MP4A

MP4A is a FourCC code used in the MP4 family of container formats to designate MPEG-4 Audio data, typically AAC. It is not a standalone codec name or a file extension, but an internal sample-entry type used in the ISO base media file format to identify audio tracks within an MP4 file.

In an MP4 file, the audio track’s sample entry in the stsd box uses the four-character code mp4a as its type. The specific decoder parameters, such as the sampling rate and channel count, are carried in the DecoderConfigDescriptor and the AudioSpecificConfig within the box, which define the AAC profile and other properties.

MP4A-adopted AAC is widely used in consumer devices and streaming. The original standard is part of ISO/IEC 14496 (MPEG-4) and ISO/IEC 14496-12 (ISO base media file format). The mp4a identifier helps software select the correct decoder and parse timing, sample rate, and channel information.

Common file extensions for MP4A-contained audio include .m4a or .mp4 when audio-only, while the mp4a FourCC is an internal tag. The presence of mp4a is orthogonal to the filename extension.

Compatibility: Most media players and editors recognize mp4a as AAC in MP4 containers, but some tools may require exact ESDS details for proper playback, especially with advanced AAC profiles.