Convert M4A to AIFF

M4A names a container, not a codec. The registry records two things it carries: AAC, which is what a purchase, a stream capture, a phone recording and almost every bounce contains, and Apple Lossless, which is what iTunes writes when someone rips a CD with the lossless setting chosen. The extension is identical and nothing on the outside of the file distinguishes them.

This converter decodes AAC. There is no Apple Lossless decoder on this path, so an ALAC file stops with a message saying nothing in it could be converted rather than producing a silent or broken AIFF. Size settles it before you waste a batch: four minutes of AAC is three to five megabytes, four minutes of ALAC is twenty to thirty. If a file is suspiciously large for its length, that is what it is.

It is worth being blunt about the shape of this. An AIFF is worth making from a lossless source, where the uncompressed file genuinely contains everything the original did. That describes the ALAC case exactly — and that is the case this tool cannot open. From an AAC source, the AIFF is a large file holding lossy audio, which is a container requirement being satisfied rather than a quality decision.

That does not make the conversion wrong. Ingest systems, samplers and delivery standards ask for PCM for their own reasons and do not care where it came from. But if you are choosing between going back to the client for a better master and converting the AAC you already have, the first option is worth one email. If the M4A is all that exists, convert it and stop worrying — nothing you do downstream will recover what the AAC removed.

Current Logic, Pro Tools and Reaper will import an M4A without complaint, so a requirement for AIFF usually comes from somewhere older or lower down the chain. Hardware samplers and groove boxes implement PCM and no decoder, because a decoder is firmware someone would have to write and maintain. Broadcast and library ingest specifications name uncompressed formats so that everything arriving is in a known state.

Studio templates are the third source, and the least defensible one — a session that was set up in 2009 to expect AIFF often still does, and nobody has revisited it. Where the requirement is a template rather than a device, importing the M4A directly is usually possible and saves both the conversion and the disk. Where it is hardware or a written specification, converting is the whole answer.

Uncompressed 16-bit stereo at 44,100 Hz costs 176,400 bytes a second — about 10 MB a minute — whether the minute is dense or silent, because every sample is written out as a plain number. An AAC file at 128 kbps costs about 0.96 MB a minute and one at 256 kbps about 1.9.

So the multiplier is decided by the source bitrate and lands between seven and ten for most material. A four-minute track supplied as a 5 MB M4A becomes about 42 MB as an AIFF. There is no quality control on this conversion because there is nothing to trade: AIFF performs no compression at all, which is the entire reason anything asks for it.

The PCM written here is signed 16-bit. From a lossless master that would be a decision worth examining, since a 24-bit source has depth that 16 bits cannot hold. From an AAC file it is not a decision at all — a lossy codec has no additional depth to preserve, and padding the output to 24 bits would produce a file half again as large with not one extra bit of information in it.

Sixteen bits at 44,100 Hz is also what most destinations for this conversion specify: CD authoring by definition, sampler slots by convention, and a good many delivery standards outright. If the session runs at 24 or 32 bits, the AIFF sits inside it as a 16-bit file and behaves normally. Session depth governs the mix, not every source dropped into it.

Nothing here resamples. Whatever rate the M4A carried is the rate the AIFF gets. Music files are usually 44,100 Hz; audio pulled out of a video, exported from a conferencing tool or recorded on a camera is usually 48,000, and that distinction reaches further than people expect because a great many M4As started life beside a picture.

It matters at exactly one point: an audio CD is 44,100 Hz and nothing else, so a 48 kHz AIFF will have to be resampled by the authoring tool or refused. It matters again in a session locked to one rate, where a mismatched import either gets converted on the fly or plays at the wrong speed depending on the software. Check the rate on the supplied file before converting a set of them.

An AIFF is a sequence of chunks and the one written here holds exactly two: COMM, which states the channel count, frame count, bit depth and sample rate, and SSND, which holds the samples. Nothing else is written — no ID3 chunk, no name, no annotation, no artwork.

M4A is unusually rich by comparison. Its metadata atoms carry title, artist, album, album artist, composer, artwork, lyrics, chapter markers and Apple's own purchase and store fields, and all of it ends here. Inside a session that is invisible and correct, because a file on a timeline is a region rather than a library entry. For a deliverable that somebody will catalogue, copy the details out before converting — the AIFF cannot be asked afterwards.

An AAC encoder does not start producing output at the first sample; it needs a short run-up, and the MP4 container records how much of the decoded result to discard so that playback begins where the recording does. Players that read that field do the right thing invisibly.

Uncompressed PCM has nowhere to record it, so a small amount of the encoder's own padding arrives as real samples at the start of the AIFF. In a timeline it is a few dozen milliseconds nobody sees. For a sampler hit, a one-shot, a stinger or anything cut to a precise length it is the first thing to check, because the file no longer begins where the sound does. Trim it in an editor after converting.

Writing AIFF costs no dependency at all. The decoded samples are muxed as a WAV and rewrapped by arithmetic — the header rebuilt big-endian, the sample rate written as the 80-bit extended float the format insists on, and each sample's bytes reversed. Unlike the MP3, AAC and FLAC targets on this site, nothing is fetched.

Decoding the AAC happens on your own machine, so a conversion moves nothing across the network in either direction. Files are capped at 100 MB each, which for an M4A is far more audio than an AIFF of the same length would be, and a hundred can be dropped in one go and taken back as a ZIP. For unreleased client material, being able to check that in the network tab is worth more than a promise in a footer.