slow motion video playback "ramp-up" stage

In iOS, when I use AVPlayerViewController to play back a slow motion video, it has a "ramp-up" stage at the start and a "ramp-down" stage at the end, and the video plays at the normal speed (i.e. not slow motion) during these stages.

My question is: are these non-slow-motion stages defined in the video file itself? (e.g. some kind of meta data?) Or, is it just a playback approach used by AVPlayerViewController ?

Thanks!

slow motion video playback "ramp-up" stage
 
 
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