21 lines
1.2 KiB
Text
21 lines
1.2 KiB
Text
Located here is a basic decoder for Apple Lossless Audio Codec files
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(ALAC). ALAC is a proprietary lossless audio compression scheme. Apple
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never released any documents on the format.
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What I provide here is a C implementation of a decoder, written from
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reverse engineering the file format. It turns out that most of the
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algorithms in the codec are fairly well known. ALAC uses an adaptive FIR
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prediction algorithm and stores the error values using a modified rice
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or golumb algorithm. Further details are in alac.c.
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Although an encoder is not provided, by using the decoder as a sort of
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specification it should be fairly trivial to write an encoder. By
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exploiting other lossless audio encoders, such as FLAC, the task will be
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much easier. Although one wouldn't be able to copy the compression
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algorithms verbatim, as adaptive compression is used in ALAC and not in
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FLAC. There are, however, a bunch of academic papers on the issue.
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The program located here will not be able to handle all ALAC files, it
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can only handle mono or stereo files. ALAC allows up to 8 channels. It
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should be trivial to finish the implementation once I find files that I
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can test it with. The ALAC decoder supports both 16 and 24 bit sample
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sizes.
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