People interested in this might find this paper interesting: The Weak-
Signal Capability of the Human Ear by
Ray Soifer, W2RS
<http://www.g1ogy.com/www.n1bug.net/tech/w2rs/humanear.html>
<http://www.g1ogy.com/www.n1bug.net/tech/w2rs/The%20Human%20Ear.pdf>
The AMSAT ZRO Tests, in which several hundred amateurs participated
in a
controlled experiment over more than seven years, established that
many good operators,
approximately the top quartile of test participants, were able to
copy by ear a sequence of
five random digits at a key-down SNR of –3.6 dB in a noise
bandwidth of 100 Hz, with a
few (4%) able to reach –6.6 dB. The median participant required a
SNR of –0.6 dB.
Given the test conditions, these findings are considered to be
accurate plus/minus
approximately 3 dB. A study of the W2RS 144 MHz EME log from 1985
to 1995, when
the author operated with 150W output to a single Yagi antenna,
yielded fairly comparable
results.
The W2RS EME log also shows that in prearranged schedules, when
operators
know what they are listening for, contacts were completed with SNRs
at least 3 dB lower
than was possible in random operation. Only four stations could be
worked on random,
out of 37 worked in total. For a good weak-signal operator in a
prearranged EME
schedule, copy by ear down to –6 or –7 dB key-down SNR in a 100 Hz
bandwidth,
equivalent to –23 or –24 dB average at 2.5 kHz, would not be
unreasonable to expect
(again, plus/minus approximately 3 dB).
Signal-processing techniques developed by SM5BSZ and WB9UWA may be
able
to improve upon the performance of the unaided ear by as much as
2-3 dB, depending
upon the characteristics of the received signals.
Note SNR is a little different from "normal":
we must define what we mean by SNR. While
any definition would probably suffice as long as it is properly
specified, this paper will,
unless otherwise noted, follow the commonly-used convention in EME
work of
specifying SNR as the ratio of key-down signal to average noise
level, in the absence of
signal, in an effective noise bandwidth of 100 Hz. (Note that this
is S/N, not (S+N)/N.)
Since the Morse code duty cycle is approximately 50%, it is assumed
here that the
average SNR, as is measured by some software and test equipment,
will be 3 dB below
key-down SNR for the same signal strength and noise level.
A wave file of the ZRO tests is out there if you want to "play
along" (though at 4kHz sample rate and, worse, 8 bit resolution is
still on the net it is not the best -- you can really hear the
quantization noise).
He also comments that this works for SSB too. Good perceptual
filtering is not just for CW :-)
[] two SSB QSOs with W5UN, which were completed despite predicted SNRs
of –7.4 and –10.7 dB, respectively (at the receiver’s bandwidth of
2.1 kHz), demonstrate
that the adaptive power of the human ear to pull weak signals out
of the noise is not
limited to CW. Although good libration peaks helped, in order to
complete the contacts
under these conditions the ear’s effective noise bandwidth had to
be significantly less
than 2.1 kHz
SM5BSZ (another VHFweak signal/EMEer) makes some interesting comments
in:
<http://www.nitehawk.com/sm5bsz/weakcom.htm>
3. If the bandwidth is too large, above 1kHz, the "signal
processor" between my ears gets overloaded and a significant loss
of performance occurs.
4. Only at bandwidths below about 50 Hz I can get any improvement
over what I get at 1kHz - but only at slow or moderate CW speeds,
and with signals that are very frequency stable. This indicates
that my "brain bandwidth" is about 50 Hz. I can work for much
longer times without loss of performance when the bandwidth is low.
I practically never use bandwidths above 100 Hz.
On Oct 22, 2007, at 8:58 AM, Steve Hunt wrote:
Many, many years ago I was involved in the design of a "survivable"
military ComSat system that would provide a low bit-rate channel under
heavy jamming conditions. We set ourselves the target that it
should do
as well as a good CW operator under the same S/No conditions. That
meant
we had to "measure" the performance of the CW operator.
We took 3 very experienced operators - the sort that copy 25wpm
random 5
letter groups whilst doing the Times cryptic crossword at the same
time
(I'm not kidding). We gave them the best receivers available at the
time, equipped with a vast array of filters.
I was fascinated as a young engineer to see that they never used the
narrowest filters, however poor the S/No. They rarely went below
1KHz. I
guess they had some pretty narrowband stuff going on in their heads :)
And by the way, the set a pretty tough target to beat!
Steve G3TXQ
--
73 DE N7WIM / G8UDP
Kevin Purcell
kevinpurcell@pobox.com
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