Is there really a solution for Audio Video Sync in OBS?

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Charles Lee

New Member
Connection, an Atem Mini Pro HDMI output with embedded audio to a BM Ultra studio Mini Recorder and then into OBS.
I then started a recording in 1080p25, made a couple of clap test in 10 minutes interval, just to see whether any audio offset is needed in OBS. After that I checked the recording in Adobe Premiere Pro and found that the audio is ahead of the video by 3 frames consistently. So I set a 120ms Sync offset in OBS Audio Advance Properties to compensate for the 3 frames difference. I then recorded again for 30 minutes with the 120ms sync offset and check it again in Premiere Pro. The audio and video sync was spot on, perfect.

I then continued with a 90 minutes recording test with the same 120ms sync offset and the result was not I had expected.
At the 30 minutes recording mark, everything was perfectly in sync, however, at the 60 minutes mark the audio started to lag behind the video by 6 frames, and by the 90 minutes mark, the audio was lagging behind the video by a good 10 frames! It seem like the audio and video sync just fluctuated throughout the recording.

With the same equipment, I made the same 90 minutes test recording in Wirecast Pro, and the audio and video sync was spot on throughout the recording. And this is even without dialling in any audio sync offset in the software. I even tested recording it on Zoom, and the audio and video sync was perfect for a good 120 minutes. I reckoned this problem is not unique, I'm sure many users in this forum have encountered this problem in OBS. Without a proper audio video sync, I'm not sure how we can maximised the use of this great piece of software really.

My setup, Mac Pro 2013, an ATEM Mini Pro connected to BM Ultrastudio Mini Recorder with embedded HDMI audio to OBS v26.1.2.
I have tried with version 11.5, 11.6,11.7 and 12 BM Desktop Video Setup to no avail.
I have also make sure that all audio setting are at 48 kHz and matched up all the video resolutions settings.
No skipped, no dropped or missing frames recorded in the Stats during the test.
The sync issue is repeatable in both 720p and 1080p25.
 
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Lawrence_SoCal

Active Member
Your experience seems about what should be expected. You are not using synchronized sources, and you are asking your PC to do that without any time sync markers during a session to keep in sync... so results would be what I expect, especially with an older, under-powered computer.

I suspect a HUGE portion of your issue is the compute power (or lack thereof) in that VERY old Mac
real-time video encoding is VERY computationally demanding. And video takes a LOT more to process than audio, hence differing process times. ONe way to alleviate that is to make sure your have plenty of resources (which you don't)
I recommend monitoring hardware resource (CPU, GPU, RAM, etc) utilization to see if your system is being maxed out with your settings (which I suspect it is).
A common recommendation would be (and this applies to any PC, not just a Mac) to route audio with video as that should enable a time-sync marker to help PC keep them together. Though you seem to be doing that... so not sure if you are adding a specific audio or video filter/effect that is re-introducing a out-of-sync issue (mitigated by having computer powerful enough to keep up), or if this is an issue of differing expectations/assumptions between Atem Mini Pro and OBS [someone else would have to speak to such). Or maybe it is the BM Ultrastudio that is a source of trouble? From a technical perspective (though not necessarily practical) it would be better for signal to come into computer, and route to OBS and a recorder at same time (vs being inline, causing its own latency)... but depends on specifics and I'm not familiar with the setup you describe, so less than $0.02 value in this last comment

Have you tried connecting the audio direct into the camera (assuming your camera allows such) that way audio and video combine in camera and coming to OBS via single HDMI signal? I say this only as a possible test. I suspect it won't help as I suspect either the under-powered 2013 Mac or something inline causing a drift in video signal processing over time. In the professional broadcast world there are things like Genlock to keep everything in sync. I quickly found this https://henrirapp.com/timecode-vs-genlock-why-timecode-is-not-enough-to-stay-in-sync/ & https://www.bhphotovideo.com/explor...ersus-sync-how-they-differ-and-why-it-matters - for what its worth
 
nottooloud

nottooloud

Member
Lawrence_SoCal said:
Your experience seems about what should be expected. You are not using synchronized sources, and you are asking your PC to do that without any time sync markers during a session to keep in sync... so results would be what I expect, especially with an older, under-powered computer.
Click to expand...

He says it works fine in Wirecast, so no, it's not his system.
 
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