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phone lines

Discussion in 'Electronic Basics' started by [email protected], May 10, 2005.

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  1. Guest

    How can a two way conversation be transmitted along two wires of my
    residential phone line?.
    Why don't the signals confuse(collide) each other as they travel
    simultaneously in opposite directions along these two pieces of copper
    wire from the pole to my phone?.

    This is not about fancy elctronics,as the same applied in the 1880s
     
  2. As far as the copper is concerned, the two signals are just added
    together. For a simple phone loop, you hear both your own voice and
    the other person's voice in your ear piece. The tricky part cane in
    when the lines got long enough that amplifiers were needed. Then the
    two signals had to be separated, so that one amplifier boosted the
    signal going one way, and another amplifier boosted the signal going
    the other way. I think the first fancy electronics invented for this
    purpose was the hybrid transformer, that performed this separation.

    Perhaps this page will help:
    http://www.du.edu/~etuttle/electron/elect61.htm
     
  3. Bob

    Bob Guest


    This hybrid, sometimes called a transhybrid or 2-to-4 wire converter is a
    nifty little gadget. Its basic operation relies on the concept of being able
    to predict what the transfer function is from the telephone's microphone to
    the phone line -- while not driving the line with a zero ohm source
    impedance (which would kill the incoming signal from the person you're
    talking to).

    In the simplest case, you assume that the phone line has some fixed
    impedance -- say 600 ohms. If you then drive the line with a 600 ohm source
    impedance driver then you know that the signal driven onto the line will be
    exactly V/2 (assuming V is at the low-impedance side of the driver). Since
    you know you're putting out V/2 on the line then you can simply subtract a
    V/2 copy from the line and use that difference signal as your receive
    signal.

    In this perfect case, you see zero (-infinity dB) of your own signal in the
    receive path when you're transmitting. However, in the real world, you
    cannot precisely predict what the line impedance will be. In practice, while
    using a simple impedance modeling network, if you get 10dB of rejection
    you're lucky. There are some fancy signal processing techniques that can do
    better than a simple passive (or opamp based) circuit, but they're
    (obviously) more complex.

    Good phone always add back some of your own transmit signal back into your
    earpiece. This is called sidetone. It replaces the signal, from your mouth,
    that was blocked by placement of the phone to your ear. The cell phones that
    I've used don't do this. To me, it sounds very unnatural. This is one of the
    reasons I can't stand using those little fuggers.

    Bob
     
  4. Guest

    Thanks for the explainations, John and Bob.
    I hate mobile phones too.
    Only have one for emergency calls.
    Hartly.
     
  5. BobG

    BobG Guest

    Good phone always add back some of your own transmit signal back into
    your
    earpiece. This is called sidetone. It replaces the signal, from your
    mouth,
    that was blocked by placement of the phone to your ear. The cell phones
    that
    I've used don't do this.
    ================================
    They would have to add analog sidetone locally...... there is a
    significant round trip delay in the digitization and packetization and
    sending and receiving process.
     
  6. John - KD5YI

    John - KD5YI Guest

    Hi, John -

    I read that page with interest. I was a bit surprised to note that the
    author, in the next-to-last paragraph, refers to Horowitz and Hill and that
    their hybrid circuit doesn't work. Do you suppose Win is aware of that? I
    don't know how to get in touch with him.

    Thanks,
    John
     
  7. Rich Grise

    Rich Grise Guest

    Post to with a subject line like, Ping Win
    Hill - What's this guy talking about?

    Tell him Rich the Newsgroup Wacko sent you. ;-D

    Cheers!
    Rich
     

  8. He is on and
    quite often.
     
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