mvhd
= Movie Header Box
解説（QuickTime File Format Specification）：
https://developer.apple.com/library/archive/documentation/QuickTime/QTFF/QTFFChap2/qtff2.html#//apple_ref/doc/uid/TP40000939-CH204-BBCGFGJG
https://developer.apple.com/library/archive/documentation/QuickTime/QTFF/art/qt_l_095.gif
なるほど！
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Movie Boxに含まれ、movieのタイムスケールや動画時間を定義する。
The movie atom usually contains a movie header atom, which defines the time scale and duration information for the entire movie, as well as its display characteristics.