Covox ADPCM

• Company: Covox
• Samples: http://samples.mplayerhq.hu/A-codecs/CovoxFormats/

This is a family of ADPCM formats used for compressing 8-bit unsigned PCM for Covox Sound Master. There are 2-,3- and 4-bit ADPCM formats.

Header

File header consists of 16 bytes:

 FF 55 FF AA FF 55 FF AA
 <type> - 1 byte
 unknown/unused - 7 bytes

Type may be one of the following:

• 0x08 - raw 8-bit unsigned PCM
• 0x82 - 2-bit ADPCM
• 0x83 - 3-bit ADPCM
• 0x84 - 4-bit ADPCM

Markers

There are two types of markers known:

• FF 55 55 55 - silence block, next 16-bit little-endian value specifies number of silence samples - 27
• FF AA AA AA - end of sound

For 3-bit ADPCM they are cut to 3 bytes (FF 55 55 and FF AA AA respectively).

After silence block ADPCM decoding state should be reset.

ADPCM decoding

This is generalized decoding procedure for all know ADPCM variants. n will denote the number of bits used in ADPCM (n=2 for 2-bit ADPCM, etc).

Decoding procedure:

1. look at the next 3 or 4 bytes, if they make a marker, do action for this marker
2. read n bytes
3. decode 8 deltas from them by taking high bits from n values and shifting them
4. apply these deltas to get decoded values

Update procedure:

high_bit = 1 << (n-1);
low_bits = (1 << (n-1)) - 1;
mul = step_table[step] * mult_table[delta & low_bits];
if(delta & high_bit)
 val -= mul;
else 
 val += mul;
val = clip(val, 0, 0xFFFF);
step = clip(step + step_update_table[delta & low_bits], 0, step_max);

ADPCM Tables

Value	4-bit ADPCM	3-bit ADPCM	2-bit ADPCM
Max. step value	72	96	80
Step update table	-8, -6, -4, -2, 2, 4, 8, 16	-4, -2, 4, 16	-6, 6
Multiplication table	0.5, 1.0, 2.0, 3.0, 4.5, 5.0, 9.0, 12.0	1.0, 1.5, 2.0, 2.5	1.0, 3.0

Step table (it has much more values than needed):

0x0080, 0x008C, 0x0098, 0x00A6, 0x00B5, 0x00C5, 0x00D7, 0x00EB,
0x0100, 0x0117, 0x0130, 0x014C, 0x016A, 0x018B, 0x01AF, 0x01D6,
0x0200, 0x022E, 0x0261, 0x0298, 0x02D4, 0x0316, 0x035D, 0x03AB,
0x0400, 0x045D, 0x04C2, 0x0530, 0x05A8, 0x062B, 0x06BA, 0x0756,
0x0800, 0x08B9, 0x0983, 0x0A60, 0x0B50, 0x0C56, 0x0D74, 0x0EAC,
0x1000, 0x1173, 0x1307, 0x14C0, 0x16A1, 0x18AD, 0x1AE9, 0x1D58,
0x2000, 0x22E5, 0x260E, 0x2980, 0x2D41, 0x315A, 0x35D1, 0x3AB0,
0x4000, 0x45CB, 0x4C1C, 0x52FF, 0x5A82, 0x62B4, 0x6BA2, 0x7560,
0x8000, 0x8B96, 0x9838, 0xA5FF, 0xB505, 0xC567, 0xD745, 0xEAC1,

0x00C0, 0x00D1, 0x00E4, 0x00F9, 0x0110, 0x0128, 0x0143, 0x0160,
0x0180, 0x01A3, 0x01C9, 0x01F2, 0x021F, 0x0250, 0x0286, 0x02C0,
0x0300, 0x0346, 0x0391, 0x03E4, 0x043E, 0x04A0, 0x050C, 0x0581,
0x0600, 0x068B, 0x0723, 0x07C8, 0x087C, 0x0941, 0x0A17, 0x0B01,
0x0C00, 0x0D16, 0x0E45, 0x0F90, 0x10F8, 0x1282, 0x142E, 0x1602,
0x1800, 0x1A2C, 0x1C8A, 0x1F20, 0x21F1, 0x2503, 0x285D, 0x2C04,

0x0240, 0x0274, 0x02AD, 0x02EB, 0x032F, 0x0378, 0x03C9, 0x0420,
0x0480, 0x04E8, 0x055A, 0x05D6, 0x065D, 0x06F1, 0x0791, 0x0841,
0x0900, 0x09D1, 0x0AB4, 0x0BAC, 0x0CBA, 0x0DE1, 0x0F23, 0x1082,
0x1200, 0x13A1, 0x1568, 0x1758, 0x1975, 0x1BC3, 0x1E46, 0x2103,
0x2400, 0x2742, 0x2AD0, 0x2EB0, 0x32E9, 0x3785, 0x3C8B, 0x4206,
0x4800, 0x4E84, 0x559F, 0x5D5F, 0x65D3, 0x6F0A, 0x7917, 0x840C,

0x02C0, 0x0300, 0x0345, 0x0391, 0x03E4, 0x043E, 0x04A0, 0x050B,
0x0580, 0x05FF, 0x068A, 0x0722, 0x07C7, 0x087B, 0x0940, 0x0A16,
0x0B00, 0x0BFF, 0x0D15, 0x0E44, 0x0F8E, 0x10F7, 0x1280, 0x142D,
0x1600, 0x17FE, 0x1A2A, 0x1C88, 0x1F1D, 0x21EE, 0x2500, 0x2859,
0x2C00, 0x2FFB, 0x3453, 0x3910, 0x3E3A, 0x43DB, 0x4A00, 0x50B2,
0x5800, 0x5FF7, 0x68A6, 0x721F, 0x7C73, 0x87B7, 0x93FF, 0xA165,

0x03C0, 0x0417, 0x0476, 0x04DD, 0x054E, 0x05C9, 0x064F, 0x06E1,
0x0780, 0x082E, 0x08EB, 0x09BA, 0x0A9B, 0x0B91, 0x0C9D, 0x0DC1,
0x0F00, 0x105C, 0x11D7, 0x1374, 0x1537, 0x1722, 0x193A, 0x1B83,
0x1E00, 0x20B7, 0x23AD, 0x26E8, 0x2A6D, 0x2E44, 0x3274, 0x3705,
0x3C00, 0x416E, 0x475A, 0x4DCF, 0x54DA, 0x5C88, 0x64E8, 0x6E0A,
0x7800, 0x82DC, 0x8EB4, 0x9B9F, 0xA9B5, 0xB911, 0xC9D1, 0xDC15