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.
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.
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".
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.
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.
"This buzz is an annoyance for sound engineers trying to make the highest quality recordings."
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.
It may be that the reason was not the sound of the motors, but the effect of the motors on your electricity supply for the audio gear.
Isolating the electricity (running stuff from batteries) might give more benefit than the audio insulation of that box.
Not just audible, either. Electric motors are very naughty when it comes to interference, as are ballasts in light fixtures. I've seen a street light throw off enough interference to disconnect a poor guy's DSL every day when it turned on, and it was only after repeated troubleshooting and hassle that an eagle-eyed install tech noticed (by waiting with the customer at the right time). It's a legend within that company now.
OT, but I've seen this come up before and I wonder if HN could explain/justify this grammatical curiosity:
> "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.
Gang is plural, so in the King's English 'a gang were accused' and of birds 'a flock were flying' I've noticed that in various US sources a 'gang' is singular even though it is composed of many individuals, sort of like a corporation is a singular noun reflecting a collection of possibly many people. One of the things that makes English fun.
In "British" English, a collective entity (such as a gang, or a company) is generally referred to using pluralized modifiers. It's a cultural clash, there's no practical reason behind it.
Last time this was posted on HN, this question went unanswered: wouldn't variations in the AC line frequency be dominated by variations in recording speed? Even digital recordings have to be perfectly clock synced or they drift out of sync, and not necessarily monotonically. I've made digital recordings just minutes long that sounded fine to the ear alone, yet even after lining up the beginning and end, the middle was noticably mismatched.
Most modern electronic recording devices have little change in speed throughout the duration of the recording. Mechanical devices, not so much, what with wow, flutter, and unpredictable speed drift.
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.
It seems to me (and I don't have any formal qualifications to answer this questions) that computer clock drift is on the order of mega- or gigahertz, while this is on the order of 50hz - 5-10 orders of magnitude slower.
It sounds cool, but I wonder how reliable this method is. What's the false positive and false negative rate? Errors here would have real consequences -- for example, according to the article, it was used as a crucial piece of evidence in putting several men behind bars for decades.
So why is an article about the Metropolitan Police forensic lab in south London (UK) illustrated with a stock photo of what appears to be a US power socket? (It might be a Euro socket, but it's certainly not a UK one.) Do they not have electricity sockets anywhere at the BBC that they could have taken a picture of?
The mains frequency is an interesting topic. While it is true that it floats still its daily average is purposefully kept at quartz-like stability so that clocks can use it for synchronization. It also needs precise phase synchronization across the whole network (otherwise generators would blow up.)
I'm not so sure if the daily average is kept so stable actually (or maybe it depends on the country).
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.
Curious, is this "mains frequency" the same ringing I often hear when in alleged silence? And, no, I'm not talking about my hearing because I've had conversations with others who witnessed it :P - more like the ring old tube TV's used to make.
The brain is designed to process signal and if there is none it will sometimes make up its own. I guess you can compare it to dark current in photo-multipliers.
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.
You're probably hearing sounds that your body is making. The sound of blood moving in your ears is pretty significant when you're in a silent environment.
The science-fiction series Babylon 5 used this as a plot device in one episode - a group of hostages were located partly by identifying the background hum of machinery in the terrorists' video of demands.
It can be defeated by jamming the relevant frequency bands, i.e. by overlaying other mains recordings with random fluctuations. No deed to remove the original.
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.
I'm a little disturbed that they are using this as evidence, yet one of the "forensic scientists" is quoted as saying
Normally this frequency, known as the mains frequency,
is about 50Hz," explains Dr Alan Cooper
That is incorrect. Normally it is exactly 50Hz. That's why the time displayed on battery-powered clocks drift over time, but clocks plugged into the wall stay correct.
It is not very exact. The total phase error can be hundreds of cycles.
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.
The general premise is that there is an arbitrary, recoverable signal which has been convolved with the data signal. The generating function for this signal is a matter of record if you have access to the utility logs, but difficult to obtain otherwise without being physically at the time and place where the effect occurred.
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.
If the hum is inaudible to humans, but can be detected in the recordings, all that is necessary to create a plausible fake recording is to use a device that is incapable of capturing the hum, or severely compresses the hum so much that the hum is not distinguishable. This may not provide a means of proving the recording is authentic, but it prevents the defense from demonstrating that it was fabricated.
I imagine you could filter out the 48-52Hz range (probably less) and make your fake recording unverifiable.
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.
> If millions of people suddenly switch on their kettle after watching their favourite soap, the demand for electricity may outstrip the supply, and the generators will pump out more electricity, and the frequency will go up.
Obviously, if a building is off the grid then there wouldn't be the same background hum, but what if the building has its own energy generation system (e.g., solar) and feeds back to the grid?
I wonder if the same technique could work for the microwave range and the cosmic microwave background. That is, is there a discernible "hum" in microwave signals from the CMB?
[+] [-] antihero|13 years ago|reply
[+] [-] dkokelley|13 years ago|reply
[+] [-] mistercow|13 years ago|reply
[+] [-] JohnLBevan|13 years ago|reply
[+] [-] jff|13 years ago|reply
[+] [-] betelnut|13 years ago|reply
[+] [-] usedtolurk|13 years ago|reply
[+] [-] dabent|13 years ago|reply
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.
[+] [-] PeterisP|13 years ago|reply
[+] [-] jspthrowaway2|13 years ago|reply
[+] [-] dkokelley|13 years ago|reply
> "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.
[+] [-] mcobrien|13 years ago|reply
http://en.m.wikipedia.org/wiki/Collective_noun#section_2
[+] [-] rcsorensen|13 years ago|reply
[+] [-] pdw|13 years ago|reply
[+] [-] zacharyvoase|13 years ago|reply
[+] [-] eze|13 years ago|reply
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.
[+] [-] tricolon|13 years ago|reply
[+] [-] Mordor|13 years ago|reply
[+] [-] ChuckMcM|13 years ago|reply
[+] [-] mhlakhani|13 years ago|reply
[+] [-] nitrogen|13 years ago|reply
[+] [-] wglb|13 years ago|reply
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.
[+] [-] mseebach|13 years ago|reply
[+] [-] wch|13 years ago|reply
[+] [-] kordless|13 years ago|reply
[+] [-] derleth|13 years ago|reply
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://antipolygraph.org/
http://www.usnews.com/news/blogs/washington-whispers/2012/09...
http://www.skepdic.com/polygrap.html
[+] [-] telent|13 years ago|reply
[+] [-] dreish|13 years ago|reply
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...
[+] [-] jcurbo|13 years ago|reply
[+] [-] praptak|13 years ago|reply
[+] [-] PeterisP|13 years ago|reply
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.
[+] [-] tobyjsullivan|13 years ago|reply
[+] [-] sneak|13 years ago|reply
You can't hear mains hum without a speaker (or transformer acting as a speaker).
[+] [-] sterna|13 years ago|reply
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.
[+] [-] tomerv|13 years ago|reply
[+] [-] VMG|13 years ago|reply
[+] [-] NickPollard|13 years ago|reply
[+] [-] pygy_|13 years ago|reply
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.
[+] [-] cjensen|13 years ago|reply
[+] [-] tlb|13 years ago|reply
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.
-- https://en.wikipedia.org/wiki/Utility_frequency
[+] [-] levlandau|13 years ago|reply
[+] [-] glimcat|13 years ago|reply
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.
[+] [-] dkokelley|13 years ago|reply
[+] [-] madoni|13 years ago|reply
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.
[+] [-] gwern|13 years ago|reply
http://en.wikipedia.org/wiki/TV_pickup
[+] [-] yaddayadda|13 years ago|reply
[+] [-] cromwellian|13 years ago|reply