File extension
.m4a
MPEG-4 audio file (M4A)
.m4a files are MPEG-4 audio files: audio-only MP4 containers usually holding AAC or Apple Lossless (ALAC) audio; related extensions include .m4b for audiobooks and .m4p for protected iTunes purchases.
Known meanings
Formats and reference material for .m4a
Details
Format notes
M4A
M4A is the conventional extension for MP4 files that contain only audio. It
was popularized by Apple’s iTunes, and the container is the same as for .mp4.
The audio is usually AAC (lossy) or Apple Lossless (ALAC). Variants include
.m4b for audiobooks and podcasts, which can carry chapters and bookmarks, and
.m4p for protected tracks sold by the early iTunes Store.
Container Structure
The container is based on the ISO base media file format, derived from the
QuickTime file format. Data is organized into boxes (atoms), each with a size and
a four-character type. A file starts with an ftyp box whose major brand is
commonly M4A (with a trailing space). The moov box holds track and sample
tables, and mdat holds the encoded audio. Tags such as title, artist, and cover
art are normally stored in the moov/udta/meta/ilst structure. When moov
comes before mdat, playback can start while the file is still downloading.
Adoption
M4A is supported by essentially all current operating systems, phones, and media
players. The media type is audio/mp4. Because the extension only tells you
that a file is audio, the actual codec (AAC, ALAC, or another) must be read
from the file.
Preservation And Security Notes
AAC in M4A is lossy; ALAC is lossless and suits archival copies. Record the codec, sample rate, and tag content. Parsers must validate box sizes and nesting depth, since malformed boxes are a common source of decoder bugs.
Further Reading
- QuickTime File Format documentation (Apple):
https://developer.apple.com/documentation/quicktime-file-format - MPEG-4 File Format (Library of Congress):
https://www.loc.gov/preservation/digital/formats/fdd/fdd000155.shtml