Why can't most iPhone apps convert to MP3?

An app running on iOS can ask the operating system a simple question: which audio formats will you encode to? The system answers with a list, through a property called kAudioFormatProperty_EncodeFormatIDs.

On iOS, that list contains AAC, Apple Lossless, and the uncompressed formats (linear PCM, and the containers that hold it). It does not contain kAudioFormatMPEGLayer3 — the identifier for MP3. It is simply absent.

There is no quality setting, no entitlement, no hidden flag that adds it. You cannot coax an encoder out of the system that the system does not have. Every iPhone app is working from the same list, which is why the overwhelming majority of them quietly do not offer MP3 output.

The usual explanation you will read is patents, and for a long time that explanation was true. MP3 was covered by a thicket of patents administered by Fraunhofer and Technicolor, and an MP3 encoder — unlike a decoder — carried real per-unit licensing costs.

That ended. The MP3 patent licensing programme was terminated in April 2017; the relevant patents have expired. Shipping an MP3 encoder today costs nobody a cent and requires nobody's permission. The gap in iOS is no longer a legal constraint, it is simply a thing Apple has not done — and given that Apple has spent twenty years pushing AAC as the successor, is unlikely to do.

Which is a defensible position, by the way. AAC really is the better codec. It just is not the codec your 2011 car stereo understands.

This trips people up constantly, and it is worth stating plainly: a Mac can create MP3s. The Music app on macOS (and iTunes before it) has been able to rip and convert to MP3 for two decades, because macOS ships an MP3 encoder that iOS does not.

So the person who tells you "just do it on your computer" is giving you real advice, not brushing you off. And the person who cannot understand why their iPhone won't do what their Mac does is not missing a setting. The two operating systems genuinely differ here.

There are exactly two roads, and every MP3-capable iPhone app is on one of them.

Road one: bundle an encoder. The app ships its own MP3 encoder inside the binary — in practice this means LAME, the long-standing open-source encoder that is, by a wide margin, the best MP3 encoder anyone has written. The app then encodes on your device using its own code, never touching Apple's missing one. This makes the app bigger and is more work to build, but the conversion is genuinely local.

Road two: upload your file. The app sends your audio to a server, which runs ffmpeg or LAME on a Linux box, and sends an MP3 back. This is far easier to build — one server handles every format, on every platform, forever — and it is why a great many "converter" apps are really just a thin client for someone else's web service.

Road two is not automatically sinister. But it is a materially different thing from what most people think they are doing when they convert a voice memo on their phone, and a lot of apps are not forthcoming about which road they took.

Often, no. If everything you own can play AAC — and everything Apple has made since 2003 can — then M4A is a smaller file at better quality, and converting it to MP3 makes it worse. See does converting a file lose quality for why a lossy-to-lossy transcode costs you twice.

But MP3 is thirty years old and effectively universal, and that universality is the entire point. You want an MP3 when something you do not control demands one:

• Car stereos and head units old enough to predate AAC.
• Gym equipment, dictaphones, older MP3 players, DJ hardware.
• Upload forms that say "MP3 only" — transcription services, podcast hosts, university portals, court filing systems.
• Windows software that technically accepts M4A and handles it badly.

In every one of these, a slightly degraded file that plays beats a pristine file that does not. That is the honest case for MP3, and it is a good one.