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How may signals travel through a cable without interfernce?

  1. Nov 16, 2011 #1
    I am an Electrical Engineering.I want to ask that how different signals move in a cable without effecting each other..e.g interference etc..
    I am not strictly speaking about it interfence but how at the end we can differentiate these signals and get one of our need.
     
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  3. Nov 16, 2011 #2

    sophiecentaur

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    To avoid interference (crosstalk) signals need to be separated in some way. They can be 'carried' on radio frequency carriers using different frequencies (just like the channels on broadcast radio and TV). This is how it used to be done on old fashioned transatlantic telephone and satellite systems. You basically have a set of low power transmitters and a set of receivers at each end of the cable. There is a limit to how close the channels can be spaced (in frequency) due to one signal getting into another receiver filter.
    This is nowadays referred to as Frequency Division Multiplexing. You can also (and this sounds really bizarre) use Time Division Multiplexing in which you switch very rapidly and regularly between a number of signals at the send end and then each receiver gets just one of the signals by looking at the appropriate times, getting a fast string of pulses (samples) of the wanted signal. You don't get anything for nothing, however. Your cable needs to be able to handle all these fast changes due to the switching so it needs to have a wide bandwidth capability.

    These days, of course, digital signals are all mixed together (multiplexed) and interleaved in all sorts of ways so that no one sees the unwanted data - just their own. But, once you get to the level of the actual signal going down the pipe, you are again effectively dealing with a very complex analogue signal which has to be looked after and protected from interference and noise in a similar way. It is said that digital signals are robust and impervious to noise and interference. This is true up to a certain level, after which it goes with a bang (frozen frame on satellite TV etc.). Old fashioned radio and telephony are always subject to interference but they tend to 'die more gracefully' as the noise and interference increase - you can hear and understand stuff that just isn't pleasant to listen to but the info can still get through. Digital systems can handle vastly more useful information than simple analogue systems, though.

    They spend millions of quids on getting systems that will reliably carry as much info as possible. And we use it for conversations like this plus the millions of other (less worthy) messages that tear around the Planet.
     
  4. Nov 16, 2011 #3
    Correct me if I am wrong as I am no expert in communication and modulation, I just join in. Look up orthogonal frequencies which is frequencies that is different in a multiple of an integer, they do not interfere with each other. This is because if you look at the sinc pulse of each frequency, each frequency lies at the zero crossing of the other and has no interaction with the other or something. I read this like 8 years ago. Look up OFDM ( orthogonal frequency division multiplexing), they explain this much better than me. Also you can find the explaination in Fourier Series expansion.
     
    Last edited: Nov 16, 2011
  5. Nov 16, 2011 #4
    The modern digital communication link is nothing digital about it!!! Look at the QAM stuff, it is a mix of two orthogonal frequencies ( 90 deg out of phase) to map each bit in a two dimension plane ( like xy plane) depend on the amplitude of each signal. When you get the QAM 256, each signal is detect and categorized into 8 different levels and this become analog, nothing digital about it.
     
  6. Nov 16, 2011 #5

    sophiecentaur

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    Hence my remark in the middle of my last post " But, once you get to the level of the actual signal going down the pipe, you are again effectively dealing with a very complex analogue signal "

    I restrained myself from spouting any more, waiting until the OP came back with some feedback about the level of answer he needed. I think too much info can scare people off. And we don't want that, do we?
     
  7. Nov 16, 2011 #6
    I reply to your post actually to support your statement that it is nothing digital about the modulation. The only thing good about this over pure analog communication is they have check sum, request to resent if error detected. But like you said, the communication can get slow and all of a sudden just complete failure with funny screen and all.
     
  8. Nov 16, 2011 #7

    sophiecentaur

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    It's quite amazing just how inefficient a straight analogue system is. Every analogue TV frame of a still shot is repeated endlessly, with the same information again and again. No wonder they can fit a dozen or more channels into the existing spectrum used by a single analogue channel.

    The combination of coding and modulation is very clever these days - and there's still a lot of juice left in that lemon, apparently. More channels and better noise performance.
    I was amused to read your "request to resend" statement (a telecoms strategy). As an ex broadcaster, that seems, to me, like a real luxury!
     
  9. Nov 16, 2011 #8

    Don't quote me 100%, yes I said that because of the telecom, every serial communication I've seen have some sort of provision of check sum and request to resent. That to me is a very strong point to make the system more robust. Just when the signal degrades, the communication will get slower and slower and more and more delay.
     
  10. Nov 16, 2011 #9

    sophiecentaur

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    Nice work when you can get it. In broadcasting, you have to plan a one way channel. Real pain.
     
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