Apple Lossless Audio Coding

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Apple Lossless Audio Coding using linear prediction with Golomb-Rice coding of the difference. Similar to FLAC, although the bitstreams are not compatible. Usually stored in an MP4 container, as a private extension since it is not part of the MPEG-4 standard.

On October 27, 2011, ALAC was open sourced under the Apache License 2.0. The source can be found at http://alac.macosforge.org/.

Extradata

Syntax Number of bits Semantics
atom size 32 (0x24), big-endian
tag 32 'alac'
??? 32 zero ?
max sample per frame 32
??? 8 zero ?
sample size 8
rice history mult 8
rice initial history 8
rice kmodifier 8
channels 8  ?
??? 16
max coded frame size 32
bitrate 32  ?
samplerate 32

Frame Header

Syntax Number of bits Semantics
channels 3 Number of channel -1 (Should be the same than in extradata ???)
 ??? 4  ??? (seem to be always 0)
 ??? 12  ??? (seem to be always 0)
hasoutputsamples 1 the number of samples is stored in the header
wasted_bytes 2 unknow (same as flac ???)
isverbatim 1 if 1, the data is not compressed
if (hasoutputsamples) {
outputsamples 32 number of samples (sometimes the value is wrong [1])
}


[1] http://www1.mplayerhq.hu/MPlayer/samples/A-codecs/lossless/luckynight.m4a need + 32 and http://www1.mplayerhq.hu/MPlayer/samples/A-codecs/lossless/ALAC/old_midi_stuff.m4a + 112

Verbatim frame

Syntax Number of bits Semantics
data sample_size * outputsamples * channels Unencoded subblock

Compressed frame

Syntax Number of bits Semantics
interlacing_shift 8 should be 0 if no channel interlacing.
interlacing_leftweight 8
for each channel {
prediction_type 4 should be 0
prediction_quantitization 4
ricemodifier 3
predictor_coef_num 5
predictor_coef_table 16 * predictor_coef_num
}
for each channel {
compressed data modfied Rice coding and Adaptive FIR filter
}

Channel Interlacing

It is a mid/side stereo with weight. channel 0 is the mid(average) channel, channel 1 is the side(difference) channel.

right = mid - ((difference * interlacing_leftweight) >> interlacing_shift)
left = right + difference

It seems we have interlacing_leftweight == interlacing_shift in most of the samples

Frame Footer

Syntax Number of bits Semantics
EOF 3 should be always 7