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

Media Data Atom TypesSound MediasiDecompressionParam Atom ('wave')The siDecompressionParam atom provides the ability to store data specific to a given audio decompressorin the SoundDescription record. As example, some audio decompression algorithms, such as Microsoft’sADPCM, require a set of out-of-band values to configure the decompressor. These are stored in an atom ofthis type.This atom contains other atoms with audio decompressor settings and is a required extension to the soundsample description for MPEG-4 audio. A 'wave' chunk for 'mp4a' typically contains (in order) at least a 'frma'atom, an 'mp4a' atom, an 'esds' atom, and a “Terminator Atom (0x00000000)” atom.The contents of other siDecompressionParam atoms are dependent on the audio decompressor.SizeAn unsigned 32-bit integer holding the size of the decompression parameters atom.TypeAn unsigned 32-bit field containing the four-character code 'wave'.Extension atomsAtoms containing the necessary out-of-band decompression parameters for the sound decompressor.For MPEG-4 audio ('mp4a'), this includes elementary stream descriptor ('esds'), format ('frma'), andterminator atoms.Format Atom ('frma')This atom shows the data format of the stored sound media.SizeAn unsigned 32-bit integer holding the size of the format atom.TypeAn unsigned 32-bit field containing the four-character code 'frma'.Data formatThe value of this field is copied from the data-format field of the sound sample description.Terminator Atom (0x00000000)This atom is present to indicate the end of the sound description. It contains no data, and has a type field ofzero (0x00000000) instead of a four-character code.SizeAn unsigned 32-bit integer holding the size of the decompression parameters atom (always set to 8).2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.185