What is an AAC file?

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.

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.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

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.

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.

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.

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.

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.

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.

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.

Most AAC audio in the world lives inside a container — an M4A, an MP4, a WebM. A file ending in `.aac` is the unusual case: the encoded audio on its own, with no container at all, wrapped only in a thin framing called ADTS that repeats a short header before every frame.

That framing was designed for broadcasting, where a receiver has to be able to start listening partway through. It works, and it means the file carries none of the things a container would provide. Almost every complaint about a bare `.aac` traces back to that single fact.

There is nowhere to put a title, an artist, an album or a cover image, because those live in container metadata and there is no container. A music library will show the file name and nothing else, and no tag editor can fix it without changing the format.

Duration is guessed rather than read. A container records the length; ADTS does not, so a player estimates it from the file size and bitrate. On variable-bitrate audio the estimate is wrong, which is why a bare AAC often shows a duration that drifts as it plays, and why seeking lands in the wrong place.

Broadcast and streaming capture, which is the main source — internet radio recorders, transport-stream extraction, and tools that pull the audio track out of a live stream. Some hardware encoders write it directly. Some conversion tools produce it when asked for AAC without being told which container.

The common thread is that something extracted or generated a stream rather than assembling a file. Nobody chooses a bare AAC for a music library, which is why one turning up there is usually a sign that a step was skipped.

None of the above is a criticism of AAC as compression. It was standardised in 1997 with the shortcomings of MP3 understood, and it sounds better at every bitrate — 128 kbps AAC is generally held to match 192 kbps MP3. It handles up to 48 channels, higher sample rates, and encodes cleanly at bitrates where MP3 falls apart.

It is the default for YouTube, Apple Music, Spotify, television broadcasting and every phone recording. The format has won comprehensively on merit; MP3 merely won earlier on compatibility.

AAC-LC is the plain profile and what nearly everything means by AAC. HE-AAC adds spectral band replication, reconstructing high frequencies from the low ones rather than transmitting them, which produces usable audio at 32 kbps or less. HE-AAC v2 adds a similar trick for stereo and goes lower still.

That is why digital radio and low-bandwidth streams sound better than their bitrates suggest. It is also why they can sound oddly glassy on cymbals and sibilance: the high frequencies are reconstructed rather than recorded. Older decoders that only know AAC-LC will play an HE-AAC file as a dull, muffled version of itself rather than failing outright.

The same AAC frames fit inside an `.m4a` unaltered, so in principle this need not touch the audio at all — `ffmpeg -c copy` does it in a second and loses nothing. The conversion offered here is not that: it hands the encoder a bitrate, which is what disqualifies the packet copy, so the audio is decoded and encoded again. What either route gains is a container: a place for the title, artist, album and artwork, a recorded duration, and a proper index so seeking works.

For a bare AAC destined for a music library or a podcast feed, do it on the desktop if the file is precious and here if convenience matters more than one generation. Converting to MP3 instead costs that generation anyway and gives up AAC’s efficiency with it.

When the destination is old. Car stereos from before roughly 2012, cheap hardware players, some public-address systems, and upload forms whose accepted-extension list was written years ago. In those cases a slightly degraded file that plays beats a better one that does not.

Understand what it costs: AAC to MP3 is lossy to lossy, and the second encoder works from audio that has already had material removed, guided by a model that does not know what the first pass took. Use 192 kbps or higher so the second generation stays inaudible, and keep the original.

AAC is patent-encumbered. Decoding is broadly licensed and appears everywhere, but shipping an encoder has historically required a licence, which is why open-source software often could not produce AAC out of the box while playing it perfectly well.

That asymmetry is a large part of why Vorbis and later Opus were created, and why Opus — royalty-free and better again at low bitrates — took over voice messaging and browser calls while AAC kept music and video. Two good codecs, divided by licensing rather than by quality.

If it is going into a music library or a podcast app, repackage it as M4A. Nothing is lost, it takes seconds, and it gets back the tags and the seeking.

If it is going to a device that only reads MP3, convert and accept the generation. If it is going onto a phone, a computer or a website, leave it alone — everything modern plays it, and every conversion from here is a step down.