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

Audio Priming - Handling Encoder Delay in AACUsing Track Structures to Represent Encoder Delay ExplicitlyIn a general caseEach instance of a sample group description atom has a type code that distinguishes different samplegroupings. There can be multiple instances of this atom if there is more than one sample groupingfor the samples in a track. At most one instance of a sample group description with a particulargrouping type exists in a track. An associated sample-to-group atom has the same value associatedwith that grouping type. The information or “payload data” is stored in the sample group descriptionatom, after the entry count, as an array of entries for which the meanings vary according to thecharacteristics of grouping type.For use in AAC encoder delay representation, there is one instance of a sample group description atom in agiven QuickTime sound track with grouping type ‘roll’. The specifics for audio data (AudioRollRecovery())are used and articulate the rolling decode dependency. Because the sample group description atom for thispurpose is describing the entirety of the AAC audio stream, the payload data field resolves to a single signed16-bit integer representing the roll distance, which is set to -1. In other words, one AAC packet (1024 encodedPCM audio samples) preceding the media sample is indicated as being of the same type as the encoded sourcedata, allowing the decode transform to operate over the required two AAC packets for the first media samplespecified in the edit list.Note The payload data value (roll distance in this use) of -1 is a typical value for existing AAC codecs,but the payload data can have other values. Codecs could use alternative values depending upontheir implementation details.Figure G-2 (page 426) shows the layout of this atom.Figure G-2The layout of a sample group description atomBytesSample Group Description atomAtom sizeType = 'sgpd'VersionFlags4413Grouping Type 4Default lengthEntry countPayload data44variable2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.426