Apple Lossless Audio Coding

• FOURCCS: alac
• Company: Apple
• Technical Description: http://craz.net/programs/itunes/alac.html
• Samples: http://mplayerhq.hu/MPlayer/samples/A-codecs/lossless/ (luckynight.m4a)

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