Bug Report Audio sync issue with Blackmagic capture cards

[James Ritson]

Hi all, I'm posting to see if any developers or users can offer support for this issue I'm experiencing. I've recorded an 8 minute long video that demonstrates the exact issue I'm having, but I'll also bullet point it below:

• Using OBS to produce a live mix output for tutorials using capture cards and an audio mixer.
• Currently using a Blackmagic Decklink 4K Extreme and two Blackmagic Intensity Pro cards.
• Audio is via the DeckLink's balanced XLR inputs.
• When OBS is first run up, there is a noticeable audio delay (visible on the audio meters). Attempting to record any output at this stage results in a huge desynchronisation between video and audio.
• This audio sync issue is solved by going into the Settings>Audio preferences and changing the primary Mix/Auxiliary device.
• For example, if the current device is "Blackmagic Decklink 4K Extreme Audio", changing it to another device and then back again will solve the issue.
• Additionally, I have the DeckLink as the default Windows recording/communication device, so you can also change it to "Default" and it will solve the issue.

Here's the video:

Currently, OBS works really well and it's been so easy to set up and get working—there's just the minor issue of having to "flip" the audio device each time OBS is run up. If you watch the video it should hopefully give you a clear idea of what's happening.

I did also have a one-off issue yesterday (so far) where I had been recording videos most of the working day (8am-2pm). I then got my colleague up and running and he tried to record a video, but everything was completely out of sync—despite the fact that I had been successfully recording videos all day. Doing the audio "flip" had no effect, and eventually I tried a reboot which solved the issue. It was bizarre, as I had just recorded a video 10-15 minutes previously that was fine (we're joking that he jinxed it, but still...)

Before we had this Windows box built and purchased the DeckLink, I was using my own PC to test and had exactly the same issue—so it's not specific to this particular setup, other than the fact that I'm using Blackmagic cards. In my PC, I had the two Intensity Pro cards plus an original Intensity, so no DeckLink at this point. For audio, I was using an M-Audio M-Track 2x2M, so the audio input was separate to the capture cards. I was originally using the audio offset feature to combat the sync issue, but with limited success, as the delay seemed to be variable.

[Edit] I should also mention that I've tried adding the devices using the Video Device Capture (I forget what it's labelled, the WDM option?) instead of using Blackmagic Device, but it made no difference—as I understand, using Blackmagic Device is preferable anyway.

I would really appreciate any thoughts or suggestions. For now, it's certainly usable, but if there's any insight into why the audio synchronisation is an issue on first run-up it would be interesting to know. I've also tried varying the order of the scenes in case the order the devices are loaded has any bearing, but I haven't noticed any difference. It's also worth noting that I have two different scene and profile setups for 16:9 (Desktop) and 4:3 (iPad)—switching scenes will also reintroduce the audio sync issue.

I've attached a log file—the audio initialisation at the end is me "flipping" the audio source from "Default" to "Blackmagic DeckLink 4K Extreme Audio" (and vice versa) to fix the sync issue.

Thanks and hope to hear back from someone—other than this issue, it's a cracking bit of software, thank you so much for the development that has gone into it.

 

[Boildown]

Your log file looks incomplete. Be sure to completely exit OBS to make sure it writes the file from memory to disk before uploading it.

 

[James Ritson]

Thanks for your reply, I've attached a log file from my colleague's recording session yesterday after I left which is definitely complete.

 

[James Ritson]

Apologies for the bump, but just wanted to try and get this seen in case anyone else is using Blackmagic cards successfully with OBS—also, now I've attached the full log file, is there anything of interest there that might suggest the initial audio lag?

 

[pkv]

hi
you seem to be confused about the various drivers allowing to acquire audio from a decklink card.
You can use either the directshow interfaces provided by blackmagic with its drivers ('blackmagic WDM capture' and the older one 'DeckLink Audio Capture') or use obs native blackmagic capture.

What you're doing in the video is using the directshow audio interfaces. Dunno why you're doing that. The audio meters should move in the Blackmagic Device source if you've set it up correctly.

As to the delay there should be none when using 'Blackmagic Device' source because the decklink input source is resyncing manually video and audio ; should there be some delay, it's added elsewhere than in obs.
This can be remedied easily though:
- go to Advanced Audio Properties ==> add there some audio delay to your source

 

[James Ritson]

Thanks for your reply—the reason I'm using the DirectShow Mic/Aux device is because the audio needs to be present in every scene, plus I've added an EQ filter on top of that. It made sense from a workflow point of view not to have to add the audio track separately to each scene. Apart from the audio input, the DeckLink device is only used once for video (the Presenter cam) using the "Blackmagic Device" input (which I assume is the OBS native capture?).

Are you suggesting I should add the DeckLink device as a video/audio input to every scene and hide the video output somehow (perhaps move it off canvas)? I'm aware you can add Audio Input Capture devices too but I suspect this is also DirectShow?

Regarding adding the audio delay source, I've mentioned this in the video and in my first post—the desync between video and audio isn't consistent, it varies between startups, so applying an audio offset is unreliable. It's only by re-initialising the audio device through Settings that they will sync up perfectly.

Possibly this makes sense: if the audio is being captured through the WDM driver it's not in sync with the actual Blackmagic device input. The only outlier is that I also had the same issue with the M-Audio interface, though I would expect that not to be in sync.

Food for thought, thanks very much, I'll give this a go when I'm back in work tomorrow morning. I've tried to make the setup easy to use, as we have three of us shooting videos at the moment and we need to quickly adjust the audio levels for our different speech volumes. This is done by adjusting the Mix/Aux level, but if the DeckLink input is contained within one scene and referenced in others it looks like this will still be doable, since the audio level will stay consistent between scenes. I also hope there isn't a noticeable "jump" in audio when moving between scenes, but I'll find out if that's the case tomorrow. My only other concern is what I would do with the video input from the DeckLink in each scene—as I mentioned above, possibly just move it out of the canvas view? Actually hiding the device/scene via the eye icon would I think disable the audio as well.

Apologies for the wall of text, just trying to cover all bases and provide as much info as possible. Thanks again for replying.

 

[pkv]

yes you could definitely hide the blackmagic source; just move it at the bottom (right-click > order > move to bottom) in the scenes where you don't need the vid.
Don't click the eye icon since this disables both video and audio