Police scientists have hailed a new technique,the capability, called "electrical network frequency analysis" (ENF) is now attracting interest from the FBI & is thought to be the exciting new frontier in digital forensics, with power lines acting as witnesses to crime.In a high profile murder trial, which was earlier this year. ENF meant prosecutors were able to show that a seized voice recording that had became vital to their case was indeed authentic. Defence lawyers suggested it could have been concocted by a witness to incriminate the accused.
The police force that ran the investigation this week declined to name the murderer in response to requests from The Register, citing undisclosed operational reasons.
ENF relies on frequency variations in the electricity supplied by the National Grid. Digital devices such as CCTV , telephone recorders & camcorders that are plugged in to or located near the mains pick up these deviations in the power supply, which are caused by peaks & troughs in demand. Battery-powered devices are not immune to to ENF analysis, as grid frequency variations can be induced in their recordings from a distance.
At the Metropolitan Police's forensics lab in Penge, south London, scientists have created a database that has recorded these deviations once every one & a half seconds for the last five years. Over a short period they form a unique signature of the electrical frequency at that time, which research has shown is the same in London as it is in Glasgow.On receipt of recordings made by the police , the scientists are then able to detect the variations in mains electricity occuring at the time the recording was made. This signature is extracted & automatically matched against their ENF database, which indicates when it was made.
Dr Alan Cooper, the leader of the Met's project, said the technique is proving invaluable in serious cases, where audio or video evidence & its authenticity is often questioned.
ENF analysis is founded on research originally carried out by Dr Catalin Grigoras, a Romanian audio forensics expert. The British team showed Dr Grigoras' findings in eastern Europe were applicable to the UK National Grid & had developed algorithms to automate the analysis.
"ENF has basically been made possible by the move to digital recording," Dr Cooper said.
"Old magnetic cassette or VHS tapes didn't keep time accurately enough to extract reliable data, but now we can analyse even cheap voice recorders.
"The Americans are very interested. It's fair to say this is the most significant development in the field since techniques were developed to analyse the Watergate Tapes."
The field of audio forensics was largely established as a result of the Watergate scandal. In 1973 a federal court commissioned a panel of audio engineers to investigate the infamous 18 minutes gap in President Nixon's Watergate Tapes, the magnetic recordings he secretly made of his White House conversations.
In contrast to the months of work on the Watergate Tapes, the computer power now cheaply available means the Met's ENF lab could authenticate a month-long digital audio or video recording in 10 to 12 minutes
