M4a is a file extension used with mp4, also known as Moving Picture Experts Group-4 (MPEG-4), files. Mp4 files are known as “container” files. Since they can contain many types of content, including audio, video, subtitles and three-dimensional (3-D) objects, it is impossible to determine the type of content contained in an mp4 file based on the standard mp4 file extension. To address this issue, Apple® created the m4a file extension to indicate audio-only files.
There are no differences in format between files with the m4a extension and those with the mp4 extension. Mp4 is the only file extension officially recognized by the International Organization for Standardization (ISO). The m4a file extension, however, is generally used for non-digital-rights-management-protected, audio-only content, while the mp4 extension is usually used for mp4 files that contain both audio and video.
M4a files can be saved locally or can be used to stream audio over the Internet. The mp4 file type is based on the Apple® QuickTime format and is identical to the Quicktime Movie (MOV) file format. However, the mp4 format also has formal support for initial object descriptors (IOP) and other MPEG specifications. Mp4 file format is the basis for other file formats such as 3GP and Motion Joint Photographic Experts Group (JPEG) 2000. The original version of the mp4 file format was released in 2001, and a second version was released in 2003.
The mp4 file format is often confused as being directly related to mp4 players. An mp4 player is the common name for a portable media player that is marketed as a next-generation MPEG Audio Layer 3 (mp3) player. Not only are mp4 players not specifically designed to play mp4 or m4a files, many are actually incapable of playing such files.
Mp4 files use audio and video compression to reduce file size. The Apple® Lossless Encoder allows files to be compressed to mp4 container format with less data loss than other “lossy” formats such as advanced audio codec (AAC) and mp3. M4a files also tend to have higher audio quality than mp3 and other audio files of the same size. Mp4 files can be played on many popular portable audio players, including the iPod.