Need help getting to bottom of audio quality problem

m5kvk

Some background first. My "remote" shack is connected at the other end of a VPN connection from my home QTH. As a result, I can run SmartSDR in three places:

1. On the PC at the remote shack (for digital modes)
2. On the home PC (using Smartlink)
3. On the home Mac (using a direct connection as SmartSDR Mac allows you to manually specify the IP address of the Flex - The Windows version cannot do this)

Audio in cases 1 and 3 is fine, but in case 2 (Windows PC accessing Flex over Smartlink), the audio is very poor. It constantly stutters and has loads of impuls noise on the stream). Network latency is reported as being good or excellent (< 28ms).

Any ideas on how I can debug this and what might be causing the stuttering?

Gareth - M5KVK

AA1SS

I don't use smartlink at all. I have it set up on Remote Hams and it works perfectly in and away from the shack . . . Sorry I coulsn't help further

Mike-VA3MW

Hi Gareth

In the case of #2 with a symptom of dropped audio. Now, we need to figure out the root cause.

• what is your ISP uplink speed?
• Have you tested with the FPS on the display reduced to zero
• Same for the display 'Rate' slider (this helps reduce upstream bandwidth
• Is the Home PC on Wifi or hard wired to the modem/router?

Latency is a good measurement tool, but it won't really help us in this situation.

What is happening is that some device between the radio and your home PC is dropping communication packets (UDP packets). When this happens, you hear audio dropouts. This is not uncommon actually as it is these communication packets that get dropped if a network is congested at some place.

The question is what part is causing the problem. The culprits tend to be (in no order) and this is from factual data we have collected.

• The switch at the radio end that communicates to the router on the modem. It is hard to diagnose since you can't log into the switch to check for dropped packets or collisions. I figured this out in my shack when I added a managed switch. You can read the details here: https://community.flexradio.com/discussion/8028378/network-cleanup-and-the-results#latest
• The router can't handle the upstream bandwidth
• You are trying to exceed the upstream bandwidth of your ISP subscription (you need at least 2mb/sec)
• Wifi is evil. That is why I asked above. It isn't very reliable for realtime voip communications. Streaming movies is much different
• The home PC is suffering from DPC (overload). https://www.sweetwater.com/sweetcare/articles/solving-dpc-latency-issues/

I hope that helps a bit for you to narrow it down.

Ha Gei

What Version are you running ?

Same result with direct PC input as source or DAX as source ?

I have a highend Lenovo laptop here, which has similar issues, even RX audio ist stuttering. It´s a Core I7 9th Gen , 32 Gig Ram , Win 10 ... no usable at all.. Other Laptops, even with lowend CPU´s work perfect on the same network . I nailed down the problem to Graphic Card issues, internal Graphic chips can make such trouble.

Quite a few People i talked to lately , told me they abandoned Smartlink even for the price of buying a MAC and the Software, as that one is far better in many cases.. I did not test myself yet, as i have no audio problems on my active machines.

Harry DL9NDW

m5kvk

@Mike-VA3MW Thanks for the ideas. Some answers to your points.

• ISP uplink averages about 20 MBit/s. Down averages about 95.
• Adjusting FPS makes no difference
• Ditto with display rate
• All devices are hard wired except the VPN between the two routers (See below for details).

Regarding the topic of network dropouts:

• In all three scenarios, traffic has to go through the network switch at the remote QTH, but only scenario 2 (Flexlink) experiences dropouts.
• The Mac on the home network goes through an almost identical network path as the PC when talking to the radio. This route is PC - Shack Switch (Unifi) - Home Router (Unifi) - VPN (OpenVPN) - Remote Router (Teltonika) - Remote Switch (Netgear) - Flex. All connections are wired except the router-router VPN.
• The only difference between the Mac and the PC is that, because the PC is using Smartlink, it addresses the radio using the its public IP address. In fact, this is also the public IP of my home network because the remote QTH connects back to the home network over the VPN. This means no traffic transits the ISP.
• The VPN is also used by the Mac, so that's not the cause.
• Thanks for the pointer to LatencyMon. Not come across that before. It reports that all is OK.

@Ha Gei Thanks for the pointer to the Graphics card. I'm running v3.4.23. For most use, I do use the Mac version of SmartSDR, but I'm curious about why the PC version is acting up so badly. It's not a widely reported issue as far as I can tell, so I'm concerned that it is telling me something about my PC that I need to know.

Mike-VA3MW

Thanks for the great input.

Based on your data rates not making a difference, then we can safely assume that this is not a network communications issue (for the most part--keep it in mind)

When you use SmartLink the audio streams are run through a compression algorithm. On a non-SmartLink connection, they are not.

What version of SmartSDR are you using? If you are not on 3.4, then you need to be as there was BIG improvements in remote audio between 3.3 and 3.4.

m5kvk

Sadly (!) I am on 3.4.23

Mike-VA3MW

At this point, my next thought is to see if you can test on another computer. At least, that is the path I would look at.

m5kvk

That’s exactly my next step. I have an HP Laptop I can try. I have an Ethernet adapter for it to eliminate that aspect. I’ll report back.