Severe Audio Stuttering and RF Power Drops in FT8 - SmartSDR+ v4.1.5 & WSJT-X v3.0.0

albert

Hi FlexRadio Support Team,

I am opening this ticket regarding a persistent modulation issue with my Flex-6500 since upgrading to SmartSDR v4.1.5.

Problem Description:When operating in FT8 using WSJT-X v3.0.0, the transmitted audio is severely "stuttered" or "hachured". This is immediately followed by correlated drops in RF output power. The issue appeared specifically after the migration to version 4.1.5.

Technical Environment:

• Radio: Flex-6500 (Firmware v4.1.5).
• Software: WSJT-X v3.0.0.
• Time Sync: Verified with NetTime (+2ms offset).
• Network: PC and Radio are on the same subnet, MTU 1500, Interrupt Moderation disabled on the NIC.

Troubleshooting already performed:

1. Clean Sweep: I have performed a complete uninstallation of all SmartSDR versions (v2, v3, and v4) and FlexVSP. I manually deleted the %APPDATA% and Program Files FlexRadio folders before a clean reinstall of v4.1.5.
2. Audio Configuration: Verified that DAX Audio TX and DAX Audio RX are strictly set to 16-bit, 48000 Hz in the Windows Sound Control Panel.
3. Split Mode: Tested with Split set to "None", "Fake It", and "Rig" with no improvement.

Despite these steps, the "Scenario B" delay we saw earlier is resolved, but the stuttering modulation makes the radio unusable in digital modes.

Could you please advise if there is a known compatibility issue with the v4.1.5 DAX drivers on certain Windows builds or if there is a specific buffer setting I should adjust?

73's

ON5AM

Mike-VA3MW

Hi

No, this is not a known problem. The first thing you need to do is a full radio reset after exporting your profiles and saving them. We do suggest doing this in the SmartSDR v4.1.5 release notes. Others have reported similar experiences and a radio reset has resolved their issues.

Give that a try.

albert

Thanks Mike,

After performing a full Factory Reset of the radio and a "Clean Sweep" of the PC software, I am stuck in the following repetitive 3-cycle loop.

• Cycle 1: The radio receives normally.
• Cycle 2: Instead of switching to Transmit as expected by WSJT-X, the radio stays in Receive mode.
• Cycle 3: The radio finally keys up (TX indicator turns red), but there is no RF carrier/output at all.

Then the sequence repeats.

Has anyone seen this "skipped cycle followed by empty carrier" behavior before?

albert

I open a ticket.

Neal Pollack AB6G

This is indeed a known problem, despite what they are saying. I submitted a ticket on this issue months ago, with 4.1.3 and 4.1.5. I added video to the ticket showing the digital power meter and the Flex waterfall display on transmit. While FT8 is transmitting, the RF power is jumping all over the place, both on the waterfall and on the external third party RF power meter.

After doing what Flex Tech Support told me to do, in order to verify that my Windows 11 PC was not to blame (Latency checks and other things), they finally said it is a known DAX driver issue, and that I should
wait for some future release to perhaps fix it.

Important note: This issue does NOT prevent making FT8 contacts, but since the waterfall shows a pulsing rise of the side skirts of the RF envelope (looks a lot like phase noise spreading worse), it is possible that this can cause noise for other people? But someone would have to hook up a nearby receiver and spectrum analyzer to verify. Anyway, the issue is not there with CW or with Phone, and it does not prevent successful FT8 usage. But it is highly annoying and disconcerting to watch it. These are, after all, very high end expensive radios, so it would be normal to expect more stable output during digital modes.

Neal

Neal Pollack AB6G

DL1DMS

Hey folks,

I am experiencing the exact same issue on a FLEX-6600 running SmartSDR v4.1.5 with WSJT-X / JTDX.

During FT8 transmission I observe:
-Brief RF power drops (2–3 times per 15s transmit cycle)
-Corresponding visible gaps in the transmitted spectrum
-No SWR increase
-No ALC spikes
-No PA temperature or current fluctuations

To isolate the problem, I performed the following tests tonight:
-Audio Path Verification
-Recorded the DAX TX stream internally using Adobe Audition.
-The recorded audio file is completely clean with no dropouts.
-This confirms WSJT-X/JTDX and DAX are delivering a continuous audio stream.

Network Tests
-Continuous ping to the radio: stable (no packet loss, 3–8 ms latency).
-Tested over both WiFi and wired LAN.
-Issue identical on both connections.

PC / OS Tests
-CPU usage normal (no spikes).
-Windows Defender real-time protection disabled.
-Reinstalled SmartSDR and DAX drivers completely.

No improvement.
-CAT / PTT Tests
-CAT control works correctly.

SmartSDR Tests
-Tested with SmartSDR minimized (no improvement).
-Verified RF power meter remains steady during dropouts.
-SWR remains stable.
-TUNE in JTDX is perfectly stable.

Additional Observations
-Issue also occurs in WSJT-X (not only JTDX).
-Does not occur with a steady tone.
-Only appears during FT8 modulation.

Based on these controlled tests, this does not appear to be:
-A PC performance issue
-A network instability
-A CAT/PTT interruption
-A power foldback condition

It appears to be a TX audio engine or DAX interaction issue introduced in SmartSDR 4.1.x.

Are there any known TX-audio instability in 4.1.5 affecting digital modes like FT8?

Please help.

73, Manuel

Mike-VA3MW

Manuel - doing a factory reset has resolved this problem in most cases. Give that a try. It worked for the original poster.