## Communication through cables and optical fibres.

Hi,
I'm in the first year of my engineering course and I've only completed the first four months of the course. As you can imagine,I don't have a very good grasp over my subjects yet,but I'm trying really hard!
In our physics syllabus,we have a few things about communication through coaxial cables,optical fibres etc. about which whenever I look up in library books I find things which are beyond my level.Please give me a basic explanation of certain things I really must have a good idea about,I don't need all the mathematical descriptions-just the intuitive idea.
Please bear with me while I cover a very short portion of my course and tolerate my dumb questions!

Firstly,I have trouble understanding how pictures,movies and sounds can be represented as electrical signals.Then,in fibre optics,they are represented as light waves and certain systems like 'decoding' and 'encoding' systems are responsible this interconversion--what is basically happening in these systems?

Also,what does 'digitization' of information mean? How can light waves carry information in the form of 'on-off pulses' called bits and bytes? (Aren't bits and bytes used in computers??)

Finally, antennae are responsible for generating waves-- after transferring the waves into the cable,does the electrical wave propgate through the oscillation of electrons of the metal or the oscillations of the atoms in the metal?

Please help me out! As you can see, I lack a basic undersanding of electrical communication--I'm in deep waters!
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 Suppose we can adequately represent the color of a single pixel in a flat panel display as various combinations of a red, green, and blue LED. Suppose also that the brightness of each LED can be on an intensity scale of 0 to 1024 (called 10-bit because 210 = 1024). So by communicating three 10-bit words containg the three color intensities is sufficient to send this information (plus parity bits, start bits, stop bits). The conversion of the analog color values to digital values is done by an ADC (analog to digital converter) like in digital cameras. Bob S

## Communication through cables and optical fibres.

Thanks MATLABdude and Bob S.
I do realise that my questions cover a very broad area of study,which makes answering them quite tiresome.I have read through the links you gave me and they have certainly helped a lot,but even after that I can't understand how light waves can be used to transmit sounds,since we can't have electromagnets or piezoelectric devices to respond to light waves.

Also,in regard to propagation through antennae, I found a basic explanation of it in http://wiki.answers.com/Q/How_does_antenna_work ,but I have not had my doubts cleared.

In regard to propagation of electromagnetic waves through coaxial cables,or optical fibres, I have a big problem in understanding modes,which infact we have in detail in our syllabus. Referring to http://www.tpub.com/neets/tm/106-10.htm , it says that there are 4 modes in which electromagnetic waves can be transmitted-TEM,TE,TM and hybrid--how can waves be transmitted when the waves in the transverse modes run along the diameter of the cable? How are these modes qualitatively different?

Lastly, in optical fibres, light travels by total internal reflection--microwaves are also electromagnetic waves--can we say that even microwaves travel by total internal reflection?

Please don't get frustrated by my endless questions,please just give me basic explanations wherever possible,I'll carry on trying to find more information on the net in the meantime.
 The current in the circuit is totally driven by the potential difference caused when the electromagnet in the diaphragm accelerates through the coil, so it is alternating, and contains all the information about the sound. If these electrons are oscillated up and down an antenna, they will radiate electromagnetic waves at the frequency of their oscillation, so the waves will in some way represent the pressure field at the diaphragm. I think this is correct...
 So I may conclude that it's the electrons in the antenna that oscillate to produce waves and not the atoms of the metal (which would cause the vibration of an air column around them,producing sound waves). Also, this variation in the electromagnetic field caused by the electromagnet cannot be caused by light waves,so my question remains as to what allows light waves to porpagate audio-visual signals. Please could someone also elaborate on modes of waves,I've already read a few pages on them (one page,I've mentioned about in my previous post),but I still can't get things clear.

Yes, you can definitely conclude that- the atoms do vibrate due to thermal excitations (in fact you can call these sound waves in a solid), but that is not what carries the alternating current- it is the free electrons of the conductors that flow back and forward radiating EM waves in the antenna.

 Quote by Urmi Roy Also, this variation in the electromagnetic field caused by the electromagnet cannot be caused by light waves
Why not?

audio -------(microphone electromagnet)---> current ---(antenna)---> EM waves ----(areal)---> current -----(loudspeaker electromagnet)----> audio waves
 Thanks for confirming that I've finally got it right, MikeyW. Also,I think I have finally understood how light waves can propagate audio-visual signals--the vibrating membrane (vibration caused due to pressure of sound waves) produce electromagnetic waves (at a particular wavelength,the electromagnetic waves may be light waves) and these light waves cause vibrations of the electrons in the recieving antenna,which again cause a sound-producing membrane to vibrate--finally reproducing the sound. Is that right? If that's okay,I can perhaps bring a few more points into consideration....most importantly,I want to repeat my question as to the basic difference between the method of propagation of light waves and microwaves....don't they both travel by total internal reflection? Also,what is the basic difference between wave guides and other cables,why are optical fibres smaller in dimensions than a coaxial cable, since the dimensions of a wave-carrier are fixed by the waves travelling within them—since wave guides and optical fibres the same kind of waves,shouln't their dimensions be alike? Finally,what enables optical fibres carry more information at a time than waveguides or coaxial cables--is it that we can have many light waves travelling through an optical fibre at a time,and why do the losses become higher at higher frequency of the wave?
 Can someone please come back to me,here!! I desperately need some help,else I'll have to cram everything in my book before the exams!!

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 Quote by Urmi Roy Can someone please come back to me,here!! I desperately need some help,else I'll have to cram everything in my book before the exams!!
Unfortunately, I think you're chasing down the individual trees, and not seeing the forest as a whole. Perhaps my experience is outmoded, or perhaps you have a program which is radically different from the way my introductory courses were scheduled, but the intro courses were there to develop the intuition that you took later on, and eventually said, "Ah ha!" to in an Epiphanic moment where everything suddenly makes sense once all the background pieces fit into place.

That said, it sounds as if you perhaps skipped some lectures or missed some notes / discussion and now no longer know what it is that you should know, nor what to focus on in your suddenly-very-large textbook. If this is indeed the case, I would suggest doing both of the following:
1) Consult the course syllabus
2) Send a polite, non-panicky well-phrased e-mail to your professor / TA asking where in the syllabus you are now at (EDIT: and where the exam cut-off point might be), and if you might be able to ask them to clarify one or two things (and actually make it one or two things; don't swamp them with everything in the course)

...Unless this is one of those survey everything (but nothing in depth) type courses. In which case, I quote a History teacher of mine from high school:
"First, I took summary notes on what I'd read, and what was given in class. Then, every week, I'd take summaries of the notes. Then, at the end, I'd summarize the summaries, and study them and the first-order summaries!"

Regarding one or two of your most recent set of questions; an optical fiber is a type of waveguide: one for light. Typically, when you have audio that travels over light, it's possible to modulate very high frequency noise with the low-frequency audio (my friends made one for their final year electronics project), but it's typically digitized first. It's a whole lot easier to send a 1 (burst of light) and 0 (no light) versus analog light levels.

Consider visible light (electromagnetic waves of optical frequency). Their wavelength is a few hundred nm (let's assume 600 nm--orangish red). Using the formula speed of light = wavelength * frequency, this works out to 5 x 10^14 Hz, 500 THz! In contrast, most coax (of the electrical variety) is usually rated to carry only a few gigahertz (10^9 Hz) The difference is enormous!
 Well, I know I have a lot to learn,and things will get clearer as I learn more,but some things,like those I asked in post no. 8 seem to me to be very basic,which I could understand to some extent even at this stage. Also,reading the last part of your last post, MATLABdude,it suddenly occurred to me that when the pressure oscillations in the air due to audio waves make an electromagnet move,producing electromagnetic waves,the frequency of the generated electromagnetic waves should match the audio waves.......but then we should get different frequencies of electromagnetic waves being produced every moment (to match with the audio waves)!! Also,pleeeeeeease briefly answer my most recent questions,I just cannot stop pondering on them,I'll try to understand as much as I can.
 P.S My problem is intensified by the fact that my teacher is very vague and doesn't seem to be too sure of certain concepts himself.
 Blog Entries: 1 Recognitions: Science Advisor To which of your questions do you refer? You asked a whole lot of them... Also, we're an internet forum. We cannot cover in a few paragraphs what you've covered in weeks and dozens of textbook pages (although we can supplement it).
 Well,you,know,I'm asking these questions from only one chapter in our entire 5-chapter syllabus in physics and our teacher spent perhaps a maximum of 5 lectures on the chapter I'm referring to,which means he spent 2 lectures on the portion including wave-guides upto optical fibres. Again,our physics book is a really quite a thin book,and the chapter in concern was only for 30 pages. Besides, my entire paragraph contains only 3 questions,really-I'll just enumerate them.... 1. What is the basic difference between the method of propagation of light waves and microwaves....don't they both travel by total internal reflection? 2.What is the basic difference between wave guides and other cables, (meaning why are optical fibres smaller in dimensions than a coaxial cable, since the dimensions of a wave-carrier are fixed by the waves travelling within them—since wave guides and optical fibres carry the same kind of waves,shouln't their dimensions be alike?) 3.What enables optical fibres to carry more information at a time than waveguides or coaxial cables (my guess is that it is because we can have many light waves travelling through an optical fibre at a time.)