Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: How does a mixer work?
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Tue, 22 Oct 2002 18:16:02 +0100
NNTP-Posting-Date: Tue, 22 Oct 2002 18:16:32 BST
"Gibbo" wrote in message
> "Kevin Aylward" email@example.com wrote:
> >"Gibbo" wrote in message
> >> "Kevin Aylward" firstname.lastname@example.org wrote:
> >> [snip]
> >> >> If you insist in filtering the carrier with an almost infinitely
> >> >narrow filter
> >> >> to prove that it is useless then try the same thing with one of
> >> >sidebands
> >> >> and see how much information you can get out of it.
> >> >>
> >> >
> >> >But this misses the point. A channel of zero BW cannot carry any
> >> >information, i.e. H = BW.log(1+S/N) bits/sec. By assumption, the
> >> >sidebands are varying, or obvously, they dont carry an
> >> >
> >> They can tell you whether or not there is a modulating signal.
> >>That is
> >> information. No amount of maths "proving" otherwise can change
> >The idea being the definition of information is the ability to use
> >information constructively. As far as transmitting, for example, a
> >random data stream, a continuous sideband is useless. Sure, you might
> >know it on, but so what? What use is this?
> It means they haven't had a power failure at the transmitter. It means
> hasn't actually sunk. It means the plane hasn't actually crashed.
> arguments, but information nevertheless.
> > If you say it can tell you
> >that it is not off, this means nothing, unless you can also have it
> >an off state as well at some point. That is, if it has two states, it
> >has log2(2), i.e. one bit of information. However, in this case the
> >system BW cannot be zero, otherwise it would take infinite time for
> >to change from one state to another.
> >Unfortunately, your concept of "information" is not very fruitful,
> >no one, and I mean no one, doing serious work in communication theory
> >and systems uses it.
> Perhaps you use the term "information" in some technical form that I
But that is exactly the point that I made. Information has a technical
definition, for a very sound reason.
>In general English useage the presence of an unchanging carrier (or
> sideband) can and does carry information.
But not Shannon information, and that's what is important in examining
the communications capacity of a channel, i.e. how much data (b/s)can be
transferred, and at what rate (f() and what probability of error.
> Not trying to be awkward. Just making a point that all that wonderful
> isn't quite as definitive as some seem to think.
The math is there to solve specific problems. Its not there to determine
whether or not some daft sod has forgotten to turn on the transmitter.
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