ALAC file format is Apple Lossless Audio Codec (ALAC) that uses the MPEG-4 compliant QuickTime container. It was introduced in 2004 as proprietary file format, until 2011 when Apple made it available open source and royalty-free. The format is similar to the FLAC (Free Lossless Audio Codec) but offers more compression comparatively. It offers better sounding alternative to AAC or MP3. Audio files encoded with ALAC codec can be opened using Apple QuickTime, Apple iTunes and Micrsoft Windows Media Player with K-Lite codec.
ALAC File Format
Files based on the ALAC codec are binary files that is available as open source on GitHub for developer’s reference. It contains the sources for the ALAC encoder and decoder. Apple has also provided a command line utility, called alacconvert, to read and write the audio data to/from Core Audio Format (CAF) and WAVE files. Following are some key points about the ALAC file format.
Bit Depths - 16, 20, 24 and 32 bits.
Sample Rate - Any arbitrary integer sample rate from 1 to 384,000 Hz. Theoretical rates of up to 4,294,967,295 (2^32 - 1) Hz could be supported.
Packet Size - The default packet size defaults to 4096 sample frames of audio per packet. Other packet sizes are certainly possible. However, non-default packet sizes are not guaranteed to work properly on all hardware devices that support Apple Lossless. Packets above 16,384 sample frames are not supported.
Supported Channels - It supports one to eight channels. Each channel has the following order of information.
stereo (Left, Right)
MPEG 3.0 B (Center, Left, Right)
MPEG 4.0 B (Center, Left, Right, Center Surround)
MPEG 5.0 D (Center, Left, Right, Left Surround, Right Surround)
MPEG 5.1 D (Center, Left, Right, Left Surround, Right Surround, Low Frequency Effects)
Apple AAC 6.1 (Center, Left, Right, Left Surround, Right Surround, Center Surround, Low Frequency Effects)
MPEG 7.1 B (Center, Left Center, Right Center, Left, Right, Left Surround, Right Surround, Low Frequency Effects)