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Police scientists have hailed a new technique that recently played a pivotal role in securing a murder conviction as the most significant development in audio forensics since Watergate.

The capability, called "electrical network frequency analysis" (ENF), is now attracting interest from the FBI and is considered the exciting new frontier in digital forensics, with power lines acting as silent witnesses to crime.

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 recorders, telephone recorders and camcorders that are plugged in to or located near the mains pick up these deviations in the power supply, which are caused by peaks and 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 digital forensics lab in Penge, south London, scientists have created a database that has recorded these deviations once every one and 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 or public, the scientists are able to detect the variations in mains electricity occuring at the time the recording was made. This signature is extracted and automatically matched against their ENF database, which indicates when it was made.

The technique can also uncover covert editing - or rule it out, as in the recent murder trial - because a spliced recording will register more than one ENF match.

The Met emphasised that ENF analysis is in its infancy as a practical tool, having been used in only around five cases to date. Proponents are optimistic about its uses in counter-terrorism investigations, for example to establish when suspects made reconnaissance videos of their targets, or to uncover editing in propaganda videos.

Dr Alan Cooper, the leader of the Met's ENF project, said the technique is proving invaluable in serious cases, where audio and video evidence and 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, and developed algorithms to automate the analysis.

"ENF has basically been made possible by the move to digital recording," Dr Cooper said.

"Old magnetic cassette and 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 and a half minute gap in President Nixon's Watergate Tapes, the magnetic recordings he secretly made of his White House conversations.

The probe gave rise to a range of new techniques that showed that in fact as many as nine separate sections of a vital tape had been erased. Their report went on to form the basis of audio forensics for decades.

In contrast to the months of painstaking 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. ®

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