Faking Forensics: Mains Audio Hum

That audio recordings can be authenticated by looking at the frequency of AC hum has been making the rounds. So… how do you fake it?

Enter a wonderful comment over at Schneier’s blog, outlining a few ways to do it… ending with the note, “Obviously you would have to plan ahead, but for entirely different reasons I have been recording the UK mains frequency delta for several years.”


“For those who want to experiment there are various ways of generating an AC signal from another AC signal where the frequencies are not the same.

First of all there is the good old fashioned rotary generator. An AC motor connected to the input supply drives a shaft connected to a generator which is wound to produce a different frequency. You used to find small versions of these used for powering up avioniics equipment running at 400Hz by ground/maintainence personnel form either 50Hz (Europe) or 60Hz (US) mains supply.

However the variation in frequency of the AC supply would unless fllywheels etc were used be seen on the ouput generator.

A second way to do it is to convert AC to DC and back to AC again there are various ways to do this and is quite often done on in-line UPS systems to prevent phase jumps when switching from mains fed to battery fed which can cause a few nasy problems that SMPSUS realy don’t like.

To get the efficiency and purity required you can use a design I came up with quite a few years ago which is to use Walsh Sequencies via class D drivers onto a multiple primary transsformer, the output is a quit pure sinewave with next to no harmonic content untill after the 16th harmonic which can be efficiently removed with a low poer transformer.

A third way to do it is with one of a variety of Variiable Frequency Transformers.

The easiest of which to understand is the phase quadrature system. To all intents and purposes a frequency difference is the equivalent of a continuously changing phase difference.

The easiest way to efficiently produce a phase difference is to add to sine waves in phase quadriture (ie one sin one cos) depending on the amplitude ratio between the two the resulting addition has a phase difference somewhere between them.

In a three phase system you can have two types of transformer a parallel or a shunt. A parallel transform has the same output phase as the input (ie either 0 or 180 degrees) a shunt transformer produces a phase quadriture (90 or -90/270 degree) output. If you take the (variable tap) output of a shunt transformer and feed it into a parallel transform wired to add the wave forms then by changing the output taps you can change the output phase of the system.

Now if you design the output of the shunt transformer to be continously variable (like that of a Variac) and drive the taps with a variable speed motor you can continuously change the amplitude at the shunt transformer output, which will cause a continuous phase change at the ouput of the parallel transformer which equates to a different frequency.

As the transformers can be made 99.9% efficient or better then the overall system will be over 99.8% efficient in power conversion. Which is kind of handy when you are talking of systems working in the 25MW and up power range as 50KW or less of heat is still quite a bit toget rid of (about what you would use to heat a six and a half thousand square foot home or office in a NYC winter.

Now obviously you could use one of the above methods to power the lights and equipment in a home garrage recording studio, or house or appartment block if you so desired and taylor the frequency changes to match that of some point in the past you have recorded somewhere (you could easily do it with a gumstick computer and an SD card for storage taking a reading every half second or so (would only need 0.165Mbyte/day storage or around 17years/GByte).

Obviously you would have to plan ahead, but for entirely different reasons I have been recording the UK mains frequency delta for several years.”


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