Question / Help Audio / Video Sync Issues on Output

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michaelc

New Member
Hi, I'm quite confused why this is the case, so perhaps someone can clear this up for me. I am capturing SDI sources in using a BlackMagic Decklink Duo 2. In OBS the video and audio look to be in sync. When I output via either the Decklink, NDI, or HDMI, there is a massive difference between the audio and video. The video is about 900ms ahead of the audio, so I have to delay the video, which isn't as easy as delaying the audio would be. Since I am using PTZ cameras, I can't just delay the video input as then OBS is useless for live camera tracking. I also found if I bring audio in from a digital sound mixer over USB, the difference is only about 600ms, and I'm not really surprised that the USB audio is faster than the SDI capture, but it's not really helping my situation.

A workaround I can do is output via NDI, open a second instance of OBS, create a scene with 2x the NDI Source, use the audio off one, and Video Delay the other as appropriate. This is a rather ugly solution, and I would far rather fix the sync in the output of OBS Studio in the first place. Thanks in advance.
 
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michaelc

New Member
I've also just realised I can do negative sync offset in the audio, which I didn't think was a thing before? This should fix my ugly workaround above, but doesn't really explain why there is such a huge sync issue on the output to begin with. Monitoring in OBS is in sync at 0ms, but output is way out.
 
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Mn Senate

New Member
We have one room which we made the four cameras in output NDI. So we can pull in those to OBS as NDI sources.

We are using a black magic decklink 4k SDI card in the computer which has OBS running on it. We use the decklink card to send out to a HELO for webstreaming and then also a router so we can watch the output on a monitor.

Looking at the OBS program monitor and listening to it, everything looks in sync. But then looking at the output from the decklink card I often get audio drifting typically about a 300ms delay. Oddly, the audio arriving late. You would expect NDI video to be the delay. It is Dante audio. Pulled in as an audio input capture in OBS.

If we add a -300ms audio delay in, the audio and video look good on the decklink output, but then my local recording on OBS is off by that delay. Sometimes it needs more than -300ms. It appears to be random.

When not using hose NDI cameras, we just capture a zoom meeting in OBS using desktop audio and it syncs up perfectly on the OBS program monitor and the decklink output. Adding in these NDI sources somehow the Decklink card and OBS will not play well together.

Restarting OBS does seem to fix it temporarily when things get way out of sync.

It's not anything with skipped or dropped frames as those have been good looking at the OBS stats window. The computer has plenty of RAM, CPU processing power and GPU resource still available while OBS is running those NDI cameras.
 
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michaelc

New Member
It's baffling. Most of the time we found it to be nearer a second out, so I thought perhaps it was something designed wrong, like the blackmagic audio was being picked up pre-buffer, and it does vary from day to day. Very frustrating. I gave up using those outputs for audio.
 
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