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Discussion in 'Beginner Electronics' started by Geoff Hackett, Sep 8, 2003.

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  1. Hello

    I am designing (for the first time), a telephone connection between two
    phones so the phones are fully functional and can dial each other etc. As
    the design progresses it is becoming more interesting to me how a commercial
    phone exchange works. Can anyone point me in the right direction where I can
    find details about the connections and electronics used in the phone

    Many thanks...
  2. Nirodac

    Nirodac Guest

    In a nut shell;

    You pickup the handset, the Central Office (CO) detects current flow on the
    lines to your house (through your phone and back to the CO). The CPU at the
    CO places two tones on your line (Dial Tone). The CO supplies 48VDC
    through 2, coils, of 200 ohms each, on your lines, this is talk battery and
    power for your touch tone calling pad.. You press a button (or short the
    line if you have rotary dial) and generate two tones that the CO detects and
    converts to a binary number, and discontinues the dial tone. After you have
    pressed all 10 digits the computer routes the call to the appropriate
    subscriber that you are calling. Depending on the system, and the age, the
    computer can either pull up relays (actually reed switches, these days) or
    convert every tone and sound you produce to digital, then send it through
    the network as packets, to be reassembled at the other end and converted to
    audio. When you hangup, you open the line (except for the ringers) and the
    CO dumps the communication path, waiting for the next call.
    Today it would be very difficult to make a call without your voice being
    digitized somewhere along the line.
    Audio on a phone line is limited to 4Khz, that's why a phone sounds like a
    phone. This was used to save band width in the old days when signals where
    multiplex together, in analogue format.
    The ringers (in North America) used about 90 VAC at various frequencies.
    Common today is 33HZ. To ring your phone the CO places 90 VAC at 33Hz on
    your line, and the capacitor and coil that make up your ringer, will
    oscillated and ring the bell. The low resistance DC path you create by
    picking up the phone causes the ringing voltage to stop, and lets the
    computer connect through the voice path.

    If your using only two phones, all you need is a coil and a battery between
    the two phones.
    Hope this helps

  3. Very useful thank you.


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