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[TowerTalk] True Or Not True #2

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Subject: [TowerTalk] True Or Not True #2
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Date: Fri, 7 Sep 2001 10:58:27 EDT
A signal to noise ratio of 0 dB doesn't mean there is no signal but all 
noise; it means that the difference in amplitude of the signal and noise is 0 
dB and that the amplitude of the signal and the noise is equal. When the 
equal amplitude signal and noise is combined, as in the audio output of a 
receiver the sum of the two will double the total power for an increase of 3 
dB and this is now called 3 dB signal plus noise to noise ratio.

Since the total power is double the change is easily detected by ear. In 
fact, if we are talking about tones such as c.w. signals, the trained ear can 
have a bandwidth related signal to noise improvement of several dB. A really 
good c.w. operator can achieve an "ear bandwidth" on the order of 30hZ. If 
the receiver bandwidth is 2kHz  the operator ear/brain "filter" can detect 
and copy signals 10log(2000/30) dB below the measured   0 dB signal to noise 
ratio. I don't have my calculator handy but this is on the order of -18 dB 
s/n ratio. 

This is impressive performance considering the cost is zero!

Tnx/73 de Gerald Williamson, K5GW
Texas Towers

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