In article , Roy L. Fuchs
wrote:
On Sun, 23 Apr 2006 12:24:51 GMT, (Don Pearce) Gave
us:
No - the wanted stuff is the signal - the rest is interference. Ever
heard of signal to noise ratio? You would call it signal to signal
ratio. Now that makes much more sense, doesn't it?
Even with s/n ratio, in an engineering analysis BOTH the signal AND
the noise are signals.
As with various of the other statements I have seen in this thread on
various sub-topics, the above seems to me to be an over-simplification.
Interesting to speculate if in this case it is the above statement that is
ambiguous, or the ways in which the terms are actually used by engineers
are ambiguous... Perhaps this supports the argument that people become
engineers because they can't communicate very well... :-)
If you go back to some of the early sources [e.g. 1] then you can find some
that describe what is observed by the receiver/destination as something
like a 'received signal' which may include some 'noise' (and some
distortion or other systematic alterations).[2]
However the sources also routinely refer to 'signal to noise' ratio.
Shannon seems to resolve this by distinguishing between the 'signal' (i.e.
what the source transmitted) and the 'received signal' (i.e. what the
destination actually observed to arrive).
So if we were to use a term like 'received signal' in the above statement
it would essentially become either a tautology or self-referential as the
signal includes the noise. Thus the problem with the statement is that it
is unclear due to the ambiguous use of 'signal'. Hence, as often is the
case with such ambiguous statements, people start arguing about the meaning
when they are simply using different definitions which the ambiguity
allows. :-)
FWIW for the above reason, when teaching Information Theory/ Comms/
Instrumentation I tended to use another approach which is common in the
area. This is to say that a 'signal' means that the pattern (or part of the
pattern) *is used to convey information content*.
Thus in the context of communications a 'signal' means that the sender and
destination have to have pre-agreed the coding/modulation system to be
employed, and the meanings of the code symbols or distinguishable patterns.
In the context of a physical scientist making observations - e.g. an
astronomer observing what can be received from a distant radio galaxy - the
'signal means that the observed pattern will be used to obtain information
about the distant source.
The status of 'signal' then stems from the deliberation or requirement that
it conveys information on a defined basis.
In both contexts what distinguishes 'signal' from 'noise' is the
information conveyance the 'signal' provides, and that 'noise' tends to
obscure, or limit, or make uncertain, the information recovery. This then
helps make clear the actual meaning in practice of terms like 'signal to
noise ratio'. (Although there may then be hours of fun for all the family
as they argue about the distinction in this phrase between assuming
'signal' means either the intended/transmitted or the 'received' signal.
:-) )
Slainte,
Jim
[1] e.g. Shannon
http://cm.bell-labs.com/cm/ms/what/s...day/paper.html
[2] Probably best at this point not to start worrying about distortion as
being 'signal' or not... ;-
--
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