Decklink SDI and OBS Video/Audio sync issues

BAnderson

New Member
Hello,

I'm doing something a little different with OBS where I'm using it more a "presenter" video software than for streaming.

I'm working in a screening room situation where I will do live two way Zoom or Teams calls from time to time. In the past, this has worked fine with Zoom. A general setup in Zoom is to send video to the screen via HDMI on the projector. It comes out of the computer into a HD Matrix which separates audio and video into two feeds. Audio from the switcher goes to a cinema processor which plays through the theatre's stage speakers and the video feed goes direct to the projector's input.

So that you can hear the audience in the room on Zoom, I have a handheld mic that goes into the computer as an input. I setup a small logitec camera in the theatre so the zoom participants can see and hear the audience. I've done it twice and works better than you would think.

I want to use OBS so I can control other scenes to the screen with audio, which I can then feed back into Zoom as a virtual camera. I can use my Decklink 8K card to output SDI to the projector's SDI input while outputting Audio via headphone jack or any other audio connection to the cinema processor. I'd figured the SDI would be a better video connection. This works fine.

But, although I get perfect video and perfect audio, they are out of sync by 200ms due to the lag in the video card in the projector and the Decklink. The sound is ahead of picture by around 7 frames. This is expected. Normally, we set sync in the video software like when we use Resolve or Baselight for playback. But there is always some way to set sync on the software side.

I don't see a setting in OBS where I can offset the monitor output to match the video on SDI. When I go to the Advanced Audio Properties and offset the SYNC for that audio output, nothing happens. From what I read, this sync function is only for the streaming output and not the monitor. Is that correct?

Do I have the audio monitoring set up wrong?
Is there a way to change this sync either through a setting in OBS or a plugin applied to that scene?

Unfortunately, unless I go into the Cinema Audio Processor as AES I can't offset those standard inputs to the processor and will need an external way of controlling it.

Also is there a way to embed the monitor audio within the SDI link feed where I can use a BMD Deembedder to AES (which is what we normally use to send the PCM to the Cinema Processor).
 

AaronD

Active Member
The Sync Delay is at the input to the encoder. The Monitor taps off before then. And there's no Delay filter. Yes, that's annoying!

Generally, OBS's audio is a mess. The devs are working on that, but it's going to be a long time coming. Meanwhile, I often recommend that people just don't.

Use a DAW for audio instead, and do EVERYTHING in there. OBS is silent, except for a "straight dumb wire" passthrough from the DAW to the stream and/or recording. Monitor Off. If you have some audio sources that originate in OBS, like a network feed or video soundtrack, send them to the Monitor Only (another "straight dumb wire"), and send the Monitor to the DAW. Everything else, input or output, connects directly to the DAW, with OBS not involved at all.

DAW = Digital Audio Workstation. Essentially a complete sound studio in one app. That's all it does, and it does it WELL! In this case, you're using the mixing console part of it, along with any plugins you might want or need. (EQ, compression, delay, etc.)

The DAW operates entirely independently from OBS, of course, but if you need them to be connected, you can use the Advanced Scene Switcher plugin in OBS to send control messages to the DAW.
My favorite DAW is Ardour (there are LOTS more to choose from!), and it accepts Open Sound Control (OSC) messages. Read the documentation for yours to see what it wants to receive, and see if Adv. SS can send it.
 
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