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Why does Handbrake by default create m4v when you want mp4? When I batch video files, the first one is mp4, while the rest are m4v. I do know that you can set the behavior in preferences but why make this the default behavior. M4v files do not work on TV's. I don't know who would want want the first one to be mp4, then the rest m4v?
MP4 == M4v. There is no difference in the file. It's literally just the text of the file extension that's different.
For the most part, there isn't compatibility issues so the default behaviour is to automatically select it when you've enabled features that certain apple devices and software require it to be m4v.
M4v extension files work fine on many TV's on the market. That said, many TV's that support MP4/M4v have poor playback support so won't play a file regardless of extension.
There is a preference to set it to always use .mp4; .m4v was required for some old Apple devices, I think we should set it to always use .mp4 as the default.