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Telephone wires

Discussion in 'Electronic Design' started by eeh, Oct 25, 2005.

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  1. eeh

    eeh Guest


    If I couple DTMF signal to one of telephone line for dialing, is there
    any difference to couple the signal to ring and tip wire?
  2. Bob

    Bob Guest

    The DTMF receiver, at the other end of the wires, will look only at the
    signal difference between tip and ring.

    As long as you couple your signal onto one of the wires without disturbing
    the (noise) signal that may be on the other then the DTMF receiver will work
    properly (as long as the amplitude, twist, and distortion products are
    within spec). In practice, this is difficult to achieve while driving only
    one of the two wires.

    Normally, any voice or DTMF signal that is applied to a telephone line is
    done so in a differential manner. That is, an equal (in amplitude) but
    opposite (in phase) signal is applied between the two wires. The receiving
    end will pick up the difference signal and (hopefully) ignore any signal
    which is equal in amplitude and phase.

    The out-of-phase part of the signal is referred to as the differential (or
    metallic) component.
    The in-phase part of the signal (with respect to earth or signal common) is
    referred to as the common-mode (or longitudinal) component.

  3. Ryan Weihl

    Ryan Weihl Guest

    depends on your local phone company
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