Re: Phone line interface for DTMF


"Bob" <nimby_NEEDSPAM@xxxxxxxxxxxxxx> wrote in message
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"Tim Wescott" <tim@xxxxxxxxxxxxxxxx> wrote in message
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jmarc wrote:
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?
jmarc...

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

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


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.


.

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