Re: Topband: FT8 on 160 - how you can make a difference
Hi all, FT8 has the advantage of being located in a single frequency. So if one arrives on a totally empty band he'll be attracted to that single "watering hole" just like insects around a lonely light at night. It happens frequently to find a desert 10m band except for some weak beacons and the FT8 segment is S7-8: I can recognize the Costas arrays by hear ;) FT8 has some theoretical SNR advantage over CW and other digimodes, but its strength comes from the single gathering point and its extremely low access effort. Personally I deem operating FT8 just a tad more rewarding than keeping a beacon active or having a KiwiSDR doing WSPR... What I hate about FT8 is its "closeness" regardless of WSJTX being open source: if your code is obfuscated, too complex to understand and you don't even publish a scientific paper to describe the details, I think the ham community should revolt against this status quo. Imagine using SSB/CW receivers without knowing the technical details and produced by a single guy in the whole world... that's crazy. Even GSM/UMTS/LTE is more documented. Personally I don't want to be an appliance operator. 73, marco / IS0KYB Il giorno dom 4 ago 2019 alle ore 15:36 George Taft via Topband < topband@contesting.com> ha scritto: > Mark etal > > Using CW, I've worked DXCC on Topband for at least the past ten "seasons" > including 2018/2019. But last year was the most skimpy with just 105 in the > log. Peak was several years ago at 155. I use July 1 - June 30 as a > "season". > > Mebbe when the new mode fad settles, CW on 160 may agn be worthy. > > 73 George > W8UVZ On Sunday, August 4, 2019, 12:02:58 AM EDT, Mark K3MSB < > mark.k3...@gmail.com> wrote: > > Jerry > > You said " Yes DX last year on 160 CW was pretty scarce" and other have > made a similar comment. > > Are you referring to new ones, or just DX in general?I worked 16 new > ones on CW last season and understand that "scarce" can be different > depending upon how many DXCC one already has worked. > > 73 Mark K3MSB > > > > > On Sat, Aug 3, 2019 at 8:51 PM K4SAV wrote: > > > NR2DX asked: "If read your post correctly you are saying that you are > > working against an ambient noise level of 20-30 db over S9 is that > > correct.? " > > > > No. The S9+20 to 30 dB is the S meter reading when all the FT8 stations > > are transmitting after the band opens a little. Receiver bandwidth was > > 1.5 kHz. Tuning to a clear frequency my noise level was about S1 with > > 200 Hz bandwidth when I made these tests. I usually used 100 Hz > > bandwidth when measuring the signal level of the FT8 signal for > > determining his actual strength. Sometimes less if there were multiple > > signals in the passband. > > > > > > W0MU asked: "Have you attempted to open a conversation with the > > creators of the mode and discuss what you are seeing?" > > > > No I have not. I have also not seen any published data from anyone > > showing actual performance. All I see are claims based on calculations. > > Theory is good but it has to agree reasonably well with actual > > measurements. If not, one of the two is in error. > > > > Yes DX last year on 160 CW was pretty scarce. Even when I was hearing > > S6 FT8 signals from Europe I would tune down to the CW portion of the > > band and usually there were no CW signals there. > > > > When I was doing these tests I was using WSJT-X in FT8 mode on 160 > > meters. I was using version 2.0.0, which was the latest version at the > > time. > > > > Jerry, K4SAV > > _ > > 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 > -- marco cogoni CRS4 http://sibamanna DOT duckdns DOT org _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: FT-8 performance
If anybody's interested in trying my version of the python FT8 decoder with a more sensible SNR implementation just go here: https://github.com/mcogoni/weakmon I modified the code to look for the lowest power bin within the input bandwidth and each individual signal is compared to this. To obtain better SNR values avoid multiple passes for decoding since it implies subtracting the strongest signals, bin by bin, from the waterfall so the relative powers get all modified for the "hidden" signals. marco / IS0KYB Il giorno gio 1 ago 2019 alle ore 21:46 Richard (Rick) Karlquist < rich...@karlquist.com> ha scritto: > It is instructive to calculate the Shannon > maximum theoretical data rate (power limited case) > (refer to wikipedia page for Shannon-Hartley theorem). > > If S/N ratio (BW=2,500 Hz) = -24 dB, > then S/N ratio (BW=1Hz) = -24 + 10 log 2,500 = -24 +34 > = +10 dB. 10 dB converted to a dimensionless ratio is 10. > > Now, channel capacity = C <= 1.44 X 10 = 14.4 bits/second. > > This rate is a little more than twice the FT-8 rate. > > Now a days, achieving 1/2 of the Shannon limit is > possible for AWGN. If your noise isn't AWGN, well > then that is another source of error. > > Thus the claim of -24 dB sensitivity seems plausible, > where the S/N is the true signal vs AWGN, as opposed > to whatever random number FT-8 reports. > > So I think the beef with FT-8 is in the way it > calculates the displayed S/N. We used to call > those "marketing specs". > > It is also notable that FT-8 uses at least twice > the average power compared to CW. If you compared > them on an average power basis (vs PEP) the FT-8 > advantage, if any, would drop 3 dB. > > On CW, you could send your call many times > in 15 seconds for "error correction" and take advantage > of QSB peaks. That tends to level the playing field. > More playing field leveling is using Super Check Partial > analogous to what FT-8 does. > > Rick N6RK > _ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > -- marco cogoni CRS4 http://sibamanna DOT duckdns DOT org _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: FT-8 performance
Hi, I agree with Jerry. I spent a few weeks trying to use FT8 to obtain antenna radiation patterns and I discovered how the SNR is computed: it's totally flawed. Basically WSJTX computes the number in two steps: the first one estimates how strong the adjacent frequency bins are with respect to the bin of interest. Then this value is corrected by estimating the baseline noise in a 2.5kHz bandwidth. The bad is that in this way the program is trying to unify a QRM concept (how strong adjacent noise is) with band noise (due to QRN or whatever wide band there is). I found out the problem because using WSPR SNR estimations lead to antenna patterns that are very similar to NEC simulated antennas but FT8 data produced WORSE patterns for the BEST antennas. This is due to the fact that best antennas receive far more signals so WSJTX gives a worse SNR. I tried to talk with Joe Taylor about this, but he said that this method is what he thinks it is best for a general use, if I have a better one, just go and modify the code yourself. I have a feeling that this way of computing the SNR was chosen because it makes you think, as Jerry pointed out, that FT8 can magically decode signals that are absolutely not hearable. That's highly dubious. What FT8 does achieve is an extreme overlapping of signals over 2.5kHz, but at the expense of not really being able to tell the SNR... If you want to dig deeper in the FT8 implementation have a look at AB1HL Robert Morris' FT8 Python implementation that is heavily commented and very very educational: https://github.com/rtmrtmrtmrtm/basicft8 He also wrote a full code/decode high performance software (not so easy to understand...): https://github.com/rtmrtmrtmrtm/weakmon Hope it helps! 73, marco / IS0KYB On 01/08/19 15:17, K4SAV wrote: I get the feeling that I must be the only person that has ever tested FT-8 to the extreme to see what it can do. It seems that everyone else just assumes it will do what the published information says. It will not. Below is a summary of my testing. First I did a bunch of testing to see if I could figure out how the software determined the S/N number. I measured the strength of a station calling CQ and subtracted that from the S meter reading for the 2500 Hz USB band and that resulted in a number very close to what FT8 reports. I repeated the test multiple times. Official documentation says FT8 can decode signals 24 dB below the noise floor. That's an interesting comment because FT8 has no way of determining the level of the noise floor. Even during the off period when no one is transmitting it can't determine the noise floor because the noise floor is not represented in the audio signal. The receiver ACG brings the noise back up when no one is transmitting. Actually for FT8 the amount of audio is greater in the off period than it is when stations are transmitting. The only way to measure the noise floor is by making the measurement in the RF world, which FT8 can't do. Actually what the program does is count everything in the 2500 Hz bandwidth as noise. (That is comprised mainly of strong signals, not noise.) It is limited to decoding signals 24 dB below that level. From that info you can guess that if you narrow the bandwidth to eliminate most of the strong signals FT8 will decode weaker signals. Yes that works. Verified it myself and others have also found this to be true. You guys that like to operate FT-8 should take note of that comment. I thought maybe it could decode stuff below the noise floor if all the signals were also below the noise floor, so I tested for that. I found out that the program poops out long before the noise floor is reached. With my tests where I decreased the gain of signals from the NE (just after sunset in the NE) by pointing my antenna west and increasing the gain such that the biggest signals were 15 dB above the noise floor, FT8 just about quits. There were probably 50 or more stations on the band and it was making one decode about every 5 or 10 minutes. FT8 has a deep search mode where it uses stored data to make guesses at call signs some times. I tested that too. Before I started WSJTX I set up a poor signal to noise ratio test. Started WSJTX in the normal mode and it did very poorly. Then I turned on deep search, increased the S/N and let the program look at the band for a little while. Then I went back to the poor S/N condition without turning FT-8 off and turned on deep search and it made a lot more decodes. Nearly all of those decodes were reported at -24 dB. I think those were guesses and it just assigns -24 dB for guesses. I was fooled by the results of a test I did when adding enough noise to the audio to cover up the signals and FT8 continued to decode the signals. However I had previously had deep search on and it had already memorized the band when I did that. It was just guessing that the same s
