On Feb 20, 2014, at 10:54 AM, Dave Hachadorian wrote:
> I updated my 2Tone to v. 14.02a this morning, and I am now looking at some
> weak signals, comparing MMTTY "Fluttered FIR" vs. 2Tone "selective" modes.
> There is indeed selective fading going on this morning, which I can see just
> looking at the amplitudes of the 2Tone mark and space filters. 2Tone is
> coping with these fades in a spectacular manner, totally running circles
> around MMTTY's fluttered fir, which I have always found to be MMTTY's best
> all-around mode.
>From my findings, there is a pretty big difference in demodulator behavior
>between Flutter (which is the profile that you use for MMTTY) and Selective
>Fading.
With Selective Fading (where Mark and Space signals fade independently), you
need some form of a Automatic Threshold Control (ATC) to equalize the Mark and
Space amplitudes (or, if you go by Leonard Kahn's work, equalizing the SNR in
the Mark and Space channels). Selective fading occurs there is multipath
through the HF channel.
I have not seen evidence that MMTTY has a decent solution when selective fading
happens.
On the other hand, Flutter is caused by severe Doppler Spreading (the Mark and
Space tones are randomly changing in frequency), typically wider than 10 Hz to
30 Hz for about 30% of the time.
There usually is also some multipath (mark and space arrive at different times
and overlaps one another) when signals are fluttering, but I have never found
any ATC circuit that works well during a flutter. The estimation error of the
threshold in an ATC algorithm often exceeds the harm done by the flutter
itself, so that the "best" thing to do in a flutter is to actually turn ATC
completely off and use a constant, unbiased threshold. I.e., assume Mark and
Space have equal amplitudes.
When there is severe Doppler Spreading (e.g., when you audibly hear a flutter),
the typical solution is to widen the demodulation filters, but you do not need
to widen the filter (at least not by much) for Selective Fading.
(I am looking at the use of a method that does not use a wider bandwidth for
flutter -- when all else is equal, wider bandwidths make SNR worse, so a narrow
ISI-free filter wins -- but have not done enough to know if it is better than
just using a wider filter.)
I suspect that a good ATC implementation can improve MMTTY under selective
fading. The lack of a good ATC should not hurt MMTTY as much relative to other
demodulators when it comes to fluttering, since many demodulators turn ATC off
for flutter copy anyway.
I know David worked a lot on both Selective Fading and Flutter modes (I don't
know if he has release his software that has the two separate profiles), so it
is definitely no surprise to me that 2Tone beats MMTTY during selective fading.
73
Chen, W7AY
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