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A Question Regarding Internet Signal Connection

  1. Jun 23, 2012 #1
    I'd like to preface by saying that I'm not excellent at physics, so I"m still trying to figure this out. I know that when you have an internet connection you have multiple "channels" open to deliver data. The same with audio wires. I was listening to a podcast and they were talking about the new movie "Brave" being shown with a new Atmos technology, which used 62 channels. Even Television channels all are present but you have to 'listen' in on the right frequency... So my question is, what is happening here? What is being sent down the wires? Electrons, and what determines how many channels you can have on a wire? and if you have one bunch of elections at a certain frequency for channel X, and another bunch of electrons vibrating at a another frequency for Channel Y, how do the two channels not interfere with each other? Especially if there are a lot of channels? I tried reading through a few wiki articles, but it was hard to get through..
     
  2. jcsd
  3. Jun 23, 2012 #2
    Newtons Apple,
    This is a huge topic...

    First, individual electrons in the wire are not "assigned' to different channels.
    We use the motion of electrons in a wire to convey a signal. The signal can be thought of as "amplitude vs. time", with the amplitude often being voltage but sometimes current. Think about Morse telegraphy back in the mid 1800s. When the telegraph key was depressed, current flowed in the wire. When it was released, current stopped. The "signal" in this case is a "current vs. time" waveform.

    Next, how do we embed useful information onto our signal? Again think about Morse, if I want to send the letter "S" I send 3 short bursts of current (dit, dit, dit). Today there are many, many different ways of embedding useful information onto signals, and a huge alphabet soup of acronyms to go with them. Most of them involve converting our signal (audio, video, image ...) into pure data. This is another big topic called coding.

    How do we provide multiple channels on the same wire? Think of an overworked telegraph operator back in the 1860s with two rich customers, each wanting to send a long message.
    Both customers want their message sent first. The telegraph operator decides to send one sentence of one message, then one sentence of the second message, and continue this ping-pong until both messages are sent. At the receiving end, both receiving customers more or less get their messages delivered at the same time. Of course there would have to be a pre-arranged code to indicate the switch, this is called "overhead". Today, the sending/receiving is automated and very fast. We can break up each channel's data and send it piecemeal, interleaved with other channel's data. The receiving end, by monitoring the overhead, knows how to reassemble the pieces. This is called time division multiplexing (TDM).

    Then there is frequency division multiplexing. Imagine two opera singers, each of which can cause a wine glass to break by singing just the right pitch. The first opera singer is singing an "A", the second is singing a "C". One wine glass is resonant at "A" and the other is resonant at "C". The air molecules that convey the sound are not either vibrating at "A" or "C", they all vibrating together in in a manner that is the sum of the tones. The "A" resonant wine glass feels both the "A" and the "C" vibrations, but because it is resonant at "A" it will build up a symphathetic vibration at "A" and eventually break. Radio & TV signals are similar, the frequencies all coexist, but tuners in the receivers are designed to be sensitive to (resonate at) a desired frequency.
     
    Last edited: Jun 23, 2012
  4. Jun 24, 2012 #3
    THanks for the thought out reply! It helps!
     
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