Hi Rex,
VK7MO wrote:
> I note that your conclusion that JT65c is no
> better than JT4 with spreading greater than 10 to 20 Hz is based on non-DS
> decoding. Based on my tests with r6536 I am inclined to agree that your
> conclusion also applies to DS decoding although I would like to do more
> testing when a build of r6559 is available.
Good! -- additional tests are certainly desirable.
> This conclusion does, however, seem to be contrary to the Text books that
> the use of higher Mary FSK should improve performance.
No question: 64-ary FSK is a more efficient modulation than 2-ary or
4-ary FSK. But it requires much more bandwidth. A JT65 submode with
information tone spacing comparable to JT4F would be something like
"JT65H", and would need ~20 kHz of bandwidth.
> ... At present
> averaging is rarely useful on JT4f DS on WSJT-X as the limit is gaining sync
> - thus any improvement in gaining sync should be of significant benefit to
> JT4f averaging.
>
> So in summary I think the next move should be to do some wider tests to
> compare the sync performance of JT4f on WSJT10 and WSJT-X to see if there
> are options to optimise this mode before you spend time on JT65d.
Agreed. I have not put recent effort into optimizing the JT4 decoder,
and you have already identified a weak spot to be worked on.
Also keep in mind: As I argued many months ago, the best mode for
microwave EME may turn out to be a wide-spaced JT9 mode, say JT9G (111
Hz tone spacing, 1 kHz total bandwidth) or JT9H (222 Hz, 2 kHz). These
submodes are already partially implemented in WSJT-X. Maybe it's time
to complete that effort.
Also, Steve and I have been investigating ways to use one of the latest
hot things in communication theory -- Low Density Parity Check (LDPC)
codes -- for possible application in WSJT-X. So far we have been
thinking about a fast mode for meteor scatter, where we need a high rate
code, high symbol rate, and bandwidth-efficient modulation.
But we could also think about a code that would work well for EME.
-- Joe, K1JT
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