Hi Dani, Charlie, and all,
As you discovered, I never created a valid scheme for computing S/N in
JT4 (as implemented in WSJT-X). I did this today, and tested it with
various amounts of simulated Doppler spread. Corrected code will be
present in the next released version of WSJT-X.
You also asked about a modified version of JT4 implemented in the
Chinese lunar satellite DSLWP-B, which (for JT4G) uses tone spacing
312.5 instead of 315.0 Hz. The distance between lowest and highest tone
is therefore too small by 7.5 Hz.
If the Doppler spread is less than a few Hz, this alignment error may
cause a sensitivity loss of 1-2 dB. For larger Doppler spreads the
sensitivity loss should be less than 1 dB.
-- 73, Joe, K1JT
On 6/17/2018 12:15 PM, Daniel Estévez wrote:
Hi all,
I've been testing the JT4G decoder in WSJT-X 1.9.1 together with Wei
Mingchuan BG2BHC, since this is the mode used by the lunar satellite
DSLWP-B. We have noted that the SNR reported by the decoder is quite
off. It is always something between -18 and -16 regardless of the actual
SNR.
I'm using this to test:
$ ./jt49sim "CQ EA4GPZ IN80" 4G 1 0 0 1 -5
$ ./jt9 -4 -b G -f 1000 000000_0001.wav
0001 -16 -0.1 1005 $* CQ EA4GPZ IN80 f
This happens both in 1.9.1 and in the latest trunk (r8748), and it also
happens with other JT4 submodes. A brief look at lib/sync4.f90 shows
that the relevant code hasn't changed since 2016, but I don't know what
is the problem.
Another related question: DSLWP-B uses 312.5Hz of tone separation
instead of the standard 315Hz. How much does this affect the decoder
performance and would it be easy to add a variation supporting 312.5Hz?
73,
Dani EA4GPZ.
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