Listing 2:ADPCM Decoder

// This routine also uses the IndexTable and StepSizeTable from Listing 1
signed int ADPCMDecoder( char code )
{
	predsample = state.prevsample;	// Restore previous values
	index = state.previndex;

	step = StepSizeTable[index];		// Find quantizer step size

	diffq = step >> 3;			// Inverse quantize ADPCM code into a
	if( code & 4 )				// diff. using the quantizer step
		diffq += step;
	if( code & 2 )
		diffq += step >> 1;
	if( code & 1 )
		diffq += step >> 2;

	if( code & 8 )				// Add the difference to predicted sample
		predsample -= diffq;
	else
		predsample += diffq;

	if( predsample > 32767 )		// Check if overflow of new predicted sample
		predsample = 32767;
	else if( predsample < -32768 )
		predsample = -32768;

	index += IndexTable[code];		// Find new quantizer step size

	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( predsample );			// Return new sample

}