Naked Science Forum
Non Life Sciences => Technology => Topic started by: neilep on 28/09/2007 15:11:04
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Dearest All (hugs ewe..mwah mwah)
see this Fibre Optic ?
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It's a thing of beauty isn't it ?..just sitting there being all busy whilst looking good !
But how much data can one strand of this stuff deliver ?...how many telephone calls ?..how does the data not get all scrambled with all the other data sent down the strand ?
I want to know and am asking you cos you know stuff !!
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It is very complicated and depends a lot on the size and type of fiber as to its functioning.
I have read this article and it seems to be pretty informative . The info must be carried through the light through out the plastic or glass lines.
Any way here you go. It doesn't answer all of your questions but some!
http://en.wikipedia.org/wiki/Optical_fiber
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Thanks karen....nice frisbee on that link !!
hopfully some exact data will come this way too but I thank you for pointing me to that wiki article.
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Your welcome. It was pretty was it not? I used to be an excellent frisbee participant and was very accurate in my throwing. My favorite time was in a big empty parking lot in town as a teenager on friday and saturday nights My girlfriend who died and I would cruise town trying to meet cute boys..LOL Then when we were done flirting we would park right in the middle of town in a big empty parking lot and we would throw frisbee. In the dark! eventually it was a routine and soon we found it quite fun because there was no need to cruise anymore, because all the guys we wanted to meet came and parked to play frisbee in the middle of town.. At first we got in trouble several times..LOL Then the police came and sent us home, but we came back and played some more and the building owner stood upo for us and let us use his lot as he said we were all good kids and what more could the police want then to have a group of nice kids playing frisbee in a parking lot untill 2:00 in the morning... We could have been doing alot worse! The police begrudgingly gave in after a petition was passed and we played for years in that lot. The kids do not do it anymore.. I remember a really cute fellow in an Bright orange Sting Ray who I really Liked .. LOL He was cute and very nice! LOL I loved his car! LOL.. We did not need fiber optic lines to meet people back then just a frisbee and a smile!
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It is very complicated and depends a lot on the size and type of fiber as to its functioning.
I have read this article and it seems to be pretty informative . The info must be carried through the light through out the plastic or glass lines.
Any way here you go. It doesn't answer all of your questions but some!
http://en.wikipedia.org/wiki/Optical_fiber
It also depends on how long the fibre is (the distance between repeaters) - although this interacts with issues of how thick the fibre is.
There is ofcourse the ultimate theoretical limit of about 1/2 the frequency of the carrier (i.e. 1/2 the frequency of light, so probably in hundreds of Teraherz region). Technological limits that causes light to smear out as it passes down the fibre (combined with the fact that the electronic devices at each end of the fibre cannot cope with such high frequencies) means that the realistic limits are many orders of magnitude less than that.
Very thin, graded (i.e. with a graduation of the refractive index from the outside to the inside of the fibre), you can reduce the amount the light wave smears out.
Ofcourse, the shorter the fibre, the less time for light to smear our as it travels along the fibre.
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George what do you mean by smear out.. do you mean for the light to fade or deminish?? I don't understand the term?
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George what do you mean by smear out.. do you mean for the light to ade or deminish?? I don't understand the term?
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Think of the above as a very crude notion of blobs of light (red dots) representing data travelling down a fibre (represented by the blue).
The first piece if fibre is very close to where the light enters the fibre, and you see the individual blobs of light distinctly separate. As you go down the fibre, you see the light spreading out, until one blob of light becomes indistinct from the next, and you cease to be able to see where the light is on, or the light is off.
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There are a couple of reasons that the light gets smeared out.
Light entering the fibre at different angles will move different distances in the same time because light going in a straight line moves more directly down the fibre.
[diagram=279_0]
So fibres are made very narrow so only light is going in a straight line.
Also different colours or wavelengths go at different speeds, which is a much more difficult problem as modulating the light into pulses spreads the wavelength of the light out, so it is impossible just to use one wavelength. This effect is called chirp.
It is possible to use a length of fibre which has the opposite dependence of speed on wavelength to unsmear the light again.
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Just want to thank you for your continued responses Dave, George & Karen....they do not go unnoticed and I appreciate the time ewe take to respond.
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I still don't understand how it keeps from getting scrambled as Neil asked earlier.....
Your welcome Neil Thanks for all the new topics they are great!
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I still don't understand how it keeps from getting scrambled as Neil asked earlier.....
Colours can be scrambled and unscrambled , can't they.
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How Much data can a strand of this carry ?
i suppose that would depend on how small the lettering was in the books you hang from it. [;D]
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How Much data can a strand of this carry ?
i suppose that would depend on how small the lettering was in the books you hang from it. [;D]
LOL !!..Michael knows the true meaning of my questions !!...I think I'll go back to doing potato prints now!! *claps hands in elated glee*
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I think that a strand of optical fibre can be made to carry just about as much data as you want; eventually.
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I think that a strand of optical fibre can be made to carry just about as much data as you want; eventually.
Well, no - you cannot get around the nyquist limit (although we are presently well short of that limit).
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I still don't understand how it keeps from getting scrambled as Neil asked earlier.....
Your welcome Neil Thanks for all the new topics they are great!
I am not sure what you mean by scrambled?
The issues about blurring (the spreading out of the wave) have been addressed, but ignoring those issues (assuming that the fibres were optically perfect, and very thing, and had equal properties for all frequencies), then it is simply a matter of first in first out (the data will come out of the fibre in the same sequence it went in).
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"Well, no - you cannot get around the nyquist limit (although we are presently well short of that limit)."
I may have misunderstood, but I thought that Nyquist's work showed that the rate o data transfer was limited, not the quantity. If you want you can send all the data you have. It may take a long time if you have a lot of data or a poor bandwidth, but you can do it eventually.
(Slight giggle at the spell checker's ideas concerning "Nyquist")
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I still don't understand how it keeps from getting scrambled as Neil asked earlier.....
Your welcome Neil Thanks for all the new topics they are great!
I am not sure what you mean by scrambled?
The issues about blurring (the spreading out of the wave) have been addressed, but ignoring those issues (assuming that the fibres were optically perfect, and very thing, and had equal properties for all frequencies), then it is simply a matter of first in first out (the data will come out of the fibre in the same sequence it went in).
That is what I meant, thanks for the explanation George I thought it could get all kinds of goofed up and wondered how it is kept straight!
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"Well, no - you cannot get around the nyquist limit (although we are presently well short of that limit)."
I may have misunderstood, but I thought that Nyquist's work showed that the rate o data transfer was limited, not the quantity. If you want you can send all the data you have. It may take a long time if you have a lot of data or a poor bandwidth, but you can do it eventually.
OK, my misunderstanding - I did not apply the word "eventually" in the way that you had intended to mean it.
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The existence of multipath transmission in optical fibres is certainly one limiting factor , as is the dispersion due to changing wave speed.
However, the ultimate information carrying capacity depends upon the way the information is coded and upon the signal to noise ratio. Every channel suffers from noise due to thermal agitation of the atoms / electrons. The actual information on a picture or piece of audio programme is not the same as the number of samples per second times the bits per sample. That is a huge over estimate.
Shannon, in the 1940s, did a lot of theoretical work on information and transmission channels. No communication channel has ever been constructed that remotely approaches the limit proposed by Shannon. We all use data compression when we send files to each other. To get the maximum useful information rate across, you need to take a lot of delay in the system. This allows you to code the information efficiently and to check and correct errors. It's fiendishly complicated but 'they' are making advances all the time. It's why you can get such a lot of programme time on a DVD or MP3 player, these days. Present systems still waste a lot of channel capacity because the coding is not ideal. It gets better all the time, however.
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We all use data compression when we send files to each other. To get the maximum useful information rate across, you need to take a lot of delay in the system. This allows you to code the information efficiently and to check and correct errors. It's fiendishly complicated but 'they' are making advances all the time. It's why you can get such a lot of programme time on a DVD or MP3 player, these days. Present systems still waste a lot of channel capacity because the coding is not ideal. It gets better all the time, however.
MP3 and DVD's (they use MPEG2, do they not) do not use lossless compression, so they are not merely making more efficient use of bandwidth (in the way that Huffman or LZW encoding do), but actually throw away data - hence the Shannon limits are not totally pertinent to them (this is compounded by massive amounts of error correction that reduce the efficiency of bandwidth usage).
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Fair enough - I quoted those well known forms of compression as the lossless compression methods are, perhaps, not so well known.
I would disagree that error correction makes for inefficiency when you bring into the account the signal to noise consideration. You may get garbage out of a system that, with a good snr, is what you would call efficient but which is passed through a noisy channel. The way you demodulate and decode your received signal is the crucial thing in its information rate capacity.
You can cram more and more into a channel and get worse and worse inter symbol interference but, as long as the snr is high enough, you can get all the info out again. That's, basically, what Shannon says and I wouldn't lightly disagree with the Daddy.
To do the above, you need to filter over a longer and longer time interval - which introduces a longer and longer delay into your channel and more complex processing..
Bandwidth Usage is not just a matter of raw data rate per Hz of channel space.
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THANK EWE again for your continued wonedrful posts !!
May I ask please what a NYQUIST limit is?...and does it have anything to do with a flavoured milk drink?
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.aussiefavourites.com.au%2Fcornershop%2Fimages%2Fnesquick250.jpg&hash=8ea2bc8ae0f1c3bb108f78f5ff540372)
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Which I love!
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NYQUIST limit?!?!?!
I have a friend who's last name is Nyquist (a quite commen Swedish name I believe).
Never knew she had a limit.
[;D]
Gosh - what an interesting thread
..... my braincells are devinitely doing backward and forward flips now.
So much to learn - so little time.......
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THANK EWE again for your continued wonedrful posts !!
May I ask please what a NYQUIST limit is?.
http://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem#Nyquist_rate
Nyquist rate
In 1927, Nyquist determined that the number of independent pulses that could be put through a telegraph channel per unit time is limited to twice the bandwidth of the channel. In symbols,
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where fp is the pulse frequency (in pulses per second) and B is the bandwidth (in hertz). The quantity 2B later came to be called the Nyquist rate, and transmitting at the limiting pulse rate of 2B pulses per second as signalling at the Nyquist rate. Nyquist published his results in 1928 as part of his paper "Certain topics in Telegraph Transmission Theory."
and does it have anything to do with a flavoured milk drink?
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.aussiefavourites.com.au%2Fcornershop%2Fimages%2Fnesquick250.jpg&hash=8ea2bc8ae0f1c3bb108f78f5ff540372)
Only insofar as Nestle is a Swiss company, as Monica is Swiss, and:
NYQUIST limit?!?!?!
I have a friend who's last name is Nyquist (a quite commen Swedish name I believe).
http://en.wikipedia.org/wiki/Harry_Nyquist
Harry Nyquist (February 7, 1889 – April 4, 1976) was an important contributor to information theory.
He was born in Nilsby, Sweden. He emigrated to the USA in 1907 and entered the University of North Dakota in 1912. He received a Ph.D. in physics at Yale University in 1917. He worked at AT&T's Department of Development and Research from 1917 to 1934, and continued when it became Bell Telephone Laboratories in that year, until his retirement in 1954.
As an engineer at Bell Laboratories, he did important work on thermal noise ("Johnson–Nyquist noise"), the stability of feedback amplifiers, telegraphy, facsimile, television, and other important communications problems. With Herbert E. Ives, he helped to develop AT&T's first facsimile machines that were made public in 1924. In 1932, he published a classical paper on stability of feedback amplifiers (H. Nyquist, "Regeneration theory", Bell System Technical Journal, vol. 11, pp. 126-147, 1932). Nyquist stability criterion can now be found in all textbooks on feedback control theory.
His early theoretical work on determining the bandwidth requirements for transmitting information, as published in "Certain factors affecting telegraph speed" (Bell System Technical Journal, 3, 324–346, 1924), laid the foundations for later advances by Claude Shannon, which led to the development of information theory.
In 1927 Nyquist determined that the number of independent pulses that could be put through a telegraph channel per unit time is limited to twice the bandwidth of the channel. Nyquist published his results in the paper Certain topics in Telegraph Transmission Theory (1928). This rule is essentially a dual of what is now known as the Nyquist–Shannon sampling theorem.
Nyquist received the IRE Medal of Honor in 1960 for "fundamental contributions to a quantitative understanding of thermal noise, data transmission and negative feedback." In October 1960 he was awarded the Stuart Ballantine Medal of the Franklin Institute "for his theoretical analyses and practical inventions in the field of communications systems during the past forty years including, particularly, his original work in the theories of telegraph transmission, thermal noise in electric conductors, and in the history of feedback systems." In 1969 he was awarded the National Academy of Engineering's fourth Founder's Medal "in recognition of his many fundamental contributions to engineering."
Nyquist lived in Pharr, Texas after his retirement, and died in Harlingen, Texas on April 4, 1976.
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As I remember, the Nyquist limit relates to the sample rate needed to re-construct a signal, perfectly. The limit is two samples for the the highest rate.
In practical terms, you need to filter the signal to be sampled with a filter which lets nothing through above half the sampling rate or you will get aliasing. (Higher input frequencies than this appear as low frequency 'beat' patterns - very disturbing)
This means that you really need a bit more than twice the highest signal frequency. Unless you are sampling a conventional TV signal, which (at least for stationary pictures) has a spectrum which is full of holes, in which case you can do sub-nyquist sampling and get away with it.
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THANK EWE again for your continued wonedrful posts !!
May I ask please what a NYQUIST limit is?...and does it have anything to do with a flavoured milk drink?
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.aussiefavourites.com.au%2Fcornershop%2Fimages%2Fnesquick250.jpg&hash=8ea2bc8ae0f1c3bb108f78f5ff540372)
Nyquist Frequency
http://mathworld.wolfram.com/NyquistFrequency.html
In order to recover all Fourier components of a periodic waveform, it is necessary to use a sampling rate nu at least twice the highest waveform frequency. The Nyquist frequency, also called the Nyquist limit, is the highest frequency that can be coded at a given sampling rate in order to be able to fully reconstruct the signal, i.e.,
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Whoops, Sophiecentaur beat me. and well explained at that!
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As I remember, the Nyquist limit relates to the sample rate needed to re-construct a signal, perfectly. The limit is two samples for the the highest rate.
I think it works both ways - as indicated by my reply above - I believe it is both the maximum information that can be pumped through a noiseless channel, and the sample rate you need to recreate the channel (strictly speaking, one is the Nyquist rate, and the other the Nyquist frequency - neither are formally known as the Nyquist limit).
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I think there are two separate issues here.
The sampling rate thing is more straightforward and relates to the initial process of sampling the original signal - say an audio programme. You can't get rid of aliasing, once you have introduced it when you need to transmit the whole of the spectrum of the source signal; it is a non-linear distortion. (TV is a special case where you can frig it a bit, due to the signal spectrum). This limit is not related to the transmission channel.
The actual capacity of a channel - which is what you would like to maximise, when you are paying for a satellite link, for instance, relates to the total information you can get through each Hz of available bandwidth. This is very much limited by the signal to noise ratio. You can instantly do better than a binary modulation system by increasing the number of levels used but it is clear that noise will start to impinge sooner. There is no real difference between using a multilevel system or using a bandwidth restricting filter in your channel. Inter symbol interference due to narrowing the bandwidth can be coped with by using appropriate temporal filtering at the receive end as long as the channel noise level is low enough
If your demodulation / decoding system is clever enough, you can squeeze a fantastic amount through. Error handling is necessary - to deal with the occasional spike, in all types of noise, but the overhead used for it is offset by the overall increase in useful capacity.
Systems get more and more complex, of course, and the problem of standardisation rears its head.
Wikkers has this to say about it.
http://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem#Comparison_of_Shannon.27s_capacity_to_Hartley.27s_law (http://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem#Comparison_of_Shannon.27s_capacity_to_Hartley.27s_law)
and here's an article about how well they are doing these days.
http://www.sciencenews.org/articles/20051105/bob8.asp (http://www.sciencenews.org/articles/20051105/bob8.asp)
The original question was about a fibre optic system and the full answer must involve the effect of noise.
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Karen W
I like your technical stuff but I have a problem with your picture of Nestle products. They (Nestle) are playing hell with the third world health by marketing formula milk where it is not safe to use because the available water is poor quality - they are 'dissing' breastfeeding in an area where it provides a useful barrier against disease for small babies.
Not relevant to Nyquist but I thought I'd get it off my chest. Is this too political?
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Karen W
I like your technical stuff but I have a problem with your picture of Nestle products. They (Nestle) are playing hell with the third world health by marketing formula milk where it is not safe to use because the available water is poor quality - they are 'dissing' breastfeeding in an area where it provides a useful barrier against disease for small babies.
Not relevant to Nyquist but I thought I'd get it off my chest. Is this too political?
Firstly, the picture was Neil's, not Karen's (Karen merely quoted Neil's post).
Secondly, this is going to go way off tangent, but the issue about Nestle is old news - very old (30 years old, to be exact), and rather over hyped even then. At present, the most recent claim against them (1999, I think) was that they supplied cheap powdered milk to maternity hospitals - this argument seems to treat third world maternity hospitals as if they were children, unable to make their own judgements as to how to use material that is provided cheap or free of charge. This really is in a different league to Microsoft supplying schools with cheap or free software, since schools would anyway use software, it may simply be a different brand - and certainly hospitals will use powdered milk, so there is nothing wrong with supplying them with such, but one has to rely on hospitals to use it sensibly and not overuse it just because it is cheap. If you cannot rely on a hospital to use that kind of judgement, I would be very worried about how hospitals are likely to be dispensing far more dangerous things, like drugs.
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The actual capacity of a channel - which is what you would like to maximise, when you are paying for a satellite link, for instance, relates to the total information you can get through each Hz of available bandwidth. This is very much limited by the signal to noise ratio.
This is the Shannon capacity, which I don't dispute at all anything about what you say.
But the Shannon capacity is, as you say, dependent upon the noise in the channel, and not a physical limit to the channel itself (i.e. new technical innovations can always be used to further reduce noise, but no amount of technical innovation can overcome the Nyquist rate limit - since that is a limit based on the carrier signal selected). The Shannon capacity will always be less than the Nyquist rate because even if you reduce noise to zero (clearly impossible, but at least asymptotically possible), it will still remain limited by the Nyquist limit for the given bandwidth.
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I have to disagree with you on what is implied by the 'Nyquist rate'. There is no reference to the number of levels in the samples to which Nyquist refers. It certainly doesn't only restrict things to two levels. You can have as many levels as you like for your samples - they can, of course, even be analogue samples. Nyquist just says that you need > = two samples per second per Hz of the highest frequency in the baseband signal. That is all. There is unlimited information, possible in each sample, if you have no noise to deal with.
The Shannon capacity relates to the total amount of information what can be carried. The actual information in each of the samples depends upon the accuracy / quantisation / number of bits per sample. So the information capacity is, potentially, much greater than half the Nyquist sampling rate.
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This is a favorite 'Channel Capacity' equation which relates the rate above the Nyquist rate that symbols can be sent. (Looking at the problem in the way another someone stated it in an earlier post.)
Where R is the Symbol rate
Bwis the Nyquist Bandwidth -Samples per second
C is the carrier power
N is the noise power (in the receiver bandwidth)
The symbol rate (Shannon limit) can be arbitrarily higher than the Nyquist frequency as long as your received noise level is low enough.
It is easy to send signals at nearly the Nyquist - defined limit. Most digital systems do just that, by design. Not many systems approach the Shannon limit.
There are systems. now, which get nearly there - space comms systems, for example.
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I have to disagree with you on what is implied by the 'Nyquist rate'. There is no reference to the number of levels in the samples to which Nyquist refers. It certainly doesn't only restrict things to two levels. You can have as many levels as you like for your samples - they can, of course, even be analogue samples. Nyquist just says that you need > = two samples per second per Hz of the highest frequency in the baseband signal. That is all.
I accept your point about the number of levels, but this does not make for infinite numbers of signals, because any modulation of the carrier will create sidebands, and you will have limits to the sidebands (although it is clear that Nyquist does not seem to imply any limit on the sidebands - maybe he has assumed that sidebands reach from zero up to the carrier frequency - which clearly in not the case ever in reality, but maybe the extreme he has assumed). Ofcourse, the closer you get to zero Hz, the less information the sideband can carry, so in the end, most of the information will be carried in the sidebands closer to the carrier frequency.
I have to admit that I don't know the maths well enough to know whether or not Nyquist takes account of the sideband limits.
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Another someone:
but this does not make for infinite numbers of signals
In the end, we are just talking about the total quantity of information; whether it's a single, high definition signal of a lot of low definition signals. Each significant bit of each sample is quite independent of the other bits (well, at least, if you don't restrict your source signal in any way) so we're talking about the total number of bits (for instance) per second of source information.
because any modulation of the carrier will create sidebands,
Yes, of course. If your modulation system gives you a flat spectrum it doesn't really matter where the notional carrier sits - because an efficient system doesn't 'waste energy' by transmitting at the carrier frequency - that is information which doesn't need to be transmitted. The information is carried over the whole spectrum of the transmitted signal.
Nyquist isn't concerned with sidebands when you're sampling, really; that is a concern in the modulation / transmission. Nor is it concerned with noise.
If you take samples (of zero duration) of a baseband signal, you end up with an infinite number of harmonics of the sampling frequency, each of which has sidebands (upper and lower) which mimic the baseband spectrum. It is, effectively, a modulation process. The fact that the lower sidebands obout the sample frequency can overlap the baseband spectrum is what can give aliases, which imposes the Nyquist limit on sample rate. For 'boxcar' samples, the spectrum is not flat.
To get things right and to eliminate the problem, your low pass filter (pre sampling) is given a cutoff frequency of FN/2 and the rolloff, to this is made symmetrical about the -3dB value. This cancels the distortions. Group delay needs to be minimised. There are practicalities in realising this filter, which limits the baseband bandwidth.
All this is before we quantise anything - so, as I said before, there is unlilimited information in each sample at this stage. This is because the more sig figs in the measurement, the more actual info there is.
Once you quantise the signal, you start to limit the information content needed for transmission.
Shannon then starts to come into it. The amount of detail you can transmit is then limited by the symbol rate, and the signal to noise ratio in the available system bandwidth. This bandwidth defining filter is 'shared' between transmitter and receiver, in order to minimise / optimise transmitter power and adjacent channel interference, when it is relevant.
If it is not relevant then the receiver filter defines the system (noise) bandwidth. I think this must be why space comms do so well, because they can afford to put all their filtering in the receiver.
Your comment about sidebands really relates to this filtering and where it is introduced.
Remember, the information carried isn't affected by the actual frequency of each sideband - each Hz of bandwidth, wherever it lies, can carry 1Hz 's worth of info. We may think logarithmically about frequency scales but a Hz is a Hz.
The spectrum occupied by an ideal system would, I suppose extend from zero to the carrier frequency and up to twice this. But things don't have to be symmetrical. AC coupling is bound to impose a lower limit to the rf spectrum -, in any case, not to mention the limit due to interference!
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Optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because light propagates through the fiber with little attenuation compared to electrical cables. This allows long distances to be spanned with few repeaters. Additionally, the light signals propagating in the fiber can be modulated at rates as high as 40 Gb/s [2], and each fiber can carry many independent channels, each by a different wavelength of light (wavelength-division multiplexing). Over short distances, such as networking within a building, fiber saves space in cable ducts because a single fiber can carry much more data than a single electrical cable. Fiber is also immune to electrical interference, which prevents cross-talk between signals in different cables and pickup of environmental noise. Also, wiretapping is more difficult compared to electrical connections, and there are concentric dual core fibers that are said to be tap-proof. Because they are non-electrical, fiber cables can bridge very high electrical potential differences and can be used in environments where explosive fumes are present, without danger of ignition.
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Additionally, the light signals propagating in the fiber can be modulated at rates as high as 40 Gb/s , and each fiber can carry many independent channels,
Yes they are already used to carry a lot of data. The question was / is , what sort of fundamental limit is there? You can be sure that it's possible to do a lot better than is presently achieved.
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We were told on a visit to this place but I forget what was said now.
http://www.porthcurno.org.uk/
Info here..
http://www.emtelle.com/?id=149