Jim,
I worked on military versions of those old, analog TDM multiplexing systems.
One was called an "FCC-18", and was quite large. We had a more modern version
of that on a second system I believe made by GTE, but the model escapes me
right now. Both were sent over tropo-scatter microwave systems that have long
been ripped out in favor of digital satellite, fiber, and microwave backbone
links.
There's one thing that you need to consider with this line of thinking -- we're
not trying to maximize profits over a fixed analog BW. We as control operators
can change our transmission characteristics to match the conditions and band
loading at hand. When there is available BW, we can use better speech
fidelity, when there's less BW, we use less speech BW. Even the latest
cellular NW are doing that! When the NW gets heavily loaded (rush hour, for
example), they reduce everyones' speech coding, even going to half-rate when
necessary. The lowest coding rates are noticeably distorted, though (that
"digitally" sound). The tradeoff, of course is that you can make or maintain
that call insead of getting "NW Busy" or dropped calls. More maintained calls
= more $$$ and happier customers, which means more $$$, etc.
Joe, N3JI
----- Original Message ----
From: Jim Tonne <tonne@comcast.net>
To: "Joe Subich, W4TV" <w4tv@subich.com>; Joe Isabella <n3ji@yahoo.com>;
amps@contesting.com
Sent: Monday, July 3, 2006 5:09:04 PM
Subject: Re: [Amps] Maximum RF output in practical application: 4-250A
I want to strengthen the information that Joe has stated:
> Bell Labs and other competent
> research facilities have long proven that communication is substantially
> unimpaired in a 2.5 to 3 KHz bandwidth and additional bandwidth does
> not contribute to significant improvement in reliability. The ITU and
> national regulatory bodies have recognized that and have designed
> most regulations around a 3 KHz bandwidth for voice.
In the days when telephone calls were sent inter-city by microwave,
those signals were in the form of single-sideband signal
frequency-modulating a 3.9 GHz carrier. Each voice signal
occupied most of a 4 kHz baseband "slot." Those signals were
sometimes generated in the 64 to 108 kHz region, sometimes at a
higher frequency and heterodyned down to that region. After
generating such a "group" those groups were again heterodyned
upwards to form a super-group and so on. The final collection of stuff
that actually modulated that 3.9 GHz carrier occupied several MHz.
(Megacycles in those days.)
Nevertheless, each individual channel had to pass - hold on to your
hats, gents - the region from 300 to 3400 Hz. Period. The response
was *quite flat* over that range. But there were NO specs above or
below that. Period. The various schemes used to generate a
channel all had the same responses, regardless of whether they
used an L-C or a crystal filter. I have designed and put into
production such equipment.
This business of "300 to 3000" and the like is a terribly casual
approximation of things.
The numbers I just stated were not come by casually, you may rest
assured, because the research was done by Ma Bell long ago. And
they had a serious financial interest in things. They wanted people
to be happy and use "long distance" a lot. But they did not want to
waste a single little bit of bandwidth to do so. (Prior to microwave
the group signal was sent via wireline.)
Listening to signals through such a channel - the broadcasters called
it a "Schedule C" - was really quite pleasant. And that is where I am
about to stop. You do NOT NEED the region above 3400 Hz to
make the "quality" any significantly better. To do so in the olden Telco
days was expensive and in the area of diminishing returns. Even the
coast-to-coast network signals only went to about 4800 Hz. And
remember that no matter how wide a signal you transmit, the
RECEIVER has to be similarly wide (or else insane amount of
pre-emphasis has to be used in the transmit end) or all is wasted.
The FAA specs (or at least DID spec) their voice-channel equipment
to go to 2400 Hz. Tones for signalling and the like were at 2700.
Sounded awful to me.
My point? A flat system response to three or so kilohertz is really
pretty nice.
- Jim WB6BLD
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|