Seeking Advice: Resolving Audio Stuttering in Remote FlexRadio Setup

SM6KZW

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

SM6KZW

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.

SM6KZW

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.

SM6KZW

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.

Paul Christy

I hope you solved your problem. If so, please let me know how you did it. I think if you ran a Maestro over the same connections, it would work fine. Problem is in Windows settings or SmartSDR. Not much incentive for them to fix that... There is a workaround. Listen to audio on DAX channel through Audacity. Quite the Kludge but it's working pretty well for me so far.

Paul

N0GN

Mike-VA3MW

This is an old thread and not an issue anymore. If you still have the issue, please make sure you are at at least SmartSDR 3.9 or 3.10

Paul Christy

Thanks Mike.

Using v3.10.15. I still have the problem and was just looking around for people who might have solved it. My configuration is way different from SM6KZW. I'm running SSDR remotely over SmartLink on new Carbon X1 laptop with business internet connections on both ends. Still have crackling in the audio stream presumably due to signal dropouts lasting on the order of milliseconds.

When I listen to the DAX channel on Audacity there's much less of a problem. Still have some black lines on the waterfall but there is substantial improvement in overall audio.

I've tried numerous suggestions from throughout the community and from the help desk for years and using Audacity is the only thing that has worked so far. I don't really like having to use 3rd party app for this. Also concerned in the future I could limit options for SO2R and other advanced techniques. One downside I've noticed so far is that using the PC microphone, the record feature on the slice doesn't work. And I can't control audio volume from the RX panel.

Mike-VA3MW

Paul, if you test with only a LAN cable, do you have the same problem? This means you have a LAN cable connected only from the radio to the PC? (you have to reboot both devices to make this work).

But, since this is remote (I'm reading and typing) the dropouts are highly likely to be caused by routers and/or switches dropping UDP packets due to some sort of congestion. This is the way the internet is designed.

These packets can get dropped anywhere in the transport layer. Any switch, any routers, and even in the NIC card of the Laptop you are using. If the laptop is on WiFi, then that amplifies things sadly.

I'm not sure how Audacity fits in here. Sorry.

Paul Christy

Thanks Mike. i use Audacity to monitor the DAX Channel. Can you suggest a specific network switch? I have a new, but cheap, Netgear switch. Willing to spend whatever it takes to get what you recommend. Thanks!

Mike-VA3MW

I use TP-Link managed switch so I can at least log in and see if something isn't correct.

Generally, look for anything in a Metal case. You may also find that a 100mb is plenty fast enough and it doesn't seem to have the autonegotiation issues some of the less expensive Gig switches have. I use a TL-SG1024DE

Paul Christy

Awright. Thanks a lot. We'll give it a try. Thanks!

John KB4DU

to second Mike’s suggestion, I converted a 100Mb access point to a switch by disabling the wi-fi. I was using a 1GB switch locally. The network quality indicator frequently showed poor network performance. Using the 100MB device fixed it.

Paul Christy

Thanks. I have Mike's switch on the way. Didn't see any 100 Mb switches. Also going to try direct connection. Also have some Cat 8 patch cables on the way