Chuck, I suspect something wasn’t quite right with your setup? At my QTH in W3
I can hear multiple FT8 signals on 1840kc USB (2.4khz bandwidth) from before my
sunset until after sunrise. They are whining/droning carriers for 13 seconds
every 15 seconds. Only for that less than 2 seconds every 15 seconds do I hear
just band noise, at all other times I hear multiple FT8 signals just fine. And
the computer can decode more signals than I can hear.
Tim N3QE
> On Dec 24, 2018, at 4:17 PM, Chuck Dietz <w5prchuck@gmail.com> wrote:
>
> I think I understand much of what you are saying, but I know that I was on
> 160 meter FT=8 two nights ago with the speaker up fairly loud. I only heard
> noise. I set the AGC off and adjusted the RF gain so that it did not
> overload. Still no hint of any signals, but I decoded two stations!
>
> Just sayin’.
>
> Chuck W5PR
>
> Sent from Mail for Windows 10
>
> From: K4SAV
> Sent: Monday, December 24, 2018 2:10 PM
> To: topband@contesting.com
> Subject: Re: Topband: FT8 - How it really works
>
> Although I have finished my FT8 testing, there is one final thought I
> would like to leave with you, and also to correct one statement I made
> earlier. Someone thought FT8 measured the noise in the interval when
> the FT8 signals were off, and I replied that would result in a real S/N
> number. That is not true as you will see in the info below. You would
> get a real S/N number if the RF was sampled, but not if the audio is
> sampled.
>
> I spent many years designing electronic circuits professionally, so I
> still think that way. So for a few minutes lets think about a circuit
> that can decode something below the noise floor .If you think about FT8
> or anything similar, from a designers point of view, you suddenly
> realize that making a statement of "the circuit can decode down to X dBs
> below the noise floor" is almost an impossible task, that is, if you are
> talking RF noise floor as most people will be assuming.
>
> Since you will be dealing with audio, not RF, the receiver will convert
> the RF into audio and compress it into something that has a lot less
> dynamic range. How much less? Say the volume is set to a level such
> that the strongest signals do not clip, then how far down is the noise?
> You can expect that to vary on each band too.
>
> Now comes a real complication. If you were taking samples in the RF
> world, you could see the noise level on your S meter and estimate it
> relative to the strongest signals. However your circuit will be dealing
> with audio. Surprisingly, when the signals disappear, the receiver AGC
> voltage drops and the receiver gain increases. That produces a lot more
> audio signal. The audio noise in the case of no signals becomes higher
> than the audio level for strong signals if you are using USB bandwidth
> and receiving something similar to FT8. That condition is not nearly as
> pronounced when using a narrow CW bandwidth. Even if you put the
> receiver into AGC slow mode it won't hold for the 3 seconds when FT8 is
> off, so you still get the increased audio in the off period. Then there
> will be a sudden increase in audio when the first signal reappears,
> until the ACG kicks in and lowers it. This happens even with fast AGC
> selected. It's fast enough that you don't notice it when listening, but
> if you put a scope on it you can see it. Yeah, all that surprised me
> too when first thinking about it. Take a close listen and see if you
> agree. If you can't hear it, put it on a scope or anything that displays
> an audio waveform and it will become very obvious.
>
> If you made a statement that this circuit can decode X dBs below the
> noise floor, most people will be thinking RF noise floor. So what is it
> in the audio world that represents the noise floor in the RF world, and
> what would your statement mean?
>
> Of course you could turn off the AGC and decrease the receiver RF gain
> and that would make the audio very low when the signals disappear. That
> would also severely limit the dynamic range for your circuit since you
> would no longer have the compression supplied by the receiver.. Your
> circuit would have to cover a much wider dynamic range, similar to what
> a receiver does. So your circuit would need what? maybe 100 dB dynamic
> range to cover the strongest signals to the weakest noise floor,
> forgetting about decoding below the noise floor. Actually that wouldn't
> really happen because receivers can't produce a dynamic range of 100 dB
> in the audio. They may do it in the RF world, but not in audio.
> Receivers have no need to do that.
>
> Jerry
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