Telephone line (north american POTS)

[Jem Berkes]

I have a question about analog telephone line interfacing. I've read about
the basics, such as voltage and current levels (high voltage on ringing)
etc., current flowing when off-hook and I understand all this.

I'm having a bit of trouble comprehending how I can put my own audio signal
onto the line when there's already audio on the line (e.g. from other
people talking). All high-voltage protection aside, I capacitively couple
my circuit's audio output onto the line right?

My audio (that I want to put on the phone line) will be coming from a low
power audio amp like the LM386. After blocking the DC output from that amp
I've got an AC voltage audio signal. Where I'm stuck is, how do I output
that signal onto the phone line when there already exists AC audio
(voltages? currents?). Is it voltage or current fluctuates that make sound?

Thanks for any help you can offer,

 

[James Meyer]

I have a question about analog telephone line interfacing. I've read about
the basics, such as voltage and current levels (high voltage on ringing)
etc., current flowing when off-hook and I understand all this.

I'm having a bit of trouble comprehending how I can put my own audio signal
onto the line when there's already audio on the line (e.g. from other
people talking). All high-voltage protection aside, I capacitively couple
my circuit's audio output onto the line right?

My audio (that I want to put on the phone line) will be coming from a low
power audio amp like the LM386. After blocking the DC output from that amp
I've got an AC voltage audio signal. Where I'm stuck is, how do I output
that signal onto the phone line when there already exists AC audio
(voltages? currents?). Is it voltage or current fluctuates that make sound?

Thanks for any help you can offer,

Consider thephone line connection as a resistor. The phone company is
providing a DC voltage across the resistor. They're also providing an AC
voltage on top of the DC. Your phone is providing Ac voltages too. All these
voltages simply sum together to make the total voltage that you see across the
phone lines.

Anything else that you want to connect to the phone line after the
original connection is made should be AC which will sum with all the other
signals.

The easiest way to add additional signals to the phone line wouuld be to
use a transformer in between the additional source and the line. A capacitor
(or two, one on either leg of the added transformer's output winding) will
prevent the added signal from changing the DC component of the line voltage.

A resistor, start with about 1K Ohms, should be connected in series with
your added signal. That will prevent the added signal's source impedance from
reducing the AC signal voltage level on the line too much when the added
connected.

Jim

 

[valentin tihomirov]

Another option is to use standard commertial analog line interface (DAA).
The transofrme should be cheaper though.

 

[Lee Leduc]

Try this URL. Lots of phone interface stuff.

http://www.epanorama.net/links/telephone.html#circuits

 

[Jem Berkes]

My audio (that I want to put on the phone line) will be coming from a

The easiest way to add additional signals to the phone line wouuld be
to use a transformer in between the additional source and the line. A
capacitor (or two, one on either leg of the added transformer's output
winding) will prevent the added signal from changing the DC component
of the line voltage.

A resistor, start with about 1K Ohms, should be connected in
series with your added signal. That will prevent the added signal's
source impedance from reducing the AC signal voltage level on the line
too much when the added connected.

Thanks for the replies! One thing I've seen mentioned repeatedly are
warnings to use unpolarized capacitors (that handle 250V) due to the
possibility of DC reversals on the line.

 

[Jem Berkes]

I didn't have any audio transformers lying around, so I made a simple
circuit that outputs the audio from the PC onto the phone line without the
extra isolation. I know this is also missing the high voltage ring
protection; once I get the transformer I can add all that in.

http://www.pc9.org/images/soundcard-phoneline.png

This appears to work very well! The volume is OK, and this doesn't decrease
the volume of everything else on the phone line when connected.

However, I do have a problem with what sounds like a mains hum. It has to
do specifically with the sound card as the input source, because when I
disconnect the sound card the hum stops. There is no hum heard through the
loudspeaker in the above circuit.

I'm guessing it has to do with the ground of the PC power supply vs the
telephone line. Will adding in the audio transformer cure that problem?

 

[Michael A. Terrell]

I didn't have any audio transformers lying around, so I made a simple
circuit that outputs the audio from the PC onto the phone line without the
extra isolation. I know this is also missing the high voltage ring
protection; once I get the transformer I can add all that in.

http://www.pc9.org/images/soundcard-phoneline.png

This appears to work very well! The volume is OK, and this doesn't decrease
the volume of everything else on the phone line when connected.

However, I do have a problem with what sounds like a mains hum. It has to
do specifically with the sound card as the input source, because when I
disconnect the sound card the hum stops. There is no hum heard through the
loudspeaker in the above circuit.

I'm guessing it has to do with the ground of the PC power supply vs the
telephone line. Will adding in the audio transformer cure that problem?

The phone line is balanced, and the sound card isn't, so you are
shorting one side of the balamced line to ground. If the phone company
detects it, they will send a tech out to find the problem, and bill you
for it, or they will disconnect the line till the problem is cleared.

 

[Jem Berkes]

http://www.pc9.org/images/soundcard-phoneline.png

The phone line is balanced, and the sound card isn't, so you are
shorting one side of the balamced line to ground. If the phone company
detects it, they will send a tech out to find the problem, and bill
you for it, or they will disconnect the line till the problem is
cleared.

I don't quite understand what you mean. If I used a transformer to isolate
the phone line from my circuit, would this still be a problem?

 

[Michael A. Terrell]

I don't quite understand what you mean. If I used a transformer to isolate
the phone line from my circuit, would this still be a problem?

That is why you need the transformer. See if you can find an old
Western electric, or Stromberg Carlson 500 or 2500 series telephone. The
network was designed to interface the transmitter and receiver to the
phone line, and provide "Sidetone" at a low volume so you can hear
yourself. The receiver should be isolated from the phone line. I have a
Stromberg Carlson telephone equipment manual around here, somewhere. It
should have the schematic of the network in it. Also. use Google to look
for schematics of "Phone patches"

 

[Kevin McMurtrie]

I have a question about analog telephone line interfacing. I've read about
the basics, such as voltage and current levels (high voltage on ringing)
etc., current flowing when off-hook and I understand all this.

I'm having a bit of trouble comprehending how I can put my own audio signal
onto the line when there's already audio on the line (e.g. from other
people talking). All high-voltage protection aside, I capacitively couple
my circuit's audio output onto the line right?

My audio (that I want to put on the phone line) will be coming from a low
power audio amp like the LM386. After blocking the DC output from that amp
I've got an AC voltage audio signal. Where I'm stuck is, how do I output
that signal onto the phone line when there already exists AC audio
(voltages? currents?). Is it voltage or current fluctuates that make sound?

Thanks for any help you can offer,

Drive it by current so it doesn't hurt the existing audio. You can do
this even through a telephone transformer, which I strongly recommend
using.

 

[Jem Berkes]

I have a question about analog telephone line interfacing. I've read

...
Drive it by current so it doesn't hurt the existing audio. You can do
this even through a telephone transformer, which I strongly recommend
using.

Thanks for the advice everyone. My experimental POTS interface seems to be
working very well; it's isolated with a transformer and I have some diodes
providing high voltage protection. I'm able to simultaneously record off
the phone line and also play audio onto the line (both using sound card),
and there is no buzz or noise in either case!

I've been using mainly web sites I've found to help me design this, but I'd
like to find the original authoritative document on the specifications.
There must be some official specification that defines the 600 ohm
impedence, DC and AC line conditions, etc.?

I would be very grateful if someone can point me in the right direction to
find such a document. I've seen mentions of CCITT and ITU-T but I started
searching and discovered that there are a TON of documents/specs!