Topband: FT8 - How it really works

[K4SAV]

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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