Dumb question RE FT8

[BoomboxDX]

Hi all,
Like many SWL's, I've heard a lot of FT8 in the HF ham bands. Sometimes the presence of FT8 can indicate the propagation of certain ham bands... Not having the decoding software, it's just sick ice cream truck sound on my radios, but I still tune into those channels and mark down the presence of FT8 in my logbook.

That said, I'm curious as to what I'm hearing. It seems that the FT8 signals are time governed, and I can hear that. They all start at the same time.

But what am I hearing? 4-5 stations transmitting simultaneously? Or 10 stations transmitting simultaneously? It sounds like I'm hearing a bunch of them, but I'm not sure.

Also, if there are so many transmitting at the same time, how does the receive software determine what it's decoding? Does it decode all 5 or 6 signals simultaneously, all being decoded at the same time? Do some FT8 signals get flooded out by the stronger signals?

And does anyone use JT65 anymore? The FT8 sounds a lot different than it used to sound in the 2010's.

Thanks for your patience.
Boombox

[autovon]

Q:But what am I hearing? 4-5 stations transmitting simultaneously? Or 10 stations transmitting simultaneously?
A: Depending on the band, you might be hearing 5, 10, 20, 30 signals or more at the same time. 

Q:Also, if there are so many transmitting at the same time, how does the receive software determine what it's decoding?
A: The mode/software is pretty amazing.  There is some very detailed reading as to how it goes about doing what it does on the WSJTX page.   Timing is very important with the software and having the computer clock synced within about 1 second is needed.

Q:Does it decode all 5 or 6 signals simultaneously, all being decoded at the same time?
A:  Yes, decodes all of them at the same time.  The user sees this decode right at the end of the 15 second cycle. It's not exactly 15 seconds, but for the purpose of basic understanding, it's 15 sec TX and 15 sec RX.

Q:Do some FT8 signals get flooded out by the stronger signals?
A: Yes, but also again the mode/software is pretty resilient.  It can decode signals that are right on top of each other often times.

[skeezix]

Each of the signals is 50 Hz

This is a screenshot from the waterfall in WSJT-X for 6m, 12m, and 15m.

[BoomboxDX]

Thank you for your replies. It really helps.
Boombox

[Charlie_Dont_Surf]

A couple more things to add:

As implied above the computer sound card acquires the entire 2700+ Hertz of bandwidth at once and processes it all at once. Decodes of the whole bandwidth are essentially instantaneous. The computer has much better dynamic range than the human ear & brain so FT8 is generally much better at low signal to noise ratio work than even CW and certainly you can pack more FT8 signals into a small space than you can CW signals. The signal to noise ratio requirement of FT8 is low enough that if there is even a glimmer of hope of any amount of propagation, you will probably hear some FT8 since it can work when conditions don't permit other modes and there are enough people using it now.

There is a lot of fancy math involved in the underlying basis for FT8. Fortunately the guy who came up with the idea is Nobel Prize winning physicist. Dr. Joe Taylor (K1JT) won the prize for his work on pulsars but he borrowed on a lot of work created for communication with deep space probes (which necessarily communicate over low signal to noise methods) to come up with the techniques. He is an EME (moonbounce) enthusiast and FT8 has it's origins in that area.

The theoretical bandwidth is around 50 Hz, as noted above, but that's only a good approximation of how much space you should leave to your neighbors. Yes, we do "step on" each other often but if both signals are strong at the receiver, it will work out, depending upon how much overlap there is. On the other hand if one signal is much weaker than the other and there is complete or sometimes only partial overlap, the weaker signal is pretty screwed. That's why we generally leave about 25 Hertz on either side of center and that seems to basically work. (Of course, not everyone hears everybody else so you can still get stepped on.) With a 2700 Hertz "swath" to work with and each transmission using 50 Hertz, that allows for 54 frequency "slots". At very busy times, for example 20 meters in the North American evenings, I have seen approximately 50 simultaneous signal decodes filling up my computer screen many times.

To get a feel for the flow of FT8 (or FT4), you can use any Kiwi SDR on the internet, turn on the FT8/FT4 decoder (in the pull down menu under "Extensions"), select the band of choice, sit back and watch.

Yes, people still use JT65 on HF. Sometimes those guys intrude into the upper ends of the FT8 area and wipe others out.