Xbox ADPCM

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.

Related links

• Explanation of IMA ADPCM block encoding
• Explanation of IMA ADPCM WAV header
• Tool to decode Xbox ADPCM files
• Sample files by FFmpeg