Joe is right on.
Back when EME was "real", i.e copied by ear, it wasn't unusual to copy signals
with a negative SNR.
A little story: I went to work for Hughes Aircraft here in Tucson when I was 25
years old. At the time I was given a hearing test as part of my physical exam.
(So they told me later).
About 15 years later, at the time I was working two-meter EME, I had to attend
a meeting in another building. After the meeting, as I exited that building and
headed to the security gate a Plant Protection Office ran to his truck, jumped
in and proceeded to turn on the siren just as I walked by. I was nearly
deafened by the sound.
The next day my ears were still ringing (to this day I have tinnitus) so I went
to the Medical Department and reported an on-the-job injury. As part of their
evaluation they gave me another hearing test. The test was almost like
listening for EME signals; detecting weak tones buried in noise and pressing a
button when I heard them. Their conclusion was that not only where my ears
fine, my hearing had actually improved since I took the first test! Clearly
impossible, but training made it seem so.
My receiver (in fact with the exception of the coax, the whole station) was HB
and designed for high linearity and low distortion. The final hardware BW was
200 Hz, provided by a passive, LC audio BPF. The final BW was set by the
tracking filter between the ears.
Wes N7WS
On 3/3/2014 12:59 PM, Joe Subich, W4TV wrote:
> If B = 2.7 KHZ, LogB = 44.3 dB
> If T = 290K (room temp), LogT = 24.6
> then Pn = -198.6+24.6+44.3 = -129.7 dBm
All of which means absolutely nothing since the proper number to use
for "B" (bandwidth) is the final detector bandwidth which can be as
little as 50 Hz *for a trained human ear*.
A conventional product detector following an IF where the selective
element is a crystal filter after the mixer has a detector bandwidth
of several KHz (the noise bandwidth of the IF) yet the real bandwidth
that determines MDS is the bandwidth of the user's ear/brain filter not
necessarily that of the IF filter or the product detector. It is quite
easy for the trained ear to detect coherent (e.g. CW) signals that have
a negative SNR of one were simply measuring signal power vs. integrated
noise power in the product detector output.
73,
... Joe, W4TV
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