Engineers Use Electrical Hum To Fight Crime 167
Hugh Pickens writes writes "A suspected terrorist has been taped planning a deadly attack and the police want to use this evidence in court, or someone has been captured on CCTV threatening an assault. Increasingly, recordings like these are playing a role in criminal investigations, but how can the police be sure that the audio evidence is genuine and has not been cleverly edited? Now Rebecca Morelle writes on BBC that a technique known as Electric Network Frequency (ENF) analysis is helping forensic scientists separate genuine, unedited recordings from those that have been tampered with and the technique has already been used in court. Any digital recording made near an electrical power source will pick up noise from the grid that will be embedded throughout the audio. This buzz is an annoyance for sound engineers trying to make the highest quality recordings, but for forensic experts, it has turned out to be an invaluable tool in the fight against crime. Due to unbalances in production and consumption of electrical energy, the ENF is known to fluctuate slightly over time rather than being stuck to its exact set point so if you look at the frequency over time, you can see minute fluctuations and the pattern of these random changes in frequency is unique over time providing a digital watermark on every recording. Forensic Scientist Philip Harrison has been logging the hum on the national grid in the UK for several years. 'Even if [the hum] is picked up at a very low level that you cannot hear, we can extract this information,' says Dr. Harrison. 'If we have we can extract [the hum] and compare it with the database, if it is a continuous recording, it will all match up nicely.'"
Re:Workaround (Score:5, Insightful)
Wait, it *is* that way.
In the future there'll be no way to determine reality, because we'll all just be disembodied heads in vats, networked into vaste reality generating machines.
Re:Interesting seeing the Skepticism (Score:4, Insightful)
One real-world use would be tying the frequency fluctuations to anchor a recording in time. Remember when Osama bin Laden would send out his tapes via courier, and the CIA would analyze them and say things like "we can tell he's still alive as of last week because he mentioned the Yemen bombing." If his tapes matched a certain frequency pattern, they might have been able to say "we know he recorded this on December 12th, because the hum matches the power grid of Pakistan's Islamabad power generating plant." That also would have proven he was in Pakistan, and might even be why the CIA insisted he was.
With continual monitoring of the world's electrical grid (that doesn't seem like an impossible task for the CIA to do), they can discover in which section of the grid such a signal originated. I was watching some of the University of Tennessee Knoxville's video footage on YouTube [youtube.com] generated by the FNET [wikipedia.org], which shows impacts to the grid rippling across the country over time. Disturbances are surprisingly visible, and the way they spread via propagation delays would act like an echolocator to pinpoint someone who sent the recording. Not only could they tie it to a specific plant, but they could tie it to a subsection of the grid with a known delay.
And they could potentially discover this location without having all the recording instruments in place in advance. If they knew the signatures of the Islamabad generator and the Lahore generator, and they knew the recordings contained signatures delayed by 2 msec from Islamabad and 5 msec from Lahore, they could start quietly plugging in line monitors around Gujrat to map out where such a grid section might be.
Of course, if bin Laden's lighting was from a private generator, which is not uncommon in that area of the world, then they wouldn't know such info. But if he was using commercial electrical power, that might have located him to within a region.