Topband: FT-8 performance

[Tim Shoppa]

We went thorugh a similar discussion here a year ago about the "cooked" S/N
statistics. Or at least they are cooked in a way that no CW operator would
cook them, by considering a bandwidth 50 times wider than the FT8 signal.
On a quiet WARC FT8 band (no interfering carriers) signals that are -18dB
according to FT8 S/N would have been easily copied by any CW operator. And
signals that are -22dB or lower probably could've been copied maybe with a
few repeats.

I know I often have a waterfall spectrum display running, and I work CW
signals all the time that I don't see at all on the waterfall.

The "work signals way down in the noise you could never work otherwise"
myth, is just part of the myth that FT8 is an effective operating mode,
when really it's the least efficient mode for a good op to work Q's or
DXCC's. See my detailed 2018 statistics by mode here:
http://n3qe.org/n3qe2018.jpg

Tim N3QE

On Thu, Aug 1, 2019 at 9:17 AM K4SAV <RadioXX at charter.net> wrote:

> I get the feeling that I must be the only person that has ever tested
> FT-8 to the extreme to see what it can do. It seems that everyone else
> just assumes it will do what the published information says.  It will
> not.  Below is a summary of my testing.
>
> First I did a bunch of testing to see if I could figure out how the
> software determined the S/N number.  I measured the strength of a
> station calling CQ and subtracted that from the S meter reading for the
> 2500 Hz USB band and that resulted in a number very close to what FT8
> reports.  I repeated the test multiple times.
>
> Official documentation says FT8 can decode signals 24 dB below the noise
> floor.  That's an interesting comment because FT8 has no way of
> determining the level of the noise floor.  Even during the off period
> when no one is transmitting it can't determine the noise floor because
> the noise floor is not represented in the audio signal.  The receiver
> ACG brings the noise back up when no one is transmitting.  Actually for
> FT8 the amount of audio is greater in the off period than it is when
> stations are transmitting.  The only way to measure the noise floor is
> by making the measurement in the RF world, which FT8 can't do.
>
> Actually what the program does is count everything in the 2500 Hz
> bandwidth as noise. (That is comprised mainly of strong signals, not
> noise.) It is limited to decoding signals 24 dB below that level.  From
> that info you can guess that if you narrow the bandwidth to eliminate
> most of the strong signals FT8 will decode weaker signals.  Yes that
> works.  Verified it myself and others have also found this to be true.
> You guys that like to operate FT-8 should take note of that comment.
>
> I thought maybe it could decode stuff below the noise floor if all the
> signals were also below the noise floor, so I tested for that.  I found
> out that the program poops out long before the noise floor is reached.
> With my tests where I decreased the gain of signals from the NE (just
> after sunset in the NE) by pointing my antenna west and increasing the
> gain such that the biggest signals were 15 dB above the noise floor, FT8
> just about quits. There were probably 50 or more stations on the band
> and it was making one decode about every 5 or 10 minutes.
>
> FT8 has a deep search mode where it uses stored data to make guesses at
> call signs some times.  I tested that too.  Before I started WSJTX I set
> up a poor signal to noise ratio test. Started WSJTX in the normal mode
> and it did very poorly.  Then I turned on deep search, increased the S/N
> and let the program look at the band for a little while.  Then I went
> back to the poor S/N condition without turning FT-8 off and turned on
> deep search and it made a lot more decodes.  Nearly all of those decodes
> were reported at -24 dB.  I think those were guesses and it just assigns
> -24 dB for guesses.
>
> I was fooled by the results of a test I did when adding enough noise to
> the audio to cover up the signals and FT8 continued to decode the
> signals.  However I had previously had deep search on and it had already
> memorized the band when I did that.  It was just guessing that the same
> station is on the same frequency as previously.
>
> I keep hearing reports from people that claim they are getting decodes
> without hearing anything in the audio. I set up conditions where that
> should have happened, but it never decoded anything.
>
> In summary. it appears that on an almost dead band, CW (with narrow
> bandwidth) has about a 15 dB advantage at decoding weak signals.  On a
> very crowded band if FT-8 is using a 2500 Hz bandwidth, CW has a huge
> (many dBs) advantage over FT-8 because FT-8 can only decode about 24 dB
> below whatever your S meter is reading.  At 2500 Hz bandwidth in the
> FT-8 band on 160 my radio usually hangs at 20 to 30 dB over S9.
>
> I was using a TS-990S receiver with no audio processing or noise
> limiting or blanking.
>
> If anyone else has run similar tests. I would love to hear about it.
>
> Jerry, K4SAV
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