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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: A proper comparison of AM to FM noise requires some mathematical > It is true that FM noise is less than with AM, >but still in principle it's there. 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. -- You must Sign in before you can post messages.
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