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The National Grid can be used to convict you

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posted on Sep, 1 2013 @ 10:46 PM
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Apparently the National Grid runs at 50hz, this signal may not be audible to the human ear in some cases....A slight buzz on a recording can give away the exact time the recording was taken.
Its pretty late here but i wanted to share this video which explains better what im going on about.....





posted on Sep, 1 2013 @ 11:02 PM
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reply to post by AmberLeaf
 

Move to the US. 60Hz here. Perfectly secure.



posted on Sep, 1 2013 @ 11:04 PM
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No.

Just no.

If I explained everything wrong with that video, it would fill 3 libraries and 7 encyclopedias.

TheRedneck



posted on Sep, 2 2013 @ 12:00 AM
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Originally posted by TheRedneck
No.

Just no.

If I explained everything wrong with that video, it would fill 3 libraries and 7 encyclopedias.

TheRedneck

It looks like it's a perfectly valid way to track the time of an audio recording to me... of course there would be certain difficulties, like pulling the signal out of low quality audio sources and storing the signal generated by every power plant for later reference. But other than that I don't see why it wouldn't work... it seems like the police have already been using this technique for a long time. I would appreciate it if you could explain briefly why you don't think it would work.



posted on Sep, 2 2013 @ 12:09 AM
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reply to post by ChaoticOrder

Briefly:

That trace is not of a sine wave. It is of a noise signal. Were power to be applied to the grid with that main signature, it would blow every motor in existence and burn out every transformer in the grid.

High frequency noise as is shown in that trace would make smart meters, cable transmission via power lines, and Internet connectivity via power lines physically impossible.

50 Hz is indeed audible. Human hearing ranges from 20 - 20,000 Hz.

Electrical appliances do not produce an audio tone correlating to the frequency of the applied power.

Stopping now... too much of my life already wasted on this.

TheRedneck



posted on Sep, 2 2013 @ 12:22 AM
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reply to post by TheRedneck
 



Electrical appliances do not produce an audio tone correlating to the frequency of the applied power.

This is a good point actually, but there are certainly some devices which do generate a hum which corresponds to the frequency of the power source the device is operating on (and even if it's too quiet to hear it can be enhanced in the recording). It seems pretty clear that this is a real technique used by the police, but I doubt they get many chances to actually use it.
edit on 2/9/2013 by ChaoticOrder because: (no reason given)



posted on Sep, 2 2013 @ 12:36 AM
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Well I just did a quick search and came across a BBC article where they explain the process in a bit more detail. They say that an audio recording device anywhere near a mains power source will be affected.

For the last seven years, at the Metropolitan Police forensic lab in south London, audio specialists have been continuously recording the sound of mains electricity.

It is an all pervasive hum that we normally cannot hear. But boost it a little, and a metallic and not very pleasant buzz fills the air.

"The power is sent out over the national grid to factories, shops and of course our homes. Normally this frequency, known as the mains frequency, is about 50Hz," explains Dr Alan Cooper, a senior digital forensic practitioner at the Met Police.

Any digital recording made anywhere near an electrical power source, be it plug socket, light or pylon, will pick up this noise and it will be embedded throughout the audio.

This buzz is an annoyance for sound engineers trying to make the highest quality recordings. But for forensic experts, it has turned out to be an invaluable tool in the fight against crime.

While the frequency of the electricity supplied by the national grid is about 50Hz, if you look at it over time, you can see minute fluctuations in the order of a few thousandths of a hertz.

"It's because the supply and demand is unpredictable," says Dr Cooper.

If millions of people suddenly switch on their kettle after watching their favourite soap, the demand for electricity may outstrip the supply, and the frequency will go down.

But when electricity generation is greater than demand, the frequency will rise.

"The grid operators will try and compensate for this, but you can sometimes see some very significant fluctuations," explains Dr Cooper.



Philip Harrison, from JP French Associates, another forensic audio laboratory that has been logging the hum for several years, says: "Even if [the hum] is picked up at a very low level that you cannot hear, we can extract this information."

It is a technique known as Electric Network Frequency (ENF) analysis, and it is helping forensic scientists to separate genuine, unedited recordings from those that have been tampered with.

Dr Harrison said: "We can extract [the hum] and compare it with the database - if it is a continuous recording, it will all match up nicely.

"If we've got some breaks in the recording, if it's been stopped and started, the profiles won't match or there will be a section missing. Or if it has come from two different recordings looking as if it is one, we'll have two different profiles within that one recording."

In the UK, because one national grid supplies the country with electricity, the fluctuations in frequency are the same the country over. So it does not matter if the recording has been made in Aberdeen or Southampton, the comparison will work.

Elsewhere around the world, it is slightly more complicated because some countries can have two or more grids. But in these cases, all it takes is for the hum to be continuously logged on each power system and for a recording to be compared against each of them.
edit on 2/9/2013 by ChaoticOrder because: (no reason given)



posted on Sep, 2 2013 @ 01:58 AM
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Originally posted by ChaoticOrder
Well I just did a quick search and came across a BBC article where they explain the process in a bit more detail. They say that an audio recording device anywhere near a mains power source will be affected.

For the last seven years, at the Metropolitan Police forensic lab in south London, audio specialists have been continuously recording the sound of mains electricity.

It is an all pervasive hum that we normally cannot hear. But boost it a little, and a metallic and not very pleasant buzz fills the air.

"The power is sent out over the national grid to factories, shops and of course our homes. Normally this frequency, known as the mains frequency, is about 50Hz," explains Dr Alan Cooper, a senior digital forensic practitioner at the Met Police.

Any digital recording made anywhere near an electrical power source, be it plug socket, light or pylon, will pick up this noise and it will be embedded throughout the audio.

This buzz is an annoyance for sound engineers trying to make the highest quality recordings. But for forensic experts, it has turned out to be an invaluable tool in the fight against crime.
.........
.........

It is a technique known as Electric Network Frequency (ENF) analysis, and it is helping forensic scientists to separate genuine, unedited recordings from those that have been tampered with.

Dr Harrison said: "We can extract [the hum] and compare it with the database - if it is a continuous recording, it will all match up nicely.

"If we've got some breaks in the recording, if it's been stopped and started, the profiles won't match or there will be a section missing. Or if it has come from two different recordings looking as if it is one, we'll have two different profiles within that one recording."

In the UK, because one national grid supplies the country with electricity, the fluctuations in frequency are the same the country over. So it does not matter if the recording has been made in Aberdeen or Southampton, the comparison will work.

Elsewhere around the world, it is slightly more complicated because some countries can have two or more grids. But in these cases, all it takes is for the hum to be continuously logged on each power system and for a recording to be compared against each of them.
edit on 2/9/2013 by ChaoticOrder because: (no reason given)


---

50 Cycle Hum for you Euros and its 60 cycle hum for us North Americans!

AND....It's the BANE of every audio engineer worth their salt!

To get rid of it we use a technology originating from Electric Guitar Manufacturing
call "The Humbucker"

See Humbucker
en.wikipedia.org...

Which can cancel it out on analog circuit by using a 180 degree out of phase
version of that same hum. MY ears are so good I hear it EVERYWHERE there
are improperly GROUNDED home mains or commercial distribution power.

---

As a forensic tool, it is one method to use to determine if audio has been
EDITED by amateurs....BUT US SMART PEOPLE KNOW to edit by hand
the waveform of mains hum on Pro-Tools or Sound Forge so that it STAYS
at a normal sinusoidal or base multi-frequency noise signature.

And my favorite suggestion to get around forensics is to record in a studio
with grounded mics using humbuckers and NOISE FLOOR gate and/or notch
filters and then MIX in mains hum or ADDED noise floor LATER on a continuous basis.

We can get around the non-audible digital signatures ADDED IN by protools
or sound forge used to watermark their audio files by using STRIPPER FILTERS
to kill the inaudible digital watermarks. This prevents forensics for identifying
edited wavefiles via software-based audio watermarking.

In my case I use MY OWN CODED audio waveform editing software
which creates custom wavefiles with NO WATERMARKING and my own custom
"atmospheric" mains hum and sound floor signatures. Ironically the MODERN human
ear has become so accustomed to sub-sonic noise and hum that IF YOU DO NOT
add it in on your Pro-Level recordings the music or voice-overs sound "UnNatural"!
edit on 2013/9/2 by StargateSG7 because: sp.



posted on Sep, 2 2013 @ 02:23 AM
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reply to post by StargateSG7
 


---

as a broadcast professional, I also see 50Hz and 60Hz hum on BOTH analog and digital
VIDEO recordings manifested as a series of wavy sinusoidal lines that move top to bottom
and side-to-side...THAT TOO can be used to determine if a video file has been edited..
...but of course....I can used Digital Signal Processing software and edge detection
using SOBEL horizontal and vertical convolution filters to find those wave forms,
edit them out via software-automated rotoscoping and then alpha key some
"real" video back into the areas of each video frame that had the video hum.
THEN i'd add in my OWN 50/60Hz video hum to make it LOOK LIKE it was unedited.

And again those Final Cut Pro, Avid, Adobe Premiere, Lightworks or Sony Vegas video
editing tools CAN ALL add their own digital watermarks to the video timelines and thus
require those video watermarks stripped out using DSP processing and edge detection
to remove them from your final masters. Add in your OWN video hum and video watermarking
using a Traveling Matte alpha key so as to fool the experts!

Again it's nice that I use my OWN CUSTOM VIDEO EDITING software to get around all those
pesky digital watermarking schemes...I even have to BYPASS Windows DirectX and Quicktime
media layers in order to prevent THAT low-level file input layer from adding the manufacturers
mandated secret watermarks. i.e. I use my OWN custom coded video file readers/filters
or presentation managers.

You just CANNOT trust government because THEY ABSOLUTELY WILL goto almost
ANY LENGTHS to invade your privacy! So go custom LINUX if possible. I build my
own OSes so I KNOW what goes in my software ON EVERY SINGLE LINE OF CODE !!!
AND I also read the assembler output to check that my compiled source code output is STILL GOOD!

It's kind of sad that I have to goto such ridiculous lengths...BUT...i'm one of the more paranoid types!



posted on Sep, 2 2013 @ 02:43 AM
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And for all you more paranoid types, COMPRESSION artifacts introduced by the
MP3 audio compression algorithms use methods of throwing away unneeded or
inaudible acoustic data that create the audio version of MACROBLOCKING:

See weblink:
en.wikipedia.org...

and the positioning and binary pattern OF THOSE audio-macroblocks
when graphed on a frequency chart will create a visible and repeatable
pattern such that it could be used to identify whether or not an MP3
file has been tampered with or edited.

The same is TRUE for VIDEO FILES, where the positioning, size and time-domain
repetitiveness of those VIDEO COMPRESSION MACROBLOCK ARTIFACTS
can be graphed to see if a VIDEO FILE has been tampered with.

---

My suggestion to get around that type of tracking is to record EVERYTHING
FULLY UNCOMPRESSED! Both audio and video at full bit-width and a high sample
rate which is THEN edited and audio/video compressed at only the LAST STAGE before final release!

----

It's kinda fun being EVERY audio/video forensic analyst's WORST NIGHTMARE!

Room Noise signatures.

Audio/Video macroblocking analysis.

Digital Watermarking detection.

Colour-Space Processing artifacts detection to determine originating software.

Reversal of Convolution filters and reversal of Iterative SFX and VFX algorithms.

sub-sonic and sub-visual artifacts detection.

you name it I've got experience is mitigating it, avoiding it AND FAKING IT!!!!
edit on 2013/9/2 by StargateSG7 because: sp.



posted on Sep, 2 2013 @ 04:35 AM
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This seems legitimate. Of course, you need the hum of the grid on your audio recording to make it work. Probably wouldn't be overly hard to implement either, I think I could do it.
edit on 2/9/13 by C0bzz because: (no reason given)



posted on Sep, 2 2013 @ 05:29 AM
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reply to post by TheRedneck
 



That trace is not of a sine wave. It is of a noise signal.

The trace at 1:27 seems to be grid frequency versus time.

The trace at 2:54 is a frequency analysis of part of the phone call. In the frequency domain there will be peaks at the frequency of the input signal. For example with a 50 Hz input signal in the time domain, there will be peaks at +/- 50 Hz which is exactly what you see in the video. You can convert a regular sine wave at 50 Hz into the frequency domain by applying the Fourier transform.

For example:


source

Of course, applying the Fourier transform will destroy time information, however the input signal could be sampled at discrete intervals, with the frequency spectra of each sample taken. Which will effectively tell the frequencies in the input signal at each point in time. The frequency of the peak in magnitude between say, 49 Hz and 51 Hz, could then be taken for each sample, thus the precise frequency of this approximately 50 Hz hum could be measured with time. Correlation code could then be developed to compare it to known frequencies of the electric power grid to find when they match, therefore finding the time at which the call was made, which is exactly what you see at 3:14.

Wouldn't be hard to develop. Also if anyone wants to pay me $75 for working MATLAB code let me know.



Electrical appliances do not produce an audio tone correlating to the frequency of the applied power.

Plenty of them do. Transformers, synchronous motors and induction motors would. Power transformers are also typically pushed into saturation which causes a source of voltage distortion leads to harmonic distortion which can also sometimes be heard.


Mains hum, electric hum, or power line hum is an audible oscillation of alternating current at the frequency of the mains electricity, which is usually 50 Hz or 60 Hz, depending on the local power-line frequency. The sound often has heavy harmonic content.

en.wikipedia.org...


Also thanks, you made me realise how my signal processing knowledge has merged relates my power systems knowledge.
edit on 2/9/13 by C0bzz because: (no reason given)
edit on 2/9/13 by C0bzz because: (no reason given)



posted on Sep, 2 2013 @ 07:34 AM
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Originally posted by TheRedneck
No.

Just no.

If I explained everything wrong with that video, it would fill 3 libraries and 7 encyclopedias.
I think you just don't understand it, which your explanation of the problems with it seems to confirm. There's nothing shown on that video that will blow any circuit breakers.

Besides, they demonstrated successfully how it works, and nailed the date and time of the test within 2 minutes (probably precisely as they suggested the investigator's clock was probably 2 minutes off), so I don't know why you're so skeptical.

However the one part of the video I find hard to come to grips with is the claim that they don't already know when phone calls are made.

The phone company records have dates and times, and my answering machine notes dates and times, so it's not like we really even need this in most cases...seems like it might only be useful for a recording made at an unknown time, where they want to determine the time.

The AC buzz is usually not very audible, but if you have a fluorescent lighting system that has seen better days, sometimes the buzz can get pretty loud.



posted on Sep, 2 2013 @ 09:18 AM
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reply to post by C0bzz

Unless something has changed in the last few years, the frequency range of the NA grid ranges between 59.9 and 60.1 Hz, and a lot of time and money is spent to keep it at precisely 60 Hz. This is necessary to keep power loss to a minimum; transformers are designed to have peak efficiency at 60 Hz. With such a tiny bandwidth, that signal would have to be multiplied quite a few times, and that introduces error each time.

I can only presume since transformer physics are not limited to America, this also applies to the 50-Hz frequency used in Europe.

Now assume that a power transformer is putting out hum from the 50/60 Hz line (which is an inefficiency in the transformer, which means it is wasted power; ever known an electric company to be OK with wasting power?). That hum, which is said to be inaudible in the video, somehow escapes the noise-reduction circuitry in the phone, is recorded digitally (more error since the digital coding will by definition quantize the signal), converted back to analog, amplified thousands of times to get a viewable trace (x% THD each time), and then compared to a log of the power grid frequency of 0.2 Hz bandwidth.

You do bring up a good point. I believe the signal compression algorithm uses Fourier transforms, which would further destroy any such indicators. Not 100% sure about that, tbh, but I do recall reading somewhere that the signal compression is similar to mp3 compression techniques, which are based on Fourier transforms. In addition, it would be far far easier to simply look at the logs of the time of the call based on number called from and to. This is all maintained by the phone companies already. Why go through all this instead of just reading the phone company logs?

It's like laying a newspaper on the ground outside your home, covered with a semi-translucent plastic sheet, pulling up Google Earth on the computer to view it, then using expensive graphics software to render the image viewable again so you can read the paper. JUST READ THE NEWSPAPER.

I repeat, no. This is nothing more than either the rantings of someone trying to drum up ratings by playing to the CT crowd or someone wanting to convince people to stay under control because their every action can be traced.

TheRedneck



posted on Sep, 2 2013 @ 09:31 AM
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Originally posted by TheRedneck
That hum, which is said to be inaudible in the video, somehow escapes the noise-reduction circuitry in the phone, is recorded digitally (more error since the digital coding will by definition quantize the signal), converted back to analog, amplified thousands of times to get a viewable trace (x% THD each time), and then compared to a log of the power grid frequency of 0.2 Hz bandwidth.
I'm totally baffled why you see problems with this...as previously posted the changes in frequency are far smaller than +/- 0.1 Hz.


Originally posted by ChaoticOrder

if you look at it over time, you can see minute fluctuations in the order of a few thousandths of a hertz.


So you seem to be implying that the frequency is so steady it will be hard to see fluctuations. Yet they are saying the frequency is even steadier than you're saying since they are looking at fluctuations orders of magnitude smaller than what you're talking about, so there's quite a disconnect here.



posted on Sep, 2 2013 @ 09:32 AM
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reply to post by Arbitrageur

Besides, they demonstrated successfully how it works, and nailed the date and time of the test within 2 minutes (probably precisely as they suggested the investigator's clock was probably 2 minutes off), so I don't know why you're so skeptical.

Did you say "it's on TV so it must be real"?

Hint: Jessica Alba didn't really turn invisible when she was Sue Storm in the Fantastic Four.


The AC buzz is usually not very audible, but if you have a fluorescent lighting system that has seen better days, sometimes the buzz can get pretty loud.

The ballasts in a fluorescent fixture do have higher frequency shifts than the AC mains, but where is the log of those shifts with time to compare them to? The conversion to high voltage within a ballast involves a good bit of phase shift and HV leakage (hum) in order to reduce size and cost, and would drown out any AC main hum. And even if you do have logs of all the ballasts and their exact frequencies, which ballast is producing the recorded hum?

As I said, there are so many things wrong here I don''t even know where to start.

TheRedneck

ETA: In response to the post directly above, look up Total Harmonic Distortion (THD). Every time a signal is amplified, a tiny amount of distortion is introduced... usually not enough to be audible, but over the amplification you describe, enough to make identification of such patterns impossible.
edit on 9/2/2013 by TheRedneck because: (no reason given)



posted on Sep, 2 2013 @ 10:10 AM
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reply to post by TheRedneck
 


The Signal Processing would be entirely digital. Therefore everything about converting back a signal to analog so it can be viewed on a scope is irrelevant.

Also all transformers that I have used will emit a hum. Actually it's pretty typical to be able to feel the vibrations physically when adjusting a continuously variable transformer.

By ballast do you mean a choke? They cause a phase shift not a "frequency shift".

What does bandwidth have to do with anything? You're not trying to send data at 50 Hz you're trying to find the exact frequency of the ~ 50 Hz signal.

Noise reduction circuitry is usually designed to filter out noise, rather than physical sound that just happens to be inaudible to humans.

How would taking the Fourier transform of a signal destroy information? It would destroy time information but not much else.
edit on 2/9/13 by C0bzz because: (no reason given)
edit on 2/9/13 by C0bzz because: (no reason given)



posted on Sep, 2 2013 @ 11:07 AM
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Also:


The Electrical Network Frequency (ENF) criterion is a novel method for digital audio recording authentication in the field of forensic science. Both an accurate frequency estimation method and a reliable frequency reference database are the key requirements for this technique. This paper briefly introduces the Frequency Monitoring Network (FNET) at UTK and analyzes the frequency characteristics of the four North American interconnections. Wide-area frequency measurements in each interconnection conform to the Gaussian distribution, but with slightly varied parameters. Short-time Fourier transform (STFT) is adopted to estimate the power system frequency signal embedded in audio files, and a procedure for using the ENF criterion, ranging from signal preprocessing to frequency estimation and frequency data matching, is proposed and then tested by two cases. Results show that the STFT can be used as an accurate ENF extraction method. Furthermore, factors which influence the accuracy of frequency estimation, such as the signal-to-noise ratio (SNR) and the recording hardware, are also discussed.

ieeexplore.ieee.org...



A novel forensic tool used for assessing the authenticity of digital audio recordings is known as the electric network frequency (ENF) criterion. It involves extracting the embedded power line (utility) frequency from said recordings and matching it to a known database to verify the time the recording was made, and its authenticity. In this paper, a nonparametric, adaptive, and high resolution technique, known as the time-recursive iterative adaptive approach, is presented as a tool for the extraction of the ENF from digital audio recordings. A comparison is made between this data dependent (adaptive) filter and the conventional short-time Fourier transform (STFT). Results show that the adaptive algorithm improves the ENF estimation accuracy in the presence of interference from other signals. To further enhance the ENF estimation accuracy, a frequency tracking method based on dynamic programming will be proposed. The algorithm uses the knowledge that the ENF is varying slowly with time to estimate with high accuracy the frequency present in the recording.

ieeexplore.ieee.org...


Lastly,


Recently, Dr Cooper was called as a witness on ENF in court for the first time.

A gang were accused of selling weapons, and undercover police had recorded an arms deal. But the defence claimed that the police had tampered with the recording and had edited together several separate recordings together to make their case.

Dr Cooper said: "The defence made the allegations and we were asked if we could authenticate the recordings.

"We carried out various forms of analysis, including the mains hum frequency analysis and we found some good quality signals, and that the alleged date and times of the recordings matched with the extracted data from the recordings themselves."

The analysis revealed that the recordings had not been tampered with - and this proved crucial to the trial. The trio - Hume Bent, Carlos Moncrieffe and Christopher McKenzie - were found guilty and jailed for a total of 33 years for being involved in the supply of firearms.

www.bbc.co.uk...
edit on 2/9/13 by C0bzz because: (no reason given)



posted on Sep, 2 2013 @ 11:14 AM
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Originally posted by TheRedneck
Did you say "it's on TV so it must be real"?

Hint: Jessica Alba didn't really turn invisible when she was Sue Storm in the Fantastic Four.
Are you comparing a BBC documentary to a science fiction show?

If you don't want to be taken seriously, compare apples and oranges like that. I try to look for factual errors in documentaries, and sometimes find them as a result of taking a complex subject and trying to dumb it down for the general public. So they can be somewhat in error, but I don't think it's intentionally so in most cases.

In this case, you're basically suggesting the demonstration is a complete hoax, and I see no reason to believe that, especially when nearly all the objections you're making don't seem to be relevant objections. You do have one point about whether the volume level of the buzz will be sufficient to appear on every recording...I think that's a fair comment, and I don't think the documentary stated that every recording ever made will always have the buzz audible even after enhancement. In some cases it will be detectable, and when it is, I see no reason why the technology won't work as claimed, and demonstrated in the video.

We also agreed about the phone company records having times, so I think that part of the presentation in the documentary was a bit muddled, implying that this information can't be gained from phone records. However the demonstration was more realistic, where a recording was presented, and they didn't know when the recording was made, until they analyzed the frequency variation. So even though they muddled the description of the application in the narrative, it was somewhat clarified by the demonstration.



posted on Sep, 2 2013 @ 12:38 PM
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reply to post by C0bzz

The Signal Processing would be entirely digital. Therefore everything about converting back a signal to analog so it can be viewed on a scope is irrelevant.

But the initial conversion to digital format is relevant. In order to digitize a signal, that signal must be broken down into quantized packets, in the process losing some of the original analog data. For voice transmission, or even more difficult signals, sufficient bits can be employed to minimize the information loss, but when the signal component is inaudible that loss can easily be enough to remove any usable information.

That trace was not a digital signal... it may have been digitized inside the scope after amplification, but the input was analog. I will concede that the D-A conversion is not absolutely necessary.


Also all transformers that I have used will emit a hum. Actually it's pretty typical to be able to feel the vibrations physically when adjusting a continuously variable transformer.

That is energy leakage you're feeling, and yes, it does exist. It is an unwanted energy waste in the transformer. Even if line transformers are the most efficient device known at this time (which I believe they are), they are not 100%. The energy lost is in the form of heat and vibration from the escaping magnetic fields.

Those magnetic fields are wasted energy and therefore are held to as low a level as humanly possible. Newer transformers tend to put out less than older ones, due to advances in shielding. Magnetic fields are also inversely proportional to the square of the distance from the source, meaning they become irrelevant after a fairly short distance.


By ballast do you mean a choke? They cause a phase shift not a "frequency shift".

I mean the high-voltage step-up transformers and associated circuitry used to power fluorescent lighting. It does act as a "choke" (inductor), and yes, it will be a phase shift instead of a frequency shift. My bad on the terminology there.

The phase shift will affect the resonance of the signal, however, and would cause interference with any power line component introduced, which would cause a slight frequency shift in the reading due to the malformation of the interfering waveforms.


What does bandwidth have to do with anything? You're not trying to send data at 50 Hz you're trying to find the exact frequency of the ~ 50 Hz signal.

The bandwidth affects the accuracy of the signal. Just as there is more detail in a 10 MPx camera image than in a 1 MPz image, there is more accuracy and thus usable information in a higher bandwidth signal than in a lower bandwidth signal.


Noise reduction circuitry is usually designed to filter out noise, rather than physical sound that just happens to be inaudible to humans.

Noise can be audible to humans. Noise filtering involves detection of constant signals (such as a 50-Hz hum) and removing that particular frequency, as well as using feedback recognition techniques to remove noise caused by feedback.


How would taking the Fourier transform of a signal destroy information? It would destroy time information but not much else.

Compression of any data using lossy compression techniques removes information. mp3 compression (which is based on Fourier Transforms) is lossy.

TheRedneck





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