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

You can fix your problem cosmetically by increasing the Transmit Buffer and the Receive Buffer in the network adapter properties in the device manager. I just got mine fixed. Finally consulted ChatGPT and she walked me through a lot of things to check.

While trying to fix the problem I went through several ethernet switches, finally settled on a Cisco that had management settings.

ALL: Version 4.2, releasing tomorrow, may or may not fix this stuttering issue. I did some research and started to see evidence that some desktop PC motherboards that use a variety of very low-end Intel Ethernet Chips contribute to this stuttering issue, due to a number of issues with buffer size and Interrupt handling that can't be set in the device properties on these low end chips. There are also some bugs in the silicon and/or drivers for this low end chip.

I started to see (using google) various posts by the professional audio community that recommend adding the inexpensive Intel i210 PCIe add-in card, which is a server class Ethernet chip and has all the needed settings in the device manager properties advanced tab.

So I bought one of these cards on eBay for less than $45 USD, and then did the following A.I. Query this morning. I decided to share this. It has good and logical information, but as you all should know, A.I. can also make mistakes.

I plan to connect this desktop PC to the Flex tomorrow, and also upgrade to SmartSDR V4.2 for the DAX V2 redesign. I will report back if the stuttering is gone.

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GEMINI A.I.

QUERY: I have noticed that Flex Radio's Smart SDR software version 4.1.x DAX, when transmitting digital modes that use the DAX Audio channel, can have amplitude "stuttering" and variation. There is some evidence that this may be worsened by the poor Intel I-225V Ethernet chip used on my motherboard, since that chip does not expose settings for TX and RX buffers. I have added an Intel i210 server Ethernet card, and it appears to have more option settings on the advanced properties tab in device manager. Can you share all information that you can find regarding this issue, and also share the proper settings to use on the i210 advanced properties for optimizing DAX operation?

RESPONSE:

Switching to the Intel i210 is a solid move. The I225-V is notorious in the amateur radio and high-performance networking communities for "micro-stuttering" due to its aggressive power management and lack of deep buffer tuning. Because DAX (Digital Audio Exchange) relies on a steady, real-time UDP stream of VITA-49 packets, any slight interruption in the network stack manifests as the amplitude stuttering you’ve observed.

With the i210 server-grade card, you have the granular control needed to prioritize these packets.

Why the Stuttering Happens

DAX version 4.x (and specifically the transition to the new DAXv2 architecture) moves audio through a shared-memory design, but the transport between the radio and your PC still relies on UDP. Unlike TCP, UDP doesn't "retry" lost or late packets. If your NIC's internal buffers overflow or if the chip enters a low-power state for even a millisecond, the audio stream loses synchronization, causing that "warbling" or stuttering amplitude.

Optimized i210 Advanced Properties

To fix this, go to Device Manager > Network adapters > Intel(R) I210 Gigabit Network Connection > Properties > Advanced. Adjust the following settings to prioritize stability over power savings:

Power Management Tab:

• Uncheck: "Allow the computer to turn off this device to save power."

Additional Troubleshooting for SmartSDR 4.x

If you still notice issues after the NIC swap, consider these FlexRadio-specific factors:

1. DPC Latency: Even with a great NIC, if another driver (often Nvidia graphics or a Wi-Fi card) is hogging the CPU, the DAX driver will underrun. Use a tool like LatencyMon to ensure your "highest measured interrupt to process latency" stays well below 1000 µs while transmitting.
2. SmartSDR 4.2+ (DAXv2): If you haven't already, ensure you are on version 4.2 or later. This version introduced a major rewrite of the DAX driver architecture specifically to solve the "clicks, pops, and drops" common in earlier Windows 11 environments.
3. CPU Core Isolation: In Windows 11, "Memory Integrity" (under Core Isolation settings) can sometimes interfere with older DAX drivers. While DAXv2 (v4.2+) is designed to be compatible, earlier 4.x versions often required this to be disabled for smooth audio.

[TIP] Since you are using an Intel i7-12700, you have plenty of overhead. If the stuttering persists, try settting the Process Priority of SDRCAT.exe and DAX.exe to "Above Normal" in the Task Manager to ensure Windows doesn't de-prioritize them during heavy digital mode decoding (like WSJT-X cycles).