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Message from discussion Where is the noise when I tune to a FM station.

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More options Oct 2 1998, 3:00 am
Newsgroups: sci.physics.electromag, sci.physics
From: b...@netcom.com (Benjamin P. Carter)
Date: 1998/10/02
Subject: Re: Where is the noise when I tune to a FM station.

berge...@iaehv.IAEhv.nl (Jos Bergervoet) writes:
> It is true that FM noise is less than with AM,
>but still in principle it's there.

A proper comparison of AM to FM noise requires some mathematical
sophistication.  This is a topic you will find in textbooks written for
electrical engineers.  It depends on Fourier analyses of the modulated
signals, to which band-limited Gaussian white noise is added.  For AM, 100
% modulation occurs when all the energy is in the sidebands, so that the
carrier disappears.  For FM, the analysis is more complicated.  100%
modulation is defined as a certain maximum allowable frequency deviation.
The signal is "wideband FM" if that frequency deviation is large compared
to the highest possible modulating frequency; otherwise the signal is
"narrowband FM".  Audio FM stations were wideband in the good old monaural
days.  Now that FM is stereophonic, the noise is much worse than it used
to be, but it is still better than the crappy audio signal in broadcast
TV, which is truly narrowband FM.  The mathematical theory verifies what
you already know about TV sound quality, which is that narrowband FM is no
better than AM.  Only wideband FM yields anything like audio high
fidelity in a noisy environment.

--
Ben Carter