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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