> Audio nulling .. that is interesting. This should have
> been easy to test as
> if he tuned both rx's to the same active frequency it
> should have nulled the
> audio..
> The theory was that the noise (static) received by both
> radios
> would cancel in the primary, leaving a clear (or nearly
> so),
> static-free signal in his headphones. Interesting....
I've loooked at this extensively over the years. (Ssince
1969 or 70!)
Here are the problems:
We have to define the noise. If it is impulse noise with a
short duty and is idential in time period on all frequencies
we can just blank it and be done. We will only lose a little
bit of receive signal if the nosie duty cycle is very low.
This is a large part of why I could work JA's on 160 through
the LORAN from Ohio. I built a phase compensated noise
blanker that sampled LORAN from the same antenna I was
using. I delayed the main receiver through a long delay line
so I had time to turn the receiver off quickly but smoothly
during each LORAN pulse (by disconnecting the antenna), and
I was already turning the receiver back on as the pulse
arriving at the receiver was stopping.
I could blank a 50 over nine LORAN signal to noise floor,
but ONLY when the LORAN signal sample was on the same
frequency and same antenna. The key to success was the pulse
was very short and I adjusted time delay so the blanking
system disconnected the receiver antenna system just as each
pulse arrived and reconnected it just as the pulses stopped
without adding switching clicks. It took about 5 vacuum
tubes and some semiconductors (diodes) and a long length of
coax to do that.
If the noise is NOT short duty with long rest period we
cannot blank it without eating up considerable signal. If
the nosie is a long duty cycle we have to subtract the noise
without subtracting signal at the same rate, or we have to
add the signal without adding the noise at the same
rate.....and this only works under **very** limited
criteria.
1.) We CANNOT phase cancel the noise on one frequency with a
receiver on a different frequency or with a receiver having
greatly different bandwidth or phase charateristics.
2.) DSP's will only work if the signal is detectable through
the noise. In other words the DSP, when selectivity is
applied, has to be able to clearly tell what is signal and
what is noise. Unfortunately the human brain does that just
as well when optimum filters are used before reaching our
ears, so the DSP can generally only enhance a signal when an
experienced weak signal operator can already copy it.
3.) The system can't subtract random "white" noise from
random "white" noise. It can only eliminate it by adding
selectivity or adding the coherent signal. This requires two
antennas, and winds up being no better than phasing two
antennas. Each antenna has to have a good pattern and about
the same S/N ratio to double the signal while the system
adds the noise randomly. In other words this is the same as
making the antenna bigger by phasing the two antennas.
It doesn't matter if we do this at RF or at audio. I can
combine the audio output of two phase-locked identical
receivers. The receivers have to share common local
oscillators and BFO's and have similar filters and stable
phase delay. Then, by changing audio phase, I can add the
signal while the random noise adds and subrtacts randomly.
The problem is the end result is absolutely no different
than changing the phase at the antenna inputs and using ONE
receiver. It is the same thing.
As a matter of fact an HP vector voltmeter measures phase
after running two different inputs through identical
receivers using a common local oscillator and it is exactly
the same as measuring the signals before conversion to the
low IF frequency!
The problem on 160 is even over a space of several hundred
feet, phase of signals become random. It is impossible to
add two skywave signals with fixed phase compensation and
have them stay in phase very long even under the most stable
conditions.
To eliminate the consant tuning and fussing to keep two
antennas several hundred feet apart running in-phase for
signals, I simply use stereo audio from the two identical
phase-locked receivers and let my brain process the signal.
This has been up on my website for years now. Unless the
receivers are perfectly phase locked and nearly identical
the full advantage is never reached.
While a DSP could do the processing, it could only do it if
the signal is sorted from the noise enough that phase can be
determined. At that point any of us who are any good at
copying a signal that is below noise could already hear the
signal clearly so the DSP is no help.
73 Tom
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