[CQ-Contest] Tip - Weak Signal Detection

Tom Rauch W8JI at contesting.com
Mon Feb 26 14:00:30 EST 2001

 > The noise floor does NOT drop - and, of course, the noise power is
 > larger compared to the original cw signal. However....
 > There is a curious psycho-acoustical phenomena dealing with noise - a
 > weak signal that is "almost free of noise" is more difficult to hear
 > than one surrounded in a reasonable amount of broad band (white/pink
 > etc) noise.  The noise provides a context that the brain needs to
 > decipher the signal.  Thus, using the SSB filter in a CW contest may
 > result in the ability to copy signals that are difficult to copy using
 > a narrow cw filter.  Of course, this is not a guarantee - it depends
 > upon the quality of signal, noise, and your personal ear/brain system.

Looking at signals here, what happens is if I have "rough noise"
narrow filters can ring and lengthen the noise pulses. In some
cases, like during QRN from thunderstorms, I find no AGC and a
wider filter sometimes works better.

If I have smooth hiss for background, then selectivity almost always

Some receivers have gain problems when using narrow filters, or
have excessive hiss after the selectivity. My FT-1000D is an
example of that. The 250 Hz filters in my 1000 are less useful than
the one's in my Drake R4C receivers because the noise figure of
the IF system after the narrow filter in my Drake is much better.

The 250 Hz narrow filters in my 751A ICOM's are almost useless,
because they ring so bad any noise gets drug out to a long
"tiiiingggg" that is matched to the frequency of CW signal, yet I can
successfully use a Sherwood 125 Hz filter in the C line and it
improves things!

Some DSP systems are particularly bad for digging weak signals
out of noise because they run out of "bits" in the A/D conversion,
and lose resolution when the signal falls into the noise. They give
an illusion of better S/N when the signal is slightly above noise, but
at the cost of being able to hear signals at or below noise.
73, Tom
(W8JI at akorn.net) 

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