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Digital and Analogue Signals

  1. Nov 2, 2004 #1
    Digital and Analogue Signals....

    When learning about digital and analogue signals, I have been told that a fibre optic cable can be used for digital signals, whilst a copper wire is usual used in the case of anaolgue elctrical signals. My question is could they be used for the other things? Ie - can you use a copper wire for a digital signal and a fibre optic cable for an analogue signal? And if not why? Its just people never suggest using a copper wire for a digital signal and vice versa.

    Thanks. :smile:
  2. jcsd
  3. Nov 2, 2004 #2
    Basically, what I am asking (if it is not clear :smile: ) is we always talk about having copper cables for analogue signals and fibre optics containing light for digital signals - why not digital signals in copper cables and analogue in fibre optic cables? After all, surely it would be possible since voltage could go on and off (to produce digital) and we could alter the intensity of light rather than just turn it on and off (for analogue). And thus why do we still use analogue with existing copper cables - since we are trying to replace them with fibre optics anyway surely we could also change the existing ones to digital in order to reduce noise?

    Thanks in advance. :rofl:
  4. Nov 2, 2004 #3


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    Yes, either could be used for either type of signal. Copper is frequently used for digital signals, I am not sure how often fiber optics are used for analog though. The issue for a analog optical signal would be the electronics used to generate and receive the signal. You would need to be able to generate a smoothly varying optical intensity, then have to demodulate it accurately.

    I think the main reason fiber is used for digital signals is simply because it is a technology which has been developed along with the digital revolution. Also digital signals are simply easier to create and decode.
  5. Nov 2, 2004 #4
    Transmitting analogue signal over optical fiber is possible but only limited to a very narrow range of application, mainly in what is called 'radio frequency over fiber'. These are short distance transmission, within a shopping complex for example, of cellular or wireless LAN signal.
    Apart from that, like what Integral had said, encoding and decoding analogue signal is too problematic.
  6. Nov 2, 2004 #5
    I just bought a new receiver for my home stereo, and it has two digital inputs. One being a fiber input, and the other is a spdif digital coax input. It obviously utilizes copper as oppose to fiber, and it says it is digital. How does it work? I am thinking about runing my dvd player's audio into the fiber input, and my digital cable recievers audio signal to the receiver alla digital coax. I have never heard of digital coax before, and I have never had a surround sound system before. Is it really a digital signal? Do digital coax cables produce better sound quality than composite cables. And is the surround sound signal actually sent through this one cable (all five speakers and the sub)? And does the same go for the fiber cable (all five speaker and the sub signals all sent through the one cable)? And should I start a new thread with all these questions? :smile:
  7. Nov 3, 2004 #6
    The reason digital signals over copper wires is used only for short distances is attenuation, or loss of amplitude and precision over distance. This occurs because electrical signals are lost as heat. It is a property that wavelike (analogue) signals don't attenuate as much as discrete (digital) signals. Digital signals over copper are, for example, used in ethernet LANs, which use Manchester encoding.
  8. Nov 6, 2004 #7
    On the topic of analogue and digital, I've been taught that "all digital signals travel further before needing to be amplified regardless of how they are transported" (ie - a digital signal in a copper cable will still travel further than an analogue signal in a copper cable) - why is this the case?
  9. Nov 6, 2004 #8


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    For long distance transmissions over copper, digital is preferable to analog.

    In a long transmission circuit, both analog & digital signals will progressively lose strength and pick up noise and static. For digital transmission, repeaters are placed at intervals along the circuit which reconstruct the original signal (omitting the noise and static) so that the original clear signal is received at the terminating end. In analog transmission, the signal can only be amplified, unfortunately all of the noise and static it has picked up also gets amplified, resulting in very poor quality for long transmissions.
  10. Nov 8, 2004 #9
    I don't know what evidence you're using, but in my networking course some years ago (also in the books we used) it was mentioned that analogue signals travel further over copper than digital signals do. And this is obvious. Although loss of energy may be the same for both, digital signals lose their pattern faster. When there are abrupt changes in voltage (and current) the discrete signals tend to "spread" and form a continuous waveform after a few hundred meters. This loss of precision violates the standards which specify the degree to which imperfections may occur. On the other hand continuous analogue signals remain in that state for extremely long distance, although they also lose energy and attenuate.
  11. Nov 8, 2004 #10
    This is simply not the case. It is known that for short distances (LANs) digital is being used, while for long distances (telephone lines, local loops, etc.) analogue carriers are being used. Of course in theory we could use repeaters every few hundred meters, but that's too expensive.
    Last edited: Nov 8, 2004
  12. Nov 8, 2004 #11


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    You are incorrect ramollari, I work for the phone company, we do NOT use analog for long distance circuits.

    I design long distance data networks for our customers, it's what I do for a living, I should know. Repeaters are used extensively, without them people wouldn't be able to communicate except over very short distances.

    This doesn't matter when it comes to a wide area communications network, the analog signal cannot be cleaned up and will continue to degrade over a long distance, analog repeaters amplify the analog signal, but as I mentioned also amplifies the noise, making analog a poor choice for long distance communications. Digital signals can be cleaned of noise and the signal regenerated back to the original state and this is why digital is used by telecommunication companies for long distance circuits.
    Last edited: Nov 8, 2004
  13. Nov 9, 2004 #12
    Evo, I disagree with what you're saying. Why then do we use the modem? To convert digital signals in the Local Area Network or in the modem serial line (e.g. RS232) to analog for long distance transmission. If long distance circuits had used digital, computer data could have been transmitted directly without the need of the modem. Maybe the technology used in your company makes an exception. I'd be curious and more than glad to know sth about that technology.
  14. Nov 9, 2004 #13
    Evo is correct

    Digital is used everywhere for Data WAN and PSTN, only the local loop (last mile technology) somethimes is analog, ie why if you use dial up you need a modem...

    The PSTN is switched digitally
    SONET & SDH are digital
    ATM is switched digitally
    FR is Switched digitally
    ISDN is digital (Integrated Services Digital Network)
    xDSL is digital (Digital Subscriber Line)

    If we used analog over WAN then ALL internet traffic coming from DSL or ISDN to the exchange at the CO would have to be Modulated again to enter the ATM circuit (or whatever) which would be insaine....

    There arent much layer 1 protocols that use analog anymore AFAIK
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