Base Media Information Header Atoms

QuickTime Movie File Format Specification, May 1996

The base media information header atom indicates that this media information atom pertains to a base media.

The base media information header atom contains the following data elements.

Size: A 32-bit integer that specifies the number of bytes in this base media information header atom.
Type: A 32-bit integer that identifies the atom type; this field must be set to ' gmhd' .