Media Header Atoms

QuickTime Movie File Format Specification, May 1996

The media header atom specifies the characteristics of a media, including time scale and duration. The media atom has an atom type of 'mdhd' .

Figure 0-19 shows the layout of a media header atom.

Figure 19 The layout of a media header atom

Media header atoms contain the following data elements.

Size: A 32-bit integer that specifies the number of bytes in this media header atom.
Type: A 32-bit integer that identifies the atom type; this field must be set to 'mdhd' .
Version: One byte that specifies the version of this movie.
Flags: Three bytes of space for media header flags. Set this field to 0.
Creation time: A 32-bit integer that specifies (in seconds since midnight, January 1, 1904) when the media atom was created.
Modification time: A 32-bit integer that specifies (in seconds since midnight, January 1, 1904) when the media atom was changed.
Time scale: A time value that indicates the time scale for this media--that is, the number of time units that pass per second in its time coordinate system.
Duration: The duration of this media in units of its time scale.
Language: A 16-bit integer that specifies the language code for this media. See "Basic Data Types" for valid language codes.
Quality: A 16-bit integer that specifies the media's playback quality--that is, its suitability for playback in a given environment. See Inside Macintosh: QuickTime for details on playback quality.