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Yes, when I compare with the local file I can't see any difference. Haven't tried with 4K video yet but it should be the same as the source.

Of course it isn't by accident that youtube re encodes video to a lower bitrate. I'm curious if they can keep it up when more en more video's get uploaded en played. Has the IPFS network enough capacity?

Technically Source is whatever the person uploads. So if the person uploaded a 720p 30fps file, then that's what the source is on that video. On the other hand, somebody could upload an 8K 120FPS file.