Apple Lossless Audio Coding: Difference between revisions
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! Syntax !! Number of bits !! Semantics | ! Syntax !! Number of bits !! Semantics | ||
|- | |- | ||
| interlacing_shift || 8 || 0 if no channel interlacing. | | interlacing_shift || 8 || should be 0 if no channel interlacing. | ||
|- | |- | ||
| interlacing_leftweight || 8 || | | interlacing_leftweight || 8 || | ||
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| predictor_coef_table || 16 * predictor_coef_num || | | predictor_coef_table || 16 * predictor_coef_num || | ||
|- | |- | ||
| | | } | ||
|- | |||
| for each channel { | |||
|- | |||
| compressed data || || modfied Rice coding and Adaptive FIR filter | |||
|- | |- | ||
| } | | } |
Revision as of 14:09, 25 May 2006
- FOURCCS: alac
- Company: Apple
- Technical Description: http://craz.net/programs/itunes/alac.html
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.
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