Re: HF DV:


Stuart Longland VK4MSL
 

On 17/03/17 20:54, Andrew O'Brien wrote:
While I would like this to succeed, the SNR required for digital voice
on most HF bands is such that it renders DV as only useful between
stations with a lot of power and/or really high gain antennas.
Well, this can be true of other systems like FM too. 1200 baud AFSK
over FM is actually pretty terrible as a modem in performance.

http://www.rowetel.com/?p=3799 is written by the Codec2 author, thus
might be seen by some as biassed, but in that, he analyses the AFSK/FM
set-up typically used in packet and assesses its theoretical
performance. The results were not great.

It was built that way because people made use of what they had at the
time, old Bell modems. It works well enough for most purposes, but it
was never a stellar performer.

Typically you need decent transmit power or high-gain antennas to make
it work well. Yet, it is used a lot in emcomms. The saving grace is
that the packet typically only needs a short burst of a few seconds to
transmit a longish sentence that would take about 30 seconds to read
out. So you can afford high power because the duty cycle is much lower.

DV would take away that gain, but then again it can be recorded,
transmitted with stronger FEC and stored for playback later, so if the
operator is distracted, the message will still be there waiting for them
when they get back to the radio. This is a trick that analogue voice is
unlikely to pull off.

As for FreeDV and D-Star HF; transceivers for both do have the advantage
that if conditions get bad, it is a single button press usually to
switch to SSB. Another button press and it's to CW. Or with a device
like the UDRC, we can use PSK-31, etc.

This is feature of amateur emcomms that we can switch and adapt as the
conditions require. Commercial radios do not give their users the same
luxury.

Procedure can dictate how you go about switching modes. Our strength in
emcomm is that we have all these modes at our disposal, and if one stops
working, we can switch modes.

This is less risky than switching frequencies, and you get to know
digital modes by ear and most applications feature a waterfall.

The application can pass through the analogue audio when there's no
decodable signal anyway, so in that case the fall-back to SSB is automatic.

A smarter application could be watching for PSK-31, slow CW, WSPR and
DV, etc, and dynamically switch between them on receive, so the only
decision an operator is making is what to use when transmitting, knowing
the other end will figure out how to receive the message.

It'd be difficult to pack this into a hand-held, although with
single-board computers like the Raspberry Pi compute module, which could
be paired up with the TLV320AIC3204 in a UDRC-compatible package that
could conceivably fit inside a hand-held radio built for it.

However, hand-helds seldom reach below 50MHz (I'll ignore 27MHz), and it
is HF we're discussing here, where we can likely afford a bit more
computing power to do such real-time processing as we can afford the
slightly bigger form factor computers that would provide the necessary
grunt.

In short, I think it is premature to discount HF DV for emergency comms.
It's no silver bullet, but rather, yet another tool in getting a
message through.
--
Stuart Longland (aka Redhatter, VK4MSL)

I haven't lost my mind...
...it's backed up on a tape somewhere.

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