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

Audio Priming - Handling Encoder Delay in AACUsing Track Structures to Represent Encoder Delay ExplicitlyUsing Track Structures to Represent Encoder Delay ExplicitlyIn QuickTime movie files (.mov) and related MPEG-4 files, AAC encoded audio is carried in a sound track as aseries of media samples—each media sample corresponding to an AAC encoded audio packet. A track usesan edit list (see “Edit List Atoms” (page 70)) to indicate the range of time from the media samples to present.The edit list atom along with additional atoms known as Sample Group Structures, introduced in “SampleGroup Structures” (page 425), can now be used to explicitly represent encoder delay.Important A complete implementation using the sample group structures is required to explicitly representthe placement of the source signal in the encoded track. An incomplete implementation will result inunspecified interpretation by Apple software and tools. In the absence of the sample group structures, theclassic solution of expecting an implicit encoding delay of 2112 samples and the edit list to start at thebeginning of encoder delay will be assumed as described in the previous section.Edit List AtomSee “Edit Atoms” (page 69) and “Edit List Atoms” (page 70) for details of edit lists in track atom structures.A sound track of AAC encoded audio uses an edit list to indicate the placement of the source signal in the timerepresented by the encoded AAC packets. The media time field of the edit list must indicate the first sampleto be presented and will correspond in time to the first audio sample following the encoder delay in that track.The edit list track duration field should be set to the duration of the source waveform in media samples. Theedit list must not extend into the encoder delay or into any remainder samples of the encoded sound track.Note that for a single waveform encoded into a sound track, the sound track requires only a single edit listatom with one entry.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.424