----- Original Message -----
From: Stewart Nelson <[email protected]>
To: <[email protected]>
Sent: Tuesday, April 17, 2001 12:41 AM
Subject: LF: [TECH] WOLF, FDK, AFK, etc.
Hi Steve and all,
Sorry that some statements on my WOLF page might be misleading.
OK
You are also correct that in the special case of dual tones symmetrically
located about the carrier frequency, in the absence of filtering, the
penalty
can be avoided.
OK
However, I don't think that this is particularly interesting,
because if the number of frequency differences is small (2 or 4), the
signal
is very similar to WOLF (BPSK with some redundancy added to aid tracking).
As FDK came well before WOLF I would prefer you said "WOLF is very similar
to FDK" :-) Just joking :-)
But either way, this can be misleading as WOLF is, as you say, the well
known BPSK mode with extra redundancy for tracking. FDK is only similar to
BPSK in terms of a possible way it can be generated. Its spectrum over a
60 second epoch (the initial character duration for now) looks identical to
a two tone SSB test signal (assuming linear) and bears no resemblance to
BPSK.
If it's large, e.g. a pair for each letter of the alphabet, I don't see a
good way to track the signal when it is very weak.
If I understand correctly, you use a tracking tone which doesn't actually
transmit character data. So although it does transmit information
(tracking frequency), so ultimately does have a beneficial effect on S/N.
What is the loss and what is the gain for this method ?
Your web page doesn't have any information on how such tracking might be
accomplished
So, IMO, although a strong FDK signal can be received without the need for
good
frequency accuracy, in the weak signal case it has needless complexity
with no real benefit.
Essentially the same as WOLF, the difference being that the tracking is
inherent in the signal which transmits character data. In FDK (or Wanjina
as I now call it to try and hose down the confusion with DFCW / VFSKCW) the
two tones are there during the 60 second character duration. By taking the
mid-frequency of the tones you get the tracking frequency. This is used to
improve the S/N during detection. FDK doesn't waste time/energy by
transmitting a separate tracking tone. I can't see that this is any worse
than WOLF.
That brings us around to AFK (known in the literature as m-ary FSK), which
is IMO the technically best system for weak signal LF. I will admit that
BPSK was chosen for WOLF, for the simple reason that many LowFERs already
had
the ability to transmit it, and it would be relatively easy for others to
gain the capability. Like almost everything else in engineering, it was
an economic tradeoff.
Understood. But I can't help thinking that if you took your own advice
you wouldn't have started on WOLF and not achieved the excellent results you
have !!! :-) Just goes to show that there is a time for theory, but reality
has the final word :-)
But I really believe that if it is possible to complete a transatlantic LF
QSO in one hour, with a signal that is about 1 Hz wide, AFK is the way to
go.
Agreed
The main problem is that it can presently only be generated with an HF SSB
transceiver feeding an LF transverter, plus taking special measures to
ensure
excellent stability.
So IMO the real challenge for AFK is to develop a very simple exciter
which can generate this format, as well as others.
An exciter is fairly easy I would have thought. As we have two channels
available from our soundcard it is a trivial task to generate two identical
tones in quadrature. Feeding this into a mixer fed with quadrature RF
signals allows generation of AFK via the Third Method. All pretty
standard stuff for Hams. An even simpler method might be via the Fourth
Method.
I just wish I had more time !!!!!
A final note. I repeat my congratulations on your success with WOLF. The
comments here and previously are not meant as criticism. I know how hard
it is to keep everything consistent as things and ideas develop over time.
73s Steve Olney (VK2ZTO/AXSO - QF56IK : Lat -33 34 07, Long +150 44 40)
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