That would be my definition of noise power also. That would not help
explain the numbers produced by FT8.
It's curious that my VFO1 - VFO2 measurement produces numbers very close
to what FT8 reports. I have no information as to why that should be,
only measurements that produce those results.
Jerry, K4SAV
On 12/19/2018 9:57 PM, Joe Subich, W4TV wrote:
Is the definition of "noise floor" being changed for FT8?
WSJT-X (and WSJT before that) defines noise as the integrated value
of noise (noise power) across the 2500 Hz (approximately based on
the receiver filter) receive bandwidth.
73,
... Joe, W4TV
On 2018-12-19 9:57 PM, K4SAV wrote:
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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