MPEG-4 audio codec

Overview

Containing profile atom: Track (sound), movie
part-ID: 0x20202020 (universal feature)
feature-code: 'mp4a'
feature-value: Unsigned int(32) where least significant 5 bits hold the AudioObjectType as found in the AudioSpecificInfo (as defined in specification ISO/IEC 14496-3, subclause 1.6) found in the esds of the MPEG-4 audio codec (QuickTime type 'mp4a') sample description

Feature Values

The least significant 5 bits hold the value. The most significant 27 bits of the feature value should be set to 0.

The list of audio object type constants is defined in specification ISO/IEC 14496-3, subclause 1.5.1.1.

Examples: AAC LC is indicated by the value 2, CELP is indicated by the value 8.

Writer Responsibilities

A writer of the MPEG-4 Audio Codec feature should record the 5 bits corresponding to the AudioObjectType found in the ES_descriptor’s audio DecoderSpecificConfig. The most significant 27 bits of the value should be set to 0.

Feature Value Algorithm

The MPEG-4 audio codec value is the 5 bits of the AudioObjectType found in the AudioSpecificInfo (a DecoderSpecificInfo). See specification ISO/IEC 14496, subclause 1.6. The AudioSpecificInfo is found in the MPEG-4 Elementary Stream Descriptor Atom within the siDecompressionParam atom of the audio sample description for the QuickTime audio codec of type 'mp4a'.

Reader Responsibilities

A reader of this feature code should compare the recorded value with the set of MPEG-4 audio decoders supported by the reader.

Comments

Because the QuickTime 'mp4a' codec may implement multiple audio decoders defined in specification ISO/IEC 14496 in the future, this feature allows the reader to determine the specific audio decoder needed to interpret the audio bit stream. The MPEG-4 Audio Codec feature should be present only if the 'mp4a' audio codec is used in a sound track.