Apple Lossless Audio Coding

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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.

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 ???
??? 12 ???
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 (hassize) {
outputsamples 32 number of samples
}

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