On Mon, 2007-10-22 at 16:58 +0100, 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, they set a pretty tough target to beat!
>
> Steve G3TXQ
>
I've seen this more than once. And I often prefer to copy a weak signal
in a bit wider bandwidth. For one thing, my ears are more frequency
agile than the super narrow filter and some filters I've had or used
(Kenwood 270 Hz, Inrad 400 Hz for Kenwood) began to ring on the CW
signals for sure by 35 wpm.
Back in the late 60s I lived a few miles from W5WXV. With a home brew
single conversion 2m receiver that used a VXO for the local oscillator
(and so super good phase noise and stability), he had a nightly sked on
2m with a station in Quincy Illinois from Garland Texas. More than 600
miles. I was maybe 20 miles north of WXV, but I rarely heard the Quincy
station. A lab tech at Collins, Al checked his hearing and detection of
weak signals in the laboratory and claimed he could detect a signal more
than 20 dB below the noise, maybe 30 dB. All just ear training and
practice. I know I didn't copy the Quincy station on the speaker in Al's
hamshack either while he was ragchewing.
In the 80's at a Central States VHF conference WB0TEM set up his
portable 1296 EME station. He was working K2UYH via the moon and
ragchewing. I copied about half the time, many with out such good ears
listened a while and walked away shaking their heads not believing there
was a signal in that noise. Yet Marc seemed like he was copying all the
time. Those only accustomed to FM were the first to walk away in
disbelief.
There is capability of significant ear processing, takes practice, and
probably could be trained in some, some can't be trained. And some of
our modern receivers demand a great deal of that training by having no
SSB or narrower filter after the front end filters so we get to listen
to all the wideband noise from the IFs, mixers, and wideband audio. I
sometimes apply a passive low pass filter in the speaker circuit that
helps some of those considerably. Crank down to a 400 Hz low pass and
shift the beat note down there and bands become deserted and very quiet.
Also removes audio and detector intermod products and harmonic
distortion. Almost too quiet.
73, Jerry, K0CQ
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