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Online experiment unable to run on PC computers, runs fine on Mac

URL of experiment: https://gitlab.pavlovia.org/rcanale/adult_goldilocks_v5_14_2020

Description of the problem:
This experiment was created in the PsychoPy builder on a Mac. When run the experiment online through either the mac it was created on or any other mac it works perfectly fine (it takes a while to download, but once it does it runs great). However, if I have someone run it on a PC, the experiment either only displays a black screen after the download has completed, or plays the trials but at a very lagged speed, sometimes skipping over video files completely. Any suggestions as to why this would be/any advice to fix it? Thanks!

Are you using the same web browser on the PC? That is, are you using Chrome in both cases? It’s very odd that there should be an OS difference for a web-based study, in principle that should be more an issue of the type of web browser than the operating system.

One of the PC’s was using chrome and one was using Internet Explorer. Both had issues displaying the .mp4 files

That’s very odd. The issue is almost certainly going to be with the video files themselves. They’re .mp4 files, but how were they encoded? Do they use h.264 compression or something else? I don’t know if there’s any particular version of an .mp4 file that works on Mac but not PC, but that’s my only guess here. I know h.264 should work on both.

I just checked the info, and it says that the codecs are H.264, but that the encoding software is Mac OS X. Do you think this could be the issue? I could try running them through an online converter.

Honestly I didn’t think that would matter, but it’s at least worth trying re-encoding them and seeing what happens.