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 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 > 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
>>
>
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