Re: 2 questions...

["Keith Doxey" <keith.doxey@xxxxxxx>]

Hi Nigel,



> -----Original Message-----
> From: Nigel Orr [mailto:nigel.orr@xxxxxxx]
>
> At 15:05 12/08/99 +0100, you wrote:
> >The distances involved in a house shouldnt cause overhearing to
any
> >significant degree even if you do use split pairs.
>
> Even of the ringing signal?  That's good, as I have a couple of places
> where I need a separate phone connection each side of a wall, and one
run
> would be preferable.
>
> How much crosstalk would you expect on cat-5 at audio
> frequencies- I've not
> seen any figures, and some real figures in dB would be useful for,
say, a
> 20m run.
>

Dont have any actual figures....just 20 years of maintaining BT PBX's.
The only times I ever noticed crosstalk were....

Split pairs as already discussed.
Low Insulation between circuits due to damp connection boxes and damaged
cables.
Long runs of very old crappy cable with little or no twist.

I did once have a customer complaining of overhearing the
"ticking" data
pulses on his systemphone located away from the main building. I
immeadiatly
assumed that it was down to the old underground cable feeding the remote
building but upon further investigation proved it to be the flat 6 wire
cord
on the phone itself. I have also noticed similar problems with systemphones
on several systems where people have shifted the phone themselves with the
aid of a 10 metre extension lead. the fact that the cable isnt twisted and
the different pairs are running parallel for long distances induces the
signal onto adjacent pairs.

A telephone line is essentially a balanced circuit and should be relatively
immune to picking up noise, where noise is particularly noticable it is
usually a sign of a fault on the line. Something as common as a block lid
blown off at the top of the pole causes the rain to get onto the terminals
creating leakage to earth unbalancing the line. Thankfully as more of the
network is put underground or replaced with newer technology the number of
line faults is falling.


> Presumably (though I haven't thought about this alot) lines emanating
from
> a home PABX might be higher impedance on the audio than from a _real_
BT
> exchange, so less immune to the ring signal, which might be at a lower
> impedance?  Any mileage in that theory?  (well it was the best I
> could come
> up with at short notice)
>
> Nigel
>

Nice try....but the only problems I encountered were from the PBX
extensions
being too short and having too low impeadance. Some Mitel systems had
problems with certain telephones causing one way transmission, high
sidetone
and other wierd problems and the fix was a plug in resistor unit which
inserted an additional 200 ohms into each leg of the line. Many of the
larger PBX's use the same or similar interface circuitry to that found in
the public exchange. Particularly manufacturers like GEC, Plessey and STC
who actually manufactured many of BT's Public exchanges.


Keith Doxey
http://www.btinternet.com/~krazy.keith
Krazy Keith's World of DIY HomeAutomation


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