---
title: MPEG-4 audio codec
framework: quicktime-file-format
role: article
role_heading: Article
path: quicktime-file-format/mpeg-4_audio_codec
---

# MPEG-4 audio codec

## Overview

Overview 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. note: A writer that records the MPEG-4 Audio Codec feature is encouraged also to write the QuickTime Audio Codec Type feature. 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.

## See Also

### Setting features

- [Table of features](quicktime-file-format/table_of_features.md)
- [Maximum video bit rate](quicktime-file-format/maximum_video_bit_rate.md)
- [Average video bit rate](quicktime-file-format/average_video_bit_rate.md)
- [Maximum audio bit rate](quicktime-file-format/maximum_audio_bit_rate.md)
- [Average audio bit rate](quicktime-file-format/average_audio_bit_rate.md)
- [QuickTime video codec type](quicktime-file-format/quicktime_video_codec_type.md)
- [QuickTime audio codec type](quicktime-file-format/quicktime_audio_codec_type.md)
- [MPEG-4 video profile](quicktime-file-format/mpeg-4_video_profile.md)
- [MPEG-4 video codec](quicktime-file-format/mpeg-4_video_codec.md)
- [MPEG-4 video object type](quicktime-file-format/mpeg-4_video_object_type.md)
- [Maximum video size in a movie](quicktime-file-format/maximum_video_size_in_a_movie.md)
- [Maximum video size in a track](quicktime-file-format/maximum_video_size_in_a_track.md)
- [Maximum video frame rate in a single track](quicktime-file-format/maximum_video_frame_rate_in_a_single_track.md)
- [Average video frame rate in a single track](quicktime-file-format/average_video_frame_rate_in_a_single_track.md)
- [Video variable frame rate indication](quicktime-file-format/video_variable_frame_rate_indication.md)