Encoding
Both file types have slightly different audio quality. M4A is encoded with Advanced Audio Coding, or AAC, or Apple Lossless Audio Codec. The M4B format is only encoded with AAC encoding.
Uses
Both formats are used for distributing different types of files. The M4A format is used for storing and encoding audio and is also used for distributing music through the iTunes store. The M4B format is used mainly for audiobooks from the iTunes store. M4B files also allow users to bookmark the position in the file, making it useful for audiobooks, which tend to be longer than music albums.
Copy-Protection
The formats use different types of copy-protection. Copy-protected M4A files from the iTunes store have a .M4P extension and may be only be played with iTunes and QuickTime. The copy-protected M4B audiobooks downloaded from the iTunes store use Apple's FairPlay DRM, which only allows authorized devices or computers to play these files.
Compatibility
Both formats are compatible with computer programs such as Winamp, Windows Media Player, and VLC player. Files in the M4A format are compatible with a small amount of portable devices. M4B, on the other hand, is not as widely supported by other devices.
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