Jim Jarvis wrote:
>The other thing I've noticed is that a slowly drifting signal
>will stand out from noise better than one which is stable.
>
>This raises the question of whether a synchronized TX & RX which
>were slowly swept would enable s/nR improvement...much as a chopper
>stabilized signal would.
>
>Anybody versed in signal processing/signal recovery techniques
>care to chime in?
>
>n2ea
>
>
Yes, a drifting signal gives the same effect as when I slowly tune
through a signal to help pick it out of the noise. One of the hams who
responded offline to my original message said he sometimes will tune
across a signal that sounds loud only to have it immediately appear
weaker when he stops on it, and then when he tunes away it sounds louder
again.
I don't believe a system where the TX and RX synchronously change
frequency would do the same thing. The idea, at least as far as my
experience goes, is to have a tone that changes in frequency in a manner
that the brain can anticipate. I really believe the predictive aspect
of a changing tone is the relevent concept. It's the frequency domain
analogy of the time-synchronous gated keying mode that some hams have
played with for years. The advantage of the frequency domain version
would seem to be that with a good steady signal on the transmit end, all
of the synchronization could be done on the receiver end. The time
domain mode requires both transmitter and receiver to be synchronized to
a common time source (WWV or locked oscillators).
My oldest son writes software for a company that does a lot of signal
processing work, mostly for the military. I've discussed this effect
with him on several occasions since it seems to me that it should be
possible to shift the BFO at a known rate and have DSP software look for
the IF or audio shift (audio is probably better because of the
percentage difference) as an additional distinction from the noise.
The DSP software would know exactly the rate of shift and know exactly
where to look for the result. So far he has other things to do and
hasn't taken the bait to try to implement it, but it still seems
potentially useful to me. I would think that the filter software could
run on a PC using RX audio fed into the sound card, with a data stream
back into the RX to shift the frequency ... it doesn't require fast
tuning of the RX. You'd have to listen to the processed signal on the
audio output of the sound card.
Again, none of this works if the background noise sounds like mating
chainsaws.
As an aside, we are sometimes able to "copy" characters that we can't
fully hear if the sender has a steady fist (or is sending machine code
from a logging program). For example, a missing dit can be filled in by
our brain when we detect an extra space that shouldn't belong there.
That's a very low-level example of time domain synchronization coupled
with our brain's knowledge of a reduced set of variables.
Dave
AB7E
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