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:doesn't work in most cases (Score:2, Informative)
usually also 40
Good job the hum is either 50 or 60Hz then :)
Re:doesn't work in most cases (Score:4, Informative)
Most (technically) respectable recordings transfer sound in the range 20Hz ~ 20kHz. Most shoddy recordings are not made using devices that employ digital FFT to get rid of low frequencies entirely. And, filtered analogue signal can still be susceptible to acquiring the hum again. From the space around, from the power supply, from your hand holding the device.
By the way, detecting the utility frequency is a standard technique used in Poland (and, I thus guess, in many other coutries) for years. And it works. Forging the frequency in software is possible (Audacity is mostly enough), if you are a profi that has access to respective databases (or has logged the uf by himself).
Re:Thanks! (Score:1, Informative)
"suspected terrorist has been taped planning a deadly attack "
When? Where? This has never happened. There are statistically, NO terrorists. The FBI is so desperate to locate any, that they manipulate informants and patsies into hopeless and ineffectual plotting. These are "terrorists" who'd otherwise be confined to grumbling on the short-bus!
I think this whole topic is one generated by the anti-terrorism industry. Resources would be better spent on pool covers, if you want to protect against the unexpected loss of innocent lives.
Re:Don't see it myself (Score:5, Informative)
The noise floor is not that simple.
If you analyse the noise, you find it's broadly spread over a wide variety of frequencies.
If you analyse 10s or so slices of signal, over the range 47-53Hz, things get considerably easier.
10 seconds means that your effective number of signal levels is not 2^16 (65536)
It's sqrt(44100*10)*65535, about 25 bits.
Throwing out the out of band noise means you lose >99.5% or so of it.
As a practical measure.
Go to pretty much any CD you have, and do a FFT in the range 47-53 or 57-63hz.
In the vast majority of cases, you will find a signal.
And it's not quite a timestamp, unless the recording is quite long.
http://www.nationalgrid.com/uk/Electricity/Data/Realtime/Frequency/Freq60.htm [nationalgrid.com] - for example - this is the UKs last hours frequency.
The graph is clearly enough to show that even if you can 'only' resolve in 10 second intervals the frequency, it's quite plausible to say if a 10 minute video can be one specific timeframe, or not.