From: GA.MCL@forsythe.stanford.edu (Mark C. Lawrence)
Newsgroups: misc.consumers.house
Subject: Re: Dual line phone causing cross-talk ??
Date: 7 Jan 1993 16:35:58 -0800
Organization: Stanford University
Distribution: usa
NNTP-Posting-Host: morrow.stanford.edu

In article <MICHAEL.93Jan7161044@sunbeam.ksc.nasa.gov>,
michael@sunbeam.ksc.nasa.gov (Michael D. Myjak x7-3286) writes:
>I would not have believed that this were possible if it wasn't
>happening to me.  It seems to go against everything I've learned in
>electronics, physics, telephoney, and electrical transmission line
>theory...  but...
>    I seem to be experiencing capacitive coupling on my phone lines!
>
>I know that capacitive coupling is not possible with DC... I know that
>there is no short either. How can this possibly be happening?

Although there is DC on the phone line, your voice is AC.  You are
experiencing both capacitive and inductive (but mostly inductive) coupling.

>I recently built and wired a new home for dual telephone lines. I
>used 3 loops (instead of home runs) throughout the house and used 26/4
>stranded, flat, unshielded untwisted telephone conductor line commonly
>called "Silver Satin". Its the usual stuff that is used to connect
>phone equipment to the wall outlets. The total wire length installed in
>the house is about 2100 linear feet.

Flat-4 wire is not suitable for long runs (over 25 feet) (as you have now
discovered the hard way).  You want wire that contains two twisted pairs,
i.e., "two pair" not "four conductor".  You have also demonstrated why "home
runs" are better than "loops".

>What is amazing to me is the volume level of the cross talk. It is
>simply astonishing! Its like there was a short or something, but there
>isn't. To give you an idea of the the magnitude of the problem,
>whenever my wife and I are using both lines simultaniously. I cannot
>not only make out my wife's voice but I can clearly make out the other
>half of the conversation...  almost as if it was the primary circuit!

To understand this a picture may help...

line 2(tip) ----------------------------------------------------------|
line 1(tip) --------------------------------------------------\phone 1 \phone 2
line 1(ring)--------------------------------------------------/        /
line 2(ring)----------------------------------------------------------|
                  <---2100 ft--->

Now what you have here is two very long skinny loops, one inside the other.
In other words, a transformer with one turn of primary and one turn of
secondary.  Naturally the primary couples energy into the secondary.  When you
use twisted pairs you prevent this effect, because (say) line 1, tip is close
to line 2, tip about as often as line 1, ring.  ("tip" and "ring" are the
designations of the two wires in a phone line, after the jacks used in
swithboards).

>... [details of test omitted]
>This was only about 20' of wire!! This is supposed to be telephone grade
>wire too! its the same stuff that your telephone equipment lines are
>made from, only without the twist.
            ^^^^^^^^^^^^^^^^^^^^^^
Well, that twist is vital, to quote Dr. Strangelove, "for reasons which
are now altogether too obvious".

>As we added in each junction box to the live circuit, we could detect
>an increase in crosstalk... we could even see the increase on the
>o'scope. With the entire house connected, the crosstalk level as
>measured on the second line was close to 50mV. A.C. However, this 50mV
>doesn't seem to correcpond to the level we are hearing.

Nice lab work; you've just done a very convincing demonstration of crosstalk
in unpaired wiring.  50mV across a phone would be quite audible; normal level
is only a few hundred mV (I think the standard is -8 dBm, or about 158 mV at
600 ohms).

Mark C. Lawrence
Systems Programmer     Internet: M.Lawrence@Forsythe.Stanford.edu
Stanford Data Center     Bitnet: M.Lawrence@STANFORD
Stanford, CA 94305-4136     Tel: (415) 723-4976