics of this mains hum will change slightly over time depending on the load of the power grid (Edit: It appears this isn't correct; see ars' comment). So apparently what they're doing is recording the power usage over time in one place, and correlating it with changes in the hum in recordings. Since they're attached to the same power grid, the variations should be similar at the two locations. If the changes line up, then the recording was probably taken at that time. Similarly, if it lines up with time T=X for part of the recording, then to time T=X+10sec later in the recording, you know that about 10 seconds of audio were edited out. Again, this is my interpretation, and I welcome corrections.