Joe, thanks for the information. I am not exactly sure what all that
means. My conclusions were based on observed data. It seems pretty
obvious to me that a signal that is more than 50 dB above the noise
floor should not receive a S/N number of -1 dB, which is what FT8
gives. I don't know how the information you provided can make a
calculation like that.
I judge that a signal reading S9+40 dB on the S meter should be more
than 50 dB above the noise floor when I can tune of to a spot where
there are no signals and the S meter reads about S2 or S3 in SSB mode or
less than S1 in a narrow bandwidth. Is the definition of "noise floor"
being changed for FT8?
Jerry, K4SAV
On 12/19/2018 7:27 PM, Joe Subich, W4TV wrote:
On 2018-12-19 4:28 PM, K4SAV wrote:
> The official documentation for FT8 says it will decode signals 24 dB
> below the noise floor. That is not a correct statement most of the
> time.
No, that is a correct statement. Signal reports in WSJT-X for FT8, JT65
and JT9 are *all* measured *with regard to the noise in 2500 Hz*. Note
that the tone filters in WSJT-X are on the order of less than 12 Hz or
so wide so the SNR *for an individual tone in the DSP filter bandwidth*
at 0 dB is -23 dB relative to the *total noise in 2500 Hz bandwidth*.
The actual filter bandwidth will change from mode to mode due to the
differences in keying rated and tone spacing ... the actual SNR limit
is shown in section 17.2.7 of the WSJT_X 2.0 User Guide.
CW operators understand this from experience ... a quality 200 Hz filter
will have ~12 dB less noise than a 2800 Hz filter. Thus a CW signal
with a 200 Hz filter will have 12 dB better SNR than the same CW signal
with a 2800 Hz filter (excluding any "processing gain" from the ear-
brain filter).
With FT8, JT65, JT9, etc. coding (forward error correction) provides
some additional SNR (called "coding gain") but the *measurement* is
based on strength of the individual tone to total noise. Thus, the
lowest accurate report is -24 dB although some signals will be decoded
at levels below that.
73,
... Joe, W4TV
On 2018-12-19 4:28 PM, K4SAV wrote:
While sitting around being bored and recovering from a gall bladder
operation, I decided to do some experiments with FT8. First thing I
did was upgrade the software to WSJT-X v2.0.
I hope this post doesn't turn into another FT8 bashing session. My
only goal was to understand how this mode works and what it can do
and what it cannot do.
The official description of FT8's signal reporting cannot be correct.
It is obviously not a signal to noise number and it is not an S meter
reading. What is it? That was the first question to answer. It's
obviously not an S/N number because how do you give a report of -1 dB
for a signal that is S9+40 dB on a quiet band. I was unable to find
any info on how the signal report was calculated so I tried to
correlate those reports to observations.
I think I have figured out a method that results in very close to the
same number that FT8 reports. Here is the experiment. I set up my
main VFO to USB 2500 Hz bandwidth and set the second VFO to CW at
about 150 Hz bandwidth. I look for a station calling CQ and tune the
second VFO to him and measure his signal strength. I also look at
the S meter for the signal level on the main VFO. I also look at the
signal report calculated by the software. For stations calling CQ
that report is calculated by the software in my computer.
The FT8 report is usually very close to the difference in signal
levels (VFO1 - VFO2). For example if the main VFO reads S9+10 and
the second VFO reads S9, the FT8 number will be -10 dB. Note that the
FT8 says that -24 dB is the lowest it can decode. With VFO1 = S9+10,
that's about S7 for the smallest signal it can decode. Observations
agree. Those numbers will vary a little depending on how your S meter
is calibrated. In order to decode a weak signal, all those close USA
stations will have to go silent.
The official documentation for FT8 says it will decode signals 24 dB
below the noise floor. That is not a correct statement most of the
time. That statement should be that FT8 will decode signals 24 dB
below the sum total of everything in a 2500 Hz bandwidth. If the
total of all signals on the band are below the noise floor, it would
be interesting to know if FT8 will decode any of them. I haven't
observed that yet in a real situation. I did however try to simulate
that condition by adding enough noise to the signals such that all
the signals were below the noise. The software did continue to
decode signals. All the reports were -24 dB. This was a very crude
test because I don't know how exactly much the signals were below the
noise. This should be of benefit to those people that have S9+ noise
on the bands they operate. They should be able to decode the
strongest signals on the band.
The (VFO1 - VFO2) test just described should always result in a
number equal to or less than zero. I notice sometimes the software
will report a small positive number. That seems to happen more often
when the bandwidth is set to something less than 2500 Hz and there
are very few signals on the band. I think this may be related to the
fact that FT8 does all its calculations using audio signals and the
receiver S meter is operating on RF. Audio shaping in the receiver
will affect the FT8 calculations. Audio processing in your computer
sound card may be a factor too. This becomes really apparent when the
radio is set to CW and the audio peaking filter is turned on. With
SSB bandwidth and flat audio response, S meter readings are a good
indication of what will be decoded. It should decode signals down to
24 dB below whatever your S meter reads.
I also narrowed the bandwidth of VFO1 and chopped out a bunch of
signals. I got S7 on VFO1. Then a station calling CQ also measured
S7 on VFO2. The FT8 report was 0 dB. Agrees.`
That test brings up a possibility. If you can narrow VFO1 to a very
narrow bandwidth hopefully containing only a very weak signal, then
you may be able to decode it. A strong signal in the passband of
VFO1 will kill the decode.
It works. I decreased the bandwidth of VFO1 to 200 Hz and it decoded
an S2 signal. I had VFO1 in USB mode with that bandwidth. My
receiver will go to zero bandwidth in USB mode. I put VFO1 into CW
mode at 100 Hz bandwidth and it decoded a signal that was moving the
meter between S0 and S1. That signal would have also been easy copy
if it was CW instead of FT8. I was using a good receiving antenna on
160 meters immediately after sunset.
While this seems to work for weak signals it is a non-starter for
normal operation. How do you tune around with a very narrow
bandwidth looking for a station calling CQ or any other station that
might be DX? It's not like CW, unless you learn to copy FT8 by ear.
You can't find him with a wide bandwidth because the software won't
decode him. He is only there when the bandwidth is very narrow.
Given the number of USA stations on FT8 that bandwidth will have to
be really narrow to keep the USA stations out of the passband. Even
50 to 100 Hz bandwidth usually doesn't do it on a crowded band and
you can't go lower than that and still decode the signal. This
doesn't sound like anything that is practical. Maybe something
useful might be to improve the copy of a weak station by narrowing
the bandwidth if you already know the station is there.
One thing you could do is set the receiver to a narrow bandwidth and
call CQ DX, listening only on your transmit frequency. However the DX
station would probably need to be receiving with a very narrow
bandwidth or he won't hear you because you are probably very weak on
his end too. I seriously doubt that he knows to do that because it
seems that no one else knows about that either. Besides it is not
often that a rare DX station will respond to a USA station calling CQ
DX. Another non-practical suggestion.
There are DX stations strong enough to be decoded that can be worked
with FT8, especially on the higher bands like 20 meters. Even on 160
meters sometimes a DX station will be strong enough to be decoded.
Just tonight right after sunset I heard a couple of European stations
on 160 running S5 to S6. Because they were so strong, I tuned down
to the CW portion of the band but I didn't hear a single signal from
anyone down there. Oh well.
Seems to me that FT8 is a very poor method of working weak signal DX.
It also seems that it isn't being used that way either. Just
listening, it seems that everyone is working very strong signals, 20
to 40 dB above the noise floor, at least as observed at my station.
Maybe this isn't the case for people that have an S9+ noise floor.
For those people, if they can't reduce the noise, FT8 may be the only
way they can do any operating.
At least I now know more about FT8 than I did before starting this
exercise. Learning stuff is never boring and it killed some time,
and my big incision feels a little better.
Jerry, K4SAV
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