The Telecom Digest for November 02, 2010
Volume 29 : Issue 295 : "text" Format
Messages in this Issue:
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Date: 1 Nov 2010 12:18:00 -0400
From: kludge@panix.com (Scott Dorsey)
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: Disconnected: Attention Passengers it's perfectly safe to use your cellphones
Message-ID: <iamp7o$jem$1@panix2.panix.com>
Richard <rng@richbonnie.com> wrote:
>
>Single-sideband, suppressed carrier (SSB-SC) would be even better for
>hearing that weak mayday message. The AM broadcast band channel uses
>double-sideband, full carrier. At night, when I hear two stations on
>the same channel, often the weaker signal is garbled becaue of phasing
>errors between the sidebands of the weaker signal and the carrier of
>the stronger. And if the carriers aren't zero-beat, there is a
>heterodyne tone. With SSB-SC, there is no distortion or tone. In my
>ham radio experience using SSB-SC, during a net check-in when many
>stations are calling at once, I can hear a several signals on top of
>each other, and make them out clearly.
Problem is that if they aren't right on frequency, they become severely
garbled. There's no carrier to use as a reference.
SSB is used for long distance aircraft HF comms, but for VHF they are less
worried about weak signals than congestion. Unicom gets very, very congested
in some places.
However, there WAS a change to the channellization a few years back... everyone
had to change their radio out for one with narrowband channels. So getting
everyone to change standards is possible, though it's an ugly prospect.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
***** Moderator's Note *****
It's true that SSB transmitters can become garbled if they're off
frequency by a large amount, but it's rare. Modern frequency-control
circuits are more than adequate to keep an SSB transmitter within the
"intelligible" range: the voice may sound a little "Donald Duck", but
it will be intelligible in a properly maintained SSB receiver. In
any case, SSB receivers usually have manual "clarifier" controls that
may be used to improve the tone of a transmitter that's "on the
edge".
The problem arrises during a transition from AM to SSB: the carrier
signals sent by AM transmitters will heterodyne with the
Beat-Frequency-Oscillator (BFO) in the SSB receivers, causing
unacceptable noise, increasing controller fatigue, and causing other
traffic - yes, possibly emergency traffic - to be missed.
It only takes a couple of hundred Hertz of frequency offset - which
is, remember, the _combination_ of the BFO and carrier frequency
errors - and the only thing a controller or other pilots will hear is
a cat screech instead of a voice.
It is because of the syncronization problem that "SELCAL" (selective
calling) signals sent to International flights when they are over
water are sent using AM, which obviates any frequency-setting problem.
Bill Horne
Moderator
Date: Tue, 02 Nov 2010 10:28:15 +1100
From: David Clayton <dcstar@myrealbox.com>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: Disconnected: Attention Passengers it's perfectly safe to use your cellphones
Message-ID: <pan.2010.11.01.23.28.12.978922@myrealbox.com>
On Sat, 30 Oct 2010 21:52:14 -0500, Robert Bonomi wrote:
.........
> A mere 50 'high in the sky' phones has the potential to 'take out' 2.5%
> of the total capacity of the cell phone system for many hundreds of
> mile around their location. (A mere 2,000 feet above ground gives a
> 'horizon' of 100 miles or so.)
So the "bottom line" is that the current cellular mobile systems are
designed on the assumption that the terminals can only connect to a
limited number of geographically close base stations, and if this
assumption is not me then the whole functionality is threatened.
One more good reason to limit (or continue to ban) handset use on flights.
--
Regards, David.
David Clayton
Melbourne, Victoria, Australia.
Knowledge is a measure of how many answers you have, intelligence is a
measure of how many questions you have.
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