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

Audio Priming - Handling Encoder Delay in AACUsing Track Structures to Represent Encoder Delay ExplicitlyIn a general caseThere may be multiple instances of sample-to-group atoms if there is more than one sample groupingfor the samples in a track. Each instance of the sample-to group atom has a grouping type code thatdistinguishes different sample groupings. Within a track there can be at most one instance of thisatom with a particular grouping type. An associated sample group description atom indicates thesame value for the grouping type.The sample-to-group atom contains a table with a sample count and group description index pairs.The sample count is the number of media samples in the run of samples with the same sample groupdescription. The group description index is an index into the array of payload data entries in theassociated sample group description atom's payload data table, the association defined by havingthe same grouping type value.For use in AAC encoder delay representation, there is one sample-to-group atom instance in a given QuickTimesound track with grouping type ‘roll’ matching the single instance of the sample group description atom.The entry count field value is set to 1, indicating one entry in the table data array. That entry is describing allthe AAC packets in the track. The sample count in the table data array is typically the same as the sample sizeatom’s number of entries field, see “Sample Size Atoms” (page 111), which represents the number of mediasamples in the track (in this use, AAC packets). For AAC encoder delay representation, the only entry in theassociated sample group description atom’s payload data table is the first, which provides the value of 1 forthe group description index.Figure G-3 (page 428) shows the layout of this atom.Figure G-3The layout of a sample-to-group atomBytesSample-to-group atomAtom sizeType = 'sbgp'VersionFlags4413Grouping type 4Entry countTable data4variableThe sample-to-group atom contains the following data elements:2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.428