Newsgroups: alt.video.equipment.broadcast,rec.radio.amateur.homebrew,sci.electronics.design,sci.engr.television.broadcast
Subject: Re: Telephone Line Audio Interface
References:  <[email protected]>
"Peter J. Bertini"  writes:

> The actual impedance of the phone line is several hundred ohms--
> if memory serves me the typical phone is designed for around
> 400 ohms.

600 ohms is the traditional telephone world impedance to which the devices 
are built to. But there are also other impedance standards, take a look at
http://www.epanorama.net/circuits/teleinterface.html#linedetails
 
> The phone line is a two-wire balanced circuit--there is NO ground
> or signal.  Both lines are signal.
> 
> A thousand ohms DC should sieze and hold the line.  Simply use
> the RadioShack telephone line transformer that they sell for a
> few dollars.  It is designed for 600 to 600 interfacing.  Once the
> transformer is across the line, it will hold and maintain your
> connection,  even if the phone is hung up.  You don't need a cap.

This is a good advice. A transformer is the best solution.
When selecting the transformer for the circuit described
above the transformer needs to be such type that it can
handle the line DC current without saturation. (usually
called "wet" telephone transformers). Not all small
tranformers can fullfull this criteria.

> You can simply drive the 600-ohm winding on the recorder side
> with the 8-ohm speaker jack audio.   The mismatch will not affect
> quality,  only the amount of maximum audio level.

Mismatch will affect the quality. Especially on long distance
phones where bad mismatch will cause echo to line. A better
match for the same application is to put a suitable resistor
between the speaker output (very low impedance) and the
transformer. In ideal system that resistance should be
600 ohms. If you take in acount transformer losses etc.
a 470 ohm resistor would provide pretty good impedace
matching.
 
> An ideal equalizer would have a 600-ohm output to better match
> the line transformer.

If the equipments have 600 ohm output, then it is the best solution.
If interfacin to anything much lower impedance, a matching resitor
is a very good idea. 

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