Listing 1: ADPCM Encoder
/* Table of index changes */
const char IndexTable[16] = {-1,-1,-1,-1,2,4,6,8,-1,-1,-1,-1,2,4,6,8};

/* Quantizer step size lookup table */
const long StepSizeTable[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};

signed int diff;               		// Diff. between sample and predicted sample
int step;                      		// Quantizer step size
signed int predsample,diffq;         		// ADPCM predictor output, Predicted diff.
char index;                      		// Index into step size table

char ADPCMEncoder( signed int sample )
{
	char code;               		// ADPCM output value

	predsample = state.prevsample;	// Restore previous values of predicted
	index = state.previndex;		// sample and quantizer step size index
	step = StepSizeTable[index];

	diff = sample - predsample;		// Compute diff. between actual sample
	if(diff >= 0)				// and the predicted sample
		code = 0;
	else
	{	code = 8;
		diff = -diff;
	}

	diffq = step >> 3;			// Quantize the diff. into 4-bit ADPCM code
	if( diff >= step )			// using the quantizer step size
	{	code |= 4;			// Inverse quantize the ADPCM code into a
		diff -= step;			// predicted diff. using the quantizer step
		diffq += step;			// size
	}
	step >>= 1;
	if( diff >= step )
	{	code |= 2;
		diff -= step;
		diffq += step;
	}
	step >>= 1;
	if( diff >= step )
	{	code |= 1;
		diffq += step;
	}

	if( code & 8 )				// Compute new predicted sample by adding
		predsample -= diffq;		// the old predicted sample to new diff.
	else
		predsample += diffq;
	if( predsample > 32767 )		// Check if overflow of the new pred. sample
		predsample = 32767;
	else if( predsample < -32768 )
		predsample = -32768;

	index += IndexTable[code];		// Find new quantizer stepsize
	if( index < 0 )			// Check if overflow of new quantizer step
		index = 0;
	if( index > 88 )
		index = 88;

	state.prevsample = predsample;	// Save values for next iteration
	state.previndex = index;
	return ( code & 0x0f );		// Return the ADPCM code
}