From: Tomi Holger Engdahl 
Subject: Re: Phone connecting to audio lead
Date: 1998/03/06
Message-ID: #1/1
References:  
Distribution: inet
Organization: Helsinki University of Technology, Finland
Newsgroups: alt.dcom.telecom,alt.support.telecommute,alt.technology.misc,comp.dcom.telecom.tech,fido.phones,rec.audio.tech,uk.tech.misc,uk.telecom


John Lundgren  writes: 
> Adrian Webb wrote:
> > I want to connect my telephone extension socket to an audio lead. On the UK
> > domestic phone lead there is 4 wires. Which ones are the audio and audio
> > shield? 

Normal analogue telephone uses two wires. In the USA style RJ-11 connector
those are the two center ones (red and green). The audio in both directions
and the telephone power are carried over this wire pair. The audio in
telehone line is balanced and shield is not normally used because it is
not necessary.

>> I've heard that telephone leads are digital and so none of them are.

Most of the telephone lines coming to houses are normal analogue telephone
lines (PSTN). That's the most common telephoeninterface in use. Nowadays
typically everything from your local telephone central are then digital.
If you have ISDN connection coming to your home then the signal
is digital to your telephone.

> > This can't be true can it? I mean a cheap 7.50ukp phone thats converts
> > digital signals into analogue. The power must be separate from audio.

Those cheap telephones are not digital. Digital telephone is much more
expensive. All cheap telephones I have seen are traditional analogue
telephone. Power and audio can be carried using one audio pair
easily: audio is transformer coupled to line and the transformers
can handle some current flowing though their without saturation.
The power is DC current and the audio is a small AC signal over it.
 
> You need to do some serious reading about how the telephone signal is
> carried from CO to your house.  Radio Shack had a good book on that, but
> I don't think that's available where you're at.  IN any case, your
> assumptions are wrong.

The writer of this article is right. Buy a book on telephone
or read UNDERSTANDING TELEPHONES document which is available at
http://massis.lcs.mit.edu/telecom-archives/archives/technical/how.phones.work
I have also described many details of telephone line interface at my
telehone line intewrfacing document at
http://www.hut.fi/Misc/Electronics/circuits/teleinterface.html
 
> To sum it up, you cannot just connect the telephone jack wires to an
> audio input.  If you do, your telephone will quit working, you may be in
> for a very, very nasty surprise if a lightning storm comes by, and you
> may get charged by the telco if they find that you have caused a major
> problem.

That's true. Whatever audio equipment you connect to telephone line make
sure that you have an isolation transformer between telephone line
and your mixer. A best solution for this isolation is an approved telephone
line audio coupler. If you connect telephoen line to your audio
equipments directly, the telephoen line might quyite work, it might
work somehow poorly for some time or you will break your equipment
inputs immediatly (normal equipments don't like 48V DC and
even 100V AC ring signal on telephone line). And in any case you will
suffer and pay for damages. Those damages can be damages to your
equipments, damages to telephone company equipments, amount of work
caused for finding the problem, maybe for the losses you cause to
company because you damaged their telephone network. And in some countries
you will also pay the fines if telephone company finds out that you
have non-approved equipments connected to public telephone network
(it is illegal in many countries). Do not mess up with telephone lines
unless you know exactly what you are doing amd live by the rules.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
I maintain electronics and PC hardware web pages at
http://www.hut.fi/Misc/Electronics/