Newsgroups: alt.video.equipment.broadcast,rec.radio.amateur.homebrew,sci.electronics.design,sci.engr.television.broadcast
Subject: Re: Telephone Line Audio Interface
References: 
"Throbber"  writes:

> I apologize for posting this to this newsgroup, but there are probably
> telephone and broadcast technicians here who could help me out. Also, a ham
> who wants to build a phone patch interface might be interested in this.

One document on the topic worth to look at is
http://www.epanorama.net/circuits/teleinterface.html
 
> Problem: I file radio news stories from remote locations in the western U.S.
> via analog voice grade lines. Sometimes my editor would like background
> audio (a missile countdown, jet taking off, etc.). I want to feed good
> quality audio directly into the phone line (I realize that an analog phone
> line is limited to 300-3,000 Hz bandwidth).
> 
> Here are my questions:
> 
> 1. What is the impedance of an analog voice grade line?

The traditional impedance matching used in telephone systems
is 600 ohms matching. The telephone equipments are typically
matched to that. With modern telephone cables the real line
impedance is typically somewhat lower, but the old 600 ohm
"standard" works still satisfactorily.

> 2. How would I match the 8-Ohm impedance from my audio recorder to the
> telephone line?

Check the circuits at
http://www.epanorama.net/circuits/teleinterface.html

> 3. The red and green wires in a telephone cord are the audio lines. Which
> one is signal and which one ground?

None of those wires is "signal and ground".
Those two lines carry the signals in balanced manner.
Neither of them is connected directly to ground and neither
of them should be connected to ground by user.
Properly designed telephone interfaces which plug
to other devices use an isolation transformer
(or other electronics which does the isolation)
between the line and the audio interface. The
transformer passes audio signal, guarantees signal
balance and provides safety isolation. Usually the transformer
has lots to do with the impedance matching also.

> 4. Could I just feed the audio directly into the line from the recorder
> using a capacitor to block the DC
> from getting into the recorder?

This does not work properly. For safety, protection agains
potential audio equipment failures and best operation
use a proper isolation tranformer between your equipment
and telephone line. A 600:600 ohm telephone transformers
are suitable for this application.

> 5. If so, what value capacitor would I use to do this?

I would not recommend capacitors. So I don't give any
recommendation here.

> 6. What kind of load (how many Ohms) do I have to put across an analog phone
> line to maintain the connection?

The typical DC resistance on telephone equipments connected
to the line is 60-300 ohms. If you try to keep the 600
ohm matching a simple resistor does not do thwe job properly.
The simplest approach for interfacing is to use a "wet" type
telephone transformer which can provide he DC path
(just connect it across the line and t keeps the phone
line in off-hook condition and passes the audio signal).

> 7. If I feed the audio into an equalizer to boost the audio below 300 Hz and
> 3,000 Hz before it goes into the phone line, would I increase the quality of
> the audio or just increase the background noise (would audio outside of the
> normal phone line bandwidth come across as noise)?

I would not recommend any strong equalization. You might get somewhat
more on frequency response, but not much. And boostin too much
can lead to problems.
 
> There is an excellent commercial product that performs this function, but it
> is very expensive.
> 
> Thanks,
> 
> Brian Webb, KD6NRP
> [email protected]
> http://home.earthlink.net/~kd6nrp
> 
> 
> 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/