Difference between revisions of "Xbox ADPCM"
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Revision as of 02:07, 28 June 2017
Xbox used it's own WAV file format to encode ADPCM data. This format is often called Xbox ADPCM.
Standard IMA ADPCM WAV files would use format code 0x0011, whereas Xbox ADPCM files use format-code 0x0069.
All Xbox ADPCM files seem to store 64 input samples per block.
This value is also stored in the 'fmt ' extra-data which is always 2 bytes, containing the Bytes 0x40, 0x00 (64 as unsigned 16-bit integer).
Because of that, all Xbox ADPCM files will have a block alignment of 36 (Mono) or 72 (Stereo).
As the decoder-setup in every block contains a predictor for each channel, there will be 65 output samples per block (65:36 compression ratio = 44.6% compression).
Aside from what was mentioned, there are no known differences to IMA ADPCM. This is probably because the APU VP will decode ADPCM in hardware. Microsoft probably had little control over the APU ADPCM implementation and had to stay compatible to standard ADPCM. Players supporting IMA ADPCM also support Xbox ADPCM. However, they might reject files due to the different format-code.
Index-Table
Same as IMA ADPCM
| +0 | +1 | +2 | +3 | +4 | +5 | +6 | +7 | |
|---|---|---|---|---|---|---|---|---|
| 0 | -1 | -1 | -1 | -1 | 2 | 4 | 6 | 8 |
| 8 | -1 | -1 | -1 | -1 | 2 | 4 | 6 | 8 |
Step-Table
Same as IMA ADPCM
| +0 | +1 | +2 | +3 | +4 | +5 | +6 | +7 | +8 | +9 | |
|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 16 | 17 |
| 10 | 19 | 21 | 23 | 25 | 28 | 31 | 34 | 37 | 41 | 45 |
| 20 | 50 | 55 | 60 | 66 | 73 | 80 | 88 | 97 | 107 | 118 |
| 30 | 130 | 143 | 157 | 173 | 190 | 209 | 230 | 253 | 279 | 307 |
| 40 | 337 | 371 | 408 | 449 | 494 | 544 | 598 | 658 | 724 | 796 |
| 50 | 876 | 963 | 1060 | 1166 | 1282 | 1411 | 1552 | 1707 | 1878 | 2066 |
| 60 | 2272 | 2499 | 2749 | 3024 | 3327 | 3660 | 4026 | 4428 | 4871 | 5358 |
| 70 | 5894 | 6484 | 7132 | 7845 | 8630 | 9493 | 10442 | 11487 | 12635 | 13899 |
| 80 | 15289 | 16818 | 18500 | 20350 | 22385 | 24623 | 27086 | 29794 | 32767 |
Algorithm
The algorithms for encoding and decoding are the same as IMA ADPCM. For an example implementation and an explanation, refer to the related links.
