How ALAC Compresses Audio
ALAC (Apple Lossless Audio Codec) is Apple's answer to FLAC: a format that shrinks audio files without losing a single sample. Like all lossless codecs, it does not discard sound the way MP3 or AAC do; instead it removes mathematical redundancy and can reconstruct the original PCM audio exactly, bit for bit, when decoded. A typical ALAC file is roughly 40 to 60 percent the size of the equivalent uncompressed WAV or AIFF, with zero quality cost.
The technique is the same family used by FLAC. The encoder predicts each audio sample from the samples just before it, then stores only the small residual, the difference between the prediction and the real value, rather than the full sample. Because audio is highly predictable over short spans, those residuals are small numbers that compress efficiently through entropy coding. On playback the decoder runs the prediction in reverse to recover the exact original samples.
ALAC audio is carried inside an MPEG-4 container, usually with an .m4a extension, the same wrapper that holds lossy AAC. This is a common source of confusion: an .m4a file can contain either lossy AAC or lossless ALAC, and you cannot tell which from the extension alone. You have to check the file's details to know whether a given M4A is the lossy or lossless kind.
From Proprietary to Open Source
Apple introduced ALAC in 2004 as a proprietary, closed format tied to iTunes and the iPod, at a time when it wanted a lossless option that fit neatly into its own ecosystem. For years this closed nature was ALAC's biggest drawback compared with the already-open FLAC, since other software could not freely implement it.
That changed in October 2011, when Apple open-sourced ALAC under a permissive license and published its reference encoder and decoder. Opening the format meant any developer could implement it without restriction, which broadened support beyond Apple's own software. Even so, FLAC had a decade's head start as the open lossless standard, so ALAC's adoption outside the Apple world remained narrower.
ALAC vs FLAC: Which Should You Choose?
ALAC and FLAC are both lossless and produce very similar file sizes, FLAC is often marginally smaller, but the difference is small and there is no audible difference at all, since both are perfect copies. The real decision is about your ecosystem, not sound quality.
Choose ALAC if you live in the Apple world: it is the lossless format that works natively across the Music app, iTunes, iPhone, iPad, and Mac, and it syncs cleanly through Apple's software without conversion. Choose FLAC for the broadest cross-platform support, since it is the de facto open standard supported by virtually every non-Apple player, network streamer, and hi-fi device. Notably, Apple's own devices historically did not play FLAC natively, which is the main reason ALAC still exists rather than everyone simply using FLAC. If your library needs to work both inside and outside Apple's ecosystem, many people keep FLAC as the master and convert to ALAC only when needed.
Limitations
ALAC's limitations are mostly about reach. Outside the Apple ecosystem its support, while now possible thanks to open-sourcing, is still less universal than FLAC's, so for maximum compatibility FLAC is the safer archival choice. Its metadata and tagging support is also less rich and standardized than FLAC's. And like any lossless format, ALAC files are several times larger than lossy MP3 or AAC, which makes them impractical for streaming or for fitting large libraries onto small devices, the reason Apple Music streams AAC rather than ALAC by default for most use.
