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Apple Lossless
Apple Lossless, also known as Apple Lossless Audio Codec (ALAC), 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 they have begun to use the abbreviated term ''ALAC'' when referring to the codec.
==Codec==
Apple Lossless supports up to 8 channels of audio at 16, 20, 24 and 32 bit depth with a maximum sample rate of 384kHz. Apple Lossless data is stored within an MP4 container with the filename extension ''.m4a''. This extension is also used by Apple for lossy AAC audio data in an MP4 container (same container, different audio encoding). However, Apple Lossless is not a variant of AAC (which is a lossy format), but rather a distinct lossless format that uses linear prediction similar to other lossless codecs. These other lossless codecs, such as FLAC and Shorten, are not natively supported by Apple's iTunes software (either the Mac OS or Windows versions) or by iOS devices, so users of iTunes software who want to use a lossless format which allows the addition of metadata (unlike WAV/AIFF or other PCM-type formats, where metadata is usually ignored) have to use ALAC. All current iOS devices can play ALAC–encoded files. 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. Furthermore, compared to some other formats, it is not as difficult to decode, making it practical for a limited-power device, such as older iOS devices.
Partly because of the use of an MP4 container, Apple Lossless does not contain integrated error checking.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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