On 8/1/2014 2:58 PM, Lostgallifreyan wrote:
Jerry Stuckle wrote in news:lrgjen$ao4$1@dont-
email.me:

Sure, but you're also talking a 5Khz deviation (actually about 12Khz
bandwidth). Commercial FM uses 75Khz deviation (typically around 180Khz
bandwidth). A huge difference.


Does that degrade the signal quality compared a really good FM broadcast? (If
it does so a lot, it amazes me how little people are willing to accept from
their portable MP3's and DAB radios, but that's another matter...) Whatever I
end up doing, I want to avoid things like catching the FM, truning to audio,
rebroadcasting with all the losses involved, then back to audio yet again!
Sounds to me like a disaster, I'd likely end up with bigger problems than I'm
trying to fix now.


I won't get into the math here, but due to the sidebands (yes, FM has
sidebands, also - the number and strength of the sidebands is
proportional to the deviation and modulating frequency), frequency
response of the modulated signal is proportional to the bandwidth of the
channel and deviation. A 5 kHz deviation can theoretically pass up to
about 5Khz of audio, but in practice it's limited to about 3 kHz (to
avoid adjacent channel interference). This is fine for voice, but does
not work well for music (try listening to music on the AM band, for
instance).

A 75 kHz deviation can easily be modulated at 15 kHz, providing for
better quality music.

Back in the late 60's and early 70's, there were some hams on 2 meters
running 15 kHz deviation. It's amazing how much better the audio
quality was, even with just voice. But that's back when there were few
repeaters; I'm glad it didn't catch on. Here in the Washington, D.C.
area, it's impossible to get coordination for a 2 meter repeater and
difficult on 440 Mhz. I can't imaging what it would be like with 1/3 of
the channels available!

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Jerry, AI0K

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