Spectral Abnormality in FT8 Mode Transmission

David Chen

I found my FT8 transmit abnormal this morning, please see the attached photo


I try to close and restart JTDX, it no use. After restart smartSDR, it looks OK now.



Any ideas?

Flex 6600 (24/7 running)
smartSDR 2.2.8
Windows 7 32 bits

73 de BA4RF,David Chen

John - AI4FR

I've read in the past where others have had this issue and fixed it by shutting down DAX and then starting it again.

N4QM

I don't know if this is part of the problem, but you appear to be way overdriving your audio chain.  The audio peak indicator is buried in the overload red zone.

Pat N6PAT

Every so often WSJT-X stops decoding. I check the WSJT-X waterfall and I see badly distorted signals. I restart DAX and all is well.

It's something I've gotten used to.

Bob K8RC

+1. It IS a DAX problem. and stopping then restarting DAX solves it.

N4QM

Interesting that I've never had that problem, Win7 and 10, 32bit and 64bit, 6400M.

Bill W2PKY

Happens when DAX has been running for generally over 24 hours.

kd5foy

I have observed the same a few times.  I stopped and restarted DAX, problem solved.

KY6LA_Howard

This is a well known defect in DAX and has been assigned a Defect Number It occurs when DAX has been running for some time and is easily corrected by stopping and restarting DAX The defect will be addressed in a future Software release. When? Flex does not publish future software release dates

Kevin N9JKP

I had it yesterday, DAX had only been running for a couple of hours.  And yes, rebooting does resolve it.

David Chen

Thank you.
I checked ALC level, it looks not reach 0db, anyway, I turn down a little bit to keep the ALC level around -5dB.

David Chen

Thank you everyone who reply my post.
Before flex fix this, let we keep one eye open on smartSDR :-)

Burch - K4QXX

I just checked my FT8 transmittion this morning and it looks like the bottom picture.  I turned on another radio here in the shack with the antenna unplugged to listen to the FT8 signal.  It sounded horrible.  I rebooted the radio, computer, stop and started DAX, rolled down my transmit level on DAX but nothing has fixed this issue.  I did recently update to the new windows 10 1803 but used the steps that Tim outlined and didn't have any issues (as far as errors).  Should I un-install SSDR and click to uninstall DAX and then reinstall? 

BurchK4QXX

Burch - K4QXX

Just tried PSK with DM780 and the signal even sounds worse. 

Bill W2PKY

How does the signal sound when the MON is turned on and listening through earphones?
Have you tried using FDX feature of the FLEX Radio to listen to the transmitted signal on another slice??

KY6LA_Howard

No need to reboot just stop and restart – DAX

KY6LA_Howard

Never use ALC to control an external amp with digital modes as ALC introduces non linearities into the transmitted signal

N4QM

Ideally you don't want the ALC meter to read at all.  ALC is a rudimentary form of compression, and compression is non-linear by definition.

Burch - K4QXX

I just shut down everything again (computer, radio, power supply).  It seems to be OK now.  It just seems like DAX is a little flaky right now.  I hate windows 10!  I work in IT and I can't tell you how many times a windows update has broken something at work.......
BurchK4QXX

N4QM

I'm wondering if you meant the Audio Level Meter and not ALC, since the ALC meter is calibrated in Volts, not dB.  You  need to try to keep the peak indicator out of the Red.

Ken - NM9P

Yes, restarting DAX usually fixes it.  But it doesn't only happen after long term DAX use.  Sometimes I have had it happen mid-QSO after only running an hour.  

I think there is also an interaction with WSJT-X that gets triggered if your CPU load pikes to 100% even if for a brief micro-second.  If I check my DXMaps or or check some other websites online while in a QSO, sometimes there is a brief glitch and I lose receive..... 

It may also be related to high DPC's.  I haven't traced it all yet.  My computer usually runds at 25-35% CPU load with occasional short spikes into the 60's and above at odd times.


Luckily, whatever it is, usually clears up with a restart of DAX.

Ken - NM9P

Alan Cerniglia

I have a similar problem..  If you look at David's waterfall, you'll notice the white horizontal line. Mine happens prob 1 out of 4 or 5 tx's.   I haven't put on headphones yet to hear if there is a noticeable audio artifact , but will soon ... Sometimes just one horz line & then 2 or 3...  Pic attached .. 

Alan Cerniglia

I also have these 4 Spurs off to the left during Tx  ..  

W9WLX_Chuck

Also--it is easier to prevent overdriving your audio chain if you run your RF power at 100%.  I generally run RF power at 100% and throttle back my final output power by reducing the audio level.

David Decoons, wo2x

Alan - If you look at the peak of your transmit signal and then look at the spurs you will see it shows them at least 60 dB down. Not an issue.

The panadapter is a double edged sword, it shows you a lot but some times too much.

Dave wo2x

Bill W2PKY

Its best to use the FDX feature to observe the transmitted signal off the air. Those spurs are not really transmitted.

N4BAA

Flex 6600 owner for about 2 weeks now and all bands and even very low power levels have these "spikes" on every FT8 transmission. It is very audible in my Bose speakers as well. NOT good as I have had several complaints about it being noticeable on the air. Any remedy?
Jose N4BAA

KD0RC

Hi Jose, what transmit level do you have DAX set for? You need to set it so that the peak level square does not go past 0dB (don't let it go into the red). On many Windows systems, this comes out to to 48.