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How does a single cable carry signals for the Telephone Intrenet and sometimes TV?

  1. May 22, 2009 #1
    how can a single copper cable carry all the different frequencies and prevent them from interfering with each other and destroying data? (I posted this question in the physics forums because it has to do with waves and frequencies)
     
  2. jcsd
  3. May 22, 2009 #2

    jtbell

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    Re: How does a single cable carry signals for the Telephone Intrenet and sometimes TV

    The different-frequency waves superpose and do not interfere with each other (or at least if they do interfere, it's in ways that can be controlled and limited). They can be separated from each other using tuners and filters, along with the information that they carry. This leads to the various schemes for modulating the information onto the waves, by varying the amplitude, frequency, and/or phase of the waves.
     
  4. May 22, 2009 #3
    Re: How does a single cable carry signals for the Telephone Intrenet and sometimes TV

    So.. superposing is different from interference?
     
  5. May 22, 2009 #4

    QuantumPion

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    Re: How does a single cable carry signals for the Telephone Intrenet and sometimes TV

    Think of it like a sound recording of a rock band. The sound of the bass, guitar, drums, and vocals are all individual sound waves when they are originally performed live. When they are recorded onto a medium, they are added together into a single wave to be reproduced by speakers. Your brain is able to differentiate the different components of the band even though all the sound is a single wave coming from a single speaker.
     
  6. May 22, 2009 #5

    jtbell

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    Re: How does a single cable carry signals for the Telephone Intrenet and sometimes TV

    If you add together (superpose) two or more waves with the same frequency, you get another single wave with that frequency, and you can't separate it back into its original waves. In that case we can call it "interference" of the original waves. If you add together two or more waves with different frequencies, you get a complex waveform that doesn't have a single frequency, and you can separate it back into its original waves. Mathematically, we describe the process using Fourier analysis. Physically we implement it with with electronic filters that pass certain frequencies and block others.
     
  7. May 22, 2009 #6
    Re: How does a single cable carry signals for the Telephone Intrenet and sometimes TV

    This may help you visualize it:
    http://www.colorado.edu/physics/2000/applets/fourier.html [Broken]
    But don't be mislead by the word "destructive" in the title. It's somewhat of a misnomer, since the waves are not really being destroyed (unless both waves are of the same frequency).
     
    Last edited by a moderator: May 4, 2017
  8. May 22, 2009 #7
    Re: How does a single cable carry signals for the Telephone Intrenet and sometimes TV

    There is another type of signal separation on a single pair of copper wires. The simplest example is simultaneous bi directional voice transmission on the standard telephone two-line pair. The two voice signal travel through each other at about 2/3 the speed of light without interfering. Directional couplers on the line can pick off either voice separately. Modems (modulator-demodulators) do this at MHz frequencies.
     
  9. May 22, 2009 #8

    rcgldr

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    Re: How does a single cable carry signals for the Telephone Intrenet and sometimes TV

    Due to a limit of saturation of electrons at any specific point in a wire, wouldn't this affect the actual peak limits from combined signals, resulting in some clipping of the combined signals?
     
  10. May 24, 2009 #9
    Re: How does a single cable carry signals for the Telephone Intrenet and sometimes TV

    Thanks guys!
     
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