Hello Igor, I agree with your analysis of the AGC issue. I have tested a few HF transceivers that can have AGC turned off and all have issues with audio distortion. As you have mentioned, using the attenuator alleviates the AGC problem a little, but decreases the sensitivity of the receiver -- witch could be necessary in certain conditions. When I am working a DX station, often there are very strong signals in the passband that dominates the AGC -- in some cases blanking out the DX station completely. Using an attenuator in these cases would not help since the signal from the DX station was already very weak.
The solution I am using is to use an SDR receiver. It provides me with complete control of the RX bandwidth and have zero issues with AGC turned off. In my case I use an SDR-IQ. For TX, I, for now, use the TX in the HF transceiver, but I am working on a homebrew project of a simple HF transmitter designed specifically for WSPR and the JT modes on HF. 73, Edson PY2SDR --- - We humans have the capability to do amazing things if we work together. - Nós seres humanos temos a capacidade de fazer coisas incríveis se trabalharmos juntos. On Thu, Nov 12, 2015 at 7:15 AM, Игорь Ч <c_i...@inbox.ru> wrote: > Hi Steve and Joe, > > I mostly agree with all your statements but one: dynamic range. I am sure > there is enough headroom in sound card and WSJT-X to upper border of the > dynamic range. > > What makes me worry is that when RF gain is used (some people do use > attenuator) to get to more linear band of the AGC operation, it supresses > receiver sensitivity putting very weak signals out of scope of the decoder. > > As RF gain control in action is similar to input attenuator signal coming > with -24 dB with maximum RF gain position will be forever lost in receiver > AF output if RF gain decrease is applied. > > For instance, in one of my shacks in countryside I have on 15m band such > low noise from antenna that I can not hear it by ears under the input > circuits noise of my receiver which has 0.11 uV sensitivity. When band is > open I get very strong signals from Europe and this antenna is working very > well on TX. Just imagine how many weak signals would disappear if I will be > using RF gain control with AGC turned on... Of course I always keep AGC > turned OFF in my transceiver. > > Another concern that worries me as owner of classic transceiver: when AGC > turned off I getting IMD in transceiver AF line output starting from signal > levels that correspond S7 on my S-meter (it does not reflect real signal > strength as I have modified 1st IF in receiver with extra low noise and > narrow bandwidth regenerative amplifier). I have also modified AF line > output to get it more linear but still have dynamic range there that is far > below of the soundcard one or WSJT-X. Of course IP connected DDC SDR have > not got AF path, and IP DDC SDR is perfect solution for keeping AGC off. > But all classic transceivers have heavy AF IMD with many IMD products in AF > output in presence of the strong signals if AGC is OFF. These IMD products > plaing the same trick with WSJT-X creating spurious candidates as does it > the AGC. > > 73, > > Igor UA3DJY > > > *Re: [wsjt-devel] high sensitivity of decoder vs TRX AGC handling > <http://sourceforge.net/p/wsjt/mailman/message/34613185/>* > From: Joe Taylor <joe@pr...> - 2015-11-12 01:24:21 > > > Hi Igor and Steve, > > Perhaps it's worth mentioning that our advice for all WSJT-related > programs has always been to disable the receiver AGC (if possible) and > turn the RF gain control well down, thereby minimizing AGC action. If > you do this and follow instructions in the User Guide about setting the > signal level coming into the sound card (or equivalent device), there's > plenty of dynamic-range "headroom" still available for nearly all > situations. > > The decoders know how to handle widely different signal levels, and they > appreciate having a reliably constant baseline noise level during an Rx > sequence. > > -- Joe, K1JT > > > > Среда, 11 ноября 2015, 18:32 -06:00 от Steven Franke < > s.j.fra...@icloud.com>: > > > Hi Igor, > > Since this may be of interest to others, I’m copying this reply to the > list. First, I’d like to echo what Joe said earlier. If you are running in > JT65A mode and if your search range is the same for all cases, then you > should see identical results on all platforms and operating systems with > the only difference being execution time. > > Having said that, I agree that r6052 and r6058 should perform differently > on different types of files. In fact, your most recent results are as I > would expect. Recall that r6052 uses a standard floating-point correlation > function for identifying candidates for further processing. If there are no > impairments to the data, then the algorithm used in r6052 should be the > most sensitive and the most accurate. As such, it is no surprise that you > see better results with r6052 for your “AGC OFF” files and for the -24dB > files generated by JTSim. > > r6058 uses a 1-bit correlation, which was introduced to mitigate the > effect of AGC-induced noise “stepdown” (for lack of a better term) at the > beginning of the record. It will also mitigate the AGC-induced “stepup” at > the end of the record. This approach turns out to be very effective in > cases where the AGC effect is severe, as in the example files that you > provided. To give an idea of what it does, most of your files produce > 100-200 (mostly spurious) candidates with the standard correlation as used > in r6052, whereas the 1-bit correlation reduces the number of candidates to > 10-30 in most cases. This dramatically reduces execution time, by as much > as a factor of 5-10 in some cases. > > The downside of using 1-bit correlation is a slight loss in sensitivity. > > Just now, in r6080, I’ve committed my latest attempt at finding the best > compromise between robustness and sensitivity. This latest version first > uses the full correlation function to identify candidates. If the number of > candidates is greater than 50, it then goes back and tries again using the > 1-bit correlation. This restores the full sensitivity of the decoder for > well-behaved data files (or even for AGC-contaminated data with a narrow > search range) while resorting to 1-bit correlation when warranted. If even > the 1-bit correlation produces more than 50 candidates (I have not yet seen > this happen), then the fallback is to reduce ntrials to 100 to keep the > decoder from appearing to “freeze”. > > For future reference, I just ran r6080 on the -24dB test files using a > setting of 8 for random erasure patterns (ntrials=10000), with 2-pass > decoding turned off, and obtained 828 decodes. > > Finally, a comment on your observation that many people have rigs that do > not allow the AGC to be turned off. I expect that many such rigs will have > an RF gain control. My recommendation is to reduce the RF gain to the point > where AGC is activated only on the strongest signals. This will minimize > the AGC-induced noise-step effect and will guarantee maximum sensitivity > from the decoder. For example, on my TS-480 I usually run with the RF gain > set at 80 (out of 100) and only 1-2% of my data files activate the 1-bit > correlation option. In any case, this latest version should “just work” > whether or not you are able to optimize your AGC settings. > > 73 Steve k9an > > > On Nov 11, 2015, at 4:12 PM, Игорь Ч <c_i...@inbox.ru > <https://e.mail.ru/compose?To=c_i...@inbox.ru>> wrote: > > > > Hi Steve, > > > > After long discussion about JT65 decoder in Russian JT65 forum I have > been completely confused by my last test results on comparison of the r6052 > and r6058 releases: > > __________________________ > > Audio files with TRX AGC OFF : > > > > release/number of decodes > > > > 6052/609 > > 6058/596 > > __________________________ > > -24 dB Audio files: > > > > release/number of decodes > > > > 6052/765 > > 6058/736 > > __________________________ > > Audio files with TRX AGC ON: > > > > release/number of decodes > > > > 6052/863 > > 6058/932 > > __________________________ > > > > There is significant difference in number of decoded signals between > 6052 and 6058. > > > > We have faced a problem: some people do use cheap/classic transceivers > where they may not turn AGC off and they must use JT65 decoder from release > 6058. Other people do use SDRs or more expensive classic transceivers but > for some reasons prefer to keep AGC turned ON - hence they also must use > decoder from r6058. > > > > But some people have possibility and like to keep AGC turned off and > they would like to get most performance from WSJT-X software, so they do > prefer to use high sensitivity JT65 decoder from r6052. > > > > What do you think if there is any way to let users to choose appropriate > JT65 decoder, could you discuss this issue with Joe, please? > > > > 73, > > > > Igor UA3DJY > > > > > > > ------------------------------------------------------------------------------ > > _______________________________________________ > wsjt-devel mailing list > wsjt-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/wsjt-devel > >
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