How ALAC Works
Like FLAC, ALAC uses linear prediction to compress audio without discarding anything- Decoding reconstructs the original samples exactly. Typical savings run 40-60% versus uncompressed PCM. ALAC files live in the MPEG-4 container with the familiar .m4a extension, which is why an "M4A" can be either small lossy AAC or much larger lossless ALAC. Because compression is lossless, an ALAC file converted back to WAV or AIFF is identical to the original- Sample for sample.
Technical Characteristics
ALAC supports bit depths of 16, 20, 24, and 32 bits and sample rates up to 384 kHz- Comfortably covering hi-res audio. Apple Music's Lossless tier streams ALAC at up to 24-bit / 192 kHz. Compression efficiency is close to FLAC (FLAC is typically a few percent smaller and faster to decode), and the MPEG-4 container provides full metadata and artwork support. AirPlay itself famously carries audio between Apple devices as ALAC, so the codec is working behind the scenes even when you never see a file.
ALAC or FLAC?
Quality is identical- Both are lossless, so the choice is purely ecosystem. Living inside Apple hardware and software? ALAC imports straight into Apple Music and syncs to iPhone. On Android, Windows-centric players, or archival setups, FLAC has broader support and slightly better tooling. Since both are lossless you can convert between them at any time with zero quality loss, so this is not a decision you can get permanently wrong. The one practical caveat is tooling: FLAC tag editors and verification utilities are more plentiful, though Apple Music covers everyday tagging needs for ALAC. See lossy vs lossless for the bigger picture.
Compatibility
ALAC is native on every Apple device and supported on Android and in many desktop players (VLC, foobar2000). Windows 10 and later play ALAC in the built-in media apps, though older Windows software needs plugins. Gaps remain in car stereos, older portable players, and some smart speakers, where lossy formats- Or FLAC- Are expected instead.
Converting ALAC on mp3.now
For small, universally playable copies of an ALAC library, use the ALAC to MP3 converter at 320 kbps- Since the source is lossless, the MP3 will be as good as MP3 gets. The MP3 to ALAC converter exists for Apple-only workflows where a library must be uniform, though remember it cannot restore detail the MP3 already discarded- The file grows without the quality improving.