My team has noticed differences in our time stamps (Begin Time and End Time) across different computers. One of us generates an .eaf file with selections based on when a bird’s foot makes contact with a branch (Begin Time). The other members of the team then open these selection files on their own computers, with the same video footage. The selections, however, are off, by >35 milliseconds (which is meaningful at our scale). The selections no longer align with the video footage. We see the same moment in our videos, but the time stamps differ considerably.
Why is this happening, and is there a solution? Is this potentially a Mac vs. PC issue?
First, to confirm my assumption: when annotator A creates an annotation starting at e.g. 8.274 ms, saves the file and transfers the file to annotator B and annotator B opens the file and hovers the mouse over that annotation, the start time in the tooltip is still 8.274? And when clicking the annotation the selection will show the correct times of the annotation, only annotator B sees a different video frame than A at that point?
If this is indeed the case, the problem might be in the encoding of the video. Some video formats, e.g. mp4, allow to encode so called B-Frames and this can result in a media player showing a different video frame for a specific media time, depending on from where a jump to that time was initiated and/or on the media framework (and maybe indeed on the operating system).
You can check if this is indeed the cause of the problem in your case, you could re-encode one of the videos with B-Frames set to 0 and/or create an I-Frame only variant, and see if the problem persists.
Please see this media encoding guide for more details.
If my assumption is completely wrong, we will have to investigate further.
Thanks very much for your response! Yes, your assumption is correct - that is indeed what happens. I’ve been able to determine that this is at least partially a Mac vs. PC issue, as Annotator A and Annotator B see the same video frames at a given time stamp if they are both using a PC, but see different frames if they are using different operating systems.
I haven’t been able to find the B-Frame values for my .mp4 files, but an encoding issue would make sense. Once I find them, I’ll try re-encoding and hopefully the problem resolves! But just knowing that the operating system makes a difference is hugely helpful.
