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M4A
What is an M4A file?
AAC audio in an MP4 container. What iTunes and voice memos produce.
What M4A is
M4A is a container: a wrapper that holds streams encoded by something else. It is used for handing a finished file over and phones.
The extension is .m4a, and the full name is MPEG-4 Audio. Both matter less than what the file can hold, which is what the rest of this page is about.
Where M4A came from
Apple published it in 2004.
Age is worth knowing here for one practical reason: the older a format is, the more programs have had time to learn it.
The specification is published, but by its owner
It is written down and implementable, though the vendor decides what the next version looks like. In practice that is enough for the file to keep opening; it is not enough to be sure of it.
M4A is a box, and the codec is what is in it
It usually holds AAC and ALAC. This is the distinction behind most "why will this not play" questions: two files with the same extension can contain entirely different encodings, and a player that refuses one may be perfectly happy with the other.
It also explains why changing container is often fast and lossless while changing codec is neither. Moving the same stream into a different box rewrites the box; re-encoding rewrites the picture.
What else it can carry
M4A can hold a layout that lets it start playing before it has finished arriving.
That matters mostly when converting: whatever the target cannot hold is dropped, usually without a warning.
How many audio channels
Up to 48. Two covers stereo and is what almost everything is; more than two is surround, and it is the part most likely to be folded down when converting to a format that does not define it.
What opens M4A
iTunes, VLC and FFmpeg read it, and so do most programs of the same kind.
If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.
Opening it in a browser
Every current browser reads it.
That makes it a safe thing to put on a page or attach to a message without wondering what the other end has installed.
What it can carry besides the content
M4A can hold document properties.
Whether any of it survives a conversion depends entirely on the target, and the honest answer is usually "some of it".
It is a delivery format
M4A is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.
The facts, in one place
| Extension | .m4a |
|---|---|
| Media type | audio/mp4 |
| Published by | Apple |
| First published | 2004 |