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 <[email protected]> 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: [email protected] > 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 > _________________ > Searchable Archives: http://www.contesting.com/_topband - Topband Reflector > > _________________ > Searchable Archives: http://www.contesting.com/_topband - Topband Reflector _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
