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.

There are always 1 (Mono) or 2 (Stereo) channels^{[citation needed]}.

All Xbox ADPCM files seem to store 64 ADPCM input nibbles 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) Bytes. As the decoder-setup in every block contains a predictor for each channel, there will be 65 samples / channel output 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.

Block format

Same as IMA ADPCM

In the following tables, the following notation is used:

• All indices start at 0
• W# denotes a 32-bit word
• B# denotes a Byte (8-bit)
• P denotes the ADPCM predictor for the block
• SI denotes the Step-Index for the block
• S# denotes a sample
• Background color denotes the channel:
  • Blue: Left = Right
  • White: Left
  • Red: Right

Mono

^[FIXME]

32-bit word	W0								W1								W2								...	W7								W8
Byte	B0		B1		B2		B3		B4		B5		B6		B7		B8		B9		B10		B11			B28		B29		B30		B31		B32		B33		B34		B35
Meaning	P = S0				SI				S2	S1	S4	S3	S6	S5	S8	S7	S10	S9	S12	S11	S14	S13	S16	S15		S50	S49	S52	S51	S54	S53	S56	S55	S58	S57	S60	S59	S62	S61	S64	S63

Stereo

^[FIXME]

32-bit word	W0								W1								W2								W3								W4
Byte	B0		B1		B2		B3		B4		B5		B6		B7		B8		B9		B10		B11		B12		B13		B14		B15		B16		B17		B18		B19
Meaning	P = S0				SI				P = S0				SI				S2	S1	S4	S3	S6	S5	S8	S7	S2	S1	S4	S3	S6	S5	S8	S7	S10	S9	S12	S11	S14	S13	S16	S15

...

32-bit word	W14								W15								W16								W17
Byte	B56		B57		B58		B59		B60		B61		B62		B63		B64		B65		B66		B67		B68		B69		B70		B71
Meaning	S50	S49	S52	S51	S54	S53	S56	S55	S50	S49	S52	S51	S54	S53	S56	S55	S58	S57	S60	S59	S62	S61	S64	S63	S58	S57	S60	S59	S62	S61	S64	S63

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 algorithms
• Explanation of IMA ADPCM WAV storage
• Tool to decode Xbox ADPCM files
• Sample files by FFmpeg