Frequency analysis of electrical network noise in forensics

Imagine at the moment the following situation: the police or intelligence appeared to record the event of interest in high enough quality, which radically changes the course of the investigation. This could be a videotape of the discrediting conversation of a famous political figure with a certain face, which not only puts an end to his career, but also faces criminal prosecution. Or the person similar to the known terrorist, acts with a televised address. Is it possible to find out the date, time and place of filming? Is it possible to indicate whether the record is authentic or not?

As a result of research of a number of forensic experts, today this is quite possible with some preliminary training. We are talking about the examination of the noise surrounding all of us electric current.

Any user of sound-reproducing equipment knows that unpleasant metallic buzzing that appears if you unscrew the volume control of the audio system to the maximum. The nature of this noise is explained by the operation of electrical networks, the main frequency of which in our country and in most countries of the world is approximately 50 Hz.
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Any digital recording made near transmission lines (power lines, wires and even sockets) will leave a trace of this noise in the audio recording. However, its frequency is not always exactly 50 Hertz, over time the frequency slightly deviates by values ​​of the order of a few thousandths of a Hertz.

This is because the mode of operation of the power grid is determined primarily by the consumer, and it is possible to predict the actions of millions of people only with some accuracy. The frequency and voltage in the network are determined not only by large consumers in the form of metallurgical enterprises, but also by entire cities of people who can smoothly change their behavior. The operators of the energy network are well aware of the daily patterns of consumption, which allows them to quickly compensate for these discrepancies, but sometimes very noticeable traces remain.

A decade ago, the Romanian audio specialist, Dr. Katalin Grigoras, who now runs the National Center for Information Forensic Examination at the University of Colorado in Denver, discovered that the imprint of electrical noise was as unique as a person’s fingerprint. The noise inevitably remaining on any audio recordings is a headache for sound engineers, but for forensic experts it has become a real boon: all devices of the same power system will record the same noise. Do not save and work from internal batteries: the microphone will be sensitive enough to catch the pickup.

If you compare the unique imprint of the noise frequency on an audio recording with a database of previously made continuous recordings of changes in the frequency of the unified power system, you can indicate the time of the original audio recording. You can also specify whether the audio was glued together from the pieces, or whether the video clip was not changed.

Noise frequency frequency can be determined even with a very weak recording, inaccessible to human perception. It was demonstrated that even fluorescent and incandescent light leaves on video recording at a frequency of 25-30 frames per second the necessary pulsations of the radiation power corresponding to a voltage change. Thus, by comparing network noise typos from a video stream and noise from an audio recording, you can establish the authenticity of the recording.

The technology was named the most notable breakthrough in forensic science since the time of the Watergate scandal, but, of course, the method has its limitations. In particular, the record must have a certain length - about 10 minutes, otherwise the accuracy of the method is small .

In the forensic laboratory of Dr. Alan Cooper, technology has already emerged from experimental status. The UK has a unified energy system, which makes it possible to conduct an examination in its various regions based on data from the behavior of the system from one point, and for seven years experts have been continuously recording. The method has already proved its effectiveness in practice in dealing with gun dealers, lawyers who said that the police fabricated evidence. The quality of the noise on the recording and their expertise allowed us to state the opposite.

With the development of the method of examination of electrical noise, it remains to expect the appearance of people selling criminal groups around the clock audio recordings of pickups of electrical networks.