Forwarded on behalf of Andy Talbot G4JNT

 

Re JT65 and LF

Forgive the second hand posting, I cannot send directly to the group as my
EMail provider has decided to throw a wobbler and stop allowing any outgoing
email to radiate.  Incoming ones still arrive though !  (perhaps the
tranmitting licence has been rescinded:-)

Anyway..
I have been looking into JT65, and its earlier incarnation JT44, in some
depth recently for use on beacons.  It is very amenable to being generated
in a simple driver based around a DDS chip.

However, I don't think that JT65 as it stands, even in its narrowest
version, has a place on LF.  It is too wide and the mode has been targetted
specifically at areas where frequency, and to a lesser extent timing,
uncertainty is appreciable such as EME, and does not make the most of the
stabilities inherent in the LF path. Coding issues notwithstanding -it does
allow 13 character random messages so the callsign prefix limitation may not
bee too bad if used sensibly.

Some years ago, Alberto introduced us to Jason, but even that is an
incoherent mode and doesn't make the most of the LF path.  But if the error
correction capabilities of JT65, were used with a reduced synchronisation
vector allowed for by better frequency accuracy, and with the tone / symbol
spacing of Jason we may be geting something formidable.

    BUT it is still incoherent.

In spite of Mike, G3XDV's reservations about the 'complexity' of GPS derived
techniques this is still clearly an obvious and straightforward way to
coherent signalling.  Any individual can have sub microsecond timing for a
few tens of pounds cost, plus the effort needed to build a small amount of
hardware.  

Dictating that any mode used on LF must be capable of interfacing with a
soundcard only, to all intents dictates incoherent modes and loses us the
6dB plus that could be recovered with a bit of effort.  

    BUt (again) there may still be a way to make soundcard / GPS simple
....

Some years ago, Andrew Senior G0TJZ wrote a chirpsounder monitoring prog
that took the output of a receiver to one side of a soundcard, and the 1 PPS
signal to the other channel. He also fed the NMEA data string from the GPS
into the serial port.  Now, the computer could get timing to the nearest
second from the NMEA data, and then look though the digitised audio samples
of the right channel to get the time to the nearest sampling interval.
Using 8kHz sampling, the received audio in the left channel could then be
correlated to UTC to an accuracy of 125us. (In fact he managed a bit better
than this by various methods).   The software measured time of flight of HF
signals from chirpsounders around the world, at any frequency at HF just by
tuning an SSB Rx roughly to the frequency wanted, then informing the
software of the exact tuning point.  It workd brilliantly, and gives a
great, simple, HF propagation tool showing the reflections from the
different layers as time progresses.   Do a Google search on G0TJZ

So, how about a dual channel signalling scheme, audio and PPS.   It may even
be possible to dispense with the NMEA data and use only the 1 PPS, provided
the PC clock can be set accurately enough.   GPS modules are cheap, simple
and easy to get going. 

Andy  G4JNT