### One person's data is another person's noise

I know it’s been forever since I’ve done a post and I’m really sorry. I got caught up in the sound issues and panorama issues. I’m going to talk about sound in this post and then make another post about panoramas. So here we go!

“One person’s data is another person’s noise.”  ― K.C. Cole

This quote pretty much sums up my experiences with the sound decoding. I was somewhat lucky in that Marisa Chan’s source code had an implementation of sound decoding that I could model off of, but at the same time, the whole function was quite cryptic. This is mostly due to the variable “naming”. And I say “naming” in the loosest sense of the word, because most were single letters:

void adpcm8_decode(void *in, void *out, int8_t stereo, int32_t n)
{
uint8_t *m1;
uint16_t *m2;
m1 = (uint8_t *)in;
m2 = (uint16_t *)out;
uint32_t a, x, j = 0;
int32_t b, i, t = {0, 0, 0, 0};

while (n)
{
a = *m1;
i = t[j+2];
x = t2[i];
b = 0;

if(a & 0x40)
b += x;
if(a & 0x20)
b += x >> 1;
if(a & 0x10)
b += x >> 2;
if(a & 8)
b += x >> 3;
if(a & 4)
b += x >> 4;
if(a & 2)
b += x >> 5;
if(a & 1)
b += x >> 6;

if(a & 0x80)
b = -b;

b += t[j];

if(b > 32767)
b = 32767;
else if(b < -32768)
b = -32768;

i += t1[(a >> 4) & 7];

if(i < 0)
i = 0;
else if(i > 88)
i = 88;

t[j] = b;
t[j+2] = i;
j = (j + 1) & stereo;
*m2 = b;

m1++;
m2++;
n--;
}
}


No offense intended towards Marisa Chan, but that makes my eyes hurt. It made understanding the algorithm that much harder. But after talking to a couple people at ScummVM and Wikipedia-ing general sound decoding, I figured out the sound is encoded using a modified Microsoft Adaptive PCM. I’ll go ahead and post my implementation and then describe the process:

const int16 RawZorkStream::_stepAdjustmentTable = {-1, -1, -1, 1, 4, 7, 10, 12};

const int32 RawZorkStream::_amplitudeLookupTable = {0x0007, 0x0008, 0x0009, 0x000A, 0x000B, 0x000C, 0x000D, 0x000E,
0x0010, 0x0011, 0x0013, 0x0015, 0x0017, 0x0019, 0x001C, 0x001F,
0x0022, 0x0025, 0x0029, 0x002D, 0x0032, 0x0037, 0x003C, 0x0042,
0x0049, 0x0050, 0x0058, 0x0061, 0x006B, 0x0076, 0x0082, 0x008F,
0x009D, 0x00AD, 0x00BE, 0x00D1, 0x00E6, 0x00FD, 0x0117, 0x0133,
0x0151, 0x0173, 0x0198, 0x01C1, 0x01EE, 0x0220, 0x0256, 0x0292,
0x02D4, 0x031C, 0x036C, 0x03C3, 0x0424, 0x048E, 0x0502, 0x0583,
0x0610, 0x06AB, 0x0756, 0x0812, 0x08E0, 0x09C3, 0x0ABD, 0x0BD0,
0x0CFF, 0x0E4C, 0x0FBA, 0x114C, 0x1307, 0x14EE, 0x1706, 0x1954,
0x1BDC, 0x1EA5, 0x21B6, 0x2515, 0x28CA, 0x2CDF, 0x315B, 0x364B,
0x3BB9, 0x41B2, 0x4844, 0x4F7E, 0x5771, 0x602F, 0x69CE, 0x7462, 0x7FFF};

int RawZorkStream::readBuffer(int16 *buffer, const int numSamples) {

// 0: Left, 1: Right
byte channel = 0;

if (_stream->eos()) {
_endOfData = true;
}

int16 index = _lastSample[channel].index;
uint32 lookUpSample = _amplitudeLookupTable[index];

int32 sample = 0;

if (encodedSample & 0x40)
sample += lookUpSample;
if (encodedSample & 0x20)
sample += lookUpSample >> 1;
if (encodedSample & 0x10)
sample += lookUpSample >> 2;
if (encodedSample & 8)
sample += lookUpSample >> 3;
if (encodedSample & 4)
sample += lookUpSample >> 4;
if (encodedSample & 2)
sample += lookUpSample >> 5;
if (encodedSample & 1)
sample += lookUpSample >> 6;
if (encodedSample & 0x80)
sample = -sample;

sample += _lastSample[channel].sample;
sample = CLIP(sample, -32768, 32767);

index += _stepAdjustmentTable[(encodedSample >> 4) & 7];
index = CLIP<int16>(index, 0, 88);

_lastSample[channel].sample = sample;
_lastSample[channel].index = index;

// Increment and wrap the channel
channel = (channel + 1) & _stereo;
}
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