Re: LF: Slow JT9 modes...

[Andy Talbot <andy.g4jnt@gmail.com>]

The only reason PSK is said to require linear amplification is that in most practical implementations to control the spectrum the amplitude has an envelope applied, so it goes to zero when the phase flips.   PSK doesn't 'have' to use amplitude tailoring to control it's spectrum.  It is possible to ramp the phase instead and that will also proviude spectrum shaping - although its shape will be very different from envelope shaping.    

With the slow PSK we're talking about for LF do the sidebands matter anyway?

Back in the early days of 73kHz several OPs used a mode called 'coherent' that was based around 10 symbols/s (10Hz)  hard switched BPSK with strong FEC.  And most people did use very hard switched - an XOR gate was typical.    

When    many amateurs hear the term "PSK"  they tend to think of PSK31 specifically, which has the most extreme amplitude tailoring possible for the mose - and hence the narrowest bandwidth.    Communications efficiency of PSK31  is therefore quite badly degraded over what could be obtained with sharp edged switching with high sideband levels.

Andy

www.g4jnt.com

On 7 September 2018 at 11:28, N1BUG <paul@n1bug.com> wrote:

Sounds interesting, Andy. Please correct me if I am wrong, but
doesn't PSK require linear amplification? If so, it would eliminate
many potential QSO partners.

Regarding EbNaut, my understanding was that OCXOs aren't good enough
and that a GPSDO or rubidium standard is needed. If that's not true,
someone please correct me on this also.

As for someone who is interested in LF either joining the JT
development team or writing code independently, if I had the
relevant skills and development tools I would leap at the
opportunity to bring back the slower JT9 submodes. Unfortunately I
have enough trouble getting my head around simple things these days,
as you've seen with my construction projects...

73,
Paul N1BUG



On 09/06/2018 09:21 AM, Andy Talbot wrote:
> As good as JT9 is, in whatever speed-flavour, at LF it throws away one
> aspect of LF propagation that isn't not really availalble for HF, phase
> stability over longer periods, at least suited to a few characters.   JT9
> is a purely power-based system with heavy FEC.
>
> So while slower versions of JT9 would be good for LF in  the short term,
> they will not make the most of what propagation can offer.  NO one on the
> WSJT-X development team is interested in LF, so it's no good looking
> there.    However, I'm sure they'd be amenable to someone else joining the
> team and adding an LF users perspective and writing / developing the
> relevant submodes.   Or do it independently using the open-source code.
>
> EBnaut carries phase signalling to the extreme, carrying no sync / timing
> info in the on-air symbols and being completely reliant on high stability
> frequency sources and people accurately starting and stopping.   This
> complex manual setting is, in my opinion its downfall;  high stability
> sources are less of a problem these days as OCXOs are now so cheap and
> readily availalble.
>
> What we need is an intermediate slow PSK or QPSK based scheme that carries
> its own timing and sync.   FEC is something that could come later, we first
> of all want a simple PSK decoder scheme that search over a modest span in
> frequency - say 0.2Hz and with unknown timing .   I suggest differential
> BPSK as a stat, where the data depends on teh phase change from one symbol
> to the next.   That removes the need for absolute clock recovery, and also,
> provided tuning error is appreciably lesss than symbol rate, even for
> carrier recovery.   Just compare one block of digitised data (of a symbol's
> length) witjh the previous one and look for a peak every so often to get
> the boundary .  Some framing will be needed in order to guarantee a few
> phase changes for alignment.
>
> A block / timed scheme like WSJT would probably work best as it reduces the
> searching for sync.  It also makes the soundcard driving software easier as
> a fixed length block can be recorded to a .WAV file more easily than
> continuous real time data reading from the s-card.
>
> Once the signalling and signal recovery is proven, then is the time to
> think about adding FEC but even a none error-corrected scheme ought to have
> a fair bit to offer in testing.
>
> The only real downside to PSK is its spectrum is you switch rapidly.   But
> its no worse than on-off keying, and if the signal is being generated by
> upconverting from a soundcard phase ramping could perhaps be used to reduce
> sidebands
>
>
>
>
>
> Andy
> www.g4jnt.com