Phone line interface for DTMF

[jmarc]

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...

 

[Tim Wescott]

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
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

 

[Bob]

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.

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

 

[scada]

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.

 

[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.

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

 

[jmarc]

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...

 

[Eeyore]

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.

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.

Graham

 

[Eeyore]

If the computer can only put out 100mV rms (for argument's sake)

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.

Graham

 

[Eeyore]

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.

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 !

Graham

 

[Bob]

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 !

Graham

Modern phones don't use an inductor but rather an inductor simulator
(gyrator).

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.

Bob

 

[martin griffith]

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.

Graham

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


martin

 

[Ross Herbert]

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...

Tomi Engdahl has a fairly comprehensive tutorial on telephone line
interfacing here for you to study.
http://www.tkk.fi/Misc/Electronics/circuits/teleinterface.html

 

[Jasen]

This is a multi-part message in MIME format.

Will you turn that off please.

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.

yes, there is a certain level needed.

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?

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

 

[Robert Baer]

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...

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.

 

[scada]

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

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?

 

[Fred Abse]

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

But maybe not from it?

 

[jmarc]

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...

 

[John Mianowski]

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...

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

 

[Jasen]

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..

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

Bye.
Jasen

 

[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

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..