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

4-bit ADPCM

Decoding procedure:

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

Update procedure:

mul = step_table[step] * mult_table[delta & 7];
if(delta & 8)
 val -= mul;
else 
 val += mul;
val = clip(val, 0, 0xFFFF);
step = clip(step + step_update_table[delta & 7], 0, 71);

Step table:

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

Step update table:

-8, -6, -4, -2, 2, 4, 8, 16};

Multiplication table:

0.5, 1.0, 2.0, 3.0, 4.5, 5.0, 9.0, 12.0};