Can someone really familiar with ADPCM fix this 4-bit to 16-bit decoder? Despite finding countless examples I'm not convinced I've got the algorithm right.
Code:
int index_table[16] =
{
-1, -1, -1, -1, 2, 4, 6, 8,
-1, -1, -1, -1, 2, 4, 6, 8
};
int step_table[89] =
{
7, 8, 9, 10, 11, 12, 13, 14, 16, 17,
19, 21, 23, 25, 28, 31, 34, 37, 41, 45,
50, 55, 60, 66, 73, 80, 88, 97, 107, 118,
130, 143, 157, 173, 190, 209, 230, 253, 279, 307,
337, 371, 408, 449, 494, 544, 598, 658, 724, 796,
876, 963, 1060, 1166, 1282, 1411, 1552, 1707, 1878, 2066,
2272, 2499, 2749, 3024, 3327, 3660, 4026, 4428, 4871, 5358,
5894, 6484, 7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,
15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767
};
int16 output = 0;
int step_index = 0;
int step = step_table[step_index];
int16 decode(uint4 input)
{
step_index += index_table[input];
if (step_index > 89) step_index = 89;
if (step_index < 0) step_index = 0;
int delta = step >> 3; /* calc delta */
if (input & 4) delta += step;
if (input & 2) delta += (step >> 1);
if (input & 1) delta += (step >> 2);
if (input & 8) output -= delta; /* sign */
else output += delta;
step = step_table[step_index]; /* next step */
if (output > 32767) output = 32767;
if (output < -32768) output = -32768;
return output;
}
{
-1, -1, -1, -1, 2, 4, 6, 8,
-1, -1, -1, -1, 2, 4, 6, 8
};
int step_table[89] =
{
7, 8, 9, 10, 11, 12, 13, 14, 16, 17,
19, 21, 23, 25, 28, 31, 34, 37, 41, 45,
50, 55, 60, 66, 73, 80, 88, 97, 107, 118,
130, 143, 157, 173, 190, 209, 230, 253, 279, 307,
337, 371, 408, 449, 494, 544, 598, 658, 724, 796,
876, 963, 1060, 1166, 1282, 1411, 1552, 1707, 1878, 2066,
2272, 2499, 2749, 3024, 3327, 3660, 4026, 4428, 4871, 5358,
5894, 6484, 7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,
15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767
};
int16 output = 0;
int step_index = 0;
int step = step_table[step_index];
int16 decode(uint4 input)
{
step_index += index_table[input];
if (step_index > 89) step_index = 89;
if (step_index < 0) step_index = 0;
int delta = step >> 3; /* calc delta */
if (input & 4) delta += step;
if (input & 2) delta += (step >> 1);
if (input & 1) delta += (step >> 2);
if (input & 8) output -= delta; /* sign */
else output += delta;
step = step_table[step_index]; /* next step */
if (output > 32767) output = 32767;
if (output < -32768) output = -32768;
return output;
}