From: alex100@axxsys.com
Subject: Re: Phone connecting to audio lead++
Date: 1998/03/04
Message-ID: #1/1
References: 
NNTP-Posting-Date: Tue, 03 Mar 1998 23:12:07 EST
Reply-To: alex100@axxsys.com
Organization: Sand Labs Research Inc.
Newsgroups: alt.dcom.telecom,alt.support.telecommute,alt.technology.misc,comp.dcom.telecom.tech,fido.phones,rec.audio.tech,uk.tech.misc,uk.telecom


The telephone line is a balanced system. In order to connect something
to it you have to observe several rules. The first and most imortant
is the GROUND isolation. Because the line caries a FLOATING
voltagage/current any ground reffered attachement will induce
severe HUM. The second the Balance of the line must be preserved
as much possible. Balance in this context is that the amount of
current/voltage flowing into one wire must be faithfully reproduced
in the other.

The simplest way to acheive this is by capacitalively coupling a
transformer  to the line. Ia two capacitors are used then the
transwormer is simetrically DC isolated not alowing line unbalace.
Also the transformer provides the isolation between the line and the
user aparatus. If at the secondary of the transformer
you attach an 1W, 5V Zener diode it will protect your equipmet fron
the ring voltages on the line.
The design could be improuved with the addition of impedance matching
circuitry and ESD protection until a complete Direct Attachment
Arangement (DAA) is achived. For one schiled in the art it is trivial.
Otherwise, as suggested somwhere else, Radio Shack or equivalent
could be used.

Have FUN. We need it so much.
Alex VonYonkers

======================================================================




"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? I've heard that telephone leads are digital and so none of them are.
>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.
>On a phone doubler, it is like audio isn't it, in that one wire splits off
>into two, sending out two audio signals for two phones? This is what i want
>to do, but one of them connecting to an audio lead.
>--
>Adrian Webb    also at:
>                       Adrian.Webb@discovery.dmpriest.com