ALAC, the Apple Lossless Audio Codec, is a lossless audio compression format that reduces the size of PCM audio files while allowing the decoder to reconstruct every original sample exactly. Among audio formats and codecs, it belongs to the same class as FLAC: smaller than WAV or AIFF, but mathematically identical to them after decoding.
Apple introduced ALAC in 2004 for its music software and portable players and released it as open source under the Apache license in 2011. It remains the native lossless format of the Apple ecosystem, which is why it appears in music libraries, lossless streaming tiers and archival rips intended for Apple devices.
How ALAC compresses audio without loss
ALAC compresses audio by predicting each sample from the samples before it and storing only the prediction error, called the residual. Because neighboring samples in real music are strongly correlated, the residuals are much smaller numbers than the samples themselves and need fewer bits to store.
The encoder works in frames of 4096 samples by default. For each frame it fits an adaptive linear predictor, can exploit similarity between left and right channels through inter-channel decorrelation, and then codes the residuals with an adaptive Golomb-Rice style entropy coder. If a frame does not compress, for example with white noise, ALAC stores it uncompressed so that the file never grows meaningfully beyond the PCM size.
Decoding reverses the process exactly. The decoder applies the same predictor, adds the stored residuals and obtains the original integer samples bit for bit. This distinguishes ALAC from AAC or MP3, which discard information permanently through perceptual coding.
Technical specifications of ALAC
ALAC supports the resolutions used in consumer and studio audio, from CD quality to high-resolution masters. The table summarizes the main parameters.
| Parameter | Supported values | Practical note |
|---|---|---|
| Bit depth | 16, 20, 24 and 32-bit integer | No floating-point audio |
| Sample rate | Up to 384 kHz | Players commonly handle up to 192 kHz |
| Channels | Up to 8 | Stereo is by far the most common |
| Container | MPEG-4, usually .m4a | Same extension as AAC files |
| Typical file size | About 40 to 70 percent of PCM | Depends strongly on the material |
| Metadata | MPEG-4 atoms | Tags and embedded cover art |
Compression efficiency varies with the music. Sparse acoustic recordings and solo piano compress well, sometimes close to 40 percent of the original size, while dense, loud, heavily limited masters and high-resolution files with ultrasonic noise compress less. Any single figure for average compression is therefore only indicative.
ALAC compared with other audio formats
ALAC is best understood alongside its neighbors in the lossless and lossy families. Its sound quality equals that of every other lossless format; the differences lie in compatibility, metadata and efficiency.
| Format | Compression | Typical size versus PCM | Main ecosystem |
|---|---|---|---|
| ALAC | Lossless | About 40 to 70 percent | Apple devices and software |
| FLAC | Lossless | Similar, often slightly smaller | Open, cross-platform players |
| WAV | None | 100 percent | Production and editing |
| AIFF | None | 100 percent | Apple-oriented production |
| AAC | Lossy | About 10 to 20 percent | Streaming and portable playback |
| MP3 | Lossy | About 10 to 20 percent | Universal legacy compatibility |
Between ALAC and FLAC, the choice is practical rather than sonic. FLAC is often marginally more efficient, decodes on almost every non-Apple platform and has a richer tagging scheme. ALAC integrates natively with Apple music software and devices. Converting one into the other is completely lossless, so neither choice locks the audio in.
Because ALAC files use the .m4a extension, they are easily confused with AAC files. The container M4A says nothing about whether the audio inside is lossless; only the codec field in the file’s properties reveals it.
When to use ALAC
ALAC is the right choice when a lossless library must play natively on Apple devices and software. For other situations, a different format is often more practical.
| Situation | Recommended format | Reason |
|---|---|---|
| Library played on Apple devices | ALAC | Native playback and full tagging |
| Mixed or non-Apple playback chain | FLAC | Broadest lossless support |
| Editing and mixing sessions | WAV or AIFF | No decoding overhead, universal in DAWs |
| Limited storage on a phone | AAC at 256 kbit/s | Much smaller, transparent for most listeners |
| Archival master copy | ALAC or FLAC | Lossless with metadata and checks |
For listening over Bluetooth headphones, ALAC offers no audible advantage, because codecs such as AAC, aptX or LDAC re-encode the signal lossily on the wireless link. The lossless chain ends at the phone unless a wired connection or a lossless wireless codec is used.
ALAC also serves as the delivery format of lossless streaming tiers in the Apple ecosystem, at resolutions from CD quality up to 24-bit and 192 kHz. High-resolution playback at the upper end usually requires an external digital-to-analog converter, because many built-in outputs and adapters are limited to 48 kHz.
Limitations and misconceptions about ALAC
The most common misconception is that lossless means better than the source. ALAC can only preserve what was in the input; converting an MP3 to ALAC produces a large file that still contains all of the MP3’s losses.
- Not floating point: 32-bit float production files must be converted to integer before encoding.
- Limited outside Apple software: many players support it, but some hardware and older devices do not.
- No built-in checksum per stream: unlike FLAC, ALAC does not store an MD5 signature of the decoded audio, so integrity checks rely on external tools.
- Higher bit depths inflate size: a 24-bit, 192 kHz file is many times larger than a 16-bit, 44.1 kHz file of the same music.
Decoding cost is low on modern hardware, so ALAC playback has a negligible effect on battery life compared with the screen, radio and amplifier of a portable device.
In everyday language
ALAC is a way to pack music files smaller without losing anything. It works like a zip file for songs. When you play the song, it unpacks into exactly the same sound that went in.
The files are usually about half the size of a CD-quality original, but still much larger than MP3 or AAC files. ALAC is Apple’s version of this idea, so it works best on Apple devices and Apple music software.
Seen in the real world
A classical music collector rips 600 CDs to keep a permanent digital copy and play it through an iPhone and a desktop music app. Uncompressed WAV would need roughly 400 GB, so the collector encodes every disc to ALAC at 16-bit and 44.1 kHz, with full tags for composer, work and performer.
The finished library takes a little over half the WAV size, plays natively on all the collector’s Apple devices and can later be converted to FLAC losslessly if the collector moves to a different player. For a phone with limited storage, a separate AAC copy is generated from the ALAC masters.