QSO credit for logging inaudible signals.
I’ve been trying to stay out of this argument, but this statement right here is
representative of the kind of rhetoric that leads to the anti-JT/FT-mode
hysteria, and shows the misinformation about the way the FT/JT modes handle and
report SNR. It isn’t Jim’s (K1TN) fault, but is likely simply due to a
misunderstanding of the WSJT-X signal reporting system.
The FT/JT modes are NOT black magic. They ARE modes that have an excellent
design and implementation. We commonly hear that these modes can work well
below the noise floor, or are “inaudible" in Jim’s words. Part of that reason
for the misinformation is the negative SNR numbers presented in the software.
Well, to borrow some info from the WSJT-X manual, and a great explanation by
Kok S Chen (W7AY), here’s what is actually happening and what that SNR figure
actually means.
“Unless you change the WSJT-X settings, the SNR is reported in an equivalent
2500 Hz noise bandwidth. The 8FSK detector in FT8 basically works with a 6.25
Hz bandwidth (unwindowed FFT), so there is a factor of 400, or 26 dB with
respect to noise in the detector's bandwidth. When you decode a "-20 dB SNR"
signal in FT8, it really has +6 dB SNR in the detector's noise floor. There is
no miracle here -- FT8 simply uses very narrow bandwidth and slow baud rate.
The FEC helps too, especially when there is QSB.”
Considering that FT4 is only good down to about -15dB (if I remember
correctly), that means that it must be around 10 or 11dB above the noise floor
for the detector to properly decode. Again, no magic involved here. The same
effect can be had with CW and/or RTTY through the use of narrow filters and the
like, albeit without the FEC coding of the FT/JT modes and the extremely narrow
bandwidths. I personally have never made a JT/FT mode contact where I couldn’t
hear the other station. It may have been very weak, but it was audible.
Again, I’m not trying to attack Jim (K1TN) here: I’m just sharing some info
that was shared with me a while back when I asked similar questions.
73
——
Dave Wright
K3DCW
FM18qt, Maryland, USA
“Real radio bounces off the sky"
On 7 June 2019 at 08:04:38, James Cain (jamesdavidcain@gmail.com) wrote:
Re the comments by others, below, I disagree. Discussions about the FT
modes absolutely belong on this reflector, because the FT modes are an
existential threat to shortwave amateur radio that some of us have known
for more than half a century.
If this reflector can "discuss" cut numbers and the morality of using Super
Check Partial, then it sure as hell can discuss QSO credit for logging
inaudible signals.
Jim Cain, K1TN
-----
"I suggest that discussions like this be moved over to the rttydigital
reflector
https://groups.io/g/rttydigital
In November of 2018, folks on the rtty Reflector got tired of seeing postings
about FT8 and FT4. The rttydigital Reflector was formed at that time for
discussions of ALL digital modes. There are a lot of contesters on that
reflector.
Dave Hachadorian, K6LL Yuma, AZ"
" … in my 24 or so years on CQ-Contest (and on Towertalk, before
CQ-Contest was born), I've seen these things come and go many times.
I would encourage the moderators to exercise their influence to
deflect the group back to more mainstream concerns. This too shall
pass.
73, Pete N4ZR"
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