DAX RX Gain After Changing AGC for Digital Operation

Ron Schwartz

When running digital modes using FLDigi or similar programs I typically turn off AGC and move the AGC/Threshold slider fully to the right in SmartSDR.  This 'redlines' the RX Gain indicator in the DAX Control Panel, suggesting a significant decrease in the RX Gain setting.  Going back to voice modes requires the DAX Control Panel gain to be changed back to around 50%.

I would appreciate advice on how others are setting their AGC threshold for digital modes and whether they are then changing the RX Gain in the DAX control panel.  Is the 'best-practice' for digital work to find the 'knee' in the noise level using the AGC slider, move the AGC slider all the way to the right, and/or turn AGC off?  I looked through the forum history but could not find any comments that linked AGC operation back to the DAX control panel.

Thanks, Ron
K2RAS

Ian1

I put mine on Fast and the slider full right in my testing this worked best for me. Ian

George KF2T

For digi modes, I run with DIGU selected, AGC OFF, with slider at the "knee" (generally around 45-52, depending on Band), and then adjust the DAX channel level for the appropriate level in the digital Application (green Diamond lights in FLDIGI, 30 db in WSJT-x, 0 db in WSJT). Use a unique DAX channel per digital app. No need for DAX on SSB, unless you have a special situation?

Al K0VM

For digital modes like WSJT and PSK ( where there is more than one signal in the pass band)  I would like AGT threshold to be reached only on the strongest of signals. Therefore I leave the leave AGT on but set the slider down into the 20-30 range,  DAX is at default value and the WSJT-X signal level indicator indicates about 10 on RX noise.    To me it seems as if its best if the digital program does most of the leveling.   If AGC is run close to threshold, strong signals supress the weaker ones. 

Just my way of doing it..

AL, K0VM

W5XZ - dan

That's curious, I see settings all over the ball park!  here, with WSJT-x, I run the rig a lot like i do on cw.  AGC-t ON, fast, < 50 typically...NB, sometimes...adjust DAX audio for '30' on the wsjt-x display..does WSJT-x have an agc function in its software? 

must just be me, but on CW, when i turn off AGC, and start digging for weak signals,somebody comes along and blows my headphones off my head! digital decoding software doesn't overload, or clip, or go into gain compression, so it doesn't need AGC?

would sure like to decode some more weak sigs, though, so will try the above suggestions.

73, w5xz, dan

Ron Schwartz

Actually, George has a point.  When I started this posting I just assumed that I should be setting the RX Gain in the DAX control panel back to a 'normal' level when I went back to voice mode.  Since voice modes do not use DAX, I really should not worry about the DAX setting when running voice.

It is interesting that there is no consensus yet on how to set AGC for digital, so I was not missing something.  I would be interested in Flex's opinion on the 'best practice'.

Ron

W5XZ - dan

well, my settings can't be THAT bad...i did pull vp8alj, falklands, out of the mud on a noisy 160m band, with modest antennas...twice, now...full credit, though, to my HiZ 4sq, on receive..

'just sayin', as the kids say?

73, w5xz, dan

Tim - W4TME

So here is my answer, not necessarily the FlexRadio answer.

It depends on two factors; the receive bandwidth filter and band QRN / QSB conditions.  It really helps to understand how AGC algorithm is implemented in the radio since it is a dual track AGC that works on both fast and slow increases in gain.  I highly recommend reading the section on Noise Mitigation in the SmartSDR for Windows Software User's Guide.

In general, I do not want a stronger signal in my receive passband "capturing" the AGC where the audio gain will decrease, making weak signal reception more difficult.  If I am using a narrow filter relative to the mode I am operating and the bandwidth of that filter is such that the probability of another strong signal entering the passband is low, then I use the Fast or Medium AGC and adjust the AGC-T to the point where I have maximum signal to noise (SNR) based on band conditions (see the section in the manual on how to do this).

If band conditions are fluctuating or I am using a wide RX filter, then I set the AGC to Slow or sometimes Off.  Again if the AGC is set to Slow, I adjust the AGC-T for maximum SNR.  If I use the Off setting, you have to be a little more careful.  When AGC is off,  it really isn't off.  It means that a fixed amount of gain will be applied to both fast and slow increases in noise.  I use these setting so that a strong signal in my passband does not capture the AGC and trigger an AF gain reduction.

George KF2T

As the Good Book says, there's more than one way to skin a cat. Or, your mileage may vary. If it works well in your unique Installation, it's good enough. Tweak on!

NX6D Dave

Jay Nation

My mileage varies from session to session. The settings get adapted for whatever tweak I'm using with that session. I use whatever works, heavy on the "whatever", and it usually works. I used to believe, I'd find some sort of magic sweetspot setting. Then I remembered I don't believe in magic.

73, Jay - NO5J  

mdavid01

For psk 31 I use AGC off, slider set to 45 or so, usually i adjust agc based on waterfall in fldigi.
Fldigi I have viewer squelch set to 1.5 with Receive Squelch level set 1/4 from bottom

I found today that while working Feld **** that I needed to crank the agc slider to 90 or so to make out the information

Jay Nation

"Today" I'm  running preamp off, agc off, threshold 16%, and DAX RX 70%, 4k passband, with WSJT-X 1.6.0 -r5881. 
Works as I expect it too.

73, Jay - NO5J

Andrew O'Brien

Has anything changed in the functioning of SmartSDR since this thread developed in 2015? I found Tim's comments quite helpful. I raise the issue because, thanks to Slicemaster, I have gone back to using Fldigi after many years . I note some of the advice from Dave W1HKJ and others involved in Fldigi , they suggest various things for setting audio levels, see Fldigi Users Manual: RX/TX Audio Levels (w1hkj.com). In that document they note

"Set your transceiver to a portion of the band with no signals; 14.065 MHz might suffice. Adjust the audio path gain; transceiver audio, external audio codec gain, and operating system gain for a noise level that is about -50 dB on the Fldigi VU meter. Then set the transceiver to an active portion of the digital band; 14.070 MHz for PSK or 14.074 MHz for JT type digital signals. Make sure that the input gain controls are not set too hot. The peak signals do should not exceed the upper/lower gray lines on the display" (see display in the linked document).

Following those steps takes quite a bit of fiddling/tweaking when taking in to account DAX sliders and the AG-T settings in SmartSDR. However, I think I am close but not sure about AGC settings mentioned in this thread. So, I thought I would check .

Andy K3UK

Mike-VA3MW

Still the same procedure.

First, set AGC-T normally as described in the manual. Unlike a superhet receiver, you really don't have to turn the AGC off as the design is hugely different than that of an AGC-T.

Next, set your DAX levels as required for the digital program. It actually isn't that critical as there is a lot of dynamic range in the software decoders.