Convert FLAC to AAC

Most routes into AAC on this site start from something already compressed, and the honest description of them is a second round of damage. This one does not. FLAC stores the exact samples that went into it, so the AAC encoder is handed the recording as it was recorded, with no earlier encoder's artefacts to spend bits describing.

That difference is larger than it sounds. A lossy encoder allocates its budget by deciding what a listener will not miss; when the input already carries the scars of a previous decision, the second encoder wastes part of its budget faithfully reproducing those scars. Starting from FLAC removes that waste entirely. It is the reason a delivery encode should always be made from the master and never from a copy somebody sent you.

A file with a `.aac` extension is not a container. It is Audio Data Transport Stream framing: a run of AAC frames, each carrying a short header that repeats the sample rate, the channel configuration and the profile. There is no index, no global header and no metadata area anywhere in it.

That shape is what certain systems want. Playout automation, segmenters that cut a stream into chunks, announcement and IVR players, and embedded hardware with a fixed parser all read elementary streams because a stream can be started at any frame and joined to another without being rebuilt. If a specification you were sent names ADTS, or names `.aac` rather than "AAC audio", it means this and not an M4A.

The Quality control resolves to a real bitrate inside the encoder, and AAC is offered three: Small file writes 96 kbps, Balanced writes 128, High quality writes 192. Four minutes of stereo lands near 2.9, 3.8 and 5.8 MB respectively. Nothing above 192 is available on this path.

Delivery specifications usually state a ceiling rather than a target, because the number came from a transport budget somebody set years ago. Pick the band at or just under the figure named, then check the result rather than assuming — a specification asking for "128 kbps AAC" is satisfied by Balanced, one asking for "up to 192" by High quality, and one asking for 256 or 320 cannot be met here at all.

FLAC carries a proper metadata system: named text fields for title, artist, album, catalogue number and anything else somebody invented, plus a dedicated picture block for artwork. ADTS has none of that. The AAC written here contains audio frames and nothing else, so every tag and every image is dropped at the boundary.

For a stream going into automation this is correct and expected — the playout system holds its own database and would ignore embedded tags anyway. For anything that will be filed, catalogued or handed to a person it is a real loss, and the answer is to convert to M4A instead, which holds the same AAC audio inside an MP4 container that has fields for all of it.

It is worth being blunt about this before you convert a hundred masters. If the file is going to a person, a library, a podcast host, a phone or anything with a play button and a title on screen, the bare AAC is the wrong deliverable. It has no duration in it, so some players report the wrong length or refuse to seek, and it has nowhere to put a name.

The M4A route produces exactly the same encoded audio with a container around it, and almost every system that says "AAC" in casual language means that. Reserve the bare stream for the cases where a document names ADTS or an elementary stream, or where the receiving system rejected an M4A and told you why. Delivering the wrong one of these two is the most common way this conversion goes wrong.

The registry records FLAC as topping out at eight channels — mono through 7.1 — and AAC as handling many more. Nothing in this direction has to be folded down, so a surround master arrives with its layout intact and a stereo master arrives as stereo. Mono stays mono, which halves the bitrate requirement for speech material and is worth keeping rather than doubling into a fake stereo pair.

Sample rate is carried across unchanged rather than normalised. A 44,100 Hz master produces a 44,100 Hz stream; a 48,000 Hz one produces 48,000. High-resolution masters are the case to watch: 96 kHz material is legal AAC but not every decoder on the receiving end will take it, and downsampling to 48 kHz in an editor before converting is usually what the specification quietly assumed.

A FLAC stream header holds an MD5 of its decoded audio, computed when the file was made. That single field is what makes FLAC an archive format rather than a smaller copy: years later a decoder can reconstruct the samples, hash them and prove the file has not rotted. AAC has no equivalent, and a corrupted AAC stream is still a structurally valid AAC stream.

The practical consequence is a filing rule rather than an audio one. Keep the FLAC as the thing you archive and treat every AAC made from it as disposable output, regenerable in a minute from the master. Deleting a master because a delivery copy exists is the mistake this conversion makes easy, and it is not recoverable — the AAC cannot be turned back into what it came from.

Chrome, Edge and Safari expose an AAC encoder through WebCodecs, and where one is present it is used directly with no download at all. Firefox does not ship one, so the first conversion to AAC in that browser fetches a WebAssembly encoder — once, and only on the pages that need it. A visitor converting a spreadsheet on this site downloads none of it.

Either way the encoding runs on your processor inside the tab. That is the property worth having for unreleased or embargoed material: you can open the network tab, run a conversion and watch the encoder come down while the audio goes nowhere. There is no queue, no account and no copy of your master sitting on somebody's disk waiting to be deleted.

Files are capped at 100 MB each and a hundred can be dropped together, coming back as a ZIP. FLAC compresses to roughly half of uncompressed PCM, so 100 MB of CD-quality stereo FLAC is around twenty minutes of audio — comfortable for tracks, stems, announcements and episodes, and not enough for a concert recording or a full interview.

Where a master exceeds the cap, split it in an editor before converting or use a desktop encoder for that one file. Saying so in advance is more useful than letting a 600 MB drop fail after it has been read into a tab. The limit is the browser's memory rather than a plan tier, which is why there is no version of this page where paying makes it larger.