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

Audio Priming - Handling Encoder Delay in AACSummary—Using Track Structures to Represent Encoder DelayGrouping type: ‘roll’Entry count: 1with this table data:Sample count: 237 (number of AAC packets representing the encoder delay, original audio samples, andremainder audio samples)Group description index: 1 (corresponds to first and only entry in the sample group description atom’spayload data table for these sample units)Other things to note from this example:●●You cannot use the edit list by itself to determine the encoder delay or remainder sample count. Thesample group atoms provide the encoder delay. The placement of the end of the edit in a compressedaudio packet allows calculation of the remainder samples.If the encoder delay was the theoretical minimum for AAC of 1024, then the media time field value in theedit list table data shown in this example would be 1024, not 2112.Summary—Using Track Structures to Represent Encoder DelayWhen using sample group structures in representing encoder delay for AAC sound tracks:●●Include a version 1 sample group description atom with grouping type set to ‘roll’. Set default lengthto 2 (bytes) for audio entries. Follow that with the payload data: the typical value is -1, meaning onepreceding AAC packet, which is the theoretical minimum decoder delay of 1024 samples.Include a version 0 sample-to-group atom with a 'roll' grouping type. By including this, you associatethe AAC packets with the corresponding sample group description atom. All AAC packets including theencoder delay must be associated with the sample group in the table data’s sample count field. Typically,the sample count for this sample-to-group atom’s table data corresponds with the number of mediasamples in the track.These two sample group structure atoms in addition to the edit list atom, properly composed, form a completeimplementation to explicitly represent the temporal position of the source audio samples in an AAC encodedtrack.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.432