OBS Audio recordings sync keeps floating compared to recorded audio by external recorder


New Member
HI guys, i know the title might be not the best ti understand, so i'll make it quick.
I'm running a live music streaming show. Sometimes we can't go live so we have to record the session and then stream it live later.

I'm the show sound engineer and as i'm using a digital mixer to mix the concert i also get a back up recording of all tracks in my DAW.
Even though i usually keep the live mix recorded by OBS a couple of weeks a go i wanted to remix a show before it goes on AIR.

What i've found is : If i sync my audio perfectly to the beginning of the audio recorded in OBS, the two things will always be slightly different.
Sometimes OBS audio will be 10 or 20 ms late , and sometimes it'll be 10 / 20 ms earlier.

Here's a few screenshots
Audio sync at beginning (upper track is my mixer LR output i usually send to OBS, lower track is OBS mp4 video/audio)
Screen Shot 2020-11-08 at 11.08.03.png

Audio sync after one song (audio from video is earlier than recorded audio, which is technically impossible unless you have a processing buffer which floats)
Screen Shot 2020-11-08 at 11.08.58.png

Audio sync at the end of the show
Screen Shot 2020-11-08 at 11.09.27.png

The OBS recorded video looks perfectly sync'd to this fluctuations.

The mixer i'm using is an SQ5 by Allen & Heath and I can really assure it keeps the sync right with the recorded audio. I'm 100% sure the issue is not there.
And either the BM ATEM television switcher we're using doesn't have anything that makes me think that it's involved in that weird audio fluttering.

What IMO is giving me this asynchronicity is the buffer , which is changing during the show, and unfortunately I still haven't tried to record the show with a fixed buffersize .

Have anyone of you ever experienced something like that?
to me 10 to 20 ms of offsync are clearly visible, and they bother me...maybe i'm just too peaky and i should just use a video recorder after my switcher if i plan to re mix the audio.

I'm recording the show at 4500 kbps, how much would you think my buffer size should be? 4500 ms?

thanks in advance to those who will try to help me!