.M4A File Format

Introduction

Apple Lossless (ALAC) is Apple’s lossless audio codec, introduced in 2004 for iTunes and the iPod. Like FLAC it decodes to exactly the original samples, at roughly half the size of uncompressed audio. Apple published the encoder and decoder as open source in 2011, and the format is now royalty-free.

ALAC has no file format of its own. It is almost always stored in an MPEG-4 file with the extension .m4a, the same extension used for AAC, so the extension alone does not say whether a file is lossless. It can also be stored in a CAF file.

Structure

An ALAC .m4a file is an MPEG-4 file made of boxes, each a 32-bit size and a four-character type. It starts with an ftyp box, usually with the brand M4A . The moov box describes the track: inside its sample description (stsd) is an alac entry carrying the codec’s settings, such as frame length, bit depth, channel count and sample rate. The sample tables give the size and position of every frame. The encoded frames themselves sit in an mdat box, and tags are stored as iTunes metadata in moov.

Also used for

• AAC audio in MPEG-4 – MP3’s successor: ADTS frames in .aac, or MPEG-4 boxes in .m4a.

Links