Sync Sample Atoms

QuickTime Movie File Format Specification, May 1996

Sync Sample Atoms

The sync sample atom identifies the key frames in the media. In a media that contains compressed data, key frames define starting points for portions of a temporally compressed sequence. The key frame is self-contained--that is, it is independent of preceding frames. Subsequent frames may depend on the key frame.

Sync sample atoms have an atom type of 'stss' . The sync sample atom contains a table of sample numbers. Each entry in the table identifies a sample that is a key frame for the media. Figure 0-34 shows the layout of a sync sample atom.

If no sync sample atom exists, then all the samples are key frames.

Figure 34 The layout of a sync sample atom

The sync sample atom contains the following data elements.

Size: A 32-bit integer that specifies the number of bytes in this sync sample atom.
Type: A 32-bit integer that identifies the atom type; this field must be set to 'stss' .
Version: A 1-byte specification of the version of this sync sample atom.
Flags: A 3-byte space for sync sample flags. Set this field to 0.
Number of entries: A 32-bit integer containing the number of entries in the sync sample table.
Sync sample table: A table of sample numbers; each sample number corresponds to a key frame. Figure 0-35 shows the layout of a sync sample table.

Figure 35 The layout of a sync sample table