Jerry The new mode FT8 is not all that new, actually, there are several aspects to consider, like detect the signal, decode the signal detected, make a decision to accept the decoded signal. The improvement on signal to noise ratio concept is very old, just the internet made it possible with time synchronization. The decode uses new algorithms and some very intelligent way to guest the decoded signal.
Check this out. 1975 Sept QST; Coherent cw test! Experiments show 20 db Signal Boost over QRM, http://www.arrl.org/files/file/Technology/tis/info/pdf/7509026.pdf The improvement on signal to noise ratio is just because a narrow bandwidth. The gates opens at the right millisecond window. On FSK the secret salvage is time synchronization. You can record the audio and play it back, the decode will happen only if you synchronize the time of the recording with the time in ms of the PC clock. I did that, and it worked, I have a SDR QS1R and using HDSDR software to record the I/Q file, RF file. I used to record rare DX expedition signal and the bandwidth is 50 KHz, I can see the FT8 guys on 1840, My question was , can I decode them from the digital file recorded several month ago? I started plaining the file at the top of the second count, and voalah!!!, The WSJT-X decoded several station, weak as -21 db. The weak signals are there, buried in the noise on my old digital recorded file. Then I decided to test my HWF, the practical result measuring cw signal is that the signal to noise increase around 20 db, 10 db due the directivity RDF 11.5 and another 10 db from the polarization filter. The Horizontal WF attenuation on vertical signals is over -90 db. The manmade noise vertical polarized is reduced below the MDS of the receiver and cannot be amplified by the receiver. The IC-7800 has two identical receivers. I connected my HWF on receiver MAIN and the TX antenna on the receiver SUB, I installed two instances of the WSJTX program, one for each receiver. After 15 minutes the number of decodes on the HWF was 20 or times more than the vertical full size vertical, my TX antenna 120 Ft high. Signals decoded around -21 db on the vertical was decoded on the HWF 0 to +1 db. Signals less the -5db decoded on the HWF was not decoded using the vertical, The HWF was decoding hundreds of signals that would be -40 db on the decode using the vertical. I think the s/n reported by the program as ball part is actually very good and close to the real s/n improvement of 2 Hz BW, depending on the mode. The only real way to increase signal to noise ratio is increasing the directivity of the RX antenna, more real RDF means real signal to noise ratio improvement. I used real because it is very easy to destroy the directivity with integration, leaking, intermodulation, low noise figure etc. One bad concept, bidirectional unterminated beverage with two lobes one in the back and one front, it just does not work because the RDF is 6 db down a terminated beverage. Same for BOG's the RDF is bad, a K9AY works better because has more RDF. A simple Flag can deliver 9 db RDF is tis easy to hide too. Two Flags in phase 11.5 db and four Flags 14 db RDF, and a very clean pattern besides real broadband from 1 MHz to 10 MHz As you can see on the ARRL 1975 article, there is nothing new about improvement of signal to noise ratio reducing the bandwidth. On the article, the test was CW at 12 wpm and 9 Hz filter BW , no ring using WWV as time source for the synchronization. That was state of the art back in the early 70's, almost 50 years ago. 73's JC N4IS -----Original Message----- From: Topband [mailto:[email protected]] On Behalf Of K4SAV Sent: Monday, December 24, 2018 3:10 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
