Seeking Advice: Resolving Audio Stuttering in Remote FlexRadio Setup

SA6TUT

Hello forum friends!

I have made some progress in setting up a proper remote radio setup. Until recently, I was operating remotely from my kitchen to a garden shed within my LAN. I have been using SmartSDR v3.7.4 on Windows 11 with my FlexRadio 6600. As a preparation for moving the radio to a remote location several miles away, I connected it to a powerful Asus Router running Merlin firmware. I then installed OpenVPN configured in TAP mode and use UDP as the protocol. The router is connected to a 4G access point. I connect to the router using the OpenVPN2 client from my PC. This setup works well, allowing me to access devices connected to the router from my PC as if they were on the same LAN. (UDP Broadcasts working) This also enables me to connect to my FlexRadio without using SmartLink.

Overall, my tests have been successful.

However, I am experiencing some issues with audio stuttering. The frequency of these interruptions seems to correlate with the quality of the internet connection at the time. The quality of my 4G connection varies, as you can see in the attached image.

I have experimented with different MTU settings and found that 1500 is the optimal value. This conclusion is based on my ability to ping the radio from my PC through the OpenVPN tunnel over 4G using the command: ping -f -l 1472 192.168.100.100

Now, I plan to see if I can improve my connection quality, as I believe this is the culprit, and where the audio stuttering issue originates.

I would appreciate any tips, comments, and suggestions from the forum.

73 de SA6TUT

SA6TUT

I can add that connecting to the radio via SmartLink works fine. Since the traffic does not go through my OpenVPN tunnel in this case, we can rule out the VPN as a factor. The "stuttering sound" issue persists in both normal and low bandwidth modes. In my humble opinion, the problem can only be due to either the router (an ASUS RT-AX86U Pro) or the 4G connection itself.

I have set up the 4G router on the roof of the garden shed to improve the signal. Despite achieving better download and upload speeds, the quality indicator in SmartSDR still shows poor quality. It sometimes goes into the red but mostly stays in the yellow.

SA6TUT

Now I have also tested connecting my FlexRadio directly to the 4G modem, which is set in bridge mode. This setup gives the FlexRadio a publicly accessible IP address. While this is a very risky approach from a security perspective, it was only for a brief test.

By connecting in this manner, I have eliminated another potential source of the problem from the equation: my Asus Router (and the VPN tunnel, of course).

Interestingly, the issue with the "stuttering sound" persists. Therefore, I must conclude that the problem lies with the 4G modem, the 4G service, my geographic location, or a combination of these factors.

73 de SA6TUT :)

KD0RC

One thing to note is that when using SmartLink, there is more audio data compression to allow for less bandwidth. When I operate remotely, I use SmartLink for SmartSDR and a VPN (SoftEther) for everything else.

SA6TUT

Thanks Len for your input. :)

I ran a test at https://www.waveform.com/tools/bufferbloat and as it seems, the Bufferbloat grade is "C" which means that the latency increases considerably during load.

Latency unloaded: 36 ms
Download active: +71 ms
Upload active: + 89 ms

David Decoons, wo2x

If you can upgrade your hotspot to 5G and have coverage, it can help. But be wary. Both T-Maybe and AT&T have started blocking UDP traffic on their 5G networks. T-Maybe here in NJ is unusable using my IPhone as a hotspot, or with SmartSDR for IOS, or using a dedicated T-Maybe 5G hotspot device.

My workaround is a BERL WiFi 6 travel router set up as PoenVPN client back to my Omaha router at home set as OpenVPN server. This uses more bandwidth but the 5G T-Maybe network seems to work fine.

Thinking of testing Verizoff eSIM on my iPhone to see if their 5G network is blocking UDP traffic.

Bottom line - cell carriers lie when they say unlimited data, as they are blocking UDP streaming.

Dave wo2x

WX7Y

If you haven't done so, In the Display settings reduce the "FPS" and "Rate" this may help with your Audio stuttering.

hope this helps

Bret / WX7Y

KI5ESV

Where are "display settings"?
Thank you

KD0RC

Assuming SmartSDR:

KI5ESV

Thank you!
Funny how one can be looking right at something and not see it.
Tried reducing both - no change in audio...
'73
Jim-

KD0RC

Hi Jim, you might want to submit a helpdesk ticket.

Skittle8972

The issue here is probably a missing feature in the firmware where a user could configure the maximum packet length when sending UDP packages (which is the default for RTP audio streams). There is a concept for TCP packages where the router could "communicate" the MTU through PMTUD and clamping. But due to the nature of UDP this doesn't work. It would be really nice if FlexRadio could simple add that as a parameter to the radio and make it configurable between 1420 and 1500 bytes. This will be important in order to secure our transceivers from malicious attacks on radios being exposed in public IP addresses just because hardware vendors (management) don't think that this is actually worth developing. But NIS2 (at least in Europe) will make them aware of that vulnerability. I guess no hardware vendor wants to be in the news when remote controlled transceivers are blocking important wireless services of a society. So please - add either a Wireguard server to the firmware or a configuration option for max RTP packet size.