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Digital Signals on Analog lines?

  1. Apr 20, 2012 #1
    Hello everyone I have a question that I've been pondering.. And it probably has a simple answer. I'm studying physics of Telecommunications at college, and we were talking about how you can't send digital square pulses over phone lines. This would require an unlimited need for bandwidth, and the square pulses would soon become rounded and all jumbled together.

    ( If this is wrong, please let me know. My school doesn't have the best science depart. so I always like to double check my professors)

    My question is this... if we can't send data like this through a phone line, which is copper. How can it work inside a computer? Isn't the data being sent from the processor to RAM or memory on copper wires as well? How can we send digital data through a USB or Serial cable but not on a phone line? whats the difference? Are these wires somehow specially made for sending binary data?
  2. jcsd
  3. Apr 20, 2012 #2


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    When transmitting a digital signal over a transmission line there is no need to preserve the exact original waveform in most cases. Limiting the bandwidth of the signal by reducing the slew rate of the transmitter is a common method. http://en.wikipedia.org/wiki/Slew_rate
    What's important is that some property of the energy can be detected as being in at least two different states. (a 1 or 0)

    An example of high speed digital data sent on copper lines would be a HDMI signal. As long as the recovered signal stays out of the center zone during signal transitions it can be recovered as a perfect signal by the receiver.
  4. Apr 21, 2012 #3


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    definately not true. Digital signals have been sent over standard copper phone lines for years. Many years gone by back in the 1980's I was installing digital networks for Telecom in New Zealand. We would have buried repeater stations ~ 5km apart to regenerate the signals

  5. Apr 22, 2012 #4
  6. Apr 22, 2012 #5
    I do not think the teacher and any of you are wrong. The teacher is theoretically correct. A transmission line acts as a low pass filter, as a result the higher frequencies (the rise, fall, and corners) of a digital pulse are stopped. A pulse soon gets spread over time and adjacent pulses do the same and they overlap. The longer the cable, the higher the spread and higher the overlaps.

    But the good news is the digital pulses are used to transmit 0s and 1s, the peak of a pulse (1) and bottom of a pulse (0) are easily detectable by electronics if samples of the received signal are taken at regular intervals.

    Computer wires are too short for such spreading effects. Even in serial (or parallel) transfer of data in computer, there is a restriction on the length of the cable.
  7. Apr 24, 2012 #6
    Well to put it simply ideal square waves are the sum of signals with infinite harmonics i.e. their frequency is infinite. This must mean that to cover all the frequencies the bandwidth must be infinite.

    With regards to digital signals to your pc through your phone line, the signal is actually analogue with different frequencies superimposed on top of a carrier signal. They use methods such as frequency shift and phase shift keying. It's basically patterns of analogue signals representing digital signals. Google frequency shift keying for more info.

    The signals on your usb cable is just simply turning a voltage source on and off to represent the bits of data. In some cases it is a differential voltage on two wires (+5v/-5v typically)

    So you see that on a phone line analogue signals are modified to represent digital signals. On a usb line you simply turn the voltage on and off to represent your bit pattern synchronised by a clock. It's a bit of a myth that digital signals are square waves and analogue signals are sine waves, both are analogue it's just the way in which they are modified to represent data.

    Hope that helps
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