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Phone line interface for DTMF

Discussion in 'Electronic Design' started by jmarc, Jun 22, 2007.

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  1. jmarc

    jmarc Guest

    Hi all,

    I would use a 600 ohms phone line transformer
    to have dc isolation, for transmitting DTMF from
    a computer.

    My very simple desing work at home, but
    doesnt work at the office. It seams I don't
    deliver enough audio level of DTMF.

    Earlier, I bought a very simple "play/record"
    device who did work (more audio DTMF level),
    but humming soon happen with differents
    configuration.... (not use transformer in)
    It is probalby caused by different ground
    voltage level, between computer and Telco gnd.

    Is someone out there, can rely me to some pretty
    schematics on the web, or have any suggest
    to do it right?

  2. Tim Wescott

    Tim Wescott Guest

    The quickest answer, if you can find one, would be an 8 ohm to 600 ohm
    coupling transformer. This will be a better match to your computer audio.

    Alternately, amplify the heck out of the computer audio before applying
    it to the the transformer -- this is less than ideal, but may be easier
    to find parts for.


    Tim Wescott
    Wescott Design Services

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at
  3. Bob

    Bob Guest

    There are two issues -- source impedance control and level control.

    The output impedance of the computer is not going to be 8 ohms so impedance
    matching will not be achieved.

    IIRC, the combined DTMF power is around +2dBm which is about 980mV rms (into
    600ohms). If the computer can only put out 100mV rms (for argument's sake),
    and knowing that he'll lose half his volts if source terminated properly,
    then he should use a 2 * 980mV/100mV turns ratio transformer (~20x), drive
    from the low-impedance speaker output, and place a 600 / (20^2) or 1.5 ohm
    resistor in series between the speaker output and the transformer to achieve
    proper source termination.

    I hope I did the math right. I used an HP calculator.

    There are many other issue in properly interfacing with telephone lines, but
    that's a whole course in itself.

  4. scada

    scada Guest

    Using the 600 ohm transformer to couple to the line, you should also put a
    capacitor in series with that transfomer. I used a 10µf cap (non-pol)
    sucessfully. Unless the transformer is specified for this use and is of the
    "wet type", the more common "dry types" will saturate from the DC line
    current. You can use a resistor (~180 ohms) to shunt the line, that will
    pull enough current to keep the line off-hook.
  5. Bob

    Bob Guest

    Yep, except that if you don't put a huge inductor in series with the 180ohm
    resistor then you destroy the source/end termination and the outgoing and
    incoming signal levels. The "wet type" solves this problem as long as its
    inductance is high enough.

    Hooking up to a phone line properly is not a simple matter.

  6. jmarc

    jmarc Guest

    Thanks for the pretty good hint!

    I did already bought a 600:8 ohms
    at the same time I bought the 600:600.
    I simply not try it already!

    I did put a 1.5 microF non polar to
    connect transformer to the line.

    I also put 16 ohms resistor to each
    audio channels to mix them into the
    transformer. May I go lower to mix
    thoses channels without burn computer
    RCA stereo outputs?

  7. Eeyore

    Eeyore Guest

    Why would a 1:8.6 step-up transformer be better ?

    Can you explain that ?

    Do you seriously want to send several watts of audio down the phone line ? The
    telco might have something to say about that ! I think the recommended level is
    about -6 to -10dBm. i.e no more than 400mV rms.

  8. Eeyore

    Eeyore Guest

    Since when has a computer sound card only been able to put out 100mV ?

    What a silly arbitrary idea.

    FYI a sound card will have little trouble outputting several volts typically.

  9. Eeyore

    Eeyore Guest

    You need to have an inductor to terminate a phone line ?

    Have you ever looked inside a modern phone ? I'd love to know where that
    inductor is !

  10. Bob

    Bob Guest

    Modern phones don't use an inductor but rather an inductor simulator

    You seem to be a very angry person. Take a stress pill and think things
    over. You've been hanging around Phil Allison too long.


  11. I used to use a Quad 303 poweramp on OB's as a line driver. Seemed to
    work/minimised xtalk onto our circuit

    But even more impressive, we ( capital radio in the 70's) got
    broadcast TX line organised, and working, to France in less than 3
    hours from first contact, from next to a famous pedestrian crossing
    in Abbey Road.
    We almost had talkback, well, by the time the show had ended.

    I think the paperwork must have been worse than the show

  12. Ross Herbert

    Ross Herbert Guest

    Tomi Engdahl has a fairly comprehensive tutorial on telephone line
    interfacing here for you to study.
  13. Jasen

    Jasen Guest

    Will you turn that off please.
    yes, there is a certain level needed.
    I've got an off-the-shelf solution:

    Use a modem. send it "atx1dt" followed by the symbols representing the DTMF you want to send

  14. Robert Baer

    Robert Baer Guest

    First, you cannot DC couple the transformer to the phone line, as Ma
    Bell uses 48V to detect on/off hook and other housekeeping chores.
    Second, here are at least two completely phone systems: "standard "
    MaBell and electronic types (Centrex, etc); they are not compatible with
    each other.
  15. scada

    scada Guest

    Very true! For my app, that was not a problem. Does anyone have a tried and
    proven gyrator circuit that will hold the phone line - that they are willing
    to share? Any links to an IC?
  16. Fred Abse

    Fred Abse Guest

    But maybe not from it?
  17. jmarc

    jmarc Guest

    I already thought using a modem.
    But with teh other solution, I can
    play music when on hold, or in background
    at same time talking on the phone..

    But this is the primary need, indeed!

    Thanks anyay! It is probalby the best
    solution, if I not going the other way!

  18. What kind of phone/phone system do you have at the office? If it's a
    modern PBX, it probably uses digital phones that aren't compatible
    with POTS.

  19. Jasen

    Jasen Guest

    you can with a voice (capable) modem too, you just need the right

  20. jmarc

    jmarc Guest

    What kind of phone/phone system do you have at the office? If it's a
    Indeed, nothing work when I connect my rig
    between the phone set and the wall. But,
    connected to the handset it almost works.
    No humming, but not enough audio level!
    The phone set is a Nortel Networks multilines,
    and probalbly sure to use digital com...

    With other coupling than Xfmr, like capacitors,
    humming come on when using a headset rig
    at the same time, and/or having a USB printer
    tied to the computer (even with a laptop)...

    I don't really bother exactly where I connect
    the rig. My goal is only to avoid ground coupling,
    for no humming, and good level of DTMF in
    order to be properly detected by Telco.. or the Pbx.

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