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Concept
Apple Lossless
Résumé
The Apple Lossless Audio Codec (ALAC), also known as Apple Lossless, or Apple Lossless Encoder (ALE), is an audio coding format, and its reference audio codec implementation, developed by Apple Inc. for lossless data compression of digital music. After initially keeping it proprietary from its inception in 2004, in late 2011 Apple made the codec available open source and royalty-free. Traditionally, Apple has referred to the codec as Apple Lossless, though more recently it has begun to use the abbreviated term ALAC when referring to the codec.
ALAC supports up to 8 channels of audio at 16, 20, 24 and 32 bit depth with a maximum sample rate of 384 kHz.
ALAC data is frequently stored within an MP4 container with the .m4a. This extension is also used by Apple for lossy AAC audio data in an MP4 container (same container, different audio encoding). The codec can also be used by the .CAF file type container, though this is much less common.
ALAC is not a variant of AAC (which is a lossy format), but rather an unrelated lossless format that uses linear prediction (similar to other lossless codecs).
ALAC also does not use any DRM scheme; but by the nature of the MP4 container, it is feasible that DRM could be applied to ALAC much in the same way it is applied to files in other QuickTime containers.
According to Apple, audio files compressed with its lossless codec will use up "about half the storage space" that the uncompressed data would require. Testers using a selection of music have found that compressed files are about 40% to 60% the size of the originals depending on the kind of music, which is similar to other lossless formats.
ALAC has been measured to require around four times as much CPU power to decode than FLAC does, with implications for battery life on limited-power devices. Still the format has been recommended for older iPod devices based on claims of lower power usage.
Partly because of the use of an MP4 container, ALAC does not contain integrated error checking.
Source officielle
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