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 tampered with or 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 anywhere near an electrical power source will pick up the noise from electricity supplied by the national grid and it 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.""