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AAC
What is an AAC file?
The MP3 successor. Better quality per megabyte, standard on Apple devices and in streaming.
What AAC is
AAC is a codec: the encoding itself rather than the box it travels in. It is used for handing a finished file over, streaming and phones.
The extension is .aac, and the full name is Advanced Audio Coding. Both matter less than what the file can hold, which is what the rest of this page is about.
Where AAC came from
MPEG published it in 1997. The specification is ISO/IEC 13818-7.
A format that has been readable for that long is a format worth trusting with something you want back in ten years.
The specification is public
Anyone can implement it without asking permission or paying a licence, which is why it turns up in so many programs and why files written twenty years ago still open.
Detail is traded for size
AAC discards detail to stay small, and what it discards does not come back. That is the right trade for something being looked at and the wrong one for something being edited: each re-save takes another bite.
Colour it can describe
Up to 16 bits per channel.
More than eight bits per channel is what stops a gradient from stepping when it is edited, which is why it matters in a working file and rarely in a finished one.
What else it can carry
AAC 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 AAC
VLC, FFmpeg and iTunes 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.
It is a delivery format
AAC 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.
What goes wrong with it
The recurring complaints: it degrades a little every time it is re-saved.
None of these is a reason to avoid the format. They are the things worth knowing before you are surprised by one, which is a different claim and a more useful one.
The facts, in one place
| Extension | .aac |
|---|---|
| Media type | audio/aac |
| Published by | MPEG |
| First published | 1997 |
| Specification | ISO/IEC 13818-7 |