MTA2 (Codec)
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MTA2 is the audio codec used in Metal Gear Solid 4. It is based off of the Maxis XA ADPCM codec, which is commonly used in Playstation games.
Containers
Format
An example of a typical MTA2 block:
00 00 00 00 00 04 00 90 00 00 00 00 00 00 00 00 00 00 00 E0 00 00 00 20 00 00 00 E0 00 00 00 E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
| Header | ||
|---|---|---|
| uint8 {1} | Track Number (0x0 unless multi-tracked) | |
| uint24 {3} | Frame Number (starts at 0) | |
| uint16 {2} | Channel Mask (see MTA2 (Container)) | |
| uint16 {2} | Length of all blocks (0x90 * channels) | |
| byte {8} | Padding | |
| Block Info | ||
| bit {6} | Sample 2 (Second raw sample, drops the last 2 bits) | |
| bit {6} | Sample 1 (First raw sample, drops the last 2 bits) | |
| bit {3} | Mod | |
| bit {5} | Shift | |
| Sample Nibbles | ||
| bit {4} | Sample Nibble (see Decoding) | |
Decoding
Nibbles are decoded in columns of 8 nibbles, left-right, top-down:
HE-AD-ER-01 HE-AD-ER-02 HE-AD-ER-03 HE-AD-ER-04 11 11 11 11 22 22 22 22 33 33 33 33 44 44 44 44 11 11 11 11 22 22 22 22 33 33 33 33 44 44 44 44 11 11 11 11 22 22 22 22 33 33 33 33 44 44 44 44 11 11 11 11 22 22 22 22 33 33 33 33 44 44 44 44 11 11 11 11 22 22 22 22 33 33 33 33 44 44 44 44 11 11 11 11 22 22 22 22 33 33 33 33 44 44 44 44 11 11 11 11 22 22 22 22 33 33 33 33 44 44 44 44
Nibbles are decoded using this formula:
output = (sample1 * c1[mod] + sample2 * c2[mod] + nibble * c3[shift] + 128) >> 8
The output is clamped from -32,768 to 32,767.
Constant Tables
c1
0, 240, 460, 392, 488, 460, 460, 240
c2
0, 0, -208, -220, -240, -240, -220, -104
c3
256, 335, 438, 573, 749, 979, 1281, 1675, 2190, 2864, 3746, 4898, 6406, 8377, 10955, 14327, 18736, 24503, 32043, 41905, 54802, 71668, 93724, 122568, 160290, 209620, 274133, 358500, 468831, 613119, 801811, 1048576
