Question / Help Magewell SDI Plus: choppy audio

[machello]

I recently acquired the Magewell SDI Plus USB capture device. Installed it and configured OBS last tuesday and everything seemed fine. Today I tested again and the audio was extremely choppy. I tried different settings (including setting OBS to the samle sample rate as the device: 48K), but to no avail. The only thing that did work was rerouting the audio with OBS: from the Magewell to "Windows Desktop" (WAVE) and then using that as the recording source. However, I then noticed the audio and video was not in sync (but I must admit: didn't look too closely when using the default setup, i.e. the one that resulted in the choppy audio). A couple of hours later I returned to the default setup and lo and behold, the audio sounded OK again. But then, after a reboot of the PC the choppiness returned and could again be solved by the rerouting trick.

It's not a peformance problem: load is about 30%. And if it helps: we're running Windows 7 and the video/audio is coming from a BMD ATEM 1 M/E.

Anyone who can shed a ligt on this?

I'm playing with the idea of replacing the Magewell with an AJA U-tap, but since U-tap is based on the same principe as the Magewell I wonder if that'd make a difference.

Any other suggestions for a replacement? I'm looking for a simple USB or PCI-E capture card with SDI support that is known to work well with OBS.

 

[Narcogen]

Please post a log with your issue! Here's how...

Magewell devices are known to work well with OBS. The only advantage to a PCI-E device over that would be bandwidth, and if you could resolve that problem the Magewell might work fine.

 

[machello]

That was my understanding also - that Magewell devices (should) work quite well with OBS.

I'll check out the logs soon. Maybe tomorrow, or else beginning of next week.

 

[machello]

I did some further testing today. To my surprise the choppy audio problem never ocurred again, not even after many hours (and after many config changes) later. But just when I thought "problem solved" the audio and video got badly out of sync. No idea why. A PC reboot seems to have cured this, but I hate getting inconsistent results like these! So far, this doesn't seem like a very stable streaming setup.

In case it helps, I've included 2 OBS logfiles.

I'm thinking of replacing the USB capture device with a Magewell capture card: the Pro Capture SDI (https://www.magewell.com/products/pro-capture-sdi). But as you stated the only advantage would probably be bandwith. Are there any other pros and cons of the Pro Capture SDI wrt OBS?

 

[machello]

Oh yes, something else I noticed: when I changed the settings of the Magewell device (within OBS) to 720p and 25fps I had to change the size of the canvas to 720p as well (the screen I'm using is a regular HD 1920x1080 screen, btw). Without that I couldn't get fullscreen video. Very strange. But this only happens when changing the "Resolution/FPS Type" setting for the device (as per Magewells instructions). With the default setting ("Device Default") I can leave the canvas at 1920x1080 and the video turns out all right.

 

[Narcogen]

I can't think of any. It's likely that the only difference between the devices is that the USB version has a USB controller connected to its PCIe bus, and communicates to your computer through that. You gain convenience and portability at the expense of potential bandwidth.

 

[machello]

This morning we had our first service at church with the Magewell capture device and results are mixed: the recording is OK (audio and video perfectly in sync, video is smooth), but the video stream was terrible. At irregular intervals the stream halts and then seems to catch up.

So, I'm definitely returning the USB capture device. Allthough, I wonder which is really at fault: the device or OBS? As said, the recording is perfect.

 

[Narcogen]

Do you have a logfile from that session? It should say definitively what the cause of the stutter is, and if it doesn't, then it may be fair to blame the device.

 

[machello]

It appears I have spoken too soon. This time, the problem may have been at the receiving end. We've a couple of ways to receive the stream. The usual way is via our website. That stream appears to work fine. The other way is through an STB based on LibreElec with a small app written by me that plays the stream. This setup worked fine so far, but apparently it can't handle the stream data generated by OBS very well. Strange.

So, the jury is still out on this one :-) But I think it's safe to say the problem lies not with the Magewell or OBS.

I checked the logfiles: no errors and nothing that points streaming errors.

Narcogen: thanks a lot for your comments!

 

[machello]

I've included the logfiles of the morning and afternoon service below. After closer inspection I do notice 2 messages about "lagged frames" in the 2nd logfile:

"Output 'adv_stream': Number of lagged frames due to rendering lag/stalls: 131 (0.1%)"

0.1% doesn't sound very dramatic, though.

 

[Narcogen]

Yeah that's nothing in terms of rendering lag.

Could you install NGINX-RTMP on the LibreElec box, and stream to that? That's what I use for local testing.

 

[machello]

You misunderstand, the LibreElec STB's are used to receive and display the stream. I've got no experience with NGINX-RTMP, but if I'm not mistaken that's a streaming server.

 

[Narcogen]

Yes, I understand. You already appear to be using custom output; if the STB is strong enough to just run a proxy then it can display the stream in just a web browser without using a custom app, and that would be a known good output profile for OBS.

I'm just saying that if the problem is described as, "my OBS stream is laggy when played back on an STB running LibreELEC using a custom app I wrote myself" the responses probably aren't going to be helpful because nobody else has that or knows what it is, and no one else can reproduce the problem in order to find the cause.

If you say, "My OBS stream is laggy when streamed to NGINX-RTMP and played in a browser" then there are more people who could at least weigh in on what might be happening, even if the problem is just that the STB hardware is the cause of the issue.

 

[machello]

Ah, I understand. The custom app does little more than calling xbmc.Player().play(), so the actual decoding etc. is done by Kodi/LibreElec.

But anyway, I was just trying to pinpoint the problem. The stream appears to work fine in a regular web browser, so the Magewell device and OBS are working OK.

I'll keep your suggestion in mind, though. I'm not going to tackle the problem with the STB straight away, but I may have to dive into that in the near future. At that point something like NGINX-RTMP might come in handy. So, thanks for that suggestion.

 

[Narcogen]

You're welcome. Good luck!