Dating Recordings by Power Line Fluctuations
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.
In the “high profile” murder trial, which took place earlier this year, ENF meant prosecutors were able to show that a seized voice recording that became vital to their case was authentic. Defence lawyers suggested it could have been concocted by a witness to incriminate the accused.
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 occurring 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.