mdat

mdat, short for Media Data Box, is a component of the ISO base media file format used by MP4 and QuickTime containers. It stores the actual media samples for the tracks described in the file’s metadata, such as encoded video frames and audio samples. The mdat box itself does not describe how the data should be decoded; that information is provided by the accompanying sample tables and track metadata elsewhere in the file.

In an MP4/QuickTime file, the media data in mdat is located relative to the file’s structure, which also includes boxes such as ftyp (file type) and moov (movie metadata). Within each track, a sample table (often within stbl) describes the size, offset, timing, and other properties of each sample. The offsets and sizes from these tables point to the actual byte ranges inside one or more mdat boxes. This separation allows the file to describe how to interpret samples without embedding control information inside the data itself.

Technical notes include that the mdat box has a 32-bit size field, but if the size is 1, a 64-bit largesize field follows to accommodate very large data payloads. A file may contain multiple mdat boxes or reference data in external locations via data reference mechanisms. While mdat holds the bulk of the media data, proper playback requires aligning its contents with the corresponding track metadata in moov/trak and the sample tables.

Understanding mdat is essential for tasks such as parsing, editing, or streaming MP4/QuickTime files, since it contains the actual media content referenced by the file’s structural metadata.