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Re: Topband: FT8 - How it really works

To: topband@contesting.com
Subject: Re: Topband: FT8 - How it really works
From: "Joe Subich, W4TV" <lists@subich.com>
Date: Thu, 20 Dec 2018 12:57:40 -0500
List-post: <mailto:topband@contesting.com>
On 2018-12-20 11:45 AM, K4SAV wrote:
> In the case of a crowded band it becomes obvious that CW is much
> superior to decoding a weak signal because all those strong signals
> limit the ability of FT8 to decode a weak signal.

That is only true if you leave AGC enabled and the strong signals
result in the AGC decreasing the overall sensitivity.  The WSJT-X
Users Guide (instructions) recommend turning off AGC.  It is
possible (depending on the transceiver design) to reduce the IF
bandwidth to remove some or all of the strong signals to reduce
overload and minimize the gain reduction by using the RF Gain
control to introduce the least gain reduction necessary to prevent
distortion.

On 160 meters, typical nighttime noise levels are as high as -90 to
-100 dBm.  Using the RF attenuator and/or RF gain to set the recovered
audio from that noise approximately 15 dB above the sound card noise
floor should provide sufficient dynamic range to decode the weakest
signal (~ 10 dB below the 200 Hz noise floor) while not distorting on
the strong (S9+40 dB) signals if one has a *quality* sound card.

73,

   ... Joe, W4TV


On 2018-12-20 11:45 AM, K4SAV wrote:
Thanks to the folks commenting on how FT8 works.

VE3KI said:
"The noise floor the wsjt-x signal is referenced to is the noise within the bandpass during the two-second period when no-one is transmitting, not the signal level when people are transmitting."

That was what I originally thought might be a possibility because that would result in a real S/N number.  However that doesn't seem to be the case.  That S9+40 dB signal I referenced would result in a huge S/N number, probably greater than 50 dB.  FT8 gives a report of -1 dB. Doing it that way would also have some problems produced by people transmitting at the wrong time and other out of band stations, however it seems that FT8 doesn't make that measurement.

Thanks to Arunas, LY2IJ .  Your comments agree 100% with what I measured.  As to your question of can FT8 decode signals below the noise floor and below the level that can be decoded by CW.  My experiment of adding audio noise which covered up the signals and the software still being able to decode signals says that under some conditions FT8 can decode signals below the noise floor.  Of course that experiment was done at audio levels, not at RF.  If you use CW you get the benefit of a much narrowed passband,  I can't run that test using audio mixing.

In a condition of only one weak signal on the band, I haven't run a test that says whether FT8 decodes better than CW or not.  NN4T said that using FT8 on 6 meter sporadic E that he observes signals being decoded with no audio in the receiver.  That is probably with a wide bandwidth, and it would be interesting to know if the signals would be audible with a narrow bandwidth.

In the case of a crowded band it becomes obvious that CW is much superior to decoding a weak signal because all those strong signals limit the ability of FT8 to decode a weak signal.  That was the basis for my conclusion that FT8 didn't seem to be useful for working weak signal DX because most of the bands are very crowded. However there may be a case where FT8 can beat CW, that is if you are tying to decode a signal on an essentially dead band.  Since FT8 seems to be able to decode below the noise floor, the noise floor in that case would just be real noise, not signals.  You could improve FT8's ability to decode by narrowing the bandwidth, although that's not normally done.  That would cut into CW's advantage obtained by using a narrow passband. I don't have a measurement with the results of that showdown of CW versus FT8 in dead band conditions but the answer would be interesting to know.

Jerry, K4SAV

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