Apple Lossless Audio Coding: Difference between revisions
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:''right = mid - ((difference * interlacing_leftweight) >> interlacing_shift)'' | :''right = mid - ((difference * interlacing_leftweight) >> interlacing_shift)'' | ||
:''left = right + difference'' | :''left = right + difference'' | ||
It seems we have interlacing_leftweight == interlacing_shift in most of the samples | |||
=== Frame Footer === | |||
{| border="1" | |||
! Syntax !! Number of bits !! Semantics | |||
|- | |||
| EOF || 3 || should be always 7 | |||
|} | |||
[[Category:Lossless Audio Codecs]] | [[Category:Lossless Audio Codecs]] | ||
[[Category:Audio Codecs]] | [[Category:Audio Codecs]] |
Revision as of 09:50, 26 May 2006
- 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
It seems we have interlacing_leftweight == interlacing_shift in most of the samples
Syntax | Number of bits | Semantics |
---|---|---|
EOF | 3 | should be always 7 |