The hum that helps to fight crime
bbc.co.uk
bbc.co.uk
- 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.
Probably doable, but a lot harder than your method.
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.
Anyway, who uses a landline these days?
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).
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.
I may be wrong but this sounds quite easy to fake...
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.
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)).
It hit me a while back when I was writing the software for a test stand for a hearing aid as a summer intern. Mysterious 60/120/180 Hz frequencies appeared on our analysis, soon to be discovered as the motors for the building's ventilation unit. It was barely noticeable to anyone, but it was quite obvious to our test equipment, even in an insulated box.
> "A gang were accused of selling weapons..."
"A gang" implies a singular entity (gang), but "were" is a pluralized use of was, as if "gang" was plural (as in, 'several gangs'). (I lack the vocabulary to properly articulate myself, since grammar is not my strong suit. I am probably not describing terms completely accurately). I've noticed this more and more in regard to singular forms of entities (typically compromised of many singular parts, such as corporations). For example: "Apple were..." or "Google have..." or "Microsoft are..." I notice that this seems to be more of a British English phenomenon.
My question is this: Why are people using what I will call pluralized modifiers on what I would consider singular nouns? What I would consider the "more correct" forms of the above examples are: "The designers at Apple were..." or "Google's board of directors have..." or "Employees of Microsoft are..."
Is this just a cultural clash between American and British grammatical conventions, or is there an elusive (to me) practical reason why one version is "better" than the other?
I apologize for thread jacking. Hopefully the more relevant comments will rise above this one.
A number of words like army, company, crowd, fleet, government, majority, mess,
number, pack, and party may refer either to a single entity or the members of the set
composing it. If the latter meaning is intended, the word (though singular in form)
may be treated as if it were a plural, in that it may take a plural verb and be
replaced with a plural pronoun: the government are considering their
position (alternatively the government is considering its position). See synesis.
I can never remember which spelling is correct for the current situation for some terms: centre/center, tire/tyre, singular or plural collective nouns, etc. I know there are two variants, I just can't remember which one is the US one and which is the UK one; stuff like colour/color is easy, as is when to use sneakers/trainers (only made that mistake once) but some words play tricks on me.
English isn't my native language, but I feel British usage is consistent with the way the word "people" is treated on both sides of the pond.
(should be "costs" in us-en, of course)
The baseline is 50 Hz, but how big are the actual variations? It's the magnitude of the variations that you need to compare against clock drift, not the baseline.
In the case I gave before, I was trying to synchronize a 48kHz USB audio interface recording with a 48kHz/30fps DV tape recording. If I lined up the beginning of the recordings, the ends were off by ~500ms (IIRC), which for a 2min clip means a 0.4% deviation. However, if I adjusted the speed of one of the clips to align the ends, the middle would be off by 500ms, suggesting a fluctuating deviation as high as 0.8% (if my middle-of-the-night mental estimation is correct).
According to [0], the UK grid is allowed to vary between 49.5Hz and 50.5Hz, or ±1%. Watching the meters at [1], [2], and [3], it looks like deviations of 0.2% are common. Depending on the frequency of the mains and recording rate deviations, it seems mains deviation could be swamped by the 0.4% variation I observed in real-world recording scenarios. Thus, I am skeptical of the forensic utility of mains frequency analysis, and would need to see evidence that forensic analysts are compensating for recording rate deviation, or arguments why it's irrelevant, before I would change my mind.
[0] http://www.nationalgrid.com/uk/Electricity/Balancing/service...
[1] http://www.nationalgrid.com/uk/Electricity/Data/Realtime/Fre...
It's been most of a decade since I worked on this stuff (in the context of radar and sonar), so that knowledge may be out of date. Still, I'd be surprised if clock (LO) jitter is the limiting factor in this type of analysis.
It is likely that what is being measured includes the shifts in the 50hz frequency from minute to minute, which would be less masked by a recording that was slightly off frequency.
Unless it's a lot more reliable than the polygraph, it shouldn't be admissible in court, either.
(The polygraph, sometimes called a 'lie detector', does not create evidence admissible in court. It has too many false positives and false negatives.)
http://www.usnews.com/news/blogs/washington-whispers/2012/09...
http://news.bbcimg.co.uk/media/images/64639000/jpg/_64639977...
This is what very nearly every power outlet in the U.S. looks like, down to the color, vertical configuration, and flat-head screw in the center:
http://img.ehowcdn.com/article-new/ds-photo/getty/article/16...
I worked at a power grid a long time ago, and I recall that in days where the grid had undersupply issues due to climate and high demand, the frequency was kept stable at 49.5-49.6 Hz for weeks or so.
The clock-sync is a nice wishlist feature; but the frequency affects power consumption which often may be more important or financially valuable.
[1] is the original source translated from danish.
[0] http://webx.dk/oz2cpu/radios/50hz-kurve122h.gif [1] http://translate.google.com/translate?sl=da&tl=en&js...
You can't hear mains hum without a speaker (or transformer acting as a speaker).
It's really bad whenever I plug speakers into the wall. Not sure if it's the mains, or the radio tower that's 500 feet away, which also "allows" me to listen to NPR at conversational volume without use of a radio.
People who have tinnitus will hear the sound you describe or something similar all the time. When I was little I had the same experience as you, but now I have developed tinnitus and never get rid of that sound any more. Sometimes it also becomes rhythmical which very annoying. The only thing that helps is to ignore it.
I'm not sure a notch filter would be enough because, even though the mains is supposed to be a perfect sine wave, some harmonics occur (sometimes with a much higher frequency). Perhaps by filtering all harmonics as well? Sound quality isn't exactly a must in these situations.
It requires a technically astute criminal, though, and most aren't.
Exactly. Many don't even use gloves. I was wondering on how precisely you could locate the hum and if you could deliberately create a hum signature for a region.
Normally this frequency, known as the mains frequency,
is about 50Hz," explains Dr Alan Cooper
In the synchronous grid of Continental Europe, the deviation between network phase time and UTC (based on International Atomic Time) is calculated at 08:00 each day in a control center in Switzerland. The target frequency is then adjusted by up to ±0.01 Hz (±0.02%) from 50 Hz as needed, to ensure a long-term frequency average of exactly 50 Hz × 60 sec × 60 min × 24 hours = 4,320,000 cycles per day.[21] In North America, whenever the error exceeds 10 seconds for the east, 3 seconds for Texas, or 2 seconds for the west, a correction of ±0.02 Hz (0.033%) is applied. Time error corrections start and end either on the hour or on the half hour.
Taking the audio from the time you want to claim the recording took place and adding it shouldn't be too difficult either, though that might leave other clues that it's not genuine.
I strongly suspect that it can be compromised under both of the following conditions:
1. You take recording A, then you take recording B at a time and place which you want to assert that recording A took place at. You recover the thumbprint from recording B, suppress the thumbprint in recording A, then apply the thumbprint from B to A.
This is not a trivial process, but you only really need a plausibly consistent result. A reasonably basic understanding of signal processing theory, a copy of MATLAB, and many pots of coffee should do the job. Then, you could automate most of it for the next guy.
2. You take many recordings at a series of locations of interest, while taking data about the power grid from nearby locations and from distribution nodes. You then attempt to predict the signal at a location from the characteristics of the surrounding area.
This is almost certainly possible, as generalization from distribution logs to the local effect is what makes their fingerprinting technique possible in the first place. It is not a trivial undertaking, and it's questionable how well it would be generalizable. But at the same time, it's largely a question of if you want the data badly enough to do the legwork, and whether you have a reasonably functional understanding of machine learning.
As for point 1, there are a number of plausible angles to approach this, but I think that the forensic adversary has a huge advantage: Synchronous detection. The approximate time of the recording is known, as is the historical record of mains frequency. That allows for the possibility of huge processing gain, which might allow for recovery even after the amplitude of the mains hum is filtered to below the quanta of the audio system. I almost think you might have to Fourier Transform the whole audio record and zero out any component at f_mains +/- delta (and harmonics). That, of course, would look pretty suspicious to a forensic analyst.
Even that might not be enough if mains hum has a determinable effect on the data compression algorithm used to store the audio data.
Edit: The point I was trying to make in the first paragraph is that if you wanted to forge a recording, you'd need to have the grid frequency data. Having it for anywhere would be good enough and not having it for anywhere simply wouldn't.
1. Take the recording of interest 2. Use any audio processing software to apply a 400Hz-4kHz bandpass filter. If you don't have audio processing software or a computer, you can play back the recording over a phone. 3. Play back the recording via cheap computer speakers (no subwoofer) or over a telephone, and re-record
Low-end speakers don't have much output below 100Hz, so the new thumbprint from location (2) will replace the old thumbprint. The harmonics of the old will still be there, but bandpassing starting at 400Hz will cut out the strongest ones and make recovery of the original thumbprint very difficult.
No and no. It's not convolved but mixed, and the signal is the same everywhere (on one grid), it's specific to time, not place, it's easy to obtain as long as you have the foresight to record it.
So yeah, somebody should record these fluctuations and put them up on the torrents for criminals and dirty cops to abuse :)