Phone line interface for DTMF

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
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

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.

Bob

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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.

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.

Bob

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?

jmarc...

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

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

Bye. Jasen

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.

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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?

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!

jmarc...

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.

JM

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

Bye. Jasen

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.

Jmarc..