Topband: FT8 - How it really works

[K4SAV]

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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