back to article Police use 24/7 power grid recordings to spot doctored audio

Forging audio recordings is a lot harder than it used to be, thanks to a new method of authenticating recordings based on the buzz of the electrical power grid at the time they were recorded. The oscillations of alternating current (AC) produce a distinct frequency – 50Hz in the UK, 60Hz in North America – that varies slightly …


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  1. keithpeter

    Well there goes another one....

    "In the kinds of amateur recordings that are often entered as evidence in court, however, typically no effort has been made to edit the power grid hum from the audio, which allows investigators to use the subtle variations in the frequency of the noise to determine whether the recording is genuine."

    So the baddies start mixing in a cocktail of phased 50Hz (UK) synthesised in audacity or pure data at a level +10db on the real signal?

    1. Simon Harris Silver badge

      Re: Well there goes another one....

      So the baddies start mixing in a cocktail of phased 50Hz (UK) synthesised in audacity or pure data at a level +10db on the real signal?

      err, If the point is to use the mains hum to authenticate a recording, wouldn't finding a wobbly 50Hz superimposed on it immediately lead to suspicion of it not being authentic? If you're a serious baddy who wants to fake recordings, wouldn't it be better to make your recordings in a heavily shielded room, do your own continuous mains frequency monitoring and add your own hum from the time you want to fake?

      1. Evil Auditor Silver badge
        Thumb Up

        Re: Well there goes another one....

        Indeed. I started to record the 50Hz signal when I first heard about it being used to verify cctv footage. Going to sell the recordings (incl. audio editing services) by the minute to anyone with sufficient funds. Sounds like a good business to me.

        Anon for obvious reason

        1. Evil Auditor Silver badge

          anon, I said!


        2. Simon Harris Silver badge

          Re: Well there goes another one....

          Don't forget for authenticity to supply the sounds of passing cars, trains, aircraft, etc.

          Oh, that's 'BBC Sound Effects no. 57: Ransom demands and death threats'

        3. Parax

          Re: Well there goes another one....

          You don't need to record it yourself, it's all recorder for you here.

    2. The Man Who Fell To Earth Silver badge

      Re: Well there goes another one....

      If one is doctoring a recording, do it while operating on batteries while on a boat in the middle of the ocean, thousands of miles from land.

      1. Terry Barnes

        Re: Well there goes another one....

        Except that the recording you're doctoring will already have the mains noise in it.

        The trick, I think, would be to use a high pass filter tuned to 100hz - although even then you'd have to be careful for harmonics.

    3. Naughtyhorse

      Re: Well there goes another one....

      thereby exposing the tape as doctored.....


  2. David Pollard

    "recording the hum of this frequency"

    Er, 'recording the frequency of this hum' seems a bit more likely.

    1. Christian Berger Silver badge

      Re: "recording the hum of this frequency"

      Well either that or phase over time. Both is equally well.

  3. Neil Barnes Silver badge

    Heard this one on R4 yesterday...

    and it did raise my skepticism level.

    Without any data on the system, I'm guessing they're using something akin to the dendrochronology and matching a short segment - presumably by frequency - against the reference.

    It strikes me that while this is possible in theory, it's going to need some pretty sophisticated DSP work to isolate the mains hum - in particular, since the mains frequency in the UK is so closely controlled, you're asking it to measure fractions of a tenth of a hertz from a device whose recording frequency control may be less than stellar. It's going to need lots of cycles...

    Equally, the general pattern of grid power usage is likely to be similar at the same time of day on different days; you're going to be looking at very small timing differences to identify different days.

    So while it's an interesting technique which *could* produce a timestamp, somehow I have the idea that it's being presented just as 'an expert says this is so' and thereby baffling the jury, rather than actually being as forensic as it is no doubt being presented.

    If I'm wrong here, then I hereby tender my humble apologies to the expert witness... and I'd like very much to see the test data and the scientific papers which have no doubt been already published...

    1. Anonymous Coward
      Anonymous Coward

      Re: Heard this one on R4 yesterday...

      While I'm not going to say the problems aren't real, this kind of technique is exceptionally easy to test. Take a thousand random recordings from the sample period, with a known but hidden date, run them through the system and see how many it gets right. Given how much DSP has come on in the last few years I'd be sorely disappointed if it was more than 1-2 errors.

    2. Ole Juul Silver badge

      Re: Heard this one on R4 yesterday...

      I have the idea that it's being presented just as 'an expert says this is so' and thereby baffling the jury

      I bet you're right. The theory sounds plausible, but to someone with a bit of actual experience and knowledge, it comes across as unlikely. It is clear that the recordings in question may often be amateurish and thus more hum prone, but by the same token they may be cheap digital with a brick wall cutoff below 100 or even higher. I note too that at least in North America, the frequency is extremely stable and the variations would not be within something which is likely significant in a badly recorded segment. One thing is for sure, now that this is talked about publicly, the concept can be more easily used by a professional forger, and work against successful law enforcement. A more sophisticated approach would be to record a hum sample in a distant place and at the time of the "incident" then use that in a later fake recording to "prove" when something took place.

      This is a can of worms. Not to put them down, but my take is that this is just law enforcement trying to look more technically skilled than they are. Maybe this is even a bluff and this story is actually more related to social engineering.

      1. Naughtyhorse

        Re: Heard this one on R4 yesterday...

        frequency is extremely stable


        as load varies (ppl using power) the the mech load on the generating sets changes, as more or less generating capacity is added/removed this also changes the loading, this causes stuff to spin up or down - F=ma and all that(we are talking very small amounts) - this is how demand is managed, control sees the frequency drop and brings more capacity online to meet it. the only other way is to.....


        well use a medium to tell you _exactly_what the load will be in 2 minutes time

        anything that changes the relationship between the load and the generating capacity will change frequency - that's what makes a 'black start' so hard to accomplish*


        black start - bringing everything back from a catastrophic failure of the grid - the large fluctuations in load as circuits are switched back in and generators brought online tends to cause other generation to drop outside of its frequency range and therefore automatically disconnect itself from the grid, this makes the original problem worse, knock out more generation and so on and so on till we are all in the dark again.

        This is why it takes to long to recover from a cascade failure, and i don't think anyone with any knowledge of the history of T&D could claim that this is an unknown phenomenon in the US :-D in fact when i think cascade failure the first thing i think of is the US! they have had a couple of doozies over the years

        As far as this technique is concerned AFAIK it's been around for donkeys years (well.. ish) and is well understood, and pretty much impossible to fake.

        (clue - your master blackhat recording tech when adding in his 'fake' hum will end up with 3 hums!, the original, his fake, and the one laid down when he did the fakery. immediately exposing the recording as doctored, and therefore unreliable.

        1. heyrick Silver badge

          Re: Heard this one on R4 yesterday...

          "As far as this technique is concerned AFAIK it's been around for donkeys years (well.. ish) and is well understood, and pretty much impossible to fake."

          I'm wondering about some of the lower quality recorders out there. I had an MP3 player that did 8000Hz mono PCM with some freaky pseudo-compression (a-law?) that gave quite small file sizes. But the quality for anything outside of the range of speech was terrible. My phone has an app to record to MP3, usually 64kbit but can opt for 32kbit. Again, mostly "adequate" for speech but sucks at other things.

          I'm just wondering if recordings made digitally with a system designed to attenuate/discard "unessential" information mightn't muck up a 50Hz signal too much to arrive at a reliable forensic result?

          1. 142

            Re: muck up a 50Hz signal too much to arrive at a reliable forensic result?

            "I'm just wondering if recordings made digitally with a system designed to attenuate/discard "unessential" information mightn't muck up a 50Hz signal too much to arrive at a reliable forensic result?"

            They'll be fine for any real world codec - you're not likely worried about 50Hz when doing this sort of analysis - that's a difficult frequency to work with, especially if you're going to be trying to verify things aurally. Of much more interest is the harmonics - 100, 150, etc, up into the low-kiloHertz (speech) range which (annoyingly for any sound engineer) tend to get picked up much more strongly than the fundamental. It's easier to pick out and analyze frequency changes in those ranges. In the event your compression algorithm for recording ditched the 200Hz range, then most likely 400Hz, or 450Hz will still be there, and you can derive the fundamental from there.

            Remember how Shazam can almost without fail pick out a recording in the background of a noisy restaurant through a crappy phone mic - that achievement is orders of magnitude more difficult than what the Met has in mind.

            You make a fair point though, as it is possible to envisage compression with the sort of frequency quantisation where this sort of analysis will not work (quantising every frequency to, say, the nearest 100Hz for example, as in a crude FFT), however that sort of codecs will make any speech sound completely robotic - think somewhere between Stephen Hawking and a Dalek.

    3. Anonymous Coward
      Anonymous Coward

      Re: Heard this one on R4 yesterday...

      heard this one on the reg >2 years ago ...

      1. LinkOfHyrule
        Thumb Up

        Re: heard this one on the reg >2 years ago

        Me too! Thumbs up coward!

        I also heard this on R4 and I thought to myself "That's that thingy I read about on the Register yonks ago!"

    4. Brangdon

      Re: Heard this one on R4 yesterday...

      I think you are missing the difference between identifying the date and time from scratch, and verifying a date and time claimed by the witness. The latter is surely a lot easier as you are just comparing two signals for equality. Even if the witness time is approximate, you'll have a relatively small range of samples to compare against.

    5. Terry Barnes

      Re: Heard this one on R4 yesterday...

      Remember that most recording these days st to solid state devices. Wow and flutter are really a thing of the past.

      I think as well as providing a form of timestamp, the system allows you to spot where a recording has been edited - sudden phase changes in the hum will point to something being cut out or inserted.

      1. Ole Juul Silver badge

        Re: Heard this one on R4 yesterday...

        Remember that most recording these days st to solid state devices. Wow and flutter are really a thing of the past.

        I think as well as providing a form of timestamp, the system allows you to spot where a recording has been edited - sudden phase changes in the hum will point to something being cut out or inserted.

        I agree that wow and flutter are a thing of the past. That makes it all the easier to make accurate forgeries. It is also not beyond the means of most people to record with hum and other external electrical input below the noise floor if they have some expertise and wish to do that. There is no need to worry about phase changes in the hum if you add it at the end or avoid edits. Awareness of what the end result is supposed to look like is the key here.

  4. VeganVegan

    Frank Zappa - Dynamo Hum!

    Zappa finally has IT relevance!

    1. Juan Inamillion

      Re: Frank Zappa - Dynamo Hum!


      That is all.

      1. jake Silver badge

        That's Dinah-Moe Humm, heathens! (was: Re: Frank Zappa - Dynamo Hum!)

        "I poked'n stroked till my wrist got numb

        An' you know I heard some Dinah-Moe Humm

        Dinah-Moe Humm"

        --Frank Zappa, December 21, 1940 – December 4, 1993. RIP

  5. Anonymous Coward
    Anonymous Coward


    Isn't this the same technique reported two years ago by el Reg : Met lab claims 'biggest breakthrough since Watergate'?

  6. Anonymous Coward
    Anonymous Coward

    Re: Isn't this the same technique reported two years ago by el Reg?

    It certainly looks that way (thank you, I remembered that article, but nothing by which to uniquely find it quickly).

    And comments on that article reference papers published in 2003.

    Still, it's always good to see the Met are on the ball.

  7. Herby Silver badge

    But it DOES get complicated...

    Here in the USA where there are basically three power grids (West, East, and Texas) the frequency doesn't change that much. That being said, the power companies make a special effort to have exactly 5,184,000 cycles of power each and every day (save leap seconds), so clocks won't accumulate errors. The power companies take considerable pride in their 60Hz they generate, and a sample every second or so is probably differentiated by the error in the sampling equipment, most likely due to the ambient noise.

    The bigger trick would be to have a recording and try to determine when it was recorded, talk about a needle in a haystack. Who knows if it will be successful.

    The other problem is when recordings are made "off grid", battery powered and away from the hum (out in the country?). Not going to get much power line grunge here. Compound that with an AC inverter run off a car battery which generates much better quality hash, and you won't be able to do much.

    Now where are those yellow stripes that are on my laser/inkjet printer.........

    1. Nuke

      @Herby - Re: But it DOES get complicated...

      The frequency is very stable in the UK too. Forget about the wow that someone mentioned here. Having said that, the UK grid no longer attempts to get an exact number of cycles each day, as there are so few synchronous clocks anymore.

      It is also the case that a mainstream recording studio will have kit that is extremely good at filtering mains hum.

      I think the point is that pirates will not have such good kit, so their mains hum will be detectable, with frequency variation more typical of an East European or Eastern country perhaps. To compare with the original they have copied, it will not be looking for a "needle in a haystack" - the prosecuting recording company will presumably have a log of when recordings were made, and then the pirate copy could be shown to have a hum variation different from that on either the master recording (if it can be detected) or from that as monitored by police at that time.

      Presumably also, if the recording companies get keen on this, they themselves could record the main frequency at the same time as the music recording, and keep it on record (to use a phrase).

      1. Anonymous Coward
        Anonymous Coward

        Re: @Herby - But it DOES get complicated...

        It's not about pirates, it's about evidence

        "I was in the pub with me mates when he was done over, the cctv and karaoke recordings will prove it"

        type of crime

        remember. most guilty people have an alibi before the crime

    2. Naughtyhorse

      Re: But it DOES get complicated...

      out in the country?

      really, if you are standing 'out in the country' and cant see a pylon or pole line, chances are you are standing on top of a cable!

      I don't know for sure, but i would expect there are _very_ few places on earth entirely free of mains derived emf.

  8. JaitcH

    So take a high-pass 200Hz filter and ...

    place it in the record path.


    Rene Christensen, "Active All-Pass Crossover Networks with Equal Resistors and Equal Capacitors", Journal of the Audio Engineering Society, Vol. 54, No. 1/2, 2006 January/February, pp. 45-53.

    Alternatively, after all editing has been completed, simply apply a 50 Hz tone and keep Plod amused.

    Might be interesting to use on the Watergate tapes with the 18 minute gap.

    1. This Side Up

      Re: So take a high-pass 200Hz filter and ...

      Usual problem. If you've taken steps to remove the mains hum, it points towards a doctored recording.

      1. YetAnotherLocksmith

        Re: So take a high-pass 200Hz filter and ...

        @This Side Up

        Hardly! Many bits of recording equipment have this built in because it is easy and cheap, and gets rid of annoying mains hum without issue. Anything with a mains filter automatically takes that step. Also, anything digital voice recorder-wise is probably only recording between 62Hz and 2kHz to save bandwidth and improve sound quality.

        1. Christian Berger Silver badge

          Re: So take a high-pass 200Hz filter and ...

          Also micrphones used close to the mouth typically have a high-pass filter since when you are close to the microphone there will be more bass in your voice.

          So it makes sense for anything below 200 Hz to be cut off.

          Of course if you record the mains independently you can just remove it and replace it with different mains hum and it would be near impossible to find that out.

        2. Naughtyhorse

          Re: So take a high-pass 200Hz filter and ...


          the hum is everywhere, any ferrous metal will pick it up.

          quoting 2 numbers for a filter is almost meaningless

          we are looking at signals way below the s/n threshold anyways

          all the filter does is _reduce_ the signal level below a threshold

          reduce != remove

          1. 142

            Re: So take a high-pass 200Hz filter and ...

            Errr... numbers aren't necessarily meaningly - a 48dB/Octave HPF 200Hz filter will push any noise at 50Hz well below the level encodable on a 16bit system.

            But anyway, that's moot point - as I've just mentioned in a different post here - of much more interest when doing this sort of analysis are the harmonics in the 500Hz-3kHz range - not the fundamental. It's utterly impossible to remove the higher harmonics of mains hum without mangling the speech.

    2. Naughtyhorse

      Re: So take a high-pass 200Hz filter and ...

      depends on the filter!

      a bog standard butterworth 3db/octave jobbie, or simml'a would see the mains hum down by 6 db, or 25% of what it was to start with.

      zillionthth order chebyshev, prolly down by a lot more :-)

      but the purpose of this type of analysis is not so much identifying hookie copies of commercial recordings, but establishing the veracity and continuity of covert recordings. the record can show that _this_ is when the recording was made, and _that_ is what the defendant said - un-edited. If the record is corrupted or contains any anomalies then it is just another piece of contaminated evidence and will be chucked out faster than OJ's sock.

    3. Nifty
      Big Brother

      Re: So take a high-pass 200Hz filter and ...

      Seemed to me from the R4 proggy that this technique used on phone call recordings too. Now these already have at least a 200Hz filter on them (heard much bass on a phone call lately?).

      I think the system relies on harmonics being detectable (an imperceptible 'buzz') as much as the fundamental freq.

      Now, back to my evil lair to fire up the portable generator, before issuing the ransom demands...

  9. Mad Chaz
    Big Brother

    What struck me is this. How can they tell what hum is what?

    Let's have a look at my own home computer. It's connected to a cheap UPS, is connected to the television and the sound system. Now imagine all those slightly out of phase 60 Hz frequencies all fudging together and trying to figure out what is what? How about taking into consideration the slight differences in frequency that might arrise in the CPU as I record ? Or how about a cheap recorder that doesn't work exactly 100% correctly as far as recording actual frequencies. Not enough to be distinguishable by human ears, but enough that they might not even keep it accuratly compared to themselves. Think cheap clock that isn't 100% stable in a digital recorder.

    That's more like trying to tell who died and when by looking at the blood patterns in sea waves if you ask me. Or maybe they just believe the bullshit the guy with the fat commission told them.

    1. Terry Barnes

      What cheap clocks that aren't stable? It's not a significant engineering problem to create a clock that keeps time. Even if time did slip, the phase change in the mains hum from a doctored recording would be easily detectable.

      I had a DAT walkman 20 years ago that produced perfect recordings using a tape mechanism. Recording to solid state storage accurately today is trivial.

      1. Andy ORourke

        "What cheap clocks that aren't stable?"

        Maybe the Cheap POS clock that's in my microwave, loses at least a minute a week!

    2. Nuke

      @ Mad Chaz

      Wrote :- "What struck me is this. How can they tell what hum is what?...Let's have a look at my own home computer. It's connected to a cheap UPS, is connected to the television and the sound system. Now imagine all those slightly out of phase 60 Hz frequencies all fudging together and trying to figure out what is what?"

      They don't need to figure out what is what. They only need to show that the mains hum on the authentic copy (if detectable) is different from that on the pirated copy (if present).

    3. Naughtyhorse

      mains out of phase??

      you getting the power for your telly from a different grid (that is, as good as saying continent!) to the one supplying your computer - neat trick that.

      (hint, no you aren't, you just misunderstand the premise - besides all the kit you mention would have SMPSU's and that a whole other can of worms)

      I think you'd need a rather expensive recorder to pitch shift on the fly as opposed to a cheap one.

      you think that slight variations in clock speed in a clock running at 100's or more probably 1000's of Mhz will have a lot of influence over a 50/60Hz hum??? (not my understanding of the behavior of crystals - a few cycles per billion if theres a massive temperature shift, but pretty stable otherwise)

  10. Anonymous Coward
    Anonymous Coward

    There are already holes in this. More and more people are getting off the grid. So if they are producing their own power, the buzz at home would be different than elsewhere. You also have where people might have solar, wind, etc. as a supplement which would also change the buzz since they are basing it on load.

    1. Anonymous Coward
      Anonymous Coward

      "as a supplement "

      Look mate, I'm far from convinced by this Metroploditan BS either, but the thing is, it's based around the frequency of the power. If you have local generation from wind, solar, etc **as a supplement to** (ie at the same time as) the incoming grid, that local generation's output by definition must be working at exactly the same frequency as the incoming grid (go read about grid tied inverters, for example). Otherwise the wiring would be on fire (or at best, fuses would be blowing and either the local generation or the incoming grid would go offline).

      1. Anonymous Coward
        Anonymous Coward

        @2nd AC :-)

        You're right about the frequency being usually the same as the one on the grid. However... Did you know that I have several electrical appliances which I can use both in Europe as well in the US ?

        Honestly; its not as dramatic as you make it. Most certainly not a totally impossible scenario.

        In fact; this could easily be another interesting excuse for a court: "But I use EU appliances from a friend...".

        1. Anonymous Coward
          Anonymous Coward

          Re: @2nd AC :-) re "same frequency"

          I don't know what you're on about, and it's not clear you do with your 50Hz vs 60Hz reference.

          Where a grid tied inverter is in use, the incoming grid, and the inverter output, have to match *exactly*, all the time. In voltage, frequency, and phase. Otherwise fuses blow, or worse.

          Doesn't matter (in principle) whether it's a nominal 50Hz setup or a 60Hz setup.

          1. YetAnotherLocksmith

            Re: @2nd AC :-) re "same frequency"

            While you are right in the main, you are wrong for this, I fear.

            The electronics that do the frequency matching of the (say) PV panels to the mains will be fairly accurate, but it is very unlikely they will be so accurate that the micro-variations in frequency the plod are claiming they can measure accurately (and 100% ID a recording from) won't turn out to be substantially different.

            I just can't see some Chinese engineer sitting there and saying "Yes, but we need it to be a perfect true sine wave running off this crystal oscillator" when honestly, a crude approximation of a sine wave will do - just look at nearly all the cheap inverters used in cars and vans.

            1. Anonymous Coward
              Anonymous Coward

              Re: @2nd AC :-) re "same frequency"

              "a crude approximation of a sine wave will do - just look at nearly all the cheap inverters used in cars and vans."

              Yes but the ones used in cars and vans are not grid-tied inverters. The ones used in cars and vans can only be used **instead of**, not **as well as**, the One True Grid. The difference matters.

              Suppose the grid is running at 50.000000 Hz and your proposed inverter is running at 50.01 Hz.

              Suppose that at some point in time the grid input and the inverter output are in sync: same voltage output at the same instant in time. That's handy really, given that the two outputs are paralleled (that's what makes it a grid tied inverter, as used in solar PV or whatever, rather than a cheapo standalone job).

              Suppose that some seconds pass. Now, your theory says the two can be at different frequencies. If (as you suggest) the frequencies are different, then the instantaneous values of the grid input and the inverter output are no longer the same voltage, even though they are both feeding the same piece of wire. In fact according to your theory there will come a point in time when one output will be (say) 120V+ and one will be 120V-. Do you see a problem there yet?

              So, how do we stop them getting out of sync like that? We design the grid tied inverter so that its instantaneous output follows the instantaneous voltage fed in from the grid. In other words:


              Are folk getting the message yet?

              1. Naughtyhorse

                Are folk getting the message yet?

                isnt it funny when types that know 'digital' encounter the real world.

        2. Naughtyhorse

          Re: @2nd AC :-)

          wiki SMPSU (Switched Mode Power Supply)

          All that most kit sees that frequency dependent these days is 4 diodes (or a bridge), 2 big assed electrolytic capacitors, a fuse, a switch, and a beefy as you like chopper transistor.

          with the loop impedance and the size of the (low esr) electrolytics, 50hz looks as close to 60hz as makes no difference.

          most of the rest of it is DC, and the bits that aren't sort their own frequencies out (much much higher than transmission frequencies)

        3. Rob Carriere


          "Did you know that I have several electrical appliances which I can use both in Europe as well in the US ?"

          No, I didn't know, but I'm hardly surprised. Any household that has electrical equipment bought in the last 20 years is likely to have such appliances. ...Which, I'm sure, the Met is well aware of...

          All of that doesn't matter, though, as the whole point of such appliances is that they adapt to the grid at hand. They happily eat 50 Hz mains in Europe and equally happily eat 60 Hz in the US. So the buzz will be the local frequency. In fact, your appliances may well make life easier for the police, as at least the cheaper kind of these power supplies tend to spit out ferocious amounts of harmonics, making the buzz much easier to pick up.

    2. Stuart Halliday

      Won't work for large areas of Scotland. We have hydrodynamic power grids fed from water and wind power....

      1. PC Paul

        No you don't. You have many hydrodynamic generators feeding The One True Grid through grid-tied inverters and other frequency locking mechanisms.

        The only way you'd have different frequency variations is if those were on their own completely isolated distribution system. That might be the case on some of the islands but the mainland is all connected (as far as I know).

      2. Naughtyhorse

        all part of UK national grid, therefore in phase.

    3. Naughtyhorse

      but the noise is in the sky man!

      it's. like, everywhere, like, all the time, man.

      no matter how far off the grid you are!

      solar, wind, biomass, batteries, or a bloke on an exercise bike.

      you still get the hum.

  11. Mike Banahan
    Big Brother

    I call bollocks on this

    I have extreme doubts about this technique. As others have pointed out, the frequency of the AC mains is usually extremely close to the nominal amount, so you need very accurate frequency measurement to spot the differences.

    Problem is, most home recordings are done through a soundcard (no shit, Sherlock?). Those soundcards typically get asked by the PC to sample at x samples per second and they deliver something like x - but their internal reference frequency is based on a nasty cheap ceramic resonator which is close to but not exactly the frequency asked. Maybe it's within 0.1% of the requested frequency, so instead of delivering 44100 samples a second it SAYS it's doing that but in a real second it's delivering 44050 instead. That goes into your file, you play that back through another sound card with a similarly off-reference oscillator and suddenly you have 0.2% (say) error in the actual frequency being replayed. A musician with perfect pitch might spot something very slightly wrong but to most people it will sound fine - just all the tones are ever so slightly off.

    I first had this brought home in no uncertain terms trying to record off-air weather fax broadcasts, where accurate time sampling is crucial or your fax comes out all slanted. A cheap outboard SoundBlaster gives frankly hilarious results and you need to be able to tweak the alignment in software to fix it.

    So, anything which takes 50 or 60Hz mains hum and thinks it can correlate that with something that was measured against a highly accurate refernce source but was actually recorded with something far worse - well, I look forward to seeing that stand up in court if the expert witness isn't a complete fuckwit.

    1. Anonymous Coward
      Anonymous Coward

      Re: I call bollocks on this

      I said that (but not so eloquently) in response to the 2010 article - regardless of what record/playback technology is used, where's the frequency reference that proves that 50.0Hz (or whatever) when recorded will be exactly 50.0Hz (or whatever) when played back? In the absence of such proof, all bets for this method's trustworthiness must surely be off.

    2. M Gale

      Re: I call bollocks on this

      Well, it may be difficult to prove when a recording was made. However, as the article states, it may still be useful to detect otherwise-undetectable edits. WHile the part of the audio you listen to might not reveal any obvious pops or clicks, the very faint hums of electrical interference in the background may well be jumping up and down like a yoyo.

      I have to wonder if more determined fraudsters are using fourier-based noise removal to get around this?

      1. Z80

        Re: I call bollocks on this

        I was sceptical about this when I heard it on R4 and I'm surprised to hear that one reference recording could cover the entire grid.

        As far as accuracy of the recording frequency is concerned though, I assume they look for patterns in the relative change of the recorded hum as it drifts up and down over the course of the recording rather than matching to the reference hum's frequency exactly. So long as the recording's sample rate was consistent they could compensate for it being out a bit?

      2. Naughtyhorse

        Re: I call bollocks on this

        the fft is how it works.

        hence absolute time domain fidelity between recording and playback devices is irrelevant.

        it started a X hz after 20 seconds went up to x+0.01 then after a further 30 seconds went to x-0.02

        calculate the difference between the recording devices value for x, and the playbacks version of x, multiply out the time changes, et voila.

    3. Christian Berger Silver badge

      No not quite

      Well the frequency error of your soundcard is in the range of less than 100 ppm. That's far better than 0.1%. Plus it's fairly consistent. It changes only very slowly. So yes, that introduces an error, but it's probably manageable.

    4. Naughtyhorse

      Re: I call bollocks on this

      i call your bollocks and raise you more bollocks!

      you do not see pitch shift on recordings carried out this way.

      you just don't


      maybe in the bad old days on tape, but even then only when the tape had been used a million times before and the recorder was on it's last legs.

      i play and record on a vast array of kit of varying qualities and tuning just isn't an issue (i have pretty good pitch, but my SA is better)

  12. This post has been deleted by its author

  13. Anonymous Coward
    Anonymous Coward

    Ah but

    my gaming rig is Steam powered.

  14. JimmyPage Silver badge

    I think the point is ...

    a massive PR campaign on behalf of the police to scare baddies into not challenging their "evidence".

    Given police claims about cannabis factories being stuffed with UV heat lamps, I'm not sure we have much to learn from their science unit.

  15. DougS Silver badge

    Isn't this only for recordings made by AC powered devices?

    If I recorded something, I'd use my phone. I'd have to, as I don't own anything I could plug into the mic input on my PC. My phone, obviously, runs on battery.

    I am highly skeptical that my recording would pick up a hum from the grid around me in my house and be audible to the extent you could tell a few hundredths of a Hertz variation for this type of forensic investigation. Or audible at all, to be honest.

    Even if this worked when I made the recording in my house, what if I'm making the recording in my backyard or on the 14th green? If you're going to tell me I'll get an audible trace from nearby power lines, no matter how "nearby" they really are, I'll be forced to question your sanity.

    I'd love to have more details on this, and know what made the recordings they were able to do this with, and where they were located at the time. I don't deny this is possible in the right circumstances, and is smart police work since keeping the record would cost nothing once it has been set up. But I doubt it is applicable very often - less often all the time as fewer grid powered recordings will be made these days.

    1. Burbage

      Re: Isn't this only for recordings made by AC powered devices?

      Well, possibly, though the way it's being sold is that it would work even if you were using a clockwork recorder on a train.

      What matters with this sort of wheeze is not whether the technique is infallible or even moderately well-proven. What matters is how readily the courts accept it. The lesson of low-copy-number DNA 'matching', accepted instantly by the courts before anyone bothered to question whether it was reliable, made any statistical sense, or was properly understood by the judiciary might have been a useful one, but there's no sign that it's been learnt, and I strongly doubt this technique will be queried much, either. I suspect the first consequences will be that any recorded evidence supplied by defendants will be swifty declared iffy and that the Met will make a small killing for the privilege of doing so.

      1. J P

        It's easier to disprove a hypothesis than to prove it*.

        I think Burbage is on about the same thing that passed through my mind - all this could really be good for is proving that a given recording _wasn't_ made at a particular time because the variance in hum is too great, in the same way that DNA matching can only [conclusively] clear people because they don't match.

        The risk of false positives is too high for a good defence lawyer to let the prosecution get away with saying that the 'hum profile' (or rather, the close approximation of it which can be extracted from recordings which almost certainly weren't made on kit designed to accurately preserve it) proves when a recording _was_ made. And of course if there's a gap in the pattern (aka doesn't match any known profile) then it's probably been fiddled with, and almost certainly won't be a 'kosher' recording made on kit connected to UK mains. But again, all that does is expand the list of known unknowns and potentially disprove a given claim, rather than being proof positive of any particular assertion.

        *If you're a scientist or a logician. A lawyer is of course required to be neither on behalf of his client.

      2. Naughtyhorse

        Re: Isn't this only for recordings made by AC powered devices?

        if the recording medium was electronic, then a clockwork recorder would still have the hum.

        the second para conflates science with statistics. saying one is 'sure' of something without sufficient evidence is always going to end in tears. - in this case, im sure the frequency record has already been sufficiently analysed to be able to say that 'we need a recording sample X seconds long to be able to have a significant probability of a match' as with DNA.

    2. Naughtyhorse

      Re: Isn't this only for recordings made by AC powered devices?

      it's not audible recording, the hum comes from the electromagnetic field produced by all those wires, the audible part comes from the nature of the recording machine - the field creates small eddy currents in the components of the recording device which are, to the device, indistinguishable from the signals coming from the microphone.

      you clearly have a smart phone - get an app that reads field strength - search your app store for 'EMF', then go play golf and prepare yourself for a shock (figuratively, that is)

      1. DougS Silver badge

        Re: Isn't this only for recordings made by AC powered devices?

        you clearly have a smart phone - get an app that reads field strength - search your app store for 'EMF', then go play golf and prepare yourself for a shock (figuratively, that is)

        Just because my phone's compass hardware can detect EMF fields doesn't mean that recording something with its microphone will result in a detectable 60 Hz hum on the recording. If you claim the microphone is affected by it, why isn't the camera?

        I checked the iTunes store for "EMF" and the only app I saw that wasn't intended for detecting ghosts was rated only two stars :)

        1. Naughtyhorse

          Re: Isn't this only for recordings made by AC powered devices?

          the hum is NOT 'recorded' with the mic. it's NOT audio, its (as you point out) EM. its superimposed on the electronic signal derived from the audio signal from your mic.

          the camera is - though sucking the data back out of a photo after jpeg compression would be CONSIDERABLY more complex.

          AH! iTunes

          nuff said, im an android man myself, plenty to choose from in our store, but then they let any old maniac write malware for droids apparently(who'd have thunk it)

  16. Anonymous Coward
    Anonymous Coward

    Well that's a relief then, we can all sleep soundly now.

    And in exactly how many hundreds or thousands of cases have criminals modified audio recordings to pervert the course of justice so far this century?

    1. Yet Another Anonymous coward Silver badge

      Re: Well that's a relief then, we can all sleep soundly now.

      How many times have the police done so to edit a recording of an interview?

      With this new technique the Met's forensic lab will be able to use the Met's archive to spot when the Met fiddles with a recording

      1. Fatman Silver badge

        Re: use the Met's archive to spot when the Met fiddles with a recording

        Like that will ever happen (the cops spotting a 'fiddled with by the cops' recording).

  17. Paul Hovnanian Silver badge

    Re: Well that's a relief then, we can all sleep soundly now.

    I think the frequency of either the police or defendants doctoring the evidence is pretty low. But this technique is a tool with which the evidence can be validated.

    The problem is; it appears that the baseline data is in the hands of the police. So they will be able to validate their recordings as required. But one can only hope that the data will be made available for the use of the accused as well.

    1. Anonymous Coward
      Anonymous Coward

      Re: Well that's a relief then, we can all sleep soundly now.

      "I think the frequency of either the police or defendants doctoring the evidence is pretty low"

      Been ignoring the news this last few months?

      Try telling that to the Hillsborough families, who have finally been able to see that two thirds of witness statements from the day were tampered with by the police, because they showed the police in a bad light, and who have finally been able to see that every subsequent inquiry till the present one has been a complete whitewash, aimed primarily at continuing the coverup of the incompetent police action on the day, and the disgraceful police tampering with evidence afterwards.

      1. Anonymous Coward
        Anonymous Coward

        Re: Hillsborough

        Written statements really aren't relevant to this discussion, are they?

        1. Fatman Silver badge

          Re: Written statements really aren't relevant to this discussion, are they?

          In a way, they are.

          If cops tamper with written statements, then it would be reasonable to expect some to tamper with electronic recordings.

          It goes to the 'mentality' of the police (cover up our fuck ups at all cost).

  18. Old Handle

    I can easily believe this is sometimes able to prove a recording is fake if it was alleged to have been made at a very specific time (say it's a phone conversation and they have call logs), but I'm really doubtful the accuracy is good enough when the time window as more than a day or so. And of course proving a recording is real is much harder, since it could be tampered with.

    But on the other hand I'm not really basing that on anything scientific.

  19. Turtle

    If only...

    "audio specialists at the Metropolitan Police forensic lab in south London have been recording the hum of this frequency continuously for the last seven years – 24 hours a day, 365 days a year..."

    If only they hadn't taken two days off, the record would be complete...

  20. Tom 35 Silver badge

    I would think

    You could use this to disprove someone saying a recording was made at a fixed date and time. If there is a miss match it's false.

    But I don't think you can use it to say something was recorded at a given time for sure. How could you say that a given pattern didn't happen more then once?

  21. Anonymous Coward
    Anonymous Coward

    Fitting the Crime.,..

    to make the Punishment.

  22. jake Silver badge

    Scaring kiddies, that's all.

    This *might* work if a kid uploaded a "mix" tape err, cd err, MP3. Maybe. But the kiddies aren't making money out of the technology. They are just sharing.

    But I doubt it'd work , even there. Too much signal loss and cross-talk in the systems the kiddies are using.

    Me, on the other hand, I have totally stable 60hz power (some old computer gear requires it). "Mains" is completely filtered out by the battery room ... If I was in the business of making money by selling other people's intellectual property, this newold technique would be utterly useless.

    Methinks the Met Police's Dr. Alan Cooper is waffling to maintain a cushy job ...

    1. This post has been deleted by its author

    2. Anonymous Coward
      Anonymous Coward

      Re: Scaring kiddies, that's all.

      Fuck me

      MP3s? File sharing? Kiddies? intellectual property?

      Do you actually read anything before deciding to post your ill thought out ramblings? Do you have any idea of what was being discussed in the article? Are you genuinely that hard of thinking?

      And it looks like we have another item to add to the list of old shit jake owns... an entire room dedicated to batteries.

      If you are going to accuse someone of waffling [1] then at least read what they are trying to tell you!

      [1] for you to accuse anyone of waffling would be like the pope accusing someone of being catholic

  23. Anonymous Coward

    Its even easier than Audacity

    This is so dumb its not even funny.

    "No your honour, I used this sound recorder optimized for voice dictation. I don't know why the police claims its false; all I did was click this "band filter" * switch here to get a better quality...".

    * Every decent audio recorder which also provides support for better voice recordings knows a simple filtering technique such as "band pass filtering". Without going into too much detail this kind of filter picks up a specific range of the audio spectrum and, well, filters everything else out. My 'fail' comes from the mere fact that Its quite common to use band pass; so basically meaning filtering out low frequencies to get rid of unwanted noise, and filter out high frequencies to get rid of unwanted hisses and such. All at the same time.

    In general the human voice sits between 80Hz and 1.2kHz (roughly). So a bandpass of, say, 150Hz to 2.5kHz wouldn't be an exception.

    Happy rumble hunting!

    1. Naughtyhorse

      Re: Its even easier than Audacity

      how many db/octave is that filter?

      six months millado!

    2. 142

      Re: Its even easier than Audacity

      What, pray tell, are you going to do about the induced noise at, say, 800Hz?

  24. Fred Flintstone Gold badge

    Another argument for alternative energy sources :)

    Try matching that off-grid pattern..

  25. Dr Paul Taylor
    Black Helicopters


    I share the skepticism, bit there is a difference between this and dendrochronology. Whereas there is no control over the climate vaiation that leads different tree rings, Plod could be getting the National Grid to introduce coded signals into the mains hum.

    1. Anonymous C0ward

      Re: fm

      And backmasking our heavy metal records.

  26. Anonymous Coward
    Anonymous Coward

    groundhog day? 1970's all over again

    this technique was first mooted in the 1970's to verify recording location, authenticity etc.

    well known was the fact that rubber belts on tape and record decks caused 'wobble' in music and voice

    thoughts turned to sychronising tape and record decks with mains frequencies, but was found to vary so much, that quality of audio and early video recordings could only be guaranteed when referenced against crystal oscillators, used on some early direct drive systems.

    so nothing new, just a reiteration of what we knew

    (record - vinyl circular disks - for those whose parents were barely out of dipers at the time)

    1. Kubla Cant Silver badge

      Re: groundhog day? 1970's all over again

      I think if you look carefully at your keyboard you'll see that there are a couple of keys labelled "Shift".

      1. Fred Flintstone Gold badge

        Re: groundhog day? 1970's all over again

        He's just being vewwy, vewwy quiet ..

        1. M Gale

          Re: groundhog day? 1970's all over again

          Vewwy, vewwy, vewwy quiet?

          Damn wabbit.

      2. Pookietoo

        Re: if you look carefully at your keyboard

        Much better all lower case than too much upper case, no?

  27. Red Bren

    Who holds the reference data?

    If you have recorded evidence of serious police misconduct, what is to stop the police from claiming its afaked because it doesn't match their reference?

    What is to stop the police from altering recorded evidence and inserting the required hum from the reference data?

    1. Stuart Halliday

      Re: Who holds the reference data?

      Let's hope they archived it and signed it with a fully independent 3rd party otherwise they should sack their IT staff.

      1. Naughtyhorse

        Re: Who holds the reference data?

        national grid.

    2. Fred Flintstone Gold badge

      Re: Who holds the reference data?

      I give that possibly one, maybe two appearances in court before some enterprising hack creates a website where you can download patterns from all over the world.

      Having said that, I lack facts here. Until I see a scientific paper that explains how it works this has about as much value to me as the use of an IP address to prove identity. That only works if you have really expensive lawyers, and even then it's dicey.

    3. Jason Bloomberg Silver badge
      Thumb Up

      Re: Who holds the reference data?

      Best point made so far; the first thing any barrister should do is call into question the police's "proof".

      Given criminal cases rest upon "beyond reasonable doubt" it is highly unlikely any case would stand or fall on the issue of a tape recording and some correlation (or not) alone and, if it did, that it would withstand an appeal. It is therefore rather moot in the scheme of thing.

      Of course the police are very likely to use claims of being indisputably able to prove their case and get a confession with the promise of going easy if one fesses-up and that's where it has best application. It's often easy to have a suspect to believe a jury won't believe them than get the jury to not believe them. It's simple social engineering.

  28. bag o' spanners

    I'm all for building enormous Faraday cages around every courtroom in the land to avoid contamination of evidence. .

  29. Pookietoo

    Speed variation, mechanical or digital

    Wow and flutter and tape stretch, instability of sample clock frequency - it is trivially easy to introduce (accidentally or deliberately) sufficiently large variations into a recording that it wouldn't match the reference, even before you look at obscuring, removing or faking the tell-tale frequency. It seems to me that an expert witness for the defence can easily discount the validity of the technique.

  30. Donald Becker
    Big Brother

    It's more useful to think of them using the phase change than the frequency.

    The grid tries really hard to keep the frequency at 60.000Hz (or 50Hz, for the countries that are a little slower).

    If the load increases, the phase lags. This indicates to the power plant that they need to throw another shovel full of coal onto the fire and let a little more steam into the turbine.

    It sounds easy to record that phase difference and match the pattern, right?

    Except that there isn't one power plant. The whole point of a grid is there are thousands connected together. So you have thousands of sources trying to push the phase a little faster. Each substation is seeing a different mix of the phase variations. All mixed together, with extra noise added by local loads being switched on and off.

    I can see how you might be able to show a proof of concept that invalidates a recording (but not authenticates its veracity) made nearby, within the same substation service area.

    The next challenge is the audio recording. Today that's largely digital. Analog audio is hard to do well on digital chips. There is quite a bit of non-linearity. Some of the distortion results in phase changes with sounds level. The audible effect is more pronounced at high frequencies, but it will overwhelm the minuscule phase difference of the power line hum. Even the sample clock of the D/A digital will have enough jitter, correlated with the processor workload and other power draws, to overwhelm any otherwise detectable hum phase pattern.

    Perhaps that's why they are publicizing this now. It's a technique that sounded promising, they invested a lot of effort, only to have progress has render it completely useless. They might as well recover whatever value they can by dissuading people from trying to forge recordings.

    1. Naughtyhorse

      you say phase lag, i say frequency change :-)

      y' cant have one without the other. in order to go out of phase, something needs to slow down/speed up

      all the power stations have to be in phase (theres kit to auto lock them out if they arent - can you imagine how loud the bang would bee if you closed 500MW onto a grid that was 180degrees out!)

      i hope your sampler is running a little faster than 60Hz btw!

    2. 142

      Good gods, Donald!

      How inaccurate do you think modern digital audio gear is??

      Mains frequency drifts, due to the phase shifts you're mentioning, between 49.9 and 50.1Hz. That's a piece of piss to record, and has been for probably 40 years! Indeed - you can audible hear the pitch change as you're working on audio with high levels of mains hum recorded over even a 15 minute period of time, if you jump back and forth between different sections. Hell, you can see it visually in the amplitude waveform, without even resorting to frequency analysis!

      Don't believe me about the frequency variation? Have a look here -

      1. 142

        Re: Good gods, Donald!

        And about phase - If I set a signal generator to give me a 50Hz sawtooth wave, and I record that 50Hz sawtooth wave, I will get a phase-perfect 50Hz facsimile of the sawtooth wave. NOT, as you seem to be implying, a phase shifted mess of a waveform.

        1. 142

          Re: Good gods, Donald!

          I may have been a tad harsh last night, lol - I've heard people the same tales about digital audio and phase before - but the reality is that unless you're building a crude ADC yourself, you'll only encounter phase issues right at the nyquist frequency.

  31. This Side Up

    Does it work with tape?

    Analogue's not a problem, but mains tape recorders with synchronous capstan motors will track any frequency variations on the supply, so cancelling out the variations in mains hum on the tape. On the other hand it should be a lot easier to detect edits in analogue recordings.

  32. Tom 7 Silver badge

    And all sound recorders are synced to ....

    some magical central source ??? I'm guessing that this will make a guaranteed timestamp once in a backhander.

  33. Suricou Raven

    Defeatable, if you know about it.

    Simply removing the hum isn't going to work - it'll still be evident that the hum was removed deliberatly, which would be very suspicious. No, you just need to make sure that the hum is gone in a way that appears accidential.

    So make your recordings on a laptop or battery-operated device, in a room with a continuous background noise (say, a moderatly loud fan) and make sure to record to a 32kbits MP3. Lets see how well they can extract the hum from *that*.

    1. Rob Carriere

      Re: Defeatable, if you know about it.

      Sure, except I expect that in a lot of these cases, the voice on the recording has to be recognizable for the recording to work. ("We have little Johny, listen to him." only flies if you recognize little Johny) And I would be _really_ worried about that fan, unless it is powered by batteries or a guy huffing and puffing on a bicycle.

      Anyhow, I'm 100% confident it is possible to defeat this technique under at least some circumstances, just as it is possible to avoid leaving fingerprints at least some of the time. Doesn't mean that it isn't a useful tool to have in the toolbox.

  34. Anonymous Coward
    Anonymous Coward

    In Audacity you can easily add a dehum filter

    I just found this nifty nyquist plugin within the audacity forum to wipe out 50 or 60 hz frequencies including most of it's harmonics. Just paste the snipplet below into a text editor and save it as dehum.ny and then place it within the nyquist folder within the Audacity installation. You'll find it then within the effects menu of Audacity:

    ;nyquist plug-in

    ;version 1

    ;type process

    ;name "50/60Hz dehummer 2.0"

    ;action "Removing Hum harmonics, ( this may take some time ) ..."

    ;info: Unintentional reverb effect. Gibbs ringing on transients.

    ;info: if you find a cure for the reverb please email me:

    ;control choice "Select mains frequency" int "0=50Hz (UK) 1=60Hz (USA) 2=choice" 0 0 2

    ;control bfreq "Base frequency" real " 10-10000 Hz " 50.0 10.0 10000.0

    ;control a "Amount of hum removed" real " 1-100 base Q=(100/a)^2" 15 1 100

    ;control v "Anti-reverberation setting" real "1-100 " 25 1 100

    ;control frac "Fraction of spectr" real " 0.01 to 1 " 1.0 .01 1.0

    (setf freq (cond ((= choice 0) 50.0) ((= choice 1) 60.0) ((= choice 2) bfreq)))

    (setq que (/ 10000.0 (* a a)))

    (setq anti (/ 10000.0 (* v v)))

    (setq mysound s)

    (setq r *sound-srate*)

    (setq itern (truncate (* (/ (/ r freq) 4) 2)))

    (setq d (/ (float itern) anti))

    (setq iter (truncate (* (float itern) frac)))

    (defun dehum (mysound freq iter)

    (dotimes (i iter mysound )

    (setf mysound (notch2 mysound (* freq (1+ i)) (* que (1+ (/ i d)))))))

    (if (arrayp s)

    (vector (dehum (aref s 0) freq iter)

    (dehum (aref s 1) freq iter))

    (dehum s freq iter))

    1. 142

      Re: In Audacity you can easily add a dehum filter

      Yip. The trouble being though, of course, when you notch out that many harmonically related frequencies from a human voice, makes it sound like they're talking in a 10 foot diameter plastic pipe...

      (Sherlock for his pipe fondness, not his sarcasm! ;-) )

  35. Secman

    Clearly, there are folks here with no idea about grid frequency maintenance. Power grid frequency wobbles a lot. Long term, it is very stable, as it is kept that way, as already noted, for the benefit of clocks. But short term... Wobbles. And as such even amongst the wow and flutter of a VHS VCR, or the clock inaccuracies of a crystal chip, there is a lot of correlation that can be done...

    Have a look at some real time grid frequency data here:

  36. Anonymous Coward
    Anonymous Coward

    I'm impressed by all the audio lurkers

    So...Mr Terrorist just needs to make their recordings using battery powered gear then, right? No hum, no mains, no long stay at Her Majesty's pleasure.

    I knew the voice memos thing on my phone was there for a reason.

  37. Anonymous Coward
    Anonymous Coward

    laptop in the garden shed for my next blackmail recording then.

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