Tracy Johnson wrote:
[log in to unmask]>
>John D. Alleyn-Day" <[log in to unmask]> Wrote:
>>...  Have you made any calls to Europe recently?  The long-distance
>>quality is almost always excellent (it's mostly digital), and it sounds
>>like next door.  It's the "last mile" where the problems lie; the >copper
wires that have been in the ground for thirty years.
>
>I fail to see the correlation here, a digital circuit can use copper
>just as an analog line does for the last leg.  And there are
>intercontinental cables that have been underwater just about as many
>years as land circuits have been underground.  Laying the blame on
>copper seems a fallacy to me, whereas the quality of the sound lies in
>the modulation, not the medium.

You're right of course,  the modulation is very important.  This is
precisely why the noise gets in on the local circuit.

With a voice circuit, the "last mile" is usually copper wire with the
analog signal directly impressed on the line.  Any noise induced in the
line within the audio band will appear directly as noise in the telephone
receiver.  A partial short to ground or from line to line, as with a wet
cable, produces a lot of noise (a good book on electronics can probably
give some kind of explanation of why this is so - I certainly don't want to
get into that here).

As soon as the voice signal gets to the central office it is, today,
usually turned into a digital signal with all the error-correcting
possibilities that this presents.  It is therefore possible to transmit
this signal to the other central office (on the other side of the world if
need be) with essentially no degradation.  At that point the signal is
converted back to an analog signal and then becomes subject again to noise
introduced into the copper circuit.

There are other possibilities for introducing noise.  If the call travels
across an old-style microwave link that is analog in nature, then rain can
introduce significant noise by interfering with the radio transmission.
These links are going out very quickly, however.  Most long-distance links
are digital and usually optical fiber, as are the modern transoceanic cables.

I'm getting on in years and I remember when transatlantic calls went by
cable with AM modulation.  I didn't make many calls (too expensive), but I
can assure you that noise and fading were always present on the line,
sometimes to the extent that it was impossible to understand the person on
the other end.  So modern digital lines are amazing in comparison.

Actually, when I was at school, I went on a trip to the International
exchange and saw (and heard) the troposcatter transatlantic phone system.
The noise was so bad that the calls were not confidential and operators
listened in all the time with stopwatches, so that you were not charged if
you had to repeat a sentence!

The fact is that there is lots of old cable in the US (and in every other
country).  If you live in a major urban area, your copper lines are
probably fairly recent and well-maintained.  But outside these areas, I
hate to think about the condition of the lines, and there is plenty of
opportunity for noise generation.  So, if you are calling from, let us say,
San Francisco to Los Angeles, you probably get excellent, virtually
noise-free transmission.  But if you're out in the middle of the Central
Valley and you phone company is a "Farmers Co-operative" your line may be
as noisy as hell, because the copper is probably very old and has been out
in the rain for 30 years.  Your phone company may hook into the backbone
via an old microwave link, which isn't going to help either.

What I am saying, of course is that the much ballyhooed Information
Superhighway" is already with us.  The phone companies have massive digital
networks crisscrossing the country; even the "off-ramps" are good.  The
problems are mainly with the "neighborhood streets", many of which are
essentially still dirt roads.  It will take a lot of time and money for
them to be upgraded.

John D. Alleyn-Day
Alleyn-Day International
408-286-6421   408-286-6474 (Fax)
[log in to unmask]       http://www.Alleyn-Day.com
"You have to be in the right place at the right time
but unfortunately I was absent the day they
explained how that's done" - Herb Caen.