Quicktime File Format (2012-08-14).pdf

Movie AtomsTrack AtomsReference type'fall''folw''forc''hint''scpt''ssrc''sync''tmcd'DescriptionIn a sound track, this references a track in a different format but with identical content,if one exists; for example, an AC3 track might reference an AAC track with identicalcontent. See “Alternate Sound Tracks” (page 335).In a sound track, this references a subtitle track that is to be used as the sound track’sdefault subtitle track. If the subtitle track is part of a subtitle track pair, this shouldreference the the forced subtitle track of the pair. This is needed only if languagetagging cannot be used. See “Relationships Across Alternate Groups” (page 336).Forced subtitle track. In the regular track of a subtitle track pair, this references theforced track. See “Subtitle Sample Data” (page 202) for more information.The referenced tracks contain the original media for this hint track.Transcript. Usually references a text track.Non-primary source. Indicates that the referenced track should send its data to thistrack, rather than presenting it. The referencing track will use the data to modifyhow it presents its data. See “Track Input Map Atoms” (page 76) for more information.Synchronization. Usually between a video and sound track. Indicates that the twotracks are synchronized. The reference can be from either track to the other, or theremay be two references.Time code. Usually references a time code track.Each track reference type atom contains the following data elements.SizeA 32-bit integer that specifies the number of bytes in this track reference type atom.TypeA 32-bit integer that identifies the atom type; this field must be set to one of the values shown in Table2-2 (page 74).Track IDsA list of track ID values (32-bit integers) specifying the related tracks. Note that this is one case wheretrack ID values can be set to 0. Unused entries in the atom may have a track ID value of 0. Setting thetrack ID to 0 may be more convenient than deleting the reference.You can determine the number of track references stored in a track reference type atom by subtracting itsheader size from its overall size and then dividing by the size, in bytes, of a track ID.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.75