WSS File Format

Introduction

WSS files are used since OFP times to store sound data and its file format is pretty much like the RIFF WAVE file format used for .wav files.

For the purposes of description here, 'Sound' in both wav or wss is recorded as PCM data in 16 bit 'samples'. The quality of that sound is determined by the frequency of those 'samples' per second, referred to as the SampleRate.

Given this: a few important paramaters are define here:

• BitsPerSample: 16
• BytesPerSample: 2 (Inferred BitsPerSample/8)

File Format

Offset	Datatype	Content	Description
0	char[4]	"WSS0"	file signature
4	ulong	CompressionType	0 == none. 8== compressed PCM data
8	ushort	format	Always 1 (WAVE_FORMAT_PCM)
10	ushort	nChannels	1=mono, 2=stereo
12	ulong	SampleRate	e.g. 44100Hz
16	ulong	BytesPerSecond	SampleRate * BlockAlign
20	ushort	BlockAlign	nChannels * BytesPerSample
22	ushort	BitsPerSecond	usually 16
24	ushort	<unknown>	unknown value, not always present
26/24	byte[fileSize-26/24]	<soundData>	here the PCM data of the sound is stored

Decompression

Bis optionally use Delta8 (byte) or Delta4 (nibble) 'Lossy' Compression for mono pcm data only.

The facility exists to handle stereo compression but it isn't employed.

Byte Compression

Compression consists of single encoded byte 'samples' versus uncompressed short 'samples'. Each byte is, effectively, extrapolated to a short, thus making the compressed BYTE array, and the resulting decompressed SHORT array the same number of elements(length). The length is the remaining file length (in bytes) after the header.

C++ code

Function returns a short array, same size as it's compressed byte equivalent

#define LOG10	2.3025850929940456840	//ln(10)
#define LOG2	1.4426950408889634070	//log2(e)
#define MAGIC_NUMBER	((LOG10*LOG2)/28.12574042515172)

short* DeCompress(const char *CompressedData,int len)
{
        short *snap,*OutputData;

	if (!(snap=OutputData=new short[len]))  return 0;
	short LastVal=0;
	for (;len--;CompressedData++)
	{
		if (*CompressedData)
		{
			double asFloat = abs(*CompressedData) *MAGIC_NUMBER;
			double rnd = Round(asFloat);
			asFloat = pow(2.0, asFloat - rnd) * pow(2, rnd);// mantissa -
			if (*CompressedData < 0) asFloat *= -1;
			int asInt = Round(asFloat)+LastVal;
			if (asInt > SHRT_MAX ) asInt = SHRT_MAX ;
			if (asInt < SHRT_MIN) asInt = SHRT_MIN;
			LastVal=(short)asInt;
		 }
		*OutputData++ = LastVal;
	}
	return snap;
}

C# code

If the <soundData> is compressed the following (C#) code can be used for decompression:

PCMData = new Int16[soundData.Length];
for (int j = 0; j < PCMData.Length; j++)
{
  SByte srcSample = (SByte)soundData[j];
  if (srcSample != 0)
  {
    double asFloat = Math.Abs(srcSample) / 28.12574042515172;
    asFloat *= 2.3025850929940456840; //ln(10)
    asFloat *= 1.4426950408889634070; //log2(e)
    double rnd = Math.Round(asFloat);
    double mantisse = Math.Pow(2.0, asFloat - rnd);
    asFloat = mantisse * Math.Pow(2, rnd);
    if (srcSample < 0) asFloat *= -1;
    Int32 asInt = (int)Math.Round(asFloat);
    asInt = (j == 0) ? asInt : (asInt + PCMData[j - 1]);
    if (asInt > short.MaxValue) asInt = short.MaxValue;
    if (asInt < short.MinValue) asInt = short.MinValue;
    PCMData[j] = (Int16)asInt;
  }
  else PCMData[j] = (j == 0) ? (Int16)0 : PCMData[j - 1];
}

Nibble Compression

Although by no means the same thing, Nibble compression is an adaptation of IMA ADPCM compression. Each nibble represents a SHORT of PCMdata. Thus the resultant output size will be four times larger (in bytes).

C++ Code

//
// all honor and glory to T_D
//

static short PCMIndex[] = {-8192, -4096, -2048, -1024, -512, -256, -64, 0, 64, 256, 512, 1024, 2048, 4096, 8192};

static short limits(short PCM)
{
 if(PCM > SHRT_MAX) PCM = SHRT_MAX;
 else if(PCM < SHRT_MIN) PCM = SHRT_MIN;
 return PCM;
}

short * DeCompressNibble(const char *NibbleData,int len,short seed=0)//len in bytes
{
  short  *snap,*pcmData;
  if (!(snap=pcmData=new short[2*len]))  throw GENERR_NOMEM;
  delta=seed;
  while (len--)
  {   
    byte nibble=*NibbleData++;
   *pcmData++ = limits(delta += PCMIndex[nibble >> 4]);
   *pcmData++ = limits(delta += PCMIndex[nibble & 0x0F]);
  }
  return snap;
}