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

Audio Priming - Handling Encoder Delay in AACUsing Track Structures to Represent Encoder Delay ExplicitlyNote Track duration uses the movie timescale instead of the media timescale used by media time.If the media timescale and movie timescale differ, the track duration may not be sample accurate.Sample Group StructuresNote This implementation of sample group structures in QuickTime is designed to be compatiblewith the ISO Sample Group Structures definition and allow for future generalization. A full descriptionof ISO 14996-12 Sample Group Structures is not required in order to provide explicit encoder delayrepresentation. A short description of the more general characteristics of these structures is includedin the descriptions below, followed by the information required for use with QuickTime.Sample group structures of roll-group type with a constant roll distance are used to represent decoderdependencies for AAC encoded media. The sample group structures are intended to serve two purposes:●●To indicate the amount of decoder delay in AAC packetsTo signal to readers parsing QuickTime movies that the sound track includes explicit information forencoder delay and remainder samples for the AAC packets encoded in the fileNote The effect of using sample group structures in the track in this manner is that the editlist’s media time and track duration fields do not include encoder delay, as specified above in“Edit List Atom” (page 424).Two sample group structure atoms are used to represent the amount of encoder delay and remainder sampleswhich must be trimmed.Sample Group Description AtomSample group description atoms give information about the characteristics of sample groups. The samplegroup description atom has an atom type of ‘sgpd’.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.425