AGC-T and FT8?

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Hi,
I keep having to change my AGC-T constantly with strong signals in the passband. If I do not, I go into the red in WSJT-X. 

I remember reading that for FT8, I should be using DIGU and AGC set to OFF. That's what I am doing, but what about AGC-T value? If I still set it to the noise knee, I get those very large swings. 

Current settings are: DIGU, AGC-off, PRE 16db, 17m, AGC-T 80, DAX 24. I am overdriving my waterfall. Everything is red, but the decoding is fine...

Any tips are appreciated.

73
Andy
KU7T
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Andy - KU7T

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Posted 2 weeks ago

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Photo of George Molnar, KF2T

George Molnar, KF2T, Elmer

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Preamp OFF. AGC off. AGC-T down to where the background on a quiet frequency starts to drop. DAX to set WSJTX “thermometer “ to 30 on same quiet frequency. Should work fine. Drop DAX further if the thermometer turns red. The waterfall sensitivity and zero are set in the waterfall window and hve nothing to do with decoding. Adjust to taste.
(Edited)
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Ken - NM9P, Elmer

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There is no knee point with AGC off. With ft8 I use AGC off and set AGC-t so that when things are quiet in the band ( in between RX cycles) the green meter in WSJT-x shows about 20-30. If it goes to the red on very strong signals drop it a little. But you don’t need to ride the control up and down. You probably need to turn the slice volume down, but you don’t really need to hear everything loudly. The WSJT-X program has lots of dynamic range.


Ken - NM9P
(Edited)
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George Molnar, KF2T, Elmer

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You’re right, of course! Too much LDS during the Free Speech movement, I guess!
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Ken - NM9P, Elmer

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Btw... AGC-T is the AGC Threshold. With AGC off, there is no threshold to set, the control is essentially just an RF gain control. You just need to adjust it so that the strongest signals don’t overload the receiver or your digital program...
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Ken - NM9P, Elmer

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George, I see that you are a Star Trek fan!
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George Molnar, KF2T, Elmer

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A logical inference.
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Andy - KU7T

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Thanks for all the responses. That seems to work better. My DAX RF gain is set very low though. Is at 5 (and the WSJT-X moves between 30 and 70 and stays green). Is that normal that I need to set the gain to 5 (on a slider between 0 and 100)? 

Thanks
Andy
KU7T
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Ken - NM9P, Elmer

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Depending on the band and conditions, I usually need AGC set anywhere from 10-35. But watch your preamp settings. You don’t usually need any added gain on 20 Meters and below. Perhaps 10 dB on17 & 15, more on 12 & 10, and full preamp on 6 Meters.
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Matt NQ6N

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Can anyone offer a detailed DSP explanation of the consequences of different AGC (and/or AGC-T) settings to the FT8 decoding process?  

Suppose I am using FT8 and I am decoding a signal with an SNR of -24 dB (near the limit of what my setup can decode). If AGC is on, decreasing the AGC-T level would seemingly risk allowing static crashes to cause the AGC to reduce gain.  Therefore, I would assume that if using AGC the best AGC-T setting would be the maximum setting, so that the decoder can do its job relatively unhindered by AGC kicking in due to static crashes or loud adjacent signals. 

So to avoid any impact of AGC artifacts on the decoding process, AGC-OFF would be optimal, except it requires a lot of attention to the audio levels to prevent strong signals or static crashes from overdriving the decoder audio input and causing clipping (information loss). 

I'm also not clear on why the FT8 decoder would benefit from additional "headroom" above the loudest audio level (when AGC is used).  Thus I set it so that the FT8 decoder audio input shows 80-90% of maximum level on a very loud signal (with rig AGC on). 

I'd be much appreciative if anyone can offer clarification or correction to any of the above. 

One side question about AGC that I've been wondering about.  My K3 has a setting called AGC-Slope which I believe changes the slope of the line that represents the ratio of the strength of the input signal relative to the amount of AGC that is applied (when it applies).  I had found that I preferred a setting that would result in "less" AGC on loud signals than the default, which made the rig sound a bit more like it had AGC off, but with most of the benefits of AGC for ear protection and clipping prevention.  Does a similar setting exist in SmartSDR?  Not sure if it's relevant or if I've accurately described what AGC-Slope does, but figured I'd ask since there is a decent chance that someone who would respond to the above question might also know the answer to this. 

73,
Matt NQ6N
(Edited)
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Ken - NM9P, Elmer

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In general the purpose of an AGC is to be able to listen to weak signals at a comfortable listening level without having your ears blown away by a stronger signal. So the AGC turns the total gain down in order to protect your ears and equalize the volume variations of a particular signal.

With digital modes like a FT8 or PSK 31 etc., you are listening to multiple signals at the same time. If you use a strong AGC to bring the weak ones up whenever a strong signal is in the past bands as well it will lower the total system gain and reduce the strength of the weak signals to the point that it might not be copyable. In any case constant variations in the signal strength of the signals below the strongest signal in the pass band would possibly confuse the digital decoding program.

Turning AGC off, and then adjusting the gain so that the strongest signals do not overload the receiver or the digital program, will result in constant signal levels and allow the digital program’s routine to detect the various signals within its own dynamic range.

It isn’t necessary for our ear to be able to hear the weakest signals at any appreciable volume level. It is only necessary for the digital program to be able to detect them.

While I would use a AGC at fast or medium for modes such as RTTY, CW, or other modes that only have one signal in the passband at a time, I prefer to use AGC off for the wideband multi signal modes like FT8, PSK31, JT9, jt65, etc., with the AGC gain carefully adjusted.
(Edited)