AAC - Advanced Audio Coding

AAC, or Advanced Audio Coding, is the lossy audio compression that came after MP3 and was meant to replace it. It was developed in the 1990s by a group of companies including Fraunhofer IIS, Dolby, AT&T, Sony and Nokia, and published by the Moving Picture Experts Group as part of the MPEG-2 and then the MPEG-4 standards. You almost certainly listen to it every day without noticing: AAC is the sound in most video files, in digital broadcasting, in streaming apps and in nearly every song bought from an online store.

A codec, not a file format

This is the thing to understand about AAC, and it explains everything else. AAC is a codec, a method for compressing and decompressing sound. It is not really a way of storing it. Like MP3 it is lossy, splitting audio into short blocks, converting them into frequencies and dropping what a listener is unlikely to hear, but it does so with larger, more flexible blocks and better tools, so it generally sounds better than MP3 at the same bitrate. Several profiles exist: plain AAC-LC is the everyday one, while HE-AAC rebuilds the high frequencies separately and holds up at very low bitrates.

Because it is only a codec, AAC audio normally travels inside a container that knows how to organise streams and tags. That container is usually MP4, which is why most AAC music files end in .m4a rather than .aac. A file that genuinely ends in .aac is typically a bare Audio Data Transport Stream: nothing but AAC frames one after another, each with a tiny header giving the profile, sample rate and channel layout. It plays, and it can be cut and joined, but it has no structure beyond the audio.

What metadata an AAC file carries

The honest answer, for a raw stream, is almost none. There is no place in it for a title, an artist or a date. The frame headers describe how to decode the audio and nothing more, so a metadata report on a bare AAC file may show little beyond the profile, the sample rate, the channel count and the length. That is not a gap in the report. There genuinely is nothing else in the file.

Two things complicate that picture. First, some software glues an ID3v2 tag, the same kind MP3 uses, onto the front of an AAC stream. It is a convention rather than part of the standard, but it works, so you may see a title, artist, album, cover art and an encoder name after all. Occasionally an APE tag is used the same way. Second, and far more often, what people call an AAC file is really an MP4 file. Then the metadata belongs to the container, and there is plenty of it: iTunes-style entries for title, artist, album artist, track number, year, genre, composer, comment, artwork and encoding tool, plus creation and modification timestamps in the container header.

That distinction matters if you care about privacy. A bare AAC stream leaks essentially nothing about you. An MP4 wrapper can carry the name of the software that made the file, a comment somebody typed, and in the case of a purchased track the buying account's email address, the account holder's name and a purchase date. So before wondering what an AAC file reveals, find out what it actually is. A metadata report tells you whether you are looking at a raw stream or a container.

When to use AAC

Use AAC when you want lossy audio that sounds good at a modest size and still plays on ordinary phones, browsers, cars and televisions. Against MP3 it gives better quality for the same number of bits and supports more channels and sample rates. Against Opus it loses on efficiency at low bitrates, but wins on hardware support. And if you intend to tag the file at all, store the AAC audio in an MP4 container as an .m4a instead of keeping a raw stream, because that is the only place the tags can live.