> >The impedance of the wires has almost nothing to do with
the
> >design impedance of the system, because the wires are not
> >large fractions of a wavelength long. The critical
parameter
> >is the source and load impedance. In a short line,
standing
> >waves have no place to stand.
>
> In a central office or studio, that's absolutely true. But
if I'm miles of wire
> from the central office it isn't.
Central offices have little to do with it Jim.
The signal is often in digital form over copper pairs, and
is converted to analog at an equipment cluster near a
distribution point. You can be 20 miles from the central
office and have a 50-foot long analog pair, and your view of
life will be that of someone 50 ft from the central office
(unless you do a ping and echo test).
My specific points in this are:
1.) Telephone systems are designed to be 600 ohms.
2.) It is impossible to predict characteristics of random
power line filters, and how those filters will affect random
systems
3.) Ability to survive poor balance and improper loading
depends on what you run through the line and how far you are
from the D/A conversion, which is almost NEVER at the
central office.
4.) My experience, and I work on this stuff every week, is
that power line filters make very poor telco RFI filters
I can't add anything else to this topic.
73 Tom
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