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.
| 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 |
|
| 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