The filter width issue clearly relates to the type of noise involved.
In my case, with a city location, the background "white" noise is at least S7
and often 10 over nine with a 2.4 kHz filter. I never use that for cw on
topband. I normally use a 250 Hz filter, and once I have a signal to copy, I
reduce the passband as much as possible, probably to 100 Hz or less with
audio filters.
Perhaps the most important action I have found, and one that relates to
Psychoacoustics (I am a psychologist with EE background), is to make sure
the cw tone is at the top of the passband so there is no noise frequency
band above the signal. High frequencies mask lower frequencies far more
than vice versa (this is a human auditory system effect as far as I know, not
a physical effect). In addition, I have my rx set so that cw signals are at
about 500 Hz audio tone, thus a 250 Hz passband yields audio from 250 to
500 Hz with all the noise below the signal. If needed for qrm reduction I can
then filter out below 500 Hz, but this adds little to weak signal
intelligibility
b/c the noise that detracts most from intelligibility (that higher in freq) is
already gone.
An additional factor is cleanliness of the signal path from distortion effects,
probably mostly intermodulation. An Icom rx had such poor audio stages
that I had to tap into the the mixer output and use outboard audio
amplification for topband work. My old Tentec is much better in that
respect.
73,
Rod VE7VV
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