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Comment by antihero | Hacker News Reader

So if one were to set up a continuous recording of mains electricity, and then provide a UI that lets you grab a slice of the hum (a time range), and sell that, you could make some rather evil money. Or, if you were the police, have your "real" recordings verified very easily.

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I don't think it's quite as easy as mixing in some of the correct hum in to your recordings. The real trick is to eliminate the authentic hum (which reveals the recording to be altered) and then substituting the correct hum. Really you would need to have someone with decent mixing/audio engineering experience to get this right.

If it's just the 50Hz frequency, it's easy to make a cheap application with a GUI that computes the FFT of a wav file, replaces the value at 50Hz using some straightforward parameters (e.g home/suburban street/city center) and IFFT back to an output wav file.

I may be wrong but this sounds quite easy to fake...

That would still sound tampered because you'd lose any bleed over from other (ambient) sounds. Having all bleed over suddenly stop at 50Hz and then a clean electrical hum would look very suspicious.

You can solve this pretty easily. First, record the victim, and tamper with the recording as needed. Call this tampered recording "sample A". You need this recording to be somewhere that ambient noise will not vary much throughout the day.

Next, go to the same room (or a similar room) at a time when the victim will not have a good alibi and when the room will have the same basic ambient noise. Record using the same recording device in the same conditions (e.g. if it was in your pocket for the original recording, keep it in your pocket for the second recording). This is "sample B".

Take your recordings into Audacity. On sample A, apply the "Equalization" effect like this: http://i.imgur.com/gxPTV.png

On sample B, apply the opposite filter like this: http://i.imgur.com/6qeYQ.png

Now mix the two samples. As long as the ambient noise does not contain easily isolated components like other people's voices, you'll have a convincing forgery.

Now you might argue that there could be other forensics techniques to detect this kind of tampering, but I would argue that if such reliable alternative techniques were available, this mains analysis technique wouldn't be particularly valuable in the first place.

I don't think that's taking into account the harmonic frequencies. The 50Hz buzz is not a sine wave. That means that even with that filter, both 50 Hz mains sounds should still be detectable in the recording. Unless I'm missing something?

Ok, so you look up the true signal for the actual recording time, subtract that from the recording, and then add the signal from the time you want it to look like. Voila: perfect bleed over.

I don't think that's taking into account the harmonic frequencies. The 50Hz buzz is not a sine wave.

And its frequency is not constant. You'd have to have some kind of adaptive comb filter to remove the original buzz with all its components. I've a feeling that would be tricky to do in a way that doesn't scream "photoshopped" (or whatever the equivalent is for audio).

It's not even that complicated and you can do it with existing software. You can take the file into something like Audacity, band stop the range surrounding 50 Hz (or 60Hz in the US), and then simply mix in the prerecorded mains sound like you would do any other mixing.

It's not necessary to mix in the frequency domain because mixing in the frequency domain is mathematically equivalent to mixing in the time domain (FFT(x + y) = FFT(x) + FFT(y)).

I think using a rolloff at 50 or eq-ing out 50hz would leave a huge valley in the eq histogram. It might make it be obvious the track was manipulated.

Perhaps you could use phase cancellation to eliminate the original 50Hz hum.

This might leave ghosting of the original signal and would probably highlight any fluctuations inherent to "the main" that the phase cancellation would miss. I think this would probably be one of the cleaner and easier ways to proceed though.

Of course, if you weren't actually near any mains electricity at the time you made the recording(s), this is not necessary.

Even more evil technique criminals could use: carry a device which emits its own fake, randomly fluctuating "mains" sound, thus tainting any legitimate recordings so that forensics analysis will incorrectly identify them as "fake".

Similar to having a bag with lots of different people's hair and skin cells to dust over a crime scene.

Worse though. A bag of hair and skin cells could throw police off your trail. A tainted recording might could make it as far as the courtroom, only to have the defense bring in an expert witness to discredit it.

If you're not worried about the time; just proving the recording is continuous simply play it back through speakers and record from a microphone near the mains supply. The current mains noise will be significant enough to drown out the noise already in the recording, and you'll have your continuity for very little effort & no money.

But you'd have to get rid of the existing hum, which is, as the article states, a challenge even for recording professionals.

Just thinking about it, couldn't you get an isolated recording of the hum, and then mix the inverse of it over the appropriate audio sections? That should leave you with a humless track.

- step 1: Set up a recorder in a relatively isolated environment, ensuring that the hum is being recorded.

- step 2: Record a 30 minute conversation with the target, ensuring that you have enough to splice together something incriminating.

- step 3: invert the hum recorded from step 1 and mix it into the track from step 2. This produces 30 minutes of humless audio with the target.

- step 4: edit the 30 minute recording to produce incriminating audio clip of about 30 seconds.

- step 5: overlay legitimate audio from 30 seconds of real hum onto faked recording.

It seems like it could work.

- step 6: Get caught for something much simpler that you overlooked, and ruin your life.

Please don't actually use this maliciously. I suppose it could be a decent defense in court if you could prove that it is possible to fake the hum.

If you've ever tried to do this you'll know that while it works in theory, getting it working in practise is much harder than you'd think. In the past I've tried to create acapellas using a song and its instrumental (both digital recordings taken from the same source) using this technique and even with the highest possible quality digital audio the results are mixed at best.

If you're doing that, then you're better off recording your audio with balanced XLR or such like and not have any hum to begin with.

I'm not sure that an XLR feed will eliminate the hum. Ideally you could record the victim somewhere isolated from the hum.

It wouldn't entirely eliminate the hum, but a balanced feed (note that XLR can be unbalanced as well) will reduce the EM interference and cross talk that adds to the background hum.

Another contributing factor is when power supplies to the equipment are different or "dirty". Often this can be resolved with something as simple as a multi-adapter with the earth pin disconnected (note, this shouldn't be attempted unless your hardware is already insulated. but then if you're trying to commit a crime, then an electric shock is likely preferable to life sentences in jail).

If the hum varies over time, I don't think you'll be able to perfectly invert it with a separate sample very easily. You'd probably have to simultaneously record it elsewhere (but close by) and you might still get artifacts--though those might end up missed in the overlaid track.

Probably doable, but a lot harder than your method.

> you might still get artifacts

Made me think, what about audio compression? Psychoacoustic algorithms might want to alter the signal, or cut off the some of the frequencies entirely. This could make the record neither valid nor invalid according to this method, since the hum would be too altered to be a valid information source.

That's a good point. Mashing to a lossy format could destroy this, depending on how lossy you get.

I infer from the article that it's not a perfectly consistent background noise across the entire grid, but rather that it's consistent enough to get a match as long as the recording is long enough. As such, even if you did have access to their recordings, or made recording of your own, you still wouldn't be able to just invert it like that.

Anyway, who uses a landline these days?

>which is, as the article states, a challenge even for recording professionals

Actually, it calls it an "annoyance", which is a good characterization of it. And that's for professionals trying to make very high quality recordings where they accurately capture the output of instruments within that frequency range while eliminating the mains interference in the same range.

If you're recording someone's voice on a portable recorder that's in your pocket or stashed somewhere in a room, you're not likely to capture much in terms of voice in that range in the first place. Even if you did, it would still be plausible that you didn't. So if you tampered with a recording and then replaced everything under 70 Hz with ambient noise recorded separately (including mains noise) in the same location, it would be pretty difficult to prove tampering.

A few minutes after posting that, I realized that a band-stop filter would work fine for spoken word. However, that might add a different set of tell-tales to the recording.

Not if there was no hum where you made the fake recording.

The police don't even have to go that far - the "expert witness" was on their staff.

There must be a host of legitimate uses for the same data. The obvious ones: 1) Verifiy or correct the date/timestamps on recordings. 2) Geo-locate based on hum signature. Could apply to recordings or "live" conversations (e.g. skype). 3) Synchronising recordings.

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