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Telephone line no noise

Discussion in 'Electronic Basics' started by Allen Bong, Sep 3, 2003.

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  1. Allen Bong

    Allen Bong Guest

    Why are good telephone lines running miles of cables don't pick up much
    noise along it's way to the telephone while it is hard to send data between
    two computers when they are a few hundred feet apart? What's the catch?

  2. And of course, voice is terribly redundant. Miss a microsecond here
    or there, and likely the receiver will still understand what is said.
    Even a whole word can be missed (provided it's not a key word) and
    the message can still be understood.

    Get a bit of noise on a digital line, and it can throw off significant
    bits of data. Unless extra data and extra error checking is in place,
    you can't make up for messed data.

  3. Baphomet

    Baphomet Guest

    Allen -

    I'm a bit out of my depth but let's give it a whack anyway. Convention
    copper telephone lines run 600 ohm balanced, and are voice bandwith limited.
    Computer lines are unbalanced with respect to ground, and have high
    bandwidth to accomodate the very fast rise and fall times of digital
    signals. If I've stepped in "it" with this response, please accept my
    apologies :)
  4. Allen Bong

    Allen Bong Guest

    You don't have to apologise for anything. I think you explained it very
    well. The catch is the "low bandwidth of the telephone line" and the "600
    ohm balanced circuit" design and the "tolorance allowed by the receiving
    end". What is the actual bandwidth allowed by a typical telephone line?

    If I reduce the baud rate of the transmission and use balanced protocol like
    RS422/RS485 would I be able to go a few hundred feet between 2 computers?

  5. Yes, RS-422 will go several thousand feet at a low Baud rate, if the
    line is properly terminated. Check the National Semiconductor website
    for application notes on RS-422. is the home page. is the search page for their
    application notes. lists all their
    application notes.

    Here is a list of serial communications related application notes
    available from National Semiconductor.

  6. bg

    bg Guest

    Differential drivers and receivers are readily available and cheap that will
    reduce line induced noise by a hundred , a thousand or more times.(+/- .2
    volt time 100 is alot of noise!!) Your friends needs would not be hard to
    accomplish. Do a google search for balanced lines, common mode rejection
  7. Allen Bong

    Allen Bong Guest

    Thank you very much Michael. Now I know I have enough evidence to convince
    my friend. And thanks to those who teach me the differences between a
    telephone line and a digital line.

  8. You're welcome, and I hope you don't get lost reading all those
    application notes! ;-)
  9. I have a huge problem with line noise. After a while windows 98 will
    just stop loading pages from the internet as if to say "this is too
    difficult". I then have to re-boot my system and try again.

    Verizon said they checked the lines outside, but I still have the
    problem. When it rains I can hear a great amount of "static", and if I
    connect to the inteernet, I'm lucky if I can get as far as one page
    before I have to reboot, so I know the problem must have something to
    do with moisture.

    Darren Harris
    Staten Island, New York.
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