I missed this when Netflix announced support for offline viewing last week, but in a post to its tech blog the company gave some details about what’s happening behind the scenes. As summarized by Janko Roettgers of Variety:
For streaming, Netflix has been using H.264/AVC almost exclusively. However, users who download Netflix shows to most Android devices instead receive content encoded with VP9… [on iOS] Netflix’s streams are encoded with H.264/AVC Main, whereas its downloads come in H.264/AVC High.
Netflix is, as you might expect, kind of maniacal when it comes to tweaking video codecs. With a streaming-video business of its scale, every small savings in bandwidth or encoding is gigantically magnified. It continues to tweak encoding settings on a scene-by-scene basis, and VP9 downloads are actually collections of smaller video files that are optimized to save bandwidth (and file size) when possible.