On 8/21/2019 8:16 AM, donov...@starpower.net wrote:
These recordings are an impressive demonstration of the benefit of
one dB of signal strength improvement in a weak signal situation.
Click on the links on this website:
www.ab7e.com/weak_signal/mdd.html
73
Frank
W3LPL
It seems to me that converting the files to .mp3
muddies the water. Dave comments that 32 kHz
is "decent fidelity". It certainly isn't (IMHO)
decent fidelity for music. I don't know about
noisy CW. The .mp3 format was optimized
for music or possibly high S/N voice, but not CW
buried in noise. The operational principle of
mp3 (or any lossy compression) is "noise gating".
For all I know, the abrupt change at low S/N may
be an artifact of the compression.
I don't understand the comment about "all audio files
were converted to .mp3 format and mixed ..." AFAIK,
mixing should be done before any compression.
Using "band noise" also muddies the water. It would
have been better to use the proverbial Additive
White Gaussian Noise (AWGN), to get reproducible results.
Dave comments that "spikey" noise is another hard to
quantify condition. I have noticed anecdotally that
160 meters tends to have spikey noise vs AWGN on say
20 meters. This results in clearly audible signals that
can't be copied by ear on 160 meters, whereas on 20
meters, I seem to be able to get solid copy on signals
that are just barely detectable. This is a poorly understood
phenomenon.
Another issue with reproducibility is the audio bandwidth.
The brain tends to be able to provide some audio filtering
of its own. IOW, I believe that you will find that
a 250 Hz bandwidth will give less than a 3 dB advantage
vs 500 Hz for the threshold of copy. At narrow bandwidths,
ringing becomes an issue. Typical brick wall filters in SDR's
may or may not be optimum for copying a CW in noise
(and in the absence of QRM). My Flex 6700 uses a
gazillion poles "because they can" :-)
Rick N6RK
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